AN2869 datasheet(13/43 Pages) STMICROELECTRONICS

13 / 43 page

AN2869

Main capacitive sensing guidelines

Doc ID 15298 Rev 6

13/43

2.2.4

Mechanical construction and PCB to panel bonding

In order to ensure stable touch detection, the PCB must always be at the same place on the

panel. The slightest variation, even as small as 100 microns, may lead to differences in the

signal which can be detected. This must be avoided to ensure the integrity of the touch

detection. The panel and other elements of the device must not be moved, or only as little as

possible, by the user’s finger. To avoid this kind of problem, glue, compression, co-convex

surfaces can be used to mechanically stabilize the PCB and the panel very close together.

In the list of the different ways to achieve this, we can put: heat staking plastic posts, screws,

ultrasonic welding, spring clips, non-conductive foam rubber pressing from behind, etc.

Figure 7.

Typical panel stack-up

Normal construction is to glue a sensor to a front panel with Pressure Sensitive Adhesive

(PSA). 3M467 or 468 PSAs work very well.

MS18974V1

Non-conductive panel

PCB

Pressure-sensitive adhesive (PSA)

e.g. 3M467

Copper electrode

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