Final Thesis Report

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Confidential

Final Thesis Report

Generic tablet design guidelines

Mechanical aspects

Prodrive B.V.

Avans University of Applied Science

Document ID: Final Thesis Report - Frank Toenders (2011116) Arvid Wouters (2008635).docx Release date: 8 June 2011

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The contents of this document are confidential information and intellectual property of Prodrive B.V. Nothing of this document may be used, duplicated or published without prior written approval of Prodrive B.V.

Document History

Rel. Date Changes

R01 13-05-2011 First version

R02 17-05-2011 Various changes to content and document structure R03 25-05-2011 Various changes to content

R04 08-06-2011 Final version for review within Prodrive

Review List

Reviewers R01 R02 R03 R04 Nard Louws x x x x Dion Cornelissen x x x x Arvid Wouters x x x x Frank Toenders x x x x Rene Eras x

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Preface

This final thesis report has been conducted during a five month period by Frank Toenders and Arvid Wouters. By finalizing this final thesis period, our mechanical engineering education at Avans Hogeschool („s-Hertogenbosch) comes to an end.

This report is written for Prodrive B.V, located in Son, the Netherlands. Prodrive – founded in 1993 – started as electronics design firm specialized in digital signal processing and motion control. Nowadays, competences have broadened to full multidisciplinary system design and production. Prodrive delivers competitive solutions in electronic-, mechanic-, mechatronic- and software design. Prodrive develops products for a wide range of markets, e.g.:

 Industrial (e.g. Pick&Place robot controller)  Medical (e.g. X-ray controller)

 Consumer and multimedia (e.g. home automation, set-top box)

 Energy, transport and security (e.g. railway safety system, HDTV security camera) Today, approximately 400 employees (+300 full-time), mostly electrical-, mechanical- and software engineers are working together in cross-functional teams.

We would like to thank all employees of Prodrive for their help and support. Special thanks to Nard Louws and Dion Cornelissen for their assistance and for making our graduation period possible. We can look back at a successful and educational period at Prodrive.

Frank Toenders and Arvid Wouters Son, June 2011

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Summary

Tablets are comparable to regular notebooks. However, instead of using a traditional keyboard, mouse, trackball or touchpad, tablets use a touchscreen for user input. For some purposes, notebooks are impractical or unwieldy. Tablets, by contrast, are smaller, thinner and lighter. Tablets – also known as a “handhelds” or “slates” – can be used for both the consumer market and the professional market. The most well-known tablet is the “iPad”, produced by Apple. The demand for tablets is growing over the past few years. Tablets are expected to displace 10 percent of the personal computers by 2014. Prodrive B.V., a developer and manufacturer of innovative electronic solutions, observed this growing interest in tablets. Therefore, Prodrive is interested in mechanical design guidelines that cover reoccurring problems when designing a tablet.

The objective for this report is: “Creation of generic tablet design guidelines, focused on the mechanical aspects”.

The guidelines will cover reoccurring design problems in the design process, e.g.:  Touchscreen integration (e.g. bezel and bezel-less)

 Snap-fit attachments (e.g. for display, PCB, front- and rear section)  Methods for cooling

 Measuring ambient temperature  Stiffness

This final thesis report is divided into the following phases:  Pre-study phase

 Definition phase  Implementation phase

In the pre-study phase, different touchscreen technologies are examined by literature research and by several meetings with distributers/manufactures of touchscreens. In the definition phase, the specifications, boundary conditions and regulations which the tablet must meet are divined. In the implementation phase, several solutions and/or suggestions to the reoccurring design problems are proposed, based on simulation and analysis.

During the final thesis period, two tablets are developed in separate projects:  Project A: bezel design in combination with a resistive touchscreen

 Project B: bezel-less design in combination with a projected capacitive touchscreen

De development of these two projects is used to face design problems and to validate the design guidelines.

By the end the final thesis project, the following products are delivered to Prodrive:  A report that includes generic tablet design guidelines

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List of Figures (Contents)

FIGURE 1-1: A CLASSIC TABLET (LEFT) AND APPLE‟S IPAD (RIGHT) ... 11

FIGURE 1-2: DEVELOPMENT PROCESS (SIMPLIFIED VERSION) ... 13

FIGURE 4-1: A BEZEL AND A BEZEL-LESS DESIGN (SIMPLIFIED VIEW)... 17

FIGURE 4-2: A BEZEL DESIGN (DETAILED) ... 17

FIGURE 4-3: A BEZEL-LESS DESIGN (DETAILED) ... 18

FIGURE 4-4: FROM TOP TO BOTTOM; CONCEPT A - CONCEPT F ... 19

FIGURE 4-5: SOLUTIONS TO SOLVE THE TOLERANCES PROBLEM ... 21

FIGURE 4-6: CORRECTED TOLERANCE ... 21

FIGURE 4-7: COVER LAYER WILL BE APPLIED TO THE TOUCHSCREEN AND DISPLAY. ... 22

FIGURE 4-8: AN EXAMPLE OF A CANTILEVER HOOK SNAP-FIT ATTACHMENT (ONE SIDE SUPPORTED) . 24 FIGURE 4-9: GENERAL DIMENSIONS OF THE CANTILEVER HOOK ... 25

FIGURE 4-10: ASSEMBLY ANGLE ... 26

FIGURE 4-11: DEFLECTION AS RESULT OF PROPER DESIGNED ASSEMBLY ANGLE ... 26

FIGURE 4-12: A TOO LARGE ASSEMBLY ANGLE COULD LEAD TO ASSEMBLY PROBLEMS ... 26

FIGURE 4-13: RETAINING ANGLE ... 26

FIGURE 4-14: DEFLECTION AS RESULT OF PROPER DESIGNED RETAINING ANGLE ... 27

FIGURE 4-15: A TOO LARGE RETAINING ANGLE LEADS TO A NON-DETACHABLE SNAP-FIT ... 27

FIGURE 4-16: THE SNAP-FIT ATTACHMENT MAY RELEASE UNDER CERTAIN CONDITIONS ... 27

FIGURE 4-17: SNAP-FIT WITH A RADIUS OF 0.05MM (STRESS APPROX. 115 MPA) ... 30

FIGURE 4-18: SNAP-FIT WITH A RADIUS OF 0.5MM (STRESS APPROX. 55 MPA) ... 30

FIGURE 4-19: THE SITUATION IN WHERE THE SNAP-FIT ATTACHMENT MUST BE DESIGNED ... 31

FIGURE 4-20: CONCEPT A ... 31 FIGURE 4-21: CONCEPT B ... 32 FIGURE 4-22: CONCEPT C... 32 FIGURE 4-23: CONCEPT D... 32 FIGURE 4-24: CONCEPT E ... 33 FIGURE 4-25: CONCEPT F ... 33

FIGURE 4-26: FINAL SNAP-FIT DESIGN ... 34

FIGURE 4-27: VARIOUS COOLING CONCEPTS ... 35

FIGURE 4-28: POWER CONSUMPTION TABLET ... 36

FIGURE 4-29: THERMAL IMAGE SIMILAR TABLET ... 37

FIGURE 4-30: ROUGHLY MAPPED PCB ... 37

FIGURE 4-31: EXPLODED VIEW DISPLAY ... 38

FIGURE 4-32: MODEL FIELD TEST ... 39

FIGURE 4-33: FIELD TEST ... 39

FIGURE 4-34: NODAL TEMPERATURES FIELD TEST 2 ... 39

FIGURE 4-35: AIR VELOCITY FIELD TEST 2 ... 39

FIGURE 4-36: THERMAL CONDUCTIVITY SCREEN ... 40

FIGURE 4-37: VENTING HOLES (PROJECT A) ... 41

FIGURE 4-38: SIMULATION RESULTS COOLING CONCEPT A ... 41

FIGURE 4-39: SIMULATION RESULTS COOLING CONCEPT C ... 42

FIGURE 4-40: CONCEPT A, VENTING HOLES (1.8MM WIDE) ... 43

FIGURE 4-41: CONCEPT B, VENTING HOLES (2.8MM WIDE) ... 43

FIGURE 4-42: CONCEPT C, VENTING HOLES ON MULTIPLE LOCATIONS (1.8MM WIDE)) ... 43

FIGURE 4-43: TEMPERATURE MEASUREMENT CONCEPTS ... 44

FIGURE 4-44: SIMULATION RESULTS TEMPERATURE MEASURING CONCEPTS ... 45

FIGURE 4-45: CROSS-SECTION OF 6 LAYER PCB... 46

FIGURE 4-46: BASELINE ... 47

FIGURE 4-47: CONCEPT 1 ... 47

FIGURE 4-51: DIFFERENT CONCEPTS FOR CONNECTION. ... 49

FIGURE 4-52: AN ADVANTAGE OF CONCEPT C. ... 49

FIGURE 4-53: A DISADVANTAGE OF CONCEPT C. ... 49

FIGURE 4-54: FROM LEFT TO RIGHT; CONCEPT A, B, C, D AND E ... 50

FIGURE 5-1: FRONT VIEW TABLET ... 51

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FIGURE 5-3: TEMPERATURE SENSORS ... 51

FIGURE 5-4: WIFI AND Z-WAVE ANTENNA... 51

FIGURE 5-5: CONNECTION HOUSING ... 52

FIGURE 5-6: CONNECTION HOUSING ... 52

FIGURE 5-7: OUTER LOOK OF THE TABLET ... 52

FIGURE 5-8: TWO CROSS-SECTION VIEWS ... 53

FIGURE 5-9: DISPLAY/TOUCHSCREEN COMBINATION (LEFT) AND THE COVER SCREEN (RIGHT) ... 53

FIGURE 5-10: FRONT SECTION OF THE HOUSING ... 53

FIGURE 5-11: DETAIL VIEWS OF THE SNAP-FIT ATTACHMENT FOR FIXATION OF THE PCB ... 53

FIGURE 5-12: REAR SECTION OF THE HOUSING ... 54

List of Figures (Appendix)

FIGURE A-1: RESISTIVE TOUCHSCREEN TECHNOLOGY ... 58

FIGURE A-2: SURFACE CAPACITIVE TECHNOLOGY ... 58

FIGURE A-3: PROJECTED CAPACITIVE TECHNOLOGY ... 58

FIGURE B-4: THE MOST COMMON USED GESTURES THAT ARE SUPPORTED BY MULTI-TOUCH. ... 64

FIGURE C-5: BEZEL-LESS DESIGN WITH A RESISTIVE TOUCHSCREEN. ... 65

FIGURE E-6: THE CANTILEVER HOOK CAN BE CONSIDERED AS A SIMPLE RECTANGULAR BEAM. ... 67

FIGURE E-7: DEFLECTION OF THE WALL; “DEFLECTION MAGNIFICATION” (RIGHT) ... 68

FIGURE E-8: THE UNDERCUT HEIGHT OF THE CANTILEVER BEAM. ... 71

FIGURE E-9: DEFLECTION OF A BEAM. ... 71

FIGURE E-10: THE COMPENSATION FACTOR AS A FUNCTION OF THE ASSEMBLY/RETAINING ANGLE. . 74

FIGURE E-11: A FREE BODY DIAGRAM WITH AN ASSEMBLY ANGLE OF 70°. ... 75

FIGURE E-12: A FREE BODY DIAGRAM WITH AN ASSEMBLY ANGLE OF 30°. ... 75

FIGURE F-13: GENERAL DIMENSIONS OF THE I-BEAM. ... 77

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List of Tables (Contents)

TABLE 1-1: BOOKS THAT ARE USED AS REFERENCE. ... 9

TABLE 1-2: WEBSITES THAT ARE USED AS REFERENCE. ... 9

TABLE 1-3: PDF DOCUMENTS THAT ARE USED AS REFERENCE1. ... 9

TABLE 2-1: COMPARISON BETWEEN TOUCHSCREEN TECHNOLOGIES ... 14

TABLE 2-2: COMPARISON BETWEEN SINGLE-, DUAL- AND MULTI-TOUCH ... 14

TABLE 3-1: FUNCTIONAL SPECIFICATIONS... 15

TABLE 3-2: NON-FUNCTIONAL SPECIFICATIONS ... 15

TABLE 4-1: CONCEPT COMPARISON ... 20

TABLE 4-2: COMPARISON BETWEEN PET- AND GLASS MATERIAL ... 22

TABLE 4-3: COMPARISON BETWEEN DIFFERENT (OPTICAL ANTI-REFLECTIVE) COATINGS ... 23

TABLE 4-4: ABBREVIATIONS FOR FIGURE 4-9 ... 25

TABLE 4-5: ABBREVIATIONS FOR EQUATION 4-1 ... 27

TABLE 4-6: COOLING CONCEPTS ... 36

TABLE 4-7: ABBREVIATIONS FOR EQUATION 4-2 ... 38

TABLE 4-8: SIMULATION RESULTS COOLING CONCEPT A ... 41

TABLE 4-9: SIMULATION RESULTS COOLING CONCEPT C ... 42

TABLE 4-10: SENSOR TEMPERATURES ... 45

TABLE 4-11: COPPER LAYER STACK UP ... 46

TABLE 4-12: MATERIAL PROPERTIES PCB ... 46

TABLE 4-13: ISOLATION METHODS ... 46

TABLE 4-14: SENSOR TEMPERATURES ... 47

TABLE 4-15: ADVANTAGES AND DISADVANTAGES DIFFERENT ALIGN METHODS ... 49

TABLE 4-16: ADVANTAGES AND DISADVANTAGES OF DIFFERENT POSITIONS ... 50

List of Tables (Appendix)

TABLE E-1: ABBREVIATIONS FOR EQUATION E-3. ... 68

TABLE E-2: VALUES FOR DEFLECTION MAGNIFICATION . ... 69

TABLE E-9: ABBREVIATIONS FOR EQUATION E-21 AND EQUATION E-22. ... 73

TABLE E-10: ABBREVIATIONS FOR FIGURE E-11 AND FIGURE E-12. ... 76

TABLE E-11: ABBREVIATIONS FOR EQUATION E-23 ... 76

TABLE F-12: ABBREVIATIONS FOR EQUATION F-24. ... 77

TABLE F-13: ABBREVIATIONS FOR EQUATION F-26. ... 77

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References

Table 1-1: Books that are used as reference.

No. Title Author/company ISBN Remarks

1.01 The first snap-fit handbook Paul Bonenberger 1-56990-388-3 2nd. edition 1.02 Sterkteleer Russell C. Hibbeler 978-90-430-1079-5 2nd. edition

Table 1-2: Websites that are used as reference.

No. Author/company Address

2.01 Danielson Europe B.V. http://www.danielsoneurope.nl 2.02 Dynapro/3M http://www.3m.com

2.03 Inotouch Technology Co., Ltd. http://www.inotouch.co.kr 2.04 EE Times design http://www.eetimes.com

2.05 Wikipedia - Emissivity http://en.wikipedia.org/wiki/Emissivity

2.06 Efunda – Radiation view factors http://www.efunda.com/formulae/heat_transfer/radiation/view_factors.cfm 2.07 Engineering toolbox – Emissivity

coefficients

http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html 2.08 Thermoworks – Emissivity

coefficients

http://www.thermoworks.com/emissivity_table.html

Table 1-3: PDF documents that are used as reference.

No. Author/company Description

3.01 Danielson Europe B.V. touch_screen_integration_guide_danielson.pdf 3.02 Dynapro/3M touch_screen_integration_guide_dynapro.pdf 3.03 Inotouch Technology Co., Ltd. touch_screen_integration_guide_inotouch_1.pdf

touch_screen_integration_guide_inotouch_2.pdf 3.04 EE Times design touch_screen_practical_considerations_1.pdf

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Abbreviations

AA CAD DS-tape DT EDD FEM LCD MT OD PC-ABS PCB PSA PSD QRD ITO SPD ST TBD TS VA Active Area Computer-Aided Design Double-Sided Tape Dual-Touch

Engineering Design Document Finite Element Method

Liquid Crystal Display Multi-Touch

Outer Dimensions

Poly Carbonate - Acrylonitrile Butadiene Styrene Printed Circuit Board

Pressure Sensitive Adhesives Pre-Study Document

Quality Results Document Indium Tin Oxide

Specification Document Single-Touch

To Be Defined Touchscreen Viewing Area

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1. Introduction

1.1. Background

In 2000, the term "tablet" was firstly introduced by Microsoft described as: "A full-function

operating system-based PC incorporating the convenient and intuitive aspects of pencil and paper into the PC experience".

Originally, the tablet-PC was based in on a notebook, equipped with a touchscreen which could be operated with a stylus. The display could be rotated to transform the notebook into a tablet-PC (Figure 1-1, left).

Nowadays, the classic tablet-PC is replaced by the more popular “slate” tablet (Figure 1-1, right). The keyboard is fully replaced by the touchscreen and can be operated with a finger or with a stylus (depending on the touchscreen technology that is used). Tablets are more and more used within the consumer- and professional market, e.g. for climate control, e-reading, browsing, multimedia applications or to operate domestic devices.

Figure 1-1: A classic tablet (left) and Apple’s iPad (right)

1.2. Problem definition

The demand for tablets is rapidly growing. Prodrive B.V. has observed this growing interest. During the design process of these tablets, many reoccurring design problems need to be tackled. Generic design guidelines are used to solve these problems which optimize the design process. This results in shorter development time and is important in today‟s fast moving market. The objective for this report is: “Creation of generic tablet design guidelines, focused on the

mechanical aspects”.

This final thesis project is a research assignment, supported by practical examples. The guidelines will be validated by the development of two tablets for two individual projects, which will be further explained in paragraph 3.1.

The guidelines will cover reoccurring design problems in the design process and are focused on themes drafted by Prodrive B.V. e.g.:

 Touchscreen integration (e.g. bezel and bezel-less)

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1.3. Structure of this document

The structure of this document is based on the Development Process used by engineers within Prodrive.

This report will focus on the following phases within the Development Process: 1) Pre-study phase

1.1) Pre-study

2) Definition phase

2.1) Definitions

3) Implementation phase

3.1) Engineering design process 3.2) Prototype design process 3.3) Qualification process

A simplified version of the Development Process can be seen on the next page in Figure 1-2.

1.1) Pre-study

 This phase is used to explore the background of the problem or to get more familiar with unknown technologies. The output of the pre-study phase is the pre-study document (PSD). The PSD document can be used as input for the definition phase.

2.1) Definitions

 This phase is used to define specifications. Project requirements are translated into specifications that the product must meet. The output of the specification phase is the specification document (SPD). The SPD document is used as input for the engineering design process.

3.1) Engineering design process

 In this process, the design concepts are created. The concept will be analyzed, for example analyzing strengths. The output of the engineering design process is the engineering design document (EDD) and a 3D CAD model. The EDD document and the 3D CAD model is used as input for the prototype design process.

3.2) Prototype design process

 In this process, a 3D CAD model is created that is suitable for prototyping. The output of the prototype design phase is a final 3D CAD model and a prototype. The prototype and the 3D CAD model is used as input for the qualifications process.

3.3) Qualification process

 In this process, the prototype is compared with the requirements specified in the SPD and the design results specified in the EDD. The output of the qualification phase is the qualification results document (QRD). After the qualification process, the implementation phase can be started over again in case of insufficient results.

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2. Pre-study phase

2.1. Background

The pre-study phase is meant to gain knowledge about the subject and/or to examine unknown technologies. Pre-studies can be useful to find the essence of one or multiple problems. For this final thesis project, the pre-study phase is used to gain more knowledge about touchscreen gestures and different touchscreen technologies that are available on the market.

2.2. Touchscreen technologies

Commonly, the following touchscreen technologies are used within tablets:  Resistive (based on pressure applied by the user to the TS)

 Projected capacitive (based on electrical charge transferred from the user to the TS)

Each technology has its advantages and disadvantages. A summarize of this comparison can be found in Table 2-1.

Table 2-1: Comparison between touchscreen technologies

Item Resistive (5-wire) Projected capacitive

Typical product example TomTom, HTC Touch Diamond iPad, BlackBerry Torch

Common sizes 1.7" to 24" Up to 10"

Endurance Approx. 30 million touches Approx. 100 million touches

Prone to breakage No (PET) Yes (glass)

Scratch resistant (without coating) Typically poor Typically very good Light transmission Approx. 81% - 85% Approx. 90% - 92%

User input Anything Conductive

Single-touch gesture Yes Yes

Dual-touch gestures No Yes

Multi-touch gestures No Yes

Visibility indoor Typically very good Typically very good Visibility direct sunlight Typically poor Typically very good

Monitor option CRT, LCD CRT, LCD, LED

Humidity level Any At least 5%

Calibration Needed regularly Not needed

Contamination resistance (functional degradation)

Resists moisture, dirt, dust, vinegar, ammonia, based food products and cleansers

May be affected by dust, vinegar, ammonia, based food products and cleansers

Note that there are many more different touchscreen technologies available. However, these technologies are not suitable to use within a tablet. Additional information about touchscreen technologies can be found in Appendix A.

Note that not every touchscreen technology supports all types of gestures (Table 2-1). A gesture is a movement of a finger (or stylus) for the purpose of communication.

Touchscreens gestures can be divided into the following three types:  Single-touch

 Dual-touch  Multi-touch

Table 2-2: Comparison between single-, dual- and multi-touch

Type of gesture Possible touch events Typical application

Single-touch 1 TomTom

Dual-touch 1 or 2 BlackBerry Storm/Torch

Multi-touch 1,2 or 3+ iPhone/iPad

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3. Definition phase

3.1. Background

Specifications have been defined for a generic tablet, which can be found in this chapter. During the final thesis period two tablet projects have been developed. By developing these two tablets, the design problems that occurred are used to establish the design guidelines.

The major difference between the two tablets is the type of touchscreen technology that is used and method of touchscreen integration:

 Project A: Bezel design with a resistive touchscreen

 Project B: Bezel-less with a projected capacitive touchscreen

Moreover, both designs feature project-specific design issues. For example, connector positions, specific mounting methods etc. Therefore, project-specific design issues are listed separately. The specifications are primary divined by Prodrive. Note that some specifications such as height, weight and depth are assumed or divined by an external design agency.

3.2. Specifications

Specifications can be divided into functional (Table 3-1) and non-functional (Table 3-2).

Table 3-1: Functional specifications

Specification Generic Project A Project B Priority

Display size 4-10" 7" 7" 1

Screen integration Bezel Bezel-less 1

Touchscreen technology Resistive Proj. capacitive 1

Max .height 120m 120m 2 Max. width 200mm 200mm 2 Max. depth 32mm 32mm 1 Max. weight 400g 400g 3 Quantity (products/year) 100.000 10.000 1 Speaker 1x 1x 3 USB connector 1x - 3 RJ45 connector 1x 1x 3 Power connector 1x 1x 3 Reset button 1x - 3 Wifi antenna 2x - 3 Z-wave antenna 1x - 3

Ambient temp. measurement Yes - 2

Table 3-2: Non-functional specifications

Specification Sub-item Generic and Project A/B

Scratch resistant screen Resistant against scratches e.g. finger nails Yes Design for manufacturing Fast and easy assembly during production Yes Possibility to disassemble during service Yes

Simple injection molding tooling Yes

Cost Use inexpensive materials and products Yes

Inexpensive injection molding tooling Yes Mechanical Sufficient stiffness of housing and connections Yes Acceptable temperatures inside the housing Yes

Shock and vibration Yes

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3.3. Regulations (norms)

There are multiple regulations a tablet may need to meet. Commonly used regulations are presented below. Only the mechanical related and most important regulations are shown.

General safety (60950)

0.2.02 Prevent short-circuit which can lead to burns and ejection of molten metal 0.2.03 Prevent fire spread from inside to outside, prevent damage to surroundings 4.2.02 Apply steady force of 10N to every part that is not an enclosure

4.2.04 Apply steady force of 250N for five seconds to the top, bottom and sides of the enclosure with a circular 30mm test tool

4.2.06 Hand-held equipment drop from 1000mm on hardwood

4.2.07 Check for hazardous part exposure after exposure of seven hour to temperature of 10°C higher than the measured temp. (not less than 70°C)

4.2.10 Wall mounted equipment should be able to withstand three times their weight for 1 minute (not less than 50N)

4.3.01 Round hazardous corners and edges

4.5.04 Max. temp. continuously touched handles, grips, nobs plastic 75°C

Audio, video and similar electronic equipment (60065)

7.1 After 4h of operation:

Accessible non-metallic parts max 60°C Lithium batteries max 40°C

12.1 Vibration test 0.35mm, 10Hz-55Hz-10Hz, 30min

Impact test commercial use, 500g 50mm steel ball,7.0 joules impact

Industrial use (IEC 60065)

7.1 Protection against heat (housing 60°C, batteries 40°C) Vibration test (30min, 0.35mm amplitude, 10-55Hz) Impact test (7J, 500g steel ball)

Drop test (3 impacts, 1.0m)

IP54

4.2 Dust-protected (ingress of dust is not totally prevented but limited to small amount) splashing-protected (water splashed from any direction shall not have harmful effects, 10/min, 5min)

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4. Engineering design process

4.1. Background

In this chapter, suggestions can be found to solve reoccurring design problems. This chapter will focus on the following design issues:

 Touchscreen integration (e.g. bezel vs. bezel-less)

 Snap-fit attachments (e.g. for display, PCB, front- and rear section)  Methods for cooling

 Measuring ambient temperature  Stiffness

4.2. Touch screen integration

There are two methods in which the touchscreen can be integrated into the tablet:  Bezel design (Figure 4-1, left)

 Bezel-less design (Figure 4-1, right)

Figure 4-1: A bezel and a bezel-less design (simplified view)

Some design considerations must be made when integrating the touchscreen into the housing. These design considerations (e.g. layer build up) varies from a bezel to bezel-less design and will be therefore discussed individually in respectively paragraph 4.2.1 and paragraph 4.2.2.

4.2.1. Bezel design

Figure 4-2: A bezel design (detailed)

In case of a bezel design, the touchscreen is clamped between the bezel and the housing. Optional, a mounting bracket can be used. A gasket secures the touchscreen in place and provides an environmental seal.

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The amount of minimum clearance varies from manufacturer to manufacturer while touchscreen dimensions will vary slightly due to inherent inaccuracies of the cutting process (especially with glass). Special attention is needed to the corners, as these are most prone to cutting

inaccuracies. Datasheets for the specific touchscreen that is used should be consulted. It is recommended to support and hold the touchscreen in place from the back, using mounting brackets, a clamping plate or the shape of the housing.

 Failure to support the touchscreen from the back can result in buckling of one or multiple layers when pressure to the touchscreen will be applied. It is not recommend using the display as a direct clamp, which may result in damage to the display.

Bezels, especially made from thermoplastic material, are sensitive to distortion or deformation.  The bezel should be stiff enough so that it does not distort or deform when clamping

pressure is applied or when exposed to the tablet‟s operating temperature range.  The bezel‟s inner side must have a smooth surface.

The touchscreen may expand or contract due to varying environmental conditions.  It is not recommend mounting the touchscreen directly to the bezel by means of an

adhesive. This can cause permanent damage (e.g., bagging, stretching, pillowing) to the touchscreen.

 It is not recommended to fill the gap between the housing and the touchscreen with any product, such as a structural adhesive, rubber or foam.

Gasket

Rubber or foamed gaskets ensure that the screen cannot be displaced. Moreover, it contributes to an environmentally seal.

 The gasket‟s material must be conformable and compressible, pH neutral and should not have chemically active corrosive and/or recycled materials made from unknown elements.  It is not recommended to use gaskets containing silicone. There is the possibility of silicone leaching from the gasket and its reactivity creating the potential for damage to the electrical connections within the touchscreen.

 The gaskets must be recessed and should not be visible from the front. Non-recessed gasket leads to a short circuit on a touchscreen.

The gasket and housing are fixed together, using a double-sided tape.

 It is recommended to use a double-side tape strip that is narrower than the width of the gasket itself.

4.2.2. Bezel-less design

Figure 4-3: A bezel-less design (detailed)

In case of a bezel-less design, the touchscreen is mounted from the front into housing. A graphic layer is applied which results in a uniformly flat surface and a smooth design.

This paragraph will focus on a bezel-less design in combination with a projected capacitive touchscreen. Background information and design considerations for a bezel-less design in combination with a resistive touchscreen can be found in Appendix C.

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Mounting

The touchscreen and display should be firmly integrated into the housing. Several mounting concepts have been analysed.

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For concept A, the touchscreen and display are firmly attached to the cover layer. The cover layer is fixed to the housings front section by double-sided tape. The double-sided tape between the display and the housing is option and will be explained later on in further detail.

For concept B, the display is supported from the back by a mounting bracket fixed with screws. A major drawback for this mounting concept is high costs (screws) and extra components. For concept C, the display is supported from the back by the housings rear section. No extra components or screws are needed in comparison to concept B. However, this concept is not preferred while it might be difficult to assembly (assembly order is pre-divined).

For concept D, the display and touchscreen are fixed by double-sided tape to the underside of the housings front section. A major disadvantage is the gap between the touchscreen and the cover layer, which leads to reflections.

For concept E, the display and touchscreen are fixed by double-sided tape into the housings front section. Concept E is basically equal to concept D. However, in some cases the

touchscreen has smaller dimensions than the display. This advantage could be used to eliminate the gap between the touchscreen and the cover layer. Note that in most cases, the touchscreen has almost equal dimensions as the display.

For concept F, the display is supported from the back by the PCB fixed with screws. Concept F is basically equal to concept B. However, instead of using a mounting bracket, the PCB is used. A major drawback for this mounting concept is high costs (screws) and possible damage to the PCB is case of vibrations etc.

Table 4-1: Concept comparison

Item

Concept A B C D E F

Costs + - + + + -

Design for manufacturing (assembly) + - + +/- +/- -

Robustness ++ +/- +/- - ++ -

Concept A is the best solution to integrate the display and the touchscreen into the housing. The main advantage of this concept is its simple design which makes it easy for assembly.

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Tolerances

The display and touchscreen will be pre-assembled. The distance between the cover layer and the pre-assembled display and touchscreen should be minimized. An air gap between display leads to reflections and will affect outdoor visibility.

To mount the cover layer and the pre-assembled display and touchscreen together, double-sided tape will be used. Additional information about optical bonding can be found in Appendix

D.

A problem that occurs within concept A is the sum of tolerances. The construction is over dimensioned.

Figure 4-5: Solutions to solve the tolerances problem

Double sided tape between the cover layer and the pre-assembled display and touchscreen is probably strong enough to keep both components together (Figure 4-5, left). Optional, an adhesive/material can be applied from the back in case the double-sided tape is not sufficient

(Figure 4-5, right).

The total thickness can be calculated as follows:

The total tolerance can be calculated as follows (worst-case scenario):

In conclusion:

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Cover layer for projected capacitive touchscreens

A smooth design (e.g. iPad-look) can be achieved by applying a cover layer to the projected capacitive touchscreen. Besides the smooth design, it improves the appearance of the display and it protects the display against scratches and/or chemicals. This cover layer could contain rounding‟s and chamfered edges and a full color decorative print. The decorative print is applied to the bottom of the cover layer.

Figure 4-7: Cover layer will be applied to the touchscreen and display.

The cover layer cannot be made of any conductive material. The use of a conductive material will shield the electrical field of the capacitive sensors and drastically reduce the sensing performance. Commonly, glass or PET material is used as cover layer. Which material is more suitable depends on the type of application and the design of the decorative print of the cover layer. A comparison between both materials can be found in Table 4-2.

Table 4-2: Comparison between PET- and glass material

Item PET Glass

Suitable for simple, rectangular designs + +

Suitable for complicated design (e.g. rounding‟s, chamfered edges etc.) + -

Overall “looks and feel” + ++

Weight + -

Prone to breakage + -

Prone to scratches (without any additional hard coating) - +

Post-processing (e.g. chamfer) + -

Thermal expension coefficient compatible with the housing + - Using a glass material may be a better solution for simple designs where a weight is not an issue. The glass is usually chemically strengthened to reduce the chance of breaking. For all other cases, a PET material is recommended. For project B, a PET material will be used. The thickness of the cover layer and the dielectric constant has major influence on the performance of the touchscreen. For best performance a thin cover layer and a high dielectric constant is recommended.

Thickness of the cover layer:

 A cover layer made of PET material is typically 3mm thick.  A cover layer made of glass material is typically 0.5 to 2mm thick.

Cover layer

Touch screen Display

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Coatings

Applying a (optical) coating to the glass or PET cover layer leads to several benefits. Three kinds of coatings are commonly used, a clear coating, a non-glare (mat) coating and an anti-reflection (AR) coating.

A clear coating is the entry-level coating. It is a good solution for indoor (without direct sunlight) applications.

Non-glare coating results into a significant reduction of light reflections. Non-glare coatings are typically used for displays in a wide range of indoor and outdoor applications. A major advantage of mat coatings is that finger prints are barely visible. A drawback of this coating is the optical performance, which is significant lower in comparison to clear or AR coating.

For a maximum reduction of light reflections, an AR coating is recommended. The AR coating works by applying a series of very thin layers of metal oxides. This results in an improvement of clarity and light transmission, and reduction of reflections. AR is especially suited for outdoor applications exposed to direct sunlight. The costs for an AR coating are approximately three times higher than a standard clear coating.

Table 4-3: Comparison between different (optical anti-reflective) coatings

Item

Type of coating Clear Non-glare AR

Optical performance + +/- ++

Anti-reflection - + ++

Finger prints - + -

Chemical resistant + + +

Costs + +/- -

Additionally, a hard coating can be applied to the cover layer which protects the cover layer against scratches. The scratch resistance of a touchscreen is defined by a pencil hardness test. Pencil hardness is a relative expression of a surface‟s ability to withstand scratching. The test is performed by pushing graphite pencils with varying hardness (9H-9B) over the surface under constant pressure and in pre-defined angle. The “H” stands for hardness, the “B” stands for blackness.

 9H: Hardest graphite pencil  HB: Average graphite pencil  9B: Softest graphite pencil

The hardest graphite pencil, which does not scratch the surface, defines the pencil hardness of the surface under test. A typical hard coating withstands a pencil hardness of 9H. Operating the tablet with a finger nail can already lead to scratches without a hard coating. Therefore, a hard coating applied to the cover layer is recommended.

Commonly, the optical anti-reflective coating is applied to both the cover and bottom face of the cover layer. The hard coating is applied to the top face only.

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4.3. Snap-fit attachments

This chapter will focus on the possibilities to assemble components within the tablet, e.g. how to attach the front- and rear housing section together. Attachment methods are:

 Mechanical attachments (e.g. snap-fits, rivets, screws)  Chemical attachments (e.g. adhesives)

 Physical attachments (e.g. by welding or soldering)

Both chemical- and physical attachment are not ideal for products produced in a high quantity. Major disadvantages are assembly time, assembly quality, etc. Several mechanical attachments are not suitable or unwanted either, for example screws. One normal-sized screw costs

approximately € 0,05. Assembly-, material and logistic costs are included in this price. Although this looks negligible, with a production quantity of approximately 100.000 pieces the total costs are € 5000,00 for using only one screw. Other drawbacks of screws are unpredictable assembly quality and extra logistic concerns.

An interesting mechanical attachment is a snap-fit attachment. A snap-fit is an economical and quick method to fix components in a certain position.

Advantages of snap-fit attachments:

 Easy and quick method of joining components together  Reduces assembly time and costs significantly

 No need of additional fastening (such as screws, welding or adhesives)  The possibility to assemble and disassemble the attachment

 Environmentally friendly (e.g. in comparison to glue) Snap-fit attachments inside a tablet can be used for:

 Fixation of the front- and back housing sections together  Fixation of the PCB

 Fixation of the screen  Fixation of the speaker

A literature study is performed concerning snap-fit attachments. Recommendations and guidelines are summarized in the following paragraph. Later on, these guidelines will be validated by finite element analyses in paragraph 4.3.2.

4.3.1. Background

Figure 4-8: An example of a cantilever hook snap-fit attachment (one side supported)

Many different types of snap-fits attachment are available, e.g. cantilever hook (also known as “barbed leg”), cylindrical and ball-socket snap-fit attachments. The cantilever hook (Figure 4-8) is the most used type of snap-fit attachment.

Cantilever hooks snap-fit attachments can be diveded into two types:

 One side support, the beam of the cantilever hook will deflect during assembling and disassembling while the other parent component is assumed to be infinitely stiff

 Two sided support, the beam is assumed to be infinitely stiff while the other component will deflect during assembling and disassembling.

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Snap-fit attachment theory is rather complicated and comprehensive. This paragraph will only focus on a one-side supported cantilever hook snap-fit attachment to fix the PCB. The guidelines in this paragraph help to improve the snap-fit design. These design guidelines can only be used to determine the initial dimensions of the snap-fit.

Figure 4-9: General dimensions of the cantilever hook Table 4-4: Abbreviations for Figure 4-9

Abbreviation Description Length Length (head) Length (beam) Length (effective) Thickness wall

Thickness beam (wall side) Thickness beam (head side)

Radius at the beam to wall intersection Width beam (wall side)

Width beam (head side) Deflection (“undercut height”)

Angle assembly

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Beam thickness

 When the beam protrudes from a wall, the beam thickness at the wall side should be about 50% to 60% of the total wall thickness

 Beams thinner than 50% could lead to filling and/or flow problems

 Beams thicker than 60% could lead to cooling problems and/or sink marks

Beam length

 A beam length between and is preferred

 A beam length of could lead to shear effects and/or bending stress concentrations at the wall side

 A beam length of could lead to warpage and/or filling problems

Assembly angle

 The greater the assembly angle , the greater the required assembly force  An assembly angle is recommended

 An assembly angle could lead to assembly problems

Figure 4-10: Assembly angle

Figure 4-11: Deflection as result of proper designed assembly angle

Figure 4-12: A too large assembly angle could lead to assembly problems Retaining angle

A major benefit of snap-fit attachments is the ease of assembly and disassembly of components. Depending on the design of the retaining angle , the snap-fit is detachable or non-detachable.

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Figure 4-14: Deflection as result of proper designed retaining angle

Figure 4-15: A too large retaining angle leads to a non-detachable snap-fit

Figure 4-16: The snap-fit attachment may release under certain conditions  The greater the retaining angle , the greater the required disassembly force

 A retaining angle is recommended for a detachable snap-fit without external load  A retaining angle is recommended for a detachable snap-fit with a relative low

external load applied

 A retaining angle is recommended for a non-detachable snap-fit or detachable with a relative high external load applied

In our case, the front- and back housing components will be rarely opened and will be only opened for service. Moreover, the PCB will be rarely removed. It is assumed that the housing will be opened approximately five times during a lifecycle. For this situation, a retaining angle of approximately is recommended. Note that non-detachable attachments may release under certain conditions when external loads are applied (Figure 4-16).

For a detachable snap-fit, it is recommended to test each individual case the permanent load which is required for assembly and disassembly. The exact assembly and retaining angles depends on many factors.

Threshold angle

For a non-detachable snap-fit attachment without any external forces, the retaining angle should be close to 90°. Any angle above 80° will behave like a 90° angle due to the friction between the feature contact surfaces, better known as the threshold angle . The

threshold angle will be used for snap-fit analysis later on in this document.

The threshold angle can be divined as: Equation 4-1

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Beam width

 To use beam theory, the beam width should be less than or equal to the beam length

Strain at the base

To reduce the strain at the base:

 It is recommended to decrease the thickness of the beam  It is recommended to increase the length of the beam

Both parameters are represented cubed in the equitation for allowable strain. Therefore, the above two recommendations are an effect method to reduce the amount of strain. Another effective way to reduce the strain at the base is to tapering the beam over its length. The strain will distribute more evenly through the beam.

 Common taper ratios ( ) range from (1.25:1) to (2:1)

Tapering beams on width ( is less effective; it is proportional to the deflection of the snap-fit attachment. A beam must have a (4:1) taper width for the level of same strain reduction as a beam with (2:1) taper in thickness.

For both projects, the housing components – which include the snap-fits – are made by injection molding (material: PC-ABS). All faces will have a draft angle (approximately 1° to 3°) which can also be considered as a taper ratio.

Stress concentrations

The most common cause of fracture of a snap-fit is the stress concentration located at base of the beam. Brittle materials are especially sensitive to fracture caused by stress concentrations. Ductile materials, by contrast will first plastic deform before fracture. Stress concentrations can be reduced by avoiding abrupt transitions between the snap-fit beam and the solid underground or wall. It is recommended using a radius.

 It is recommended using a radius of 50% of the thickness of the beam.

 It is not recommend using a greater radius of 50%, which may lead to sink marks.

Mold design considerations

For a first injection molded prototype, safe tolerances are recommended. By removing material out of the mold, the final production will contain more thermoplastic material. By this method, a snap-fit can be made stiffer if needed. Making a snap-fit attachment less-stiff requires more material in the mold. This is solved by means of welding, but this is not desirable.

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Material

Although snap-fits are not limited to thermoplastic components, thermoplastic material is ideal for snap-fit attachments due to the materials high flexibility and the ability to be easily and inexpensively molded into complex geometries. Other advantages include the relatively high elongation, low coefficient of friction and sufficient strength and rigidity.

Analyses in where thermoplastics are used are often based on three assumptions:  Elastic linearity – the stress-strain curve is linear in the elastic area

 Homogeneity – material‟s composition is consistent

 Isotropy – physical properties at a point are equal for any direction

Thermoplastics do not meet these assumptions. Thermoplastic materials are not isotropic and part composition depends on many factors, including raw material mixing, mold flow and cooling. Moreover, most thermoplastics are not linear over the elastic area of the stress-strain curve. For calculations, non-homogeneity and non-isotropy behavior can be compensated with safety factors. To compensate the non-linear behavior in the elastic area, the secant modulus can be used. The secant modulus is the gradient of the line through the origin and a point (usually a 1% strain) on the stress-strain curve.

Unlike steel, the shape of the stress-strain curves for thermoplastics may be quite different from material to material. Tough thermoplastics are the preferred materials for snap-fit attachments. Toughness is a measure of a material‟s resistance to impact loads and is represented by the area under the stress-strain curve. A brittle (steep stress-strain curve) or flexible (elongated stress-strain curve) material for a snap-fit attachment is undesirable.

Under all circumstances, the snap-fit attachment may only deform elastically and return to its original shape when the applied load is removed.

When deflections occur rapidly during assembly or disassembly, the dynamic strain should be used for analysis instead of static strain.

The maximum strain will occur at the base of beam. The maximum strain depends on the

yield point of a material. The yield point, also known as the yield strength, is an important value when designing a snap-fit attachment. The yield point determines when the snap-fill will permanent plastically deform.

 A typical for PC-ABS is 2.5%

Note that is a different value than the tensile elongation (break)

Other material properties that can influence calculations are:

 Additives – added chemicals to enhance certain functional of processing capabilities  Creep – long-term increase in strain under a sustained load

 Temperature effects – mechanical properties are strongly temperature depended.  Fatigue – occurs when cyclic loads are applied

 Stress relaxation – long-term decrease in stress under a constant strain  Load time – Behavior of thermoplastic is strongly time depended

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4.3.2. Stress analysis

Background

A finite element analysis can be a useful tool when performing a stress analysis for snap-fit attachments. A finite element analysis is especially useful when dealing with complex parts, e.g. shapes and/or cross sections.

Unfortunately, it is rather complex to simulate snap-fit attachments. Some factors could not or could be difficult be added to the simulation. For example in case of fixing the PCB:

 Sharp edges of the PCB will locally deform the cantilever hook  Friction between the PCB and the snap-fit is difficult to predict  The displacement of the PCB is often a non-linear movement

However, a finite element analysis can be useful to check the stress concentrations. When the undercut height is known, an enforced displacement can be applied to the snap-fit. The result can be compared to the maximum allowable stress.

 For PC-ABS, the allowable stress concentration should be approx. 60 MPa

Note that small local stress concentrations may not lead directly to problems. These stress concentrations occur for a small period and occur at a small area.

Figure 4-17: Snap-fit with a radius of 0.05mm (Stress approx. 115 MPa)

Figure 4-18: Snap-fit with a radius of 0.5mm (Stress approx. 55 MPa)

The figures above (Figure 4-17 and Figure 4-18) illustrate the effect of using a radius for the transition from the beam to the solid underground. In this example, the maximum stress is approx. two times lower for the snap-fit with a radius.

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A practical example

In this paragraph, the development of a snap-fit attachment can be seen. Several finite element analyses are used to determine the dimensions for a suitable snap-fit attachment.

Figure 4-19: The situation in where the snap-fit attachment must be designed

In the figure above (Figure 4-19) the situation can be seen in where the snap-fit must be designed. The snap-fit will be used to fix the PCB in a certain position.

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Figure 4-21: Concept B

A gap is designed in front of the snap-fit. This gap is needed to form the snap-fit without a slide in the injection-molding tooling. Stress concentrations are still too high (approx. 450 MPa).

Figure 4-22: Concept C

A radius is applied. Stress concentrations are still too high (approx. 450 MPa).

Figure 4-23: Concept D

In this design, the snap-fit length is increased by designing a recess in the wall. When practical testing with first out of tool prototype indicates that the snap-fit is not stiff enough, the recess can easily be removed or reduced in depth by removing material out of the mold. Stress

concentrations are significantly reduced. The maximum stress concentration is approx. 105 MPa.

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Figure 4-24: Concept E

In this design, the depth of the recess is increased. The result is a reduction of the maximum stress concentration that occurs, which is now 95 MPa.

Figure 4-25: Concept F

Stress concentrations in the designs above are still too high. By increasing the recess depth significantly and expand it horizontally, the length of the snap-fit will increase. This results in more flexibility. Like the previous design, the recess can easily be removed or reduced in depth by removing material out of the mold in case the snap-fit is not stiff enough.

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Figure 4-26: Final snap-fit design

Additionally, edge blends are applied to reduce local stress concentrations. The finite element analyses indicate that the snap-fit is sufficient with a maximum stress concentration approx. 63 MPa.

Although this value exceeds the allowable stress for PC-ABS (60 MPa), the stress that occurs in the final snap-fit design is still acceptable. Small, local deformations will not lead to direct problems.

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4.4. Cooling methods project A

4.4.1. Background

Various components in a tablet dissipate heat. It is important that this heat is abducted from the tablet housing to prevent overheating of electrical components.

4.4.2. Boundary conditions

The maximum temperature of an electrical component depends on multiple factors and is specified in the data sheet provided by the manufacturer. For both projects, not all electrical components have been specified during the final thesis period so a general maximum

temperature of 70°C is defined for the components mounted on the PCB. Moreover, the General Safety Norm (60950) states that continuously touched handles, grips, nobs (plastic) may not exceed 75°C. The Audio And Video Norm (60065) states that accessible non-metallic parts should not exceed 60°C.

The lifetime of some electrical components (e.g. electrolytic capacitors) strongly depends on the environmental temperature. Therefore, the goal is to achieve low internal temperatures. This can be achieved by various cooling methods, presented in the following paragraph.

4.4.3. Concepts

There are multiple cooling concepts; which are presented in Figure 4-27. The block indicated with an “H” represents the hot components in the tablet.

Concept A: closed enclosure Concept B: direct contact

Concept C: venting holes Concept D: internal heat sink

Concept E: internal fan

Figure 4-27: Various cooling concepts

The method of cooling must be; cheap, silent, durable, small and easy to implement in multiple tablets, should provide sufficient cooling. In Table 4-6, the cooling concepts are compared.

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Table 4-6: Cooling concepts C o n c e p t C h e a p S ile n t D u ra b le S mall S im p le im p le men ta tion C o o lin g p e rf o rman c e R e mar k s

A + + - + + - Probably insufficient cooling due to indirect heat transfer

B - + + + +/- + Expensive due to highly thermal conductive materials (metals / gap pad). Not all components can be cooled in this way

C + + + +/- +/- + Venting holes have influence on “look and feel”

D - + +/- - +/- +/- Heat sink is not effective in high internal ambient temperatures E - - - ++ Fan is not appreciable due to high cost, noise and short lifetime Concept A and concept C feasible and will be reviewed by thermal simulation later on in this report.

4.4.4. Thermal loads

The following thermal model is based on project A.

The thermal loads in the model are based on the power consumption of the tablet. There are three power consumption profiles: active, dimmed and sleep.

- In active state (user gives input on display) the tablet can consume up to 7.2 watt in a worst case scenario. This load case is used to check which cooling concept is preferred. - In dimmed mode (most common) power consumption is reduced to 4 watt by dimming

the LCD screen and reducing the load on the CPU, Wifi and power supply. In this mode ambient temperature measurements are taken. This load case is used to determine the best temperature measurement concept.

- In sleep mode the LCD screen, wifi and z-wave are completely turned off. This load is rarely used.

The power consumption can be divided into separate function groups (Figure 4-28)

Figure 4-28: Power consumption tablet

Power supply; 800 Wifi; 792 CPU; 753 LCD (in screen); 472 Remaining; 250 ETH; 225 USB; 190 Touch screen; 179 Z-Wave; 122

Audio; 100 LCD (at pcb); 94 DDR; 72 NAND Flash; 18

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To check the assumption that the most power consuming components produce the most heat a thermal image was taken from a similar 7” tablet product with the same features. Most hot spots appearing in Figure 4-29 are the components in the high power consuming groups shown in

Figure 4-28.

Figure 4-29: Thermal image similar tablet

A PCB engineer is consulted to roughly map the PCB components layout. The main objective during this process is to create an area without components for the placement of temperature sensors. Another objective is to keep the most power consuming components far away from the area reserved for the temperature sensors. The temperature sensors will be positioned at the bottom left corner.

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4.4.5. Simplified thermal model display

The thermal conductivity is known for most materials; therefore the plastic housing can easily be modeled and simulated using the thermal properties given by the manufacturer of the plastic. A display consists of various layers of multiple materials. Therefore it is much more complex to simulation than the plastic housing. To get a better overview of the materials that are used, a 3.80mm thick display was taken apart. In Figure 4-31, an exploded view of the display can be seen.

Figure 4-31: Exploded view display

The materials that are used in the display can be divided in three categories:  Metals, the casing is made from 0.30 mm galvanized steel (43.0 W/mK)

 Glass, the actual LCD screen is made of ITO glass and contains liquid crystals. The glass sheet is 1.30mm thick. (0.96 W/mK)

 Plastics, the backlight module with all its polarizing and reflective sheets is made from a transparent plastic. All these layers combined result in a thickness of 1.75mm (0.15 W/mK) If the thermal contact between the layers was perfect the thermal conductivity could be

calculated: Equation 4-2 ( ) ( ) ( ) ( ) ( ) ( )

Table 4-7: Abbreviations for Equation 4-2

Abbreviation Description Unit

Thermal conductivity [W/mK]

Thickness layer [mm]

However the sum of all layers in the screen ad up to 3.65mm, this is 0.15mm less than the combined thickness of the screen. This indicates that there is an air gap in-between the layers of the screen and that therefore the thermal contact between the separate layers cannot be perfect and that thereby the calculation is inaccurate. The thermal conductivity is expected to be lower than the calculated value.

The thermal conductivity of the screen will be determined by comparing a field test to a simulation.

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Field test

The goal of this field test is to generate values that can be used as an input for the thermal simulation.

In this test the heat is transferred from the resistors directly to the rear of the screen. An aluminum plate (Figure 4-32, yellow) is used to distribute the heat generated by the resistors (Figure 4-32, red) evenly over a large area of the screen (Figure 4-32, green). The Styrofoam (Figure 4-32, gray) will prevent major losses of heat. A load of 5watt was applied to the resistors.

Figure 4-32: Model field test Figure 4-33: Field test

Temperatures are measured using thermocouples and a data logger. The test was conducted for a couple of hours to provide sufficient stabilize time.

Simulation

A model is created based on the dimensions used in the field test.

The screen is modeled as one component of “screen material”. The thermal conductivity of the “screen material” is varied until the temperatures of the simulation matches with field test results. The results of the simulation are shown in Figure 4-34, Figure 4-35 and Figure 4-36.

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The temperature target lines are plotted as horizontal lines in the graph. These lines show the temperatures measured in the field test. The curved lines show the simulated temperatures (with the varying thermal conductivity of the “screen material”).

Figure 4-36: Thermal conductivity screen

The heat sink average temperature and the ambient average temperature reach their target value when the thermal conductivity of the screen is approximately 0.15 W/mK. The screen top average does not reach the target value. This value comes close to the target value between 0.20 and 1.00 W/mK.

Conclusion

The field test results do not perfectly match the simulation results. At 0.15 W/mK, the deviation is -0.9% for the screen top average temperature, -1.1% for the internal air temperature and -5.2% for the screen top average temperature.

In conclusion, the thermal conductivity of the LCD can be generalized to be approximately 0.15 W/mk.

4.4.6. Simulation cooling concepts

The main goal in this paragraph is to decide which cooling concept (A or C), chosen in

paragraph 4.4.3 is preferred. Two models are set up:  A model with venting holes

 A model without venting holes

The size of the venting holes is refined during the simulations. In this process the venting holes are repositioned / enlarged until sufficient temperatures are obtained. Note that the shape and size of the venting holes have to match the overall look and feel of the tablet, which is

determined by an external design agency in both projects. In Figure 4-37 the rear of the tablet is shown with the venting holes.

30,0 35,0 40,0 45,0 50,0 55,0 60,0 65,0 0 0,2 0,4 0,6 0,8 1 Tem p e ratu re ( °C) Thermal resistance (W/mK)

Thermal conductivity screen

Screen simulated Heatsink simulated Air internal simulated Screen top measured Heatsink measured Air internal measured

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Figure 4-37: Venting holes (project A)

The simulation is based on the thermal worst case scenario. In the simulation the tablet has been running in active mode until the temperatures stabilized. This is a very unlikely load case because normally the tablet only functions in active mode for a couple of minutes when the user gives input to the screen. Although is it an unlikely load case the tablet should be able meet the requirements.

Results cooling concept A

The simulation results are presented in Figure 4-38 and Table 4-8.

Figure 4-38: Simulation results cooling concept A Table 4-8: Simulation results cooling concept A

Position Temperature (°C)

Outside housing 62.2

PCB & components 75.5

LCD 51.3

Results cooling concept C

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Figure 4-39: Simulation results cooling concept C Table 4-9: Simulation results cooling concept C

Position Temperature (°C)

Outside housing 55.2

PCB & components 69.0

LCD 47.1

Conclusion

The housing and component temperatures simulated in cooling concept A exceed the maximum specified temperatures (60°C housing, 70°C components).

The housing en component temperatures simulated in cooling concept C stay below the maximum specified temperatures after fine-tuning of the venting hole size. Even though the venting holes at the bottom of the tablet are spaced far apart the PCB is still sufficiently cooled. Cooling concept C (venting holes) is the preferred method to cool the tablet.

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4.5. Cooling methods project B

Project B, by contrast, is less elaborated during the final thesis project, especially methods for cooling. However, some concepts are reviewed by using thermal simulation software. In this simulation a load of 2 watt was applied to the LCD screen and a load of 4 watt was applied to the PCB. In the simulation the ambient temperature was 20°C. Results of these simulations are shown in Figure 4-40, Figure 4-41 and Figure 4-42.

Figure 4-40: Concept A, venting holes (1.8mm wide)

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4.6. Ambient temperature measurement

The primary task of the tablet developed in project A is to control the heating in a domestic environment. The tablet measures the ambient temperature by an internal temperature sensor. It is of great importance that this sensor measures the ambient temperature accurately. This proved to be difficult due to the high power dissipation of the tablet.

4.6.1. Temperature measurement concepts

There are different methods to integrate the temperature sensor in the housing. Three sensors defining 3 concepts are set up suitable for tablet use. The concepts are shown in Figure 4-43.

Figure 4-43: Temperature measurement concepts

 Sensor A: isolated vented sensor enclosure. This concept is used in many electronic thermostats. The sensor is placed in a containment isolating the sensor from the air inside the tablet. This enclosure is vented to the outside by slots or holes in the housing.

 Sensor B: contact with housing. In this concept the temperature sensor measures the temperature of the housing. The sensor is placed in a containment isolating the sensor from the air inside the tablet. The back housing is designed to have a protruding block of plastic which is nearly touching the sensor. The gap between this block and the sensor is filled with a flexible and compressible thermal conductive material (gap pad).

 Sensor C: chimney effect. The sensor is placed behind one of the venting holes near the bottom of the tablet. As air is warmed up inside the tablet it starts to rise, exiting via the top ventilation holes. At the same time cold air (ambient temperature) is drawn into the tablet via the bottom ventilation holes. This flow of cold air cools the sensor.

Simulation

All three concepts are integrated in the same simulation to prevent differences between simulations. The tablet in this simulation is in dimmed mode consuming approximately 4 watt. This load case has been chosen because temperature measurements are only taken when the tablet is in dimmed mode or sleep mode.

The ventilation holes from cooling concept C are used in the simulation model.

A

B

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Results

In Figure 4-44 the temperature sensors and PCB are shown. What strikes first are the

temperatures of the sensors; all sensors are above 34°C. This means that all sensors measure minimal 14°C above ambient temperature. This indicates that none of the temperature sensors will measure the ambient temperature accurately.

Figure 4-44: Simulation results temperature measuring concepts

The main reason why the sensors get so hot is the conduction of heat via the PCB. Solutions for this problem can be found in paragraph 4.6.2.

In order to determine which cooling concept has the best performance temperatures of the sensor and the PCB near it where measured in the simulation. These results can be found in

Table 4-10. The difference between the sensor temperature and the board temperature is in the

third column. This ΔTSensor-PCB increases when more heat is abducted from the sensor. Table 4-10: Sensor temperatures

Temp at sensor (°C) Temp at PCB (°C) ΔTSensor-PCB (°C)

Sensor A 36.2 36.4 0.2

Sensor B 35.1 35.8 0.7

Sensor C 34.4 35.3 1.1

Conclusion

Sensor B (contact with housing) and Sensor C (chimney effect) have the highest ΔTSensor-PCB

which implies that these measuring concepts abduct the most heat from the PCB resulting in a cooler sensor. Therefore these concepts are preferred for measuring the ambient temperature.

4.6.2. Isolating temperature sensor from PCB

A temperature sensor needs to be electronically connected to the PCB. This connection automatically introduces a thermal connection between the PCB and the sensor. The PCB is warmed up by the components mounted on the PCB. To prevent introduction of heat by the PCB to the sensor the sensor should be thermally insulated.

PCB construction