Mechanical construction considerations

AN3431

12/18

Doc ID 019017 Rev 1

A nonelastic adhesive should be used, together with a backing support material, to merge

the different layers (conductive top cover, spacer layer, and PCB layer) together

permanently.

If the conductive top cover is too stiff and the adhesive is elastic, it may cause ‘mechanical

problem 2’ (see Figure 6, part 2). In this situation, if a force is exerted to button 2, the

conductive top cover over button 1 may lift.

Figure 6.

Mechanical problem 2

1.

No force applied

2.

Force applied (incorrect movement)

3.

Force applied correct movement

Table 3.

Mechanical problem 2 analysis

Possible mechanical problems in Figure 6

Electrical consequences

Adhesive did not create a permanent adhesion

between layers - consequently, conductive top

cover layer deforms

Increased capacitance on button 2 due to touch

1.

Decreased capacitance on button 1 causes a latch-on effect on this button when the force is no longer

exerted to button 2. This is due to a reference adaptation.

Adhesive layer is too elastic and conductive top

cover is too stiff

Buttons are too close to each other