Homebrew PCBs

My next project is probably going to require a solder paste stencil.
So, I've been pondering what to do.  I could buy one, but that's
expensive, adds delays, and isn't as much fun as doing it myself ;-)

I've tried drilling a transparency sheet (plastic) but the holes were
very rough.  Plus, I don't have drills small enough for the SMD pads
I'll be using (11 mils wide).  So I thought of etched brass, since the
toner transfer kits say they can be used for that.

After a little research and some thought, there seem to be two
common techniques:

1. TT on one side, paint the other side, etch in FeCl or H2O2, clean.

2. TT on both sides for faster etching and cleaner edges.

But I thought of a third, based on how pcb-pool makes PCBs.  I have no
idea if this would work, but it goes like this:

* TT on both sides, with toner *only* where you want paste.
  I.e. little dots of toner all over the place.

* Electroplate some tin or other metal onto the sheet.

* Remove the toner, leaving exposed brass where the paste goes.

* Etch.

Not sure if this has any practical advantages over original option #2,
though, and it's probably a lot more work to set up correctly.

But anyway, has anyone had success etching their own paste stencils?

DJ

On Sat, 02 Dec 2006 17:47:01 +0100, DJ Delorie <dj@...> wrote:

> My next project is probably going to require a solder paste stencil.
> So, I've been pondering what to do.  I could buy one, but that's
> expensive, adds delays, and isn't as much fun as doing it myself ;-)
> I've tried drilling a transparency sheet (plastic) but the holes were
> very rough.  Plus, I don't have drills small enough for the SMD pads
> I'll be using (11 mils wide).  So I thought of etched brass, since the
> toner transfer kits say they can be used for that.
> After a little research and some thought, there seem to be two
> common techniques:
> 1. TT on one side, paint the other side, etch in FeCl or H2O2, clean.
> 2. TT on both sides for faster etching and cleaner edges.
> But I thought of a third, based on how pcb-pool makes PCBs.  I have no
> idea if this would work, but it goes like this:
> * TT on both sides, with toner *only* where you want paste.
>   I.e. little dots of toner all over the place.
> * Electroplate some tin or other metal onto the sheet.
> * Remove the toner, leaving exposed brass where the paste goes.
> * Etch.
> Not sure if this has any practical advantages over original option #2,
> though, and it's probably a lot more work to set up correctly.
> But anyway, has anyone had success etching their own paste stencils?
> DJ


Someone recently posted PICs about using CNC drilled brass for stencils.  
The holes are of course round but that is not a problem.

How much are you going to do? You know you can just bulk-paste a row of  
pins with a bead over all of them?

ST

"Stefan Trethan" <stefan_trethan@...> writes:
> Someone recently posted PICs about using CNC drilled brass for
> stencils.  The holes are of course round but that is not a problem.

"Plus, I don't have drills small enough for the SMD pads."

> How much are you going to do? You know you can just bulk-paste a row of  
> pins with a bead over all of them?

Yeah, I've done that for TQFPs but the next project has an SMD
connector with the pins underneath, if there's shorts I won't be able
to see or fix them.  The paste for that is about 11x40 mils, far from
round.  So I want to be very careful about putting the right amount of
paste in the right place.

Plus I have a hard time making a bead narrow enough for 0.5mm parts
without lots of bridges.

On Sat, 02 Dec 2006 22:46:24 +0100, DJ Delorie <dj@...> wrote:

>> Someone recently posted PICs about using CNC drilled brass for
>> stencils.  The holes are of course round but that is not a problem.

> "Plus, I don't have drills small enough for the SMD pads."
>

Some people have so many they sell them. ;-)

>> How much are you going to do? You know you can just bulk-paste a row of
>> pins with a bead over all of them?

> Yeah, I've done that for TQFPs but the next project has an SMD
> connector with the pins underneath, if there's shorts I won't be able
> to see or fix them.

ouch.

> The paste for that is about 11x40 mils, far from
> round.  So I want to be very careful about putting the right amount of
> paste in the right place.
> Plus I have a hard time making a bead narrow enough for 0.5mm parts
> without lots of bridges.

Does the connector have air between the pins or is the space filled? I  
have found that the flush packages like a QFN actually repell bridges  
between pins, those were much easier than they looked.

I'm not sure about this etching business quite frankly. I doubt it will  
work well without pro etching equipment you'll see lotsa undercut with the  
thick stencils. It might also be extremely tricky to align two sides of TT  
precisely enough for the doublesided version you described, since laser  
printers have distortion.

Someone here does etch brass sheet for model railroads, i hope he speaks  
up.

ST

"Stefan Trethan" <stefan_trethan@...> writes:
> Some people have so many they sell them. ;-)

Smaller than 13 mil?  That's the smallest I have, and I'm not sure my
drill press is precise enough to not snap anything smaller anyway.

Stephen Lane wrote:
    Using the Epson Durabrite Inks straight on to clean brass with the 
opposite side coated with conformal coating (spray varnish) I get min of 
5 to 6 thou between "pad holes"
on the boards/stencils/gobos I make. beauty about brass stencils is you 
don't have to use modified printer as brass shim for stencil will tape 
straight to paper carrier & feed straight thru the printer. I 
contributed basic method in a thred  "Re: Easiest inkjet" on 4/11/06 
with my name (please search old messages on forum"
    I hope it helps but its how I do it

Regard
Stephen Lane