Membrane Switch Construction
A basic membrane switch includes a graphic overlay layer, spacer layer, static layer, backside adhesive layer, printed circuit board connector, and stainless steel snap domes
Graphic Overlay Layer: The top layer of a membrane switch is a graphic overlay. The graphic layer is typically a 2nd surface printed polyester material (or silicone rubber keypad). Graphic overlays, and silicone rubber key pads can be processed in a variety of ways, and have a variety of features. Click here for a full discussion of graphic overlay layer design issues and options.
Spacer Layer: The spacer layer adheres to the static layer on one side and the graphic layer on the other. The spacer layer:
- is a “sandwich” of polyester with adhesive on both sides
- is typically designed to be .003” to .005” smaller than the dome height so as to allow a slight preload on the dome which results in a superior tactile feel.
- includes venting for air displacement during button actuation, as well as pockets to contain the metal domes in the desired position.
Static Layer: The static layer contains the circuitry in a basic membrane switch. It includes:
- “Interdigitated” switch pole pairs for each push buttonink
- Screen printed conductive silver traces leading to the tail, and connector from the switch pole pairs.
- Dielectric insulating ink. Insulating ink is printed in areas susceptible to shorting, and where conductive trace routing requirements necessitate cross-overs.
- Resting points on one of the switch poles for the metal dome.
Back Side Adhesive: Adhesive on the backside of the membrane switch is specified to achieve optimal adhesion to the surface the switch is applied to. For many surfaces, and environmental conditions a .002” adhesive optimized for high surface energy materials is optimal (e.g., 3M467MP). For other surfaces, and environments another adhesive will be more appropriate. Melrose can assist you in specifying the correct adhesive for you application if you make us familiar with the surface the membrane switch will be applied to (texture & material), and the environmental conditions to which it will be exposed.
3M is a major supplier of adhesives to Melrose. They have an excellent web site that Melrose staff may refer to for “out of the ordinary” backside adhesive requirements (as well as for adhesives used in the membrane switch “sandwich”). You may also find it informative.
Printed Circuit Board Connector: Melrose supplies membrane switches with tail connectors compatible with standard .100” pitch thermoplastic printed circuit board headers, zif headers, as well as solder tabs. For additional details on connector and solder tab options click here.
- 2.54mm (.100”) pitch male and female connectors are available with single or double rows, locking or not, polarized or not. Rated to UL94V0.
- Zif connectors. Melrose supplies tails configured for zif connectors of different styles, pitches, and thicknesses. (e.g. .0,5mm, 1mm, 1,25mm, 1,27mm, 2,54mm)
- Solder Tabs: Melrose supplies membrane switches with solder tabs (long or short, single or double retention).
Metal Domes: Membrane switches from Melrose can be designed with a wide range of stainless steel snap domes. Most applications call for 4 legged 12 mm square domes with trip forces of 280, 300, 400 or 450 grams. However other dome sizes are used in special situations.
- Oblong, triangular or round snap domes to achieve a uniform dome coverage across these key geometries.
- Small snap domes (i.e., 6mm) to achieve higher key densities, or minimize trace routing complexity on the static layer
- Large snap domes as large as 20 mm for large keys
- High Trip Force snap domes with trip forces as high as 2200 grams (nearly 5 lbs). Trip forces in excess of 5 lbs can be accomplished by “overlaying” multiple domes.
A sampling of available snap dome sizes, shapes, and trip forces can be seen at this link.
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