Re: gEDA-user: wacky (or not?) solder stencil ideas

[DJ Delorie <dj@xxxxxxxxxxx>]

> Yesterday while I was at the TechShop experimenting with the laser 
> cutter

You suck ;-)

> 2) Slap it in the laser cutter, and print the paste layer on it with 
> just enough laser power to blast away the blue paint.  (The CO2 laser is 
> the wrong wavelength for cutting metal, it just reflects off.)

Huh?  I thought CO2 was what they cut metal stencils with.

> Anyway, DJ, I've looked over your web site where you do basically this 
> with the toner transfer method.  Got any comments?

The trick to etching paste masks is to etch from both sides to reduce
undercut.  That means careful alignment of both sides, which is tricky
with toner transfer (mostly because you can't see through the paper to
align it).  Otherwise, the mask'd side is undercut by N mils, where N
is the thickness of the foil.

The best method I've seen so far is to take generic aluminum foil,
laminate a photoresist on both sides, and expose/develop.  Now, DIP it
in FeCl - the aluminum etches almost instantly.  Use as-is - don't
strip the photoresist!  The combination of foil and resists is just the
right thickness for solder paste stencils.

This works because the foil is so thin you can just pre-align the two
mask prints (which is easy because they're transparent), then slip the
foil between them for a near-perfect alignment.

> Topic two: I've been trying to get a process that makes stencils in 3 
> mil drafting mylar.

Have you tried plain old 20lb copy paper?

> Third wacky idea: TechShop has one of those PCB mils.

I've seen someone do it with just a CNC drill - one drill hole for
each pad, leaves a spot of paste.  Won't work for TSSOP, but will for
0603 et al.

For fine-pitch parts, it's almost easier to etch a slot along the pins
and rely on surface tension to put the solder where it's needed.


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