Homebrew PCBs

Does it work any better if you hold at 175C for 5 minutes or so?  The
times I've had trouble with lead-free reflow is when I don't get the
board up to temp on the soak.  Most parts (Si especially) should be fine
with a few minutes unpowered at 175C.  (Most newer IGBTs even work at
250C though the packaging doesn't like it much).

--Adam Shea.

On 03/09/11 02:33, John Wilson wrote:
> 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
> 
> 
> ------------------------------------
> 
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