Jon Elson wrote:
>
>
> --- In Homebrew_PCBs@yahoogroups.com
> <mailto:Homebrew_PCBs%40yahoogroups.com>, "Brian" <blue_eagle74@...> wrote:
> >
> > Never thought of this. Cool idea. You said to use thin foil.
>
> I've been doing something similar, but using brass shim stock. I
> started with 5 mil but that was getting too much solder paste on the
> fine-pitch stuff, and also I was having some sideways etching problems.
> I changed to .003" shim stock, and have had great results so far. I
> clean the shim stock with fine sandpaper on both sides to remove oil
> or plastic preservative coatings. I laminate both sides of the shim
> with DuPont Riston etch resist dry film. I then expose with aligned
> mirror-image films and etch like a circuit board in a Kepro spray
> etcher. This has a heater that brings the etchant temperature to 35 -
> 40 C. I have had a little problem in that the board holder in the
> etcher is designed to hold rigid things like PC boards, and the .003"
> shim stock is too flexible, so I will have to provide a frame next
> time I do it. But, the results have been quite good. I have done
> down to 0.5mm pitch chips. I had some breakdown of the "bars" between
> adjacent apertures with the .005" shim, but it is working great on the
> .003" material.
>
> Jon
I had attempt at stencils. The main difference between PCB and stencil
your trying to etch 100um~150um instead of 35um. Any professional who
etches stencils, will double side etch. I was trying to etch for QFN
packages with 0.5mm pitch, and could not get the required accuracy. All
the other component pads were usable. It was only this particular IC
that caused me problems. Major factors affecting the accuracy of the
stencil were limitation on my inkjet printer resolution and etch
undercut. I tried compensating for etch undercut and even tried double
sided etch, but failed to get consistent apertures for the 0.5mm pitch
pads. In the end I paid $180 for a brass stencil , which I mounted to my
own frame.
For parts like 0603, 0402 , 0.65mm pitch TQFP, then I think home brew
stencil etching is quite feasible.
I was new to stencils and solder past printing so maybe someone can
prove me wrong.