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.