Holland Membrane User Manual

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

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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.

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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

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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

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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.

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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

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