|Re: HP15C- LE bad button|
Message #13 Posted by robert rozee on 9 Oct 2011, 10:57 a.m.,
in response to message #1 by Djj
an excellent analysis.
i worked on a product several years ago that used a very similar keyboard arrangement. there was a relatively small number of units produced per year (amounting to around 10 per day) selling to the customer for about $5k each. hence assembly was quite 'manual' orientated with many QC checks.
the white plastic sheet has adhesive applied with a stencil. there is a dot of adhesive (perhaps 3/16th of an inch across) that holds each of the domes in place, as well as adhesive in all areas well clear of the domes. in our case, a 'path' was also left clear of adhesive such that no closed air pockets were formed around keys.
as supplied to us, we received the white plastic sheet with the domes pre-applied. a waxed-paper (light blue coloured) backing sheet was also there to cover (just) the areas with exposed adhesive. the application process in the factory consisted of peeling off the blue backing sheet - being careful to not disturb the domes - and using an alignment jig to apply the white plastic sheet + attached domes to the keyboard PCB.
the possible failure modes were:
1. a dome could fall off, rendering a key completely non-functional
2. a dome could be moved out of position slightly, producing a key that would often operate ok but wouldn't 'click' due to the button pressing it other than in the dead centre
3. a stray scrap of the white plastic sheet, blue backing sheet, or a piece of loose adhesive, could get between dome and PCB during assembly, resulting in a key that would click but either not register, or only register if pressed hard.
the above three cases to seem to mirror closely the problems seen with the 12C/15C-LE.