AN3431

Mechanical construction considerations

Doc ID 019017 Rev 1

11/18

2.2

Application rigidity and adhesion of layers

The PresSense should be in a neutral position where no points on the back of the PCB

should be stressed. The PCB should have uniform pressure across the system and should

not be allowed to deform/move if a force is exerted onto the conductive top cover.

It is strongly recommended to have a backing support of rigid insulating material pressing

the PCB uniformly against the spacer layer.

Figure 5 (part 2) shows ‘mechanical problem 1’ where the system deforms when a force is

applied.

Figure 5.

Mechanical problem 1

1.

No force applied

2.

Force applied (incorrect movement)

3.

Force applied (correct movement)

Table 2.

Mechanical problem 1 analysis

Possible mechanical problems in Figure 5

Electrical consequences

Spacer layer not made from nondeformable material -

entire system deforms downwards.

Increased capacitance on buttons 1 and 2

with possible false detection on both

Mechanical support behind PCB is not in a neutral

position - whole structure (conductive top cover, spacer

layer, and PCB layer) deforms downwards.