Holland Membrane Leitfaden

Membrane Switch Design

Guide

Introduction

Membrane switch technology has become a reliable front panel solution where

environmental concerns or frequent cleaning are an issue. The sealed nature of the

technology coupled with its reliability and ability to offer tremendous aesthetic flexibility

make it the solution of choice for many industries.

The success of any membrane switch design effort is greatly enhanced by

effective communication between the design engineers and the membrane switch

vendor. A mutual understanding of the technology and nomenclature is critical. This

design guide is a tool to facilitate communicating those requirements.

Overlay Materials

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

Polycarbonate is a commonly used material because it is easy to print on, die cut and

emboss – making it a very cost effective alternative. The disadvantage of polycarbonate

is that it begins to show signs of wear 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 hardcoat 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. However, polyester, due to its memory properties, frequently requires

hydroforming rather than embossing. Hydroforming is more expensive for both tooling

and unit cost. Polyester is also more difficult to die cut resulting in more frequent

reblading of steel rule dies.

Both polyester and polycarbonate are available with a variety of textures and

hardcoats. In their uncoated glossy form both materials are very susceptible to

scratching. For this reason we recommend that gloss materials receive a hardcoat.

Material suppliers have developed specialty materials that offer some or all of the

properties mentioned above. Appendix “A” lists many of the commonly requested

specialty materials. For additional technical data or specific applications, please contact

your Holland Nameplate sales engineer.

Artwork

Holland Nameplate 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. Always provide Holland

Nameplate with a hard copy drawing as well. Proofs of the artwork will be sent for

approval to our customer prior to production.

Color Matching

There are many systems that a customer can use to communicate color

requirements to us.

PantoneMatching System (PMS) is the most popular color standard. It

identifies colors by specific numbers. This system was originally devised for use in the

offset printing industry, but has become a common tool for all types of printing. The

Pantone System is very popular because it is inexpensive and simple to use. There are

over 1000 colors displayed in a Pantone color guide that can be purchased for about

$75.00. The disadvantage of this system is that there are slight variations in the colors

from sample book to sample book. The colors also fade with time, and books need to be

replaced annually. These samples are printed with offset inks on white paper, and

therefore we may not be able to exactly match a color by screen printing or painting on a

particular substrate.

The Munsell Color System identifies colors in terms of three attributes: hue,

value, and chroma. The physical samples to which we match using this system are

opaque, pigmented films. There are over 1500 Munsell color samples available. Holland

Nameplate’s files contain many, but not all, of these color samples. We are able to order

samples for colors we do not have on file.

CIE (International Committee on Illumination) The CIELAB color system is the

most widely recognized system for describing colors with numbers. The advantage of

using a numeric system is that it is objective, and computers can be used to match

colors and quantify how close a color match is. We require a physical color chip (2" x 2"

minimum) as well as the numeric color coordinates to match a color using this system.

Customer supplied color samples: We can match the color of a customer -

supplied sample if required. Our preference is that the sample be at least 2" x 2" on

painted metal. The sample should also be opaque. It should be kept in mind that colors

will appear different when printed on different substrates. This is especially true in the

case of subsurface printing on membrane switch overlays.

Color samples are supplied to customers when requested. We supply a sample

of the actual ink to be used in production applied to the same substrate from which the

part will be made. We provide this service at no charge in conjunction with an order.

Ultraviolet Hardcoats

The most durable hardcoats are those that are cured by exposure to ultraviolet

light. These coatings are called UV hard-coats. Hardcoats can be selectively added to

many of the materials to produce parts with a velvet textured background, and gloss or

anti-glare windows.

Embossing

In many applications it is desirable to emboss or hydroform the keys of a switch.

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.

Hydroforming can be used to attain higher embossments, up to 2-3 times

material thickness. Three dimensional dies can also be built. Overlays can be

hydroformed with domes in them to provide tactile feedback. Hydroforming tools are

significantly more expensive than embossing tools.

We have samples of both hydroforming and embossing on polyester and

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

Holland Nameplate sales engineer to request hydroforming and embossing samples.

Cosmetic Inspection

In evaluating the cosmetic attributes of a membrane switch, it is important that

the vendor and the customer have a common understanding of how the part will be

inspected, and what the criteria for evaluation will be. To facilitate this Holland

Nameplate has developed a standard cosmetic inspection specification. In the absence

of a customer specification we will use this standard.

This standard is also useful in setting a standard format for specifying cosmetic

inspection requirements. It is recommended that in cases where the customer desires to

establish his own inspection criteria, the format of our specification be used.

The key requirements include viewing time, viewing angle, viewing distance, and

defect size.

Mechanical Tolerances

Steel rule dies are usually used to fabricate the various layers of a membrane

switch. Standard tolerances should be +/- .015". Tolerances of +/- .010" can be held on

critical dimensions such as the perimeter or cutouts. Hole center to hole center tolerances

of +/- .005" can be held. Tolerances on very large parts will be greater. Tighter tolerances

can be held by laser cutting or with the use of hard tooling.

The switch layers under the overlay will typically be fabricated smaller than the

overlay. This allows for die cutting and assembly tolerances. All layers will typically be

.015" inset from the overlay at all edges and cutouts.

Laser Cutting

The various layers of a membrane switch can be cut out by using a numerically

controlled laser. This technology offers two advantages. Tighter mechanical tolerances

can be held, and no tooling is required. While laser cutting is a more expensive process

than die cutting, in many low and medium volume applications it is quite cost effective.

Pinouts

The schematic or pinout of a switch may be specified by the customer, but if it is

initially unimportant to the customer, then the pinout should be unspecified. As with any

circuit layout, more freedom will allow us to produce a more efficient layout. This has the

advantage of shorter development time and a simpler circuit layout, which could

nominally affect switch reliability. Membrane switches can be designed with a common

bus or in a matrix. Matrix layouts are desirable for keyboards with many keys to simplify

the interconnect.

ESD/RFI Shielding

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 the 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 shielding requirements.

Tail Exit Point

Flexible membrane switches are connected by means of a flexible tail that is cut

from the circuit material. We have a great deal of flexibility in selecting the exit point. The

tail cannot exit under or within .125" of the active keypad area.

Interconnect

The flexible tail that exits a membrane switch usually has single row traces on

.100" centers. This tail can be connected to a circuit board with many different single row

connectors designed for flex circuits. Holland Nameplate is tooled to crimp AMP, Berg,

and Molex connectors onto flex circuits.

The most common type of Berg connector used is the Berg Clincher (65801).

This is a female single row connector with contacts on .100" centers. The Clincher

system is also available in a male version, part number 66226. The AMP system is a two

component system. The contacts are specified separately from the housing. The flex

circuit can also be terminated with solder tabs that can be directly soldered onto the

circuit board. The AMP part number used is 88997-2.

The lowest cost alternative is to use a ZIF (Zero Insertion Force) connector.

AMP and Molex both offer ZIF connectors. When using a ZIF connector the membrane

switch is shipped with exposed contacts on the end of the tail. The customer then

inserts the tail into the ZIF connector. ZIF connectors are available with locking

mechanisms in both .100" and .050" centers. When using a ZIF connector, you should

specify either the connector or the requirements for the connector.

Screen Printed Flex Circuits

The typical flex circuits used in membrane switches are made with screen

printed silver-filled epoxy ink. The process is carefully controlled to insure maximum

conductivity, adhesion, and flexibility.

This type of circuit does not utilize feed thrus, so circuits are single sided. Traces

can cross through use of a dielectric crossover. This increases the number of printing

operations, and consequently increases cost. A dielectric material is screen printed in

the area the trace will be crossed over, and then silver jumpers are printed on top of the

dielectric.

Creasing of Flex Circuits

In many applications the tail of a membrane switch needs to be bent at or near

the exit point. Extreme caution should be exercised when bending flex circuits. While it is

likely that continuity will be maintained in a trace that is creased sharply, the trace will

most likely be damaged, and be a potential area of future failures. It is recommended

that a .010" radius be used to wrap the flex tail around if a bend needs to be put into the

tail. A folded piece of card stock works well.

It is also possible to have Holland Nameplate form the tails with a bend to a

customer’s specification. This allows Holland Nameplate to inspect and test the parts

after the bend is in the tail. Always inform Holland Nameplate’s Engineering Department

when a crease is going to be put into the tail of a membrane switch, so that this fact can

be taken into consideration during design.

Rigid Membrane Switches

It is often desirable to build a membrane switch onto a rigid circuit board instead

of using screen printed flex circuits. This construction provides a rigid panel, and allows

for the easy addition of other components, such as LEDs and resistors. These types of

membrane switches are usually supplied with a standard header with pins on .100"

centers.

Domes

Because of the relatively short travel of membrane switches, it is often necessary

to provide users with some type of feedback. Feedback can be visual, audible, or tactile.

Visual or audible feedback should be a consideration in the electronics design.

Domes can be added to a membrane switch to provide tactile feedback. There

are two types of domes that we use in membrane switches, stainless steel and

polyester. There is no significant difference in reliability between these two dome

technologies.

Many people prefer the feel of stainless steel domes. Stainless steel domes also

require lower initial tooling costs. Stainless steel domes are almost always used on

printed circuit board based membrane switches.

Polyester domes are usually formed into the top circuit of the membrane switch.

Polyester domes require relatively expensive machined tools that are built specifically for

each design.

As volumes increase, polyester domes become more cost effective because they

do not need to be assembled individually. As a general rule of thumb, it makes economic

sense to consider polyester domes if the total dome usage per order is expected to

exceed 2500. A drawback of polyester domes is that they tend to relax and lose their

tactile feel at elevated temperatures and are not recommended for applications that will

experience temperatures above 55˚C (131˚F).

Actuation Force

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

range from 6 to 24 ounces. Polyester domes usually have greater actuation force than

do stainless steel domes. Polyester domes have typical actuation forces of 14 to 24

ounces, and stainless steel domes are in the 12 to 18 ounce range. The tolerance on

actuation force is +/- 3 ounces.

Electrical Performance

The great variety of membrane switch designs makes it difficult to outline a

general set of specifications that covers all membrane switches. Listed below are some

basic performance specifications. If an application has specific requirements other than

the ones below, the customer should be certain to communicate them to Holland

Nameplate.

Loop Resistance

The loop resistance of a switch is a function of trace width and length. In almost all

applications the maximum loop resistance is less than 100 Ohms.

Open Circuit Resistance

50 MegaOhms minimum

Contact Rating

100 Milliamps at 28 VDC Maximum

Maximum Load

1.5 VA Nominal

Contact Bounce

5 Milliseconds Nominal

Operating Temperature Range

-45˚C to + 75˚C (See notes on polyester domes)

Life Cycle Testing

Holland Nameplate performs life cycle testing to insure that our customers have

a realistic expectation as to the typical number of cycles a particular design can endure.

Any specific life requirements should be communicated to Holland Nameplate’s

engineering department before the design begins.

Windows

The overlay materials used in membrane switches begin the process clear.

Colors are screen printed on the back of the overlay material. Areas that do not have

color printed on them become windows.

As mentioned in the section on hardcoats, window areas can have a variety of

hardcoats or textures added to them. It is recommended that small discrete LED

annunciator windows have the same texture as the background. Larger windows for

LEDs, LCDs, or VFDs may need a window with less light diffusing characteristics.

Window coatings in general are a trade-off between anti-glare characteristics and optical

clarity. The closer the display is to the overlay the less effect the coating will have on

display readability.

If LED displays are within .062" of the overlay, a velvet texture will usually yield

acceptable readability. For distances greater than .062", we recommend a matte finish

for the window coating.

If LCD displays are within .125" of the overlay, an anti-glare coating should yield

acceptable readability. For distances greater than .125" a gloss coating may be

necessary.

Insertable Legends

In some applications it is desirable to have the ability to customize some of the

legends, either when the switch is assembled to the end product or in the field. This

allows a customer to use one standard membrane switch for many different models of

similar instruments. It also allows customizing a membrane switch for specific

applications.

Switches can also be designed with legend cards that can be changed by the

end user. One very successful application is a scoreboard with an insertable legend card

for each sport. This allows the user to re-legend the switch for the sport that is being

played. Consequently, the switch can be kept relatively small with only a few keys.

To design a switch with insertable legends, a clear window is put into the area

that will have insertable legends. An oversized pocket is put behind this clear area so

that the legend card can be inserted and seen through the window. The entire overlay

has the same texture, which helps hide the fact that legends are being viewed through a

window. The diffusing properties of the texture are not a problem for viewing the legends

because the legend cards are held directly against the overlay by the pocket.

It is also recommended that the background color be different than the color

surrounding the window. This will also help disguise the legend card. The fact that a

membrane switch has insertable legends can be almost completely transparent to the

user if implemented correctly.

Thick Film Resistors

Screen printed carbon thick film resistors can be incorporated into a membrane

switch flex circuit. The tolerances of the resistors determine their cost. Tolerances of +/-

10% can be held if required. In most cases these are relatively expensive resistors.

Embedded LEDs

It is relatively common to include small surface mounted LEDs into membrane

switches. This is a very simple way to add annunciators to a switch.

A standard membrane switch is not thick enough to accommodate the package

size of most surface mounted 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 switches 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.

Data sheets and engineering samples for embedded LEDs are available from

Holland Nameplate sales engineers.

Mounting Adhesives

In most cases flexible membrane switches are shipped to our customers with a

pressure sensitive adhesive on the back side. The most commonly used adhesive is

3M’s 467MP. This is an excellent adhesive for bonding to smooth metal and high

surface energy plastic surfaces. For rougher surfaces we recommend 3M’s 468MP. If

you are uncertain as to the correct adhesive for your mounting surface, contact a

Holland Nameplate sales engineer.

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