Mechanical construction considerations

AN3431

10/18

Doc ID 019017 Rev 1

It can be seen from Table 1 :

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Aluminium has a very favorable

most abundant metal in the earth's crust.

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The other listed metals can also be used, but the higher force required should be

considered. Designing with stringent design parameters can still yield a soft touch on

these metals.

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Polycarbonate is a very workable plastic which is used as an electronic component and

construction material, in automotive, aircraft and security components, and in niche

applications.

2.1.3

Spacer layer

The spacer layer is used to create an air-gap between the top cover and the sensor pad on

the PCB. When a user presses the top cover (above the sensor pad area), the slight

deflection of the overlay material into the air-gap increases the capacitance measured.

Although the thickness of the spacer layer is the main design consideration, it is also

important to consider the material. A rigid material, such as FR-4 or a nondeformable

plastic, should be used for the spacer layer so that it does not deform with the top cover if a

force is applied on the overlay material. A rigid spacer layer also keeps the PCB secure.

The deflection required in the overlay material should be at least 2-5 %. This yields a

0.9-2 % change in the capacitance of the button (deflection is not linearly equivalent to

change in capacitance due to parasitic capacitances). A larger deflection is better because it

yields a higher signal-to-noise ratio (SNR).

Spacer layer recommendations

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The recommended spacer layer thickness is 0.1-0.5 mm.

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The spacer layer can be a nondeformable plastic film with holes over the buttons.

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The conductive top cover can have blind holes above the sensor pads. This also

decreases the cost of spacer layer manufacturing, fitting and adhesion, as it is directly

incorporated into top cover. For example,

–

0.7 mm Aluminium with 0.2 mm deep blind holes above sensor pads

–

1 mm plastic/perspex with 0.2-0.5 mm blind holes above sensor pads

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The holes in the spacer layer are recommended to be at least 110-130 % of the button

size. This increases the area which is deflected towards the sensor pad, in essence,

increasing the change in capacitance.

2.1.4

PCB layer

The PCB layer can be made with an extra mechanical tier over the top routed tier. The PCB

can be 0.1-0.5 mm thick with cut-out/blind holes above the sensor pads. This decreases the

cost of the spacer layer manufacturing, fitting and adhesion, as it is directly incorporated into

the PCB.

2.1.5

Mechanical bracket/support layer

This layer can be added to ensure that the module does not bend or move when a force is

applied to a button. To get a stable and robust solution, only local deflection over the button

is permitted.