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

Hello there! This is probably a dumb question but I can't find anything in
the list archives (but then again maybe I'm searching for the wrong word).

I'm trying to get the hang of reflow soldering of small 2-layer PCBs with
no solder mask, at Pb-free temps using a toaster oven. The problem I'm
having is that I can't get the solder to melt w/o scorching the boards.
I assume that means I'm being an idiot in some way, but come to think of
it, all of the examples I've found on the web showing how great toaster
oven reflow is seem to show boards ∗with∗ soldermask (which might protect
the board from heat somewhat and/or make the scorching hard to see) and/or
using Sn/Pb paste (which melts at a lower temp). So maybe what I'm trying
to do is unrealistic? I've done a whole series of the boards at different
max temps and the progression is obvious:

Looks perfect = doesn't work.
Very faint suntan = doesn't work.
A bit scorched = works fine.
Cajun style (burned to a crisp) = works fine!

So basically my question is: has anyone had good luck reflowing boards
at lead-free temperatures w/o soldermask? If so, was there anything tricky
about it?

I've had decent success soldering these boards with a hot air pencil (set to
325 C but I don't believe it) and a preheater. The only problem is shorts
underneath one of the components, so a lot of boards fail when I test them,
and I figure real reflowing is the answer. I sell these boards so they
have to be Pb-free, and I stuff them by hand so I've never bothered with
solder mask (just being careful has been good enough -- until this new rev
with the SMT 7-seg LED that gets the shorts).

Details in case they matter:
- PCBs = from Advanced Circuits (regular FR4 laminate IIRC)
- paste = Ameritronics ZeroLead (96.5 Sn / 3 Ag / 0.5 Cu -- MP = 217-219 C)
- soak = 150 C to 217 C (max rate = 0.6 C per second)
- reflow temp = 230 C max (max rate = 1.6 C per second)
- oven = Black & Decker Infrawave driven by Arduino with SparkFun thermocouple
and MAX6675 chip and my own software -- yes I know that sounds fishy but
my cheapskate Harbor Freight IR thermometer agrees with the measured temps
and so (eventually) does a regular oven thermometer, so I'm fairly sure
the temps are accurate enough

There's an article in QST (ham magazine) by someone who got excellent results
just running the toaster by hand (with a thermocouple-equipped multimeter).
So how stupid do I have to be to ruin piles of PCBs (and stink up my house)
with precise computer control? Lay it on me!

Thanks,

John Wilson
D Bit