Resistive Touch Screen Design Guide

Resistive Touch Screen Integration Guide Version 1.1

Colophon

Editor: Danielson Europe BV., Marketing Department Author: Danielson Europe BV.Design: Danielson Europe BV., Marketing Department

© 2009 by Danielson Europe BV., De Nieuwe Haven 12, 7772 BC Hardenberg, The Netherlands

All rights reserved. No part of this publication may be reproduced in any manner without permission. All images are © the artists, reproduced with the kind permission of the artists and/or their representatives. All literal and printer´s errors are barred.

Every effort has been made to contact copyright holders and to ensure that all the information presented is correct. If proper copyright acknowledgment has not been made, or for clarifications and corrections, please contact the publishers and we will correct the information in future reprintings, if any.

Bringing your ideas to life

Table of contentS

CHAPTER PAGE

Chapter 1 - Resistive Touch Screens 1 1.1 Poly Laminate - or Film-On-Glass construction 1 1.2 Integral or bonded tail 2 1.3 Solid copper or silver Polymer tail 3 1.4 ZIF connections 3 1.5 Circuit vias or twin tail design 4 1.6 Simple or buffered flex construction 4 Chapter 2 - Methods of integration 5 Chapter 3 - Rear mount integration 7 3.1 Bezel design 7 3.1.1 Metal bezels 8 3.1.2 Plastic bezels 8 Chapter 4 - Clamps and mounting brackets 9 Chapter 5 - Gaskets 11 Chapter 6 - Front mount integration 13 Chapter 7 - Tail connection 15 7.1 Silver Polymer tails 15 7.2 Copper tails 15 7.3 Integral tails 16 7.4 Bonded tails 16 7.5 Tails with crimp connectors 16 7.6 ZIF tails 16

Bringing your ideas to life1

Chapter 1 Resistive Touch Screens

Resistive Touch Screens are in effect a lamination of circuit layers made from varying materials. Although in itself extremely robust some care should be taken when integrating a resistive Touch Screen into a bezel or housing.

Danielson offers resistive Touch Screens in a variety of constructions and designs, depending on the demands dictated by the application. Before continuing you should verify which type of Touch Screen construction you are using. Please refer to the following explanations to identify your exact Touch Screen construction.

1.1 Poly Laminate or Film-On-Glass construction

Danielson offers two fundamentally different Touch Screen constructions. A Film-On-Glass construction consists of two circuit layers, whereby the bottom layer is an ITO coated piece of glass, to which the sensing circuitry has been applied. The top circuit on the other hand is made from an ITO coated Polyester sheet. Both circuits are bonded via an adhesive layer in the perimeter of the Touch Screen. This construction, as shown in Figure 1.1 offers the highest transmission and optical clarity.

Top Film (flex layer) Spacer Dots Top Circuit

Spacer AdhesiveBottom Circuit Glass

Tail

Connector

Figure 1.1 Side view of a typical Film-On-Glass construction


For those applications where special glass is required Danielson offers a Poly Laminate construction. In this type of Touch Screen the bottom circuit consists of ITO coated Polyester, which is laminated onto a glass plate by means of an optical adhesive. The glass can therefore be thermally or chemically strengthened offering up to three to four times the strength of normal glass. The fact that the circuit consists of a non-breakable polymer material means that even in the event of the glass cracking the sensor will remain electrically functional (Figure 1.2).

Top Film (flex layer)Spacer Dots Top Circuit

Spacer Adhesive

Bottom Circuit

Glass

Tail

ConnectorPolyester

Optically clear adhesive

Figure 1.2 Side view of a typical Poly-Laminate construction

1.2 Integral or bonded tail

There are two tail constructions available in the Danielson Touch Screen product range. For Touch Screens with integral tails the tail is in fact part of one or both flexible circuit layer(s) (Figure 1.3).

Flexible circuit layersare cut to yield two tailsthat lead to one connector

Figure 1.3 How layers of the Touch Screen lead to one connector

Silicon Bead

Conductive AdhesiveBonded Tail

Strain Relief

Conductive AdhesiveBonded Tail

Figure 1.4 A typical bonded tail construction

Bonded tails, primarily used where copper- or particularly long tails are required, are separate tails which are electrically and mechanically connected to the Touch Screen’s bottom (stable) circuit. The traces on the tail hereby mate with the Touch Screen circuitry at some tail connection point. Generally these tail constructions are additionally mechanically secured by means of a strain relief tape or silicon bead, as illustrated in Figure 1.4.


1.3 Solid copper or silver Polymer tail

Most Danielson Touch Screen tail constructions are made of Polymer silver. This material consists of silver flakes that are suspended in a Polymer resin which is applied directly to the tail substrate and is then dried resulting in a thin, silver filled – and therefore electrically conductive – plastic trace. These traces are typically gray in colour but also may appear black. In this case a carbon overprint has been added to protect the silver (only used for extremely harsh environments).

Copper tails consist of solid metal copper sheet material that is laminated to a flexible Polymer substrate and is subsequently etched to reveal individual copper traces.

1.4 Tail connections

Two types of connection options are available. Zero Insertion Force (ZIF) connections are contact terminals on the tail substrate itself. In the case of Polymer silver tails these terminals are typically black in colour, due to a carbon ink overprint which is designed to prevent migration of the silver conductive material below. Terminals on copper tails are typically tin - or gold plated to prevent oxidation of the copper. These terminals are designed to be used with appropriate Zero Insertion Force (ZIF) terminals that are available from most connector manufacturers in a variety of pitches and surface treatment qualities.

FCC terminal connectors consist of metal terminals that are punched through the conductive material and substrate of the tail and are assembled in a plastic housing. They are designed to be used with standard 2.54 mm or 1.27 mm pitch PCB headers. This type of terminal provides for the most reliable Touch Screen connection of any Danielson Touch Screen.


1.5 Circuit interconnection or twin tail design

Most Touch Screen designs make use of circuit vias to achieve electrical interconnection between the Touch Screen’s top and bottom layer in order to route the traces to a single tail, in a similar way that the circuits in multi-layer printed circuit boards are connected.

These connections are typically made by filling a special conductive material into small holes of the adhesive spacer and can be identified by two adjacent silver rectangles – also referred to as interconnect pads – on both top - and bottom circuit layers. Figure 1.5 illustrates the cross section of such an interconnection.

Electrically filled materialis injected to connect two circuit layers

Top circuit

Bottom circuitFigure 1.5 How an electrical interconnection between the Touch Screen’s circuit layers is achieved by means of a circuit via

In some cases Poly-Laminate constructions are equipped with a twin tail design meaning that each layer leads to an individual tail that is then terminated in a separate clincher connector, as shown in Figure 1.6.

Flexible circuit layers are cut to yield two tails that lead to one connector

Figure 1.6 How the layers of the Touch Screen lead to one connector

1.6 Simple or buffered flex construction

The majority of Touch Screen sensors and all stock products use a simple flex construction, which means that the top layer of the Touch Screen consists of a Polyester substrate that has the Touch Screen circuit on its lower side and is hard-coated on the side that is exposed to the user.

Some designs, however make use of a buffer layer that is bonded to the Touch Screen’s top circuit layer by means of an optical adhesive. These constructions are typically used in conjunction with graphic printing on the back of the buffer layer. Such custom products can be provided by Danielson complete with a graphic overlay that is bonded directly to the Touch Screen’s surface.


Chapter 2 Methods of integration

There are two fundamental ways in which a Touch Screen can be integrated into a bezel. The simplest method is to clamp a Touch Screen between a bezel and the display. This integration method is referred to as rear mount integration. The Touch Screen is hereby mounted behind a bezel with a rectangular cut out in the dimensions of the Touch Screen’s viewing area.

The Touch Screen is positioned behind this cut out and typically clamped between some mounting plate or directly by the LCD. Rubber or foamed rubber gaskets hereby ensure that the screen is environmentally sealed and cannot be displaced. Figure 2.1 shows a typical side view of a rear mount Touch Screen integration. Refer to “Chapter 3 Rear mount integration” for details on how to assemble your Touch Screen into such a bezel.

Bezel or Housing

Touch Screen

Mounting Clamp Display GasketsFigure 2.1 A typical rear mount construction, in which the Touch Screen is sandwiched in between the bezel and a clamping plate, with two gaskets securing it in place and providing for an environmental seal

Alternatively you may choose front mount Touch Screen integration. The Touch Screen hereby is assembled into a sill that is molded or machined into the front of the bezel. A flexible graphic overlay is then applied to the surface of the bezel and the Touch Screen’s perimeter in order to seal the gap between the bezel and the Touch Screen. Figure 2.2 shows the side view of a typical front mounted Touch Screen.

Bezel Touch Screen Graphic Overlay

Structural Adhesive LC-Display

Figure 2.2 A typical front mount Touch Screen whereby the Touch Screen is set into a well in the surface of the bezel and covered by a graphic overlay


Please note that for such a method of Touch Screen integration great care needs to be taken with regards to the bezel design (flatness), the selection of bonding materials and assembly. If you are unfamiliar with this type of Touch Screen integration you should consider leaving this job to an expert that is specialized in this form of Touch Screen assembly.

Keep in mind that Danielson will also supply complete sub-assemblies including the bezel, Touch Screen and graphic overlay. It is also possible to optically bond oversized graphic overlays directly to the Touch Screen, thus eliminating the step between a graphic overlay and the Touch Screen’s viewing area.


Chapter 3 Rear mount integration

No matter what material the bezel is made of it is vital that the area in which the Touch Screen is mounted remains flat at all times and cannot distort. Also make sure that the material is sturdy enough so that it does not deform when clamping pressure is applied.

When designing the bezel make sure that you integrate some form of positioning system for the Touch Screen, e.g. a well into which the Touch Screen can be positioned. Keep in mind that every Touch Screen’s dimensions will vary slightly due to the inherent inaccuracies of the cutting process of glass. Pay special attention especially to the areas in the Touch Screens’ corners, as these are most prone to cutting inaccuracies. If you decide on a cavity based positioning system make sure that the well is slightly larger than the Touch Screen’s outer dimensions. Typically your Touch Screen’s outer dimensions will vary by 0.3 mm thus requiring an effective well gap of 0.2 mm – 0.3 mm around the Touch Screen’s overall size. Therefore design a positioning well into your bezel as illustrated in Figure 3.1.

3.1 Bezel design

0.3mm

0.3mm

Figure 3.1 A positioning well should be slightly larger than the Touch Screen itself


The ideal bezel for any resistive Touch Screen is made from milled or cast metal, due to the inherent stability and sturdiness of this material. When utilizing a milled bezel ensure that the material is not exposed to excessive stress during the milling process, which can cause the material to distort.

For cast metal bezels make sure that the contact area for the Touch Screen’s sealing gasket is smooth and free from protrusions that may affect the seal. It is recommended that this area should either be sand-blasted or milled to provide for a flat, smooth surface.

3.1.1 Metal bezels

3.1.2 Plastic bezels

The most important rule for a plastic bezel design is that it must not be able to deform or distort. This means that careful attention needs to be paid when designing the bezel itself. Keep in mind that the Touch Screen’s mounting area must be flat and free from distortion at all times. Also make sure that the bezel does not deform when mounted to the housing and that it does not distort when exposed to the application’s storage and operating temperature range.


Chapter 4 Clamps and mounting brackets

The most simple way to integrate a Touch Screen is to place it between two gaskets which are applied to the bezel on the front and a clamp (or the display on the back). Danielson does not recommend using the display as a clamp as excessive mechanical force exerted onto this device may result in damage to the display or will lead to warranty disputes with the supplier of the LCD.

If possible consider using mounting brackets or a clamping plate to hold the Touch Screen sensor in place, as shown in Figure 4.1. When designing such a system, make sure that the clamping pressure is the same at any point of the Touch Screen’s perimeter.

Figure 4.1 A Touch Screen held in place by two gaskets between a clamping plate and the bezel

Note that the Touch Screen’s spacer adhesive is not designed to hold the weight of the screen’s stable circuit and subsequently the glass. Therefore whatever clamping method is used it is vital that the Touch Screen is supported from the back. Failure to support the Touch Screen from the back can result in delamination of the complete product when operated (Figure 4.2). If you are using a resistive sensor with circuit interconnections, force exerted onto the Touch Screen may cause these interconnection points to break, causing the Touch Screen to lose its calibration or rendering the product completely non-functional, as shown in Figure 4.3.

Adhesive Tape

Delamination from spacer adhesive

Figure 4.2 A Touch Screen will delaminate when not supported from behind

Bezel or Housing

Touch Screen

Mounting Clamp Display Gaskets


Adhesive Tape

Loss of circuit via interconnection

Figure 4.3 An interconnect will break when not supported from behind

Also, do not attempt to provide a support for the glass by filling the gap between a mounting well and the Touch Screen as illustrated in Figure 4.4. This prevents the layers of the Touch Screen from expanding and contracting with varying environmental conditions and can cause the Touch Screen to fail. Also, strong adhesives will not allow the bezel material to expand and contract with changing environmental conditions and will cause the bezel to distort relative to the glass, which can lead to permanent destruction of the Touch Screen sensor.

Structural adhesivee.g. an epoxy

Mounting the Touch Screen from the sideprevents layers from expanding

Figure 4.4 The Touch Screen’s top layer can distort when the gap between the bezel’s positioning well and the touch sensor is filled with a solid material


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Chapter 5 Gaskets

Typically three types of gaskets are used to mount and environmentally seal a Touch Screen: foam gaskets, O-ring gaskets and adhesive gaskets.

Under no circumstances should your Touch Screen be permanently mounted to the bezel by means of an adhesive. The Touch Screen is designed in such a way that the individual layers are able to expand and contract with changing environmental conditions without distorting. By preventing the top layer from being able to move relative to the stable and spacer layer the Touch Screen’s top film will distort. The result may be a pillow effect that degrades the optical appearance of the product but can also lead to an electrical short between the layers.

Danielson recommends using foam gaskets that are either die cut and placed or dispensed directly onto the bezel. Select a closed or open cell foam that – when clamping pressure is applied – will form an environmental seal. Recess the gaskets that when clamping pressure is applied it will not encroach on the Touch Screen’s active area and cause a short. As a rule of thumb a clamped gasket should be recessed by > 0.5 mm from the spacer’s inner perimeter, as shown in Figure 5.1.

Non-recessed gasketleads to touch screenshort circuit

Recessed gasket prevents shorting

0.5 mm

Figure 5.1 Gaskets that overlap the adhesive spacer cause short circuits

Attach one gasket to the back of the bezel by means of a suitable adhesive, keeping in mind that most pressure sensitive adhesives are prone to fluid ingression. Therefore when using an adhesive tape make sure that the adhesive strip is more narrow than the width of the gasket itself, as illustrated in Figure 5.2. Alternatively dispense a gasket directly to the bezel.

Gasket seals the areaaround the adhesive

Mounting adhesive

Figure 5.2 An adhesive strip used to secure a gasket should be undersized


Another method of sealing the Touch Screen against the elements is to use an O-ring gasket, which is placed into a milled or molded groove of the bezel, as shown in Figure 5.3. Make sure that O-ring gaskets are relatively soft, to allow for the layers in the Touch Screen to expand and contract when exposed to varying environmental conditions.

If the Touch Screen you are using has interconnection vias please make sure that the O-ring gasket does not exert excessive force in these areas. Ideally position the O-ring in an area where it does not sit directly on top of the interconnect pads.

O-Ring gasket

Do not run gasketover interconnect

Figure 5.3 An O-ring gasket should ideally be positioned relative to the Touch Screen’s spacer

As with die-cut gaskets it is important that O-ring gaskets do not encroach in the Touch Screen’s spacer opening which will result in a short circuit.

Avoid using adhesive gaskets for sealing the gap between the Touch Screen and bezel, as these will not allow for an environmental seal. You may however, although this is not recommended, use a mounting adhesive to bond the Touch Screen to a clamp or the display.


Chapter 6 Front mount integration

Danielson resistive Touch Screens can be mounted onto the front of a bezel in order to eliminate the gap which is taken up by a sealing gasket. To accomplish this, the bezel needs to yield a recess into which the Touch Screen is placed.

Although in some cases possible, it is highly recommended not to use plastic as a bezel material, as the majority of Polymer materials – even when fiber reinforced – will distort. Instead use a cast, milled or welded metal bezel which is free from internal stresses.

In order to integrate a Touch Screen from the front, design the bezel with a well for the Touch Screen to be placed in which is ~0.3 mm to 0.5 mm deeper than the actual overall thickness of the entire Touch Screen assembly. Then place the Touch Screen, assembled to the graphics overlay upside down onto a perfectly flat surface. Remove the liner from the graphic overlay’s adhesive and assemble it to the bezel. Next fill the gap between the Touch Screen and the bezel (still upside down) in order to even out the materials’ tolerances. Your final construction should somewhat resemble an assembly shown in Figure 6.1.

Figure 6.1 A typical front mount integration into a metal bezel

It is important that the structural mounting adhesive allows for some movement of the materials, whilst preventing the Touch Screen to move vertically against the bezel. Also try to avoid mounting the Touch Screen from the side, as shown in Figure 6.2.

Mounting adhesive

Figure 6.2 A Touch Screen secured in place from the side (not recommended)

Bezel Touch Screen Graphic Overlay

Structural Adhesive LC-Display


Entirely filling the gap between the bezel and the Touch Screen will prevent the Polymer/adhesive layers of the touch sensor and the graphic overlay from expanding and contracting with temperature, which in turn may lead to an unsightly pillowing or ripple effect of the Touch Screen’s top layer as shown in Figure 6.3.

Adhesive prevents movement of layers

Distortion of touch screen’s top layer

Figure 6.3 An adhesive dispensed into the gap between bezel and Touch Screen prevents the layers from expanding resulting in a distortion of the top film

It is also important that the mounting adhesive for the graphic overlay has a low shear strength and some degree of flexibility, to prevent continuous expansion and contraction of the graphic overlay material from affecting the Touch Screen’s top layer. Choose a relatively thick acrylic Pressure Sensitive Adhesive (PSA) as a mounting adhesive for your graphic overlay. If it cannot be guaranteed that the overlay material does not inflict stress onto a Touch Screen’s interconnection, as shown in Figure 6.4, this side mount integration option should not be employed.

Loss of electrical interconnection

Pressure onTouch Screen

Flexible mounting adhesive

Figure 6.4 A flexible side mount system can lead to the broken Touch Screen interconnection vias


Chapter 7 Tail connection

When routing the tail of a Touch Screen to the controlling electronics, please keep in mind that the Touch Screen controller relies on accurate analogue readings from the sensor in order to determine the exact touch location. Therefore try to avoid that the Touch Screen and its tail are in the close perimeter of any high voltage sources, such as display backlight inverters.

Depending on the type of tail and material used, refer to the following procedures when designing your tail connection.

7.1 Silver Polymer tails

If your Touch Screen comes with a tail comprised of silver filled Polymer conductive you should under no circumstances crease or bend the tail with a radius of < 4.0 mm. In such a case the material is stretched causing the silver particles to lose contact. This will cause the Touch Screen to lose its calibration or to fail completely (Figure 7.1).

r = > 4 mm

Figure 7.1 A silver Polymer tail should be bent by more than a 4.0 mm radius

7.2 Copper tails

Unlike silver Polymer tails the traces on copper tails consist of solid metal, which can be creased. In fact these tails are bonded to the Touch Screen by means of electrically conductive adhesives. It is desirable to relieve this bond area from any strain. Danielson therefore recommends applying a 90º crease to the tail, as shown in Figure 7.2 in order to reduce the strain to the tail connection area.

Tail ConnectionCrease

Figure 7.2 A copper tail should be creased in order to prevent strain to tail connection


7.3 Integral tails

Touch Screens using Danielson’s integral tail design provide the most robust and easy to handle tail connections. As the tail is part of at least one of the flexible Polyester layers. In such a case the traces are comprised of Polymer silver. Refer to the instructions in “Chapter 7.1 Silver Polymer Tails” for details on how to handle such a tail design.

7.4 Bonded tails

If your Touch Screen comes with a bonded tail make sure that no excessive strain is applied to the tail connection area. It is advisable not to bend the tail directly where it is bonded to the Touch Screen. Therefore run the tail straight for 5 mm to 6 mm before initiating a bend to route it to the controller circuit.

7.5 Tails with crimp connectors

Crimp connection is the most reliable way of connecting a Touch Screen, as the connector will securely attach to a PCB header. You should however make sure that the pins on the PCB header will not oxidize or change in resistance, as this may cause the Touch Screen to lose its calibration over time. Some care should be taken, that the tail is routed to the header as straight as possible in order to prevent creases or strain on the Touch Screen tail.

7.6 ZIF tails

Special attention needs to be paid to the connection of Zero Insertion Force (ZIF) tails. ZIF connector pads on the tail typically consist of tin or gold plated copper (copper tails) or carbon covered silver Polymer ink (silver tails). Especially silver tails should not be connected and disconnected more than necessary, as every insertion and clamping process will cause the carbon overcoat to wear. Once in place it is vital that the tail cannot move relative to the ZIF header on the PCB. Failure to do so will result in varying contact resistance that – depending on the Touch Screen technology employed – may result in loss of calibration.

It is especially important that ZIF tails are not under tension when connected. Make sure that ZIF tails are routed to the header in a straight fashion. Any tension that (even over time) may cause the tail to skew in the connector may cause the contact pads to wear, short or lose connection entirely, as illustrated in Figure 7.3.

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Figure 7.3 A ZIF connector must be laterally perfectly aligned to its mating connector

Please note that the instructions provided within this document are recommendations only. Due to the wide variety of bezel, adhesive and sealing materials available Danielson is not in a position to test every construction possible. Danielson is only able to warrant the Touch Screen sensor as-is. It is up to the customer who integrates the sensor in a specific fashion to test the Touch Screen in an integrated state for the application’s particular environment variables. If you have further questions concerning the integration of Danielson resistive Touch Screens please contact your Danielson Sales or Applications Engineer or your local sales office.

Danielson Europe BV

De Nieuwe Haven 12 7772 BC Hardenberg The Netherlands T: +31 (0)523 281 200F: +31 (0)523 281 202 E: [email protected]

Danielson (UK) Ltd.

29 Pembroke RoadStocklake, AylesburyBucks, HP20 1DBUnited Kingdom

T: +44 (0)1296 319 000F: +44 (0)1296 319 012E: [email protected]

www.danielsoneurope.com


www.danielsoneurope.com

Resistive Touch Screen Design Guide

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