Environmental Design Considerations
There are a number of areas that you will need to consider:
- RFI, (Radio Frequency I
nterference), EMI, (Electro Magnetic I
nterference), and ESD, (Electro Static Discharge).
- Temperature constraints.
- Humidity constraints.
RFI, EMI, and ESD.
These areas should be considered at the fundamental design stage. Whilst a membrane keypad does not normally generate RFI, the space that
it occupies may either be a leakage point for RFI out, (generated by some part of the internal equipment itself), or for leakage in.
Shielding can be effected in a number of ways, including printing on either a matrix of silver ink, or a full silver ink layer.
Naturally, the less ink used the less the cost involved.
Transparent windows can be screened by using Indium Tin Oxide,
ITO, which is deposited onto the window. The screening is printed as above, and then the whole layer is earthed to a point on your main circuit board, using a track to conduct to it.
Normal operating temperatures are -40 to +80 degrees celsius. It is always prudent to test not only the theoretical temperatures, but
also run practical tests, if you feel that these operating limits are close to your calculations. Don't forget that your user's ambient temperatures may be different to those that you are used to!!
If the keypad is subject to large extremes, but still within these limits, it would be advisable to specify these.
Humidity constraints are not normally a problem, but do be aware that if relative humidity is to exceed 75-80% then do specify this.
Only you the designer can know what problems you are likely to encounter, but do be aware that certain chemicals
can attack both plastics and adhesives.
Again, treat the keypad as another part of your design, which needs just as much specifying as all the other components, and you
won't go far wrong. Involve us at the outset, rather than as an afterthought!!!