Homebrew PCBs

On Mon, 20 Oct 2008 15:19:41 -0000, you wrote:

>Hi guys,
>
>I tried my first photo process board this weekend and the process 
>failed completely.  I am using the dry film, negative resist from MG 
>chemicals.

Negative resist?  I'm familiar with the positive resist on precoated
boards.
>
>The failure was in the developing of the resist.  While developing, 
>all the resist washed off.  It was developed as I could see the 
>traces in the resist, but faintly.
>
>The developer solution was a best guess on dilution so this is 
>probably my main failure point.  
>

probably not.  For the positive resist it's about 10:1 water to
developer.

>I am wondering what actually holds the resist to the board though.  
>Per the instructions, I am suppose to peel off the backing, apply 
>resist to the board and then run it through a laminator twice.  Is 
>this what causes the dry film to stick to the board or is it the 
>exposure process?
>
>I also suspect that using vellum for the artwork is not a good 
>thing.  Using some scraps, I noticed that exposure is much different 
>using transparncy than vellum.  So my next attempt will be using 
>transparencies and to carefully measure the developer.

Very much so, however, I think you have a different problem.

Assuming that the resist is a negative resist, and if it is
unpolymerized ( which means that just out of the can it washes away in
the developer), it "hardens" from the top down.  Insufficient exposure
causes there to be a layer of hardened resist over the top of a layer
of unexposed resist.

That unexposed resist will wash off.

I'd suggest you do an exposure test, steps of two, or take their
recommended exposure and go around that.  You may see the difference.

The transmission of your media of the UV is very different from velum
to a transparency, so that's possibly a cause of your problems.

Harvey

>
>Chris
>