Holland Membrane - Manual (Page 10)

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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.