Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

I just now made up a simple single-sided circuit board with 12 mil
tracks and 14mil spacing. Its all SMD. (Except for the DB-9 and 10-
pin header.)

I used the Dyna-Art paper. (They are called Pulsar, now.) Needless to
say, this stuff has been sitting around on my shelf un-noticed for too
long. But, I figured I should give TT a try for this small board
(since I don't have any of the other method like EDM or CNC milling
working - yet.)

Anyway, the short of it is - the TT process went well. I used an
ancient HP-5L with a real HP C3906A toner cartridge. I cleaned the
board rather thoroughly with Comet and steel wool. I used moderate
pressure on an iron for 5 minutes. (The Pulsar instructions say 3
minutes - but that didn't work.) All the toner ended up on the
copper.

I etched with Sodium Persulfate (it goes from clear to light blue,
letting you easily monitor the process.) I used an aquarium bubbler in
a shallow tank made from a small tupperware dish. I had the board
horizontal the whole time. Not a good idea. The etching process took
over 1.5 hours. Instead of the 0.5 indicated on the Sodium
Persulfate. A couple of things might have worked against me. I had
started with a rather hot etchant bath (kitchen tap water warm) but
with the large flat surface, it quickly cooled. I was being stingy
with the etchant...but it would be more effective I think to etch with
the board held vertically in a thin tank, rather than horizontally in
a shallow tank. Even with the bubbler, it was clear that the agitation
was incomplete. Since the copper nearest the bubbler etched much∗
faster that the copper elsewhere. I ended up having to change the
orientation of the board several times during the etching process.
Disposable chopsticks made a good tool for doing this. :-)

All-in-all, though, I was quite happy with the result.

But, I wouldn't leave well enough alone.

I decided I really wanted a tinned board. So, I picked up some
plumber solder paste at the local hardware store.

Except they didn't have the lead-based "Solder-It" brand. Instead,
they had some Silver-based stuff. It is called "Flo-Temp" Lead-free
instant Plumbing solder∗∗∗. By Alpha Metals.

Anyway, I spread on a thin layer using a chopstick. Well, not thin
enough I think. I then hit it with the hot air gun (part of my SMD
soldering station.) I watched it go through all the transitions that
Stefan recounted. It seemed to be doing pretty much what I wanted.
(It was fun watching the image of the board re-emerge as the stuff
heated, melted, and grabbed onto the copper leaving the etched areas
free.)

But I wasn't pleased with how it looked. There was too much solder
blobbed up on most of the board (some parts were OK...just at the
edges...where I had probably accidently laid down less.) So I tried
Stefan's suggested method for removing the excess. With a paper
towel. Except, it didn't really want to come off. I used a slightly
damp paper towel, btw. I shot the board again with the hot air gun -
just before applying the towel. I suspect the wet towel might have
cooled the board, I mean the solder, off too quickly, so it froze in
place, instead of wiping aside.

Any hints as to what to do next? On certain parts of the board, it
looks tantilizingly like a professional job, with just the thinnest
coating of silver barely hiding the copper. But most of the board
looks like hell.

At least there aren't any (or many) shorts. So, although its ugly, it
should work.

Should I try throwing it in the over to get even heat, and then a
swipe with a dry paper towel? It is a little hard getting the solder
across the whole board to be shiny, at the same time, with the hot air
gun. I have a little toaster oven which is destined someday to be a
reflow oven...but right now it hasn't any temperature regulation other
than what it came with. Should I just set the oven to 450, wait for
it to reflow, and then wipe off the excess?

Or should I quit while I'm ahead...before I screw it up further...and
over heat it so I pull up a trace? Nah...full speed ahead. Tomorrow
I'll fire up that toaster oven.

Jim

∗well, actually just a bit faster. Obviously with a 1.5 hour etch
time, this wasn't going anywhere fast.

∗∗Actually, I think there was one other thing working against me. I
am not certain that I cleaned the Pulsar papers "glue" off of the
board completely enough before etching. Their procedure is to wipe
the board once with the "depleted" paper after making the transfer,
and then rinse under running water. I am not sure, but I thought I saw
some residue still on the board. But, I figured it would come off in
the bath. Finally, perhaps the etchant is old? I cannot remember how
many years ago I purchased it. I am not clear on its shelf life.

∗∗∗It says it melts at 430F. Regular lead-tin solder melts at 370F.