samIam said:
What has worked for you guys in the past?
I've been using Pulsar film, available from Digikey (182-1003-ND and
182-1021-ND) to do a dozen or so boards so far, including some
fine-pitch SMT designs, though all single-sided.
The process is fairly straightforward, but has taken me a lot of
experimentation and frustration to get it right. As a result I'm going
to write up what I've learned one of these days soon, so others don't
have to figure the same stuff out the hard way. Here's the quick
version, off the top of my head, with as much detail as I can in this
format:
0) Design your board carefully, according to a number of rules that I
still need to codify. Some easy ones: use copper pours wherever
possible to remove less copper; make sure you set your trace/gap numbers
to a comfortable level, I haven't gone below 10mil/10mil yet though I
easily could; set the pad annulus to 15+mil so you have some drilling
slop; don't let the copper pour or other traces get too close to the
*inside* of any SMT chip pads, you can't wick that stuff away.
1) *Laser* print the *positive* artwork onto the transfer paper, cut to
size and taped (Avery laser label bits) to a piece of backing paper
(both to feed into the laser, and for positioning).
2) Thoroughly rough up (Scotchbrite pad / steel wool) every sq. mm of
the board, then wash very carefully, possibly using acetone as the last
step (to remove oils *and* copper dust). Do not touch the copper after
this step.
3) Laminate/iron the artwork to the board. Pulsar resells a GBC
laminator that has both high heat and pressure, and will accept up to
0.03125" boards (not 0.0625", big deal, IMO) for ~$50.
4) Dump the board paper-side up onto the *surface* of a bowl of water
and wait (tick tock) for the paper to literally fall off (takes longer,
but much more consistent results, don't rush this step).
5) Carefully wash everything off the toner surface, and make sure there
are no oils left either.
INSPECTION: if there are any missing areas, clean the copper off with
acetone, go all the way back to step 2 and start over. DO NOT SKIP ANY
OF STEP 2, do it all over again.
6) Laminate/iron the green film onto the board, let cool, and carefully
peel back the plastic.
INSPECTION: if the film wrinkled and left visible toner on the board, or
if any toner is visible on the peeled off plastic, go back to step 2 and
start over, as per previous inspection step.
7) Grab some gloves, a sponge (combo glove+sponge works great for me),
and a few ounces of FeCl or equivalent. Get some FeCl on the tip of the
sponge, and lightly *scrub* the board all over until you can see all the
way through the copper-free areas when held against a light.
8) Thoroughly clean the film and toner off with acetone, scrub it down
with steel wool, and clean it again. Cleaning is your friend.
9) Dump the board in some Liquid Tin (MG Chemicals 421-500ml) for
5-10min. When removing the board from the liquid tin, do not touch the
copper+tin surface until you've washed it off and let it sit for a
minute. I kept getting fingerprints on the tin and finally realized it
came from *above*, not underneath the plating. The stuff is soft. It's
also extremely nasty stuff, you *MUST* take even small amounts to a
disposal site. MG says even 50ml diluted into 5 gallons of water is too
nasty to dump down the drain. You can use extremely fine-grade steel
wool, *very* lightly, to clean up the surface a little bit, but as soon
as it shows a little copper tint to it, you're done. Very nice color
effect though.
10) Use appropriate drill bits, such as the re-ground ones you can get
cheap in sets of 25. Only problem is that there no guarantee of size
selection with those. This would be a good time to thank yourself for
cranking up the pad annulus and leaving yourself some room to mess up.
11) SMT components can be soldered by hand, but I've had far better luck
using solder paste and a toaster oven. Digikey has it (KE1507-ND) but
as a rather high price ($42 for 35g), and you'll need to find a syringe
tip to use, the smaller the better. I'm still getting a feel for
exactly how much paste to use for different parts, and the
time/temperature constant of my toaster oven is abysmal, so the process
is still evolving, but it's worked fairly well so far.
I've taken a few closeup pics of some of my most recent boards, both
good and bad:
http://www.omegacs.net/misc/pcbs/
It's not in a decent gallery because the software I was using didn't put
up with an upgrade and I haven't figured out what's broken about it yet.
How about double sided boards?
I haven't done any yet, but I plan on doing some attempts eventually.
The process would be to put the toner and film on one side, drill 3+
reference holes, then transfer the other side. The problem may be what
the laminator will do to the first side when going through the second
time. It may be necessary to use even more extra dimensions on the PCB
and transfer films and carefully tack the two pieces of transfer paper
down through the drilling process, and laminate the two sides at the
same time.
The through-hole plating problem is another one I need to research.
Pulsar recommended going to International Eyelets (ask for a catalog,
you get it within a few days) who make nothing but "PCB eyelets"
designed to do through-holes. They're going to be noticably bigger than
your average via, but comparable to a normal through-hole component pad.
Absolute smallest one their catalog shows a 0.030" hole diameter,
0.020" finished hole, with a 0.046" flange/pad. That's actually not too
bad. I'm going to get some trail stuff pretty soon and see what happens.
