Homebrew PCBs

I haven't measured the film, but I'd guess about 1 or 2 mils thick. Without 
filling the holes the film bows in or out depending of board temperature, 
which weakens the film at the edges of the holes. Some times it tears a 
little as a result, letting in the etchant. Filling the holes solves that 
problem. I use the film I have. After developing I re-expose it, which 
hardens the film a great deal. Any flexing at all is a Bad Thing.
--
Phil M.

----- Original Message ----- 
From: "cunningfellow" <andrewm1973@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Wednesday, May 22, 2013 1:19 PM
Subject: [Homebrew_PCBs] Re: printer


>> Phil wrote:
>>
>> <SNIP>
>>
>> Before applying the film I  fill the holes
>> with a mixture of drywall plaster and sugar,
>> let it dry, and sand the board lightly to
>> remove any plaster from the surface. This
>> provides a flat surface for the resist film,
>> and is easy to remove with water and a
>> water-pik (dental cleaner) after etching.
>> The sugar resists the etchant in case any
>> does come into contact, which otherwise
>> would solidify the plaster making it
>> difficult to remove.
>
> Have you had problems with tenting?  How thick
> is negative active film you are using?
>
> Thicker film and a good contrast artwork should
> make a tent that is plenty strong enough to
> survive even vigorous hand developing/etching.
>
>
>
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