custom membrane switch design guide - sigma 6 · pdf filethe circuit layer includes the...

Custom Membrane Switch Design Guide Updated 12/05- All Rights Reserved

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7323 Engineer Road, San Diego, CA 92111

858-279-4300

www.sigma6electronics.com

This design guide is intended as an outline for communicating the requirements of our

customers to Sigma 6 Electronics, a mutual understanding of technology is critical.

A membrane switch is a momentary switch device in which at least one contact is on, or

made of, a flexible substrate. The top layer of the switch is the graphic interface between

the user and the machine. While other layers make the switch work, it is a well designed

attractive graphic layer that impresses your customers first. The other layers of the

membrane switch need to support the graphic layer by providing the qualities and

functionality you need. The user of a machine judges the product based on both the

appearance and functionality of your switch. As such, the membrane switch can be the

most important part of a product. Sigma 6 Electronics works with customers to design the

optimal switch that meets their needs. We can maximize a switch for appearance,

functionality, and price. Sigma 6 Electronics prides itself on quality testing each switch to

your specifications so that there are no surprises in the field.

Our in-house production capabilities were designed for flexible manufacturing allowing

us to provide customers rapid prototyping, production ramp-up as product demand grows,

and even efficient production of long product runs. For larger runs Sigma 6 Electronics

has developed close ties with offshore partners.

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Membrane Switch Design Guide

Table of Contents

▫ Standard Membrane Switch Construction

1. Materials

2. Artwork

3. Embossing

4. EMI/ESD/RFI Shielding

5. Tactile Feel (Domes)

6. Embedded LEDs

7. Rear Adhesive

8. Sub-panels

9. Backlighting

10. Connectors

11. Prototyping

12. Membrane

Appendix “A” Electrical and Mechanical Specifications Standard

Appendix “B” Membrane Switch Checklist

Appendix “C” Glossary

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Membrane Switch- Standard Construction

Membrane Keypad

▫ The Graphic presentation layer of the membrane switch is a graphic overlay.

The configuration is dependant on the function of the device. The graphic

presentation layer is typically a printed material, such as polyester or

polycarbonate.

▫ Graphic adhesive layer facilitates the assembly of the graphic presentation layer to

the circuit layer, which receives the mechanical pressure applied to the membrane

switch domes.

This layer is typically made from polyester type material with adhesive on both

sides, and is normally designed a few thousands of an inch smaller than the dome

height. This is done to allow a small preload on the dome to facilitate a tactile feel

on the selection pads. Sometimes, as dictated by the membrane switch

application, this layer will include venting of the domes to allow the trapped air

within the dome to escape during actuation/pressing of the pad-dome.

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▫ The Circuit or PCB layer includes the membrane switch electronics, as well as the

dome feature.

The circuit layer includes the integrated switch pole pairs for each button-dome

combination of the membrane switch.

▫ Adhesive separator layer is an optional layer to facilitate an additional circuitry

required for some Membrane switch applications.

This layer will accommodate the interconnection to the dome circuit layer. This

layer will have adhesive on both sides for assembly.

▫ Optional second circuit or PCB layer incorporates additional circuitry required by

the membrane switch application.

This layer typically includes the necessary flex cable or interface pins to integrate

the membrane switch.

▫ Adhesive layer on the mounting surface of the membrane switch is provided as a

design option to mount the membrane switch to the target installation

▫ Metal domes used on membrane switches can be designed with a wide range of

stainless steel shapes and degree of tactile responsiveness.

Typical applications may have 4 legs and me 12 mm square with trip or snap

forces between 200 to 500 grams.

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1. Materials

A variety of overlay materials are used in membrane switch applications. A commonly

used material is polycarbonate. It is easy to print on, die cut and emboss. For these

reasons it is very cost effective. The disadvantage of polycarbonate is that it begins to

show signs of wear and tear sooner than some of the alternate materials. In most

applications polycarbonate overlays will last a minimum of 100,000 cycles during life

cycle testing. Uncoated polycarbonate is also susceptible to damage from a variety of

chemicals. If a polycarbonate overlay is going to be in an environment that will subject it

to chemicals, a Neltex hard coat should be used to protect the overlay.

Polyester is a more robust material that has superior life cycle and chemical resistance

properties. In life cycle testing, polyester shows no signs of wear at 1,000,000 cycles. It

often needs to be hydro formed instead of embossed because of its memory.

Hydro forming is more expensive for both tooling and unit costs than is embossing.

Polyester is also more difficult to die cut. Dies will require more frequent reblading. Both

polyester and polycarbonate are available with a variety textures and hard coats. In their

uncoated glossy form both materials are very susceptible to scratching. For this reason

gloss materials should always be hard coated, Contact your Sigma 6 Electronics engineer

for additional technical data on any of these materials.

Generally, for excellent tactile feel in a switch with stainless steel domes we recommend

an overlay thickness between .006’ and .008.” Thicker overlays will decrease the tactile

feedback and thinner material with decrease the life.

▪ Polyester- .005 to .010 gloss or Matte

▪ Polycarbonate- .005 to .020 Gloss or Matte

2. Artwork

Sigma 6 Electronics offers complete artwork layout services. The customer should

specify type styles, colors, and sizes of all copy. Artwork should be provided to us for all

logos or special symbols. We use the latest version of AutoCAD with Autoscript to

generate our drawings and artwork. It is helpful for customers to supply us with an

AutoCAD DWG file, or as an alternate, a DXF or IGES file.

We can convert other files for our use as well. Always provide Sigma 6 Electronics with

a hard copy drawing as well. The artwork will be plotted at Sigma 6 Electronics on our

photoplotter that is capable of holding plus or minus .0005” tolerances over a 20” x 26”

area. Proofs of artwork will be sent to our customer prior to production.

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Color Matching

Color samples provide the best method of color matching. If you have a sample, please

provide it. If a perfect match is necessary, please let your sales professional know so that

a sample can be provided to you for approval.

Textured Background

The surface can be printed by us or supplied on the material by our supplier. Extended

use keypads should not have a printed texture since it will show wear.

If you do not have artwork, our graphic artists and engineering team also design the

graphic overlay and circuitry for your project. Common file types accepted by Sigma 6

Electronics are:

A. Adobe Illustrator- AI or EPS

B. CorelDraw- CDR Adobe

C. AutoCAD-DWG or DXF

3. Embossing

Embossing is capable of greatly enhancing the look of an overlay and improving the

users’ experience. There are two ways to emboss. First, you can use a male or female die.

This allows for a clean finish but has height limitations. The second method is hydro-

forming; it allows for greater embossing heights but is more expensive. Typically, a

material is embossed 1 to 1.5 times the thickness of the base material. It is possible to

emboss more than this but the durability of the keypad is sacrificed.

The phrase “Plateau Embossing” is used to describe keys that are raised and flat on the

top. The term “Rim Embossing” is used to describe raising only the border of a key.

Embossing is typically .010” high and two-dimensional. Hydro-forming can be used to

attain higher embossments, up to 2-3 times material thickness. Three-dimensional dies

can also be built. Overlays can be hydro-formed with domes in them to provide tactile

feedback. Hydro-forming tools are significantly more expensive that embossing tools.

We have samples of both hydro-forming and embossing on polyester and polycarbonate

that will help illustrate the different results that can be attained. Contact a Sigma 6

Electronics sales engineer to request hydro-forming and embossing samples.

4. EMI/ESD/RFI Shielding

Printed conductive inks, aluminum foil, copper foil or Indium Tin Oxide (ITO) is

employed in membrane switch designs to minimize the impact of electromagnetic

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interference (EMI), electric static discharge (ESD), or radio frequency interference (RFI).

Several options are available for shielding membrane switches. The most common are

printed carbon, printed silver, and aluminum foil. From a functional standpoint, the main

difference among these materials is their conductivity. Either carbon or silver can be

printed on top of the top circuit to act as a shield. These shields have the advantage of not

adding any additional layers to the switch construction. Carbon shields are less expensive

than silver shields. Silver is usually printed in a grid pattern to reduce cost. A layer of

aluminum foil can also be added above the top circuit. This material is the most

conductive shield available. However, it does add two layers to the switch construction.

The shield is usually connected to the ground through the connector, or by means of a tab

with a slot for a fastener.

The customer should express their shielding requirements in Ohms per square inch. The

entire product packaging must be considered when specifying these requirements.

5. Tactile Feel (Domes)

The dome type, the material, part thickness, and embossing are all important in

determining the actuation force needed to activate the switch. For instance, in a

machinery application where panels might be bumped, you might want the actuation

force to be high. However, in a health care setting you might want the safety switch to

require a low actuation force.

There are two types of domes: Stainless steel and Mylar.

▪ Stainless steel domes are the most common. They offer the longest

actuation life and a more tactile feel. Sigma 6 Electronics maintains an

inventory of domes ranging from 3mm to 12mm in size.

▪ Mylar domes can be formed into the upper switch or a dome layer can be

added on top of the upper switch.

Actuation Force

The customer may specify the switch actuation force. Typical actuation forces range from

6 to 24 ounces. Mylar domes usually have greater actuation force than do stainless steel

domes. Mylar domes have typical actuation forces of 14 to 24 ounces. Stainless steel

domes are in the 12 to 18 ounce range. The tolerance on actuation force is +/- 3 ounces.

Our engineering staff will proved expert advice on the type of dome best suited for your

application.

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6. Embedded LEDs

IT is relatively common to include small surface mounted LEDs into membrane switches.

This is a very simple way to add annunciations to switch. A standard membrane switch is

not thick enough to accommodate the package size of most surface mount LEDs. For this

reason, the overlay may be embossed in the window area, or extra fillers may be added to

the switch construction. Silver conductive epoxies are used to mount the LEDs to the

circuit. These epoxies are not very flexible, and consequently switched with embedded

LEDs should not be bent in the LED areas prior to being installed. Stiffeners can be

added to the switch construction to add rigidity when needed.

7. Rear Adhesive

Adhesion qualities are affected by the substrate surface energy (high or low), types of

materials used, the required bonding requirements, environmental concerns and a number

of other factors. Your sales associate will help you decide what is best.

8. Sub-Panels

In some applications it is desirable to have Sigma 6 Electronics supply the membrane

switch mounted to a rigid subpanel. The most commonly used material for such

subpanels is aluminum. These subpanels can be supplied with a variety of hardware

installed. It is important to keep in mind that the subpanel must have different mechanical

dimensions to allow for assembly tolerances. The subpanel should be .020” smaller than

the membrane in both height and width. All cutouts and holes should be .030’ larger.

Cutouts behind windows should be .060” larger than the window. These general

guidelines are intended to help insure that the subpanel is not visible after assembly.

Other subpanel materials are:

▪ Aluminum- Anodized, sealed, unsealed, dyed, or un-dyed

▪ Cold Roll Steel-Plated with, Chrome, Zinc, or other plating

▪ Stainless Steel- Brushed, Sanded, other finishes.

▪ Plastic Panel

▪ Printed Circuit Board- Single or multi-Layer

9. Backlighting

We offer several backlighting solutions for viewing legends in no light and low light

applications. The most common backlighting methods used are Fiber Optics,

Electroluminescent panels, and LEDs. Our ability to manufacture a switch, which meets

our customer’s expectations, is dependent on how well we understand our customer’s

requirements. The customer should be sure to supply us with as much detail about is

design requirements as possible. Appendix “B” is a checklist of issues that should be

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addressed in communicating a customer’s complete requirements. It is helpful for the

customer to supply a copy of this checklist with notes on topics not covered elsewhere in

this documentation.

10. Connectors

Sigma 6 Electronics recommends the Nicomatic connector. We have found them to be of

high quality and long lasting. However, we can supply almost any standard connector

method.

Nicomatic Connector Specifications:

▪ .100” pitch Male, Female, and Solder tab

▪ Plating-Tin or Gold

▪ ZIF-Zero Insertion Force Connector

▪ This connector is designed for installation of ZIF Connectors

11. Prototyping

A prototype is a cost-effective method for creating and executing your design with a

minimum investment in tooling and development time. Often there are changes in size,

colors, hole locations and shape after a prototype run.

To assist in prototype efforts, Sigma 6 Electronics has new equipment for laser cutting

and digitally printing that can make first runs very affordable.

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Appendix A Membrane Switch - Electrical and Mechanical Specifications

A. Electrical

1. Configuration – momentary SPST normally open

2. Current Rating – 30V – 100MA to 120V – 10MA maximum

3. Breakdown – 220 V RMS

4. Closed – 100 OHMS typical contact resistance

5. Open – 10 MEG OHMS contact resistance minimum

6. Capacitance – 30 Picofarads

7. Life;

a. Flat Switch – approximately 5 million cycles

b. Domed (embossed) layer – approximately 5 million cycles

c. Domed circuit – approximately 1 million cycles

d. Metal Dome – approximately 5 million cycles

B. Mechanical & Environmental

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Appendix B

Membrane Switch Checklist

The following items need to be communicated to Sigma6 before an order can be

processed.

___ Mechanical dimensions of the finished part

___ Tolerance specified (+/- .015")

Type of switch

___ Non Tactile

___ Tactile with stainless steel domes

___ Tactile with polyester domes

___ Tail exit point

___ Tail length

Tail termination

___ Berg Clincher

___ AMP

___ Solder Tabs

___ Exposed silver for ZIF

___ Other _______________

Pinout

___ Determined by Sigma6

___ Customer Specified

Shielding

___ None required

___ Printed silver grid

___ Printed carbon

___ Aluminum foil

___ Other ________________

Overlay Material

___ Velvet textured polycarbonate (8B35)

___ Gloss polycarbonate with hard coats (8010)

___ Velvet textured polyester with hard coat (Autotype)

___ Gloss polyester with hard coats (Autoflex)

___ Other _________________

Logos and special graphics

___ Customer supplied

___ None required

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Windows/Lens coating

___ Velvet textured (enunciator, LED)

___ Anti Glare (LED, VF, LCD)

___ Gloss, water clear (LCD)

___ Window insert

Embossing

___ Key rims

___ Plateau

___ LED windows

Hydro forming

___ Key rims

___ Plateau

___ LED windows

Insertable legends

___ Not required

___ Specified

Support panel

___ LED windows

___ Not required

___ Drawing included with material specified

Artwork

___ Sigma6 generated

___ Customer supplied (attached to order)

___ Colors specified

Color matching

___ Custom (samples supplied)

___ Pantone Matching System

___ Federal Standard No. 595a

___ Copy size, style, and location specified

Switch Sealing Requirements

___ Standard splash resistant

___ NEMA 4

Switch Venting Requirements

___ Standard internal venting

___ Externally vented

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Appendix C

Glossary

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