I can only have experience with negative dry film resists.
Generally the longer its been on there the more difficult it
is to remove an invisible residue (scum) left behind on the
copper surface after developing. They call this "resist lock
on". I did not notice any change with the stripping and
exposure between 1 week old boards and 2 years old. The
residue can be removed with vigorous developing (>35°C &
lots of brushing), but you run into the chance of damaging
the exposed resist pattern.
Leon Heller wrote:
> It's a good idea to use the pre-coated PCB material fairly soon after buying
> it, as the shelf-life is limited. I find I get rather poor results when
> using stock much older than about six months. I tend to accumulate a lot of
> odd-sized pieces of unknown vintage, so I need some sort of FIFO technique.
>
> Leon
> --
> Leon Heller, G1HSM
> leon_heller@...
> http://www.geocities.com/leon_heller
>
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