You Wrote:

>Clewer,Brian wrote:
>>
>> Hi all,
>> I have a photo sensitive PCB held behind a glass plate
>
>> it appeared to work at first but it then took all the
>> photo sensitive layer off.
>
>Errm, ordinary glass absorbs UV. The glass in UV EPROMs
>is fused quartz (which doesn't)..
>
>Perhaps your glass plate is absorbing UV, hence the
>uniformity of your etching.
>
>Is your resist UV-softened or hardened?
>This will dictate whether your symptoms are caused by
>too much or too little UV.


Yep, I realise that glass does infact absorb UV rays but I think in this
case the UV wave length is the sort where a glass plate will not make too
much difference.  The UV tubes are not the kind I would erase my eproms
with 'cause it wouldn't work.  My eprom eraser tubes are made from quarts
but the PCB tubes have a white coating on the inside of the tube (I guess
to convert the energy into a different wave length).  I don't know if
they are soft or hard wavelength tubes, but they are certainly different
to my eprom eraser.  By the way, the glass plate is only about 1/8 of an
inch thick.

Brian.