Homebrew PCBs

Brian,

The foil actually only is suposed to serve as a carrier for the dry film 
laminate on both sides of it. The one I use is less than one mil thick, 
and I suggest to use the thinnest one you can get hold of as long as you 
manage to laminate it which is not all that hard. Just make sure to feed 
your laminator in a direction in which the foil was not originally bent 
cause otherwise there is the risk that the foil wraps itself around one 
of the rols of the laminator (been there, had to disassemble/reassemble 
the laminator :-( ). The extremly thin aluminium foil is also the reason 
why the aluminium is etched away almost instantly (with some impressive 
bubbling, smoke and even some heat :-) ) when dipping it into Fe3Cl at 
those spots where the dry film laminate was developped away. You truly 
don't need any air agitation for this etching process. Dipping is 
enough. I usually dip it shortly, then rinse and check if still 
aluminium is there. Somethimes it's necessairy to stick it back into the 
developement bath, then back into the etchant. After two or three cycles 
the stencil will be perfect! The dry film then gives the alluminium at 
those spots where it's really fine some aditional strenght. I do have a 
PC microscope but for some reasons it does not work at the moment, so 
the biggest magnification I can do here are my 8 times magnifying 
googles and using them I can't tell if there are spots where only 
dryfilm is present or if the aluminium is still there. The all important 
point though is that even if the aluminium shold be gone, the silk 
screenig of the solderpaste works without problems for me.

I think though that my end resulting stencil is probably not so far away 
from a mylar stencil. I never had one, but I figure structures that fine 
will break easily anyways no matter what material is used. So care must 
be taken not to bend the stencil sharply. I was however amazed to see 
how strong it was. The stencil kind of sucked itself to the PCB surface 
and I've ben able to rub in all directions to truly evenly spread the 
paste. Raising it thereafter was also no problem and thereafter all fine 
structures at the stencil were still ok. Needless to say that they were 
also present on the PCB.

You may have noted that I took a plastified card of a cardgame as the 
spatula/squeegee. They are IMHO fairly well suited for the job, can even 
be cleaned thereafter (reused many times) and dirt cheap (just make sure 
you take your sons yugioh (spelling?) or pokemon cards AFTER he grew out 
of it!). Since they are made of cardboard, they do not scratch up the 
stencil and by bending them more and less you can easily press the 
solderpaste down to the PCB.

The stencil ideally should be 4 mils thick to end up with the propper 
amount of solder paste on the pads. Since using my method you end up 
with a stencil of that thickness, subsequent soldering is no problem.

I've so far not been able to clean the stencil after use cause all the 
fluids I tried so far destroyed the dryfilm laminate. It's no problem to 
use the stencil for multiple PCBs while you are at it. Just if the paste 
dries it's (so far) over. However that's really no problem for me cause 
I a) anyways only produce prototypes (5 pieces of one PCB is a hughe 
quantity! :-) ) and b) since it only takes 15 minutes to creae a new 
stencil and since the material costs is close to nothing that doesn't 
bother me. The time saved compared to hand soldering, and the perfectly 
looking result way outweight it.

As mentioned elsewhere I'm in the process of creating a website with 
tutorials on how I create my PCBs and how all the equipement I use was 
made with descriptions and pictures for those intersted in building part 
of it on their own etc. etc. I then also will upload sample pictures of 
projects done this way.

Markus

Brian schrieb:
>
> Never thought of this. Cool idea. You said to use thin foil, when you
> apply the paste do you have a problem with the foil between the pads
> breaking? and if so does it matter since when it reflows it sucks up to
> the pads? Also, using thin foil, is there any problem with having
> enough solder to make a reliable solder joint? Do you have a picture of
> the final board?
>
> Brian
>
> --- In Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>, Markus Zingg <homebrew-pcb@...>
> wrote:
> >
> > Hi group
> >
> > Under the folowing path in the files section you find a description
> on
> > how to create your own fine pitch stencils. There you also see how to
> > apply solder paste with it. The example given is made with one of the
> > (four layer) PCB's I created here at home.
> >
> > Files </group/Homebrew_PCBs/files/> > PhotoEtching
> > </group/Homebrew_PCBs/files/PhotoEtching/> > solderpaste and stencil
> >
> > I hope this helps others. Enjoy!
> >
> > Markus
> >
>
>