--- In
Homebrew_PCBs@yahoogroups.com, John Wilson <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 tried to do this, and found that controlling temperature with a thermocouple in air severely overheated the boards. It has nothing to do with soldermask. The boards turned out black in spots and with delaminations.
The fix was to poke the thermocouple into a through hole near the middle of the board. This way, the temp controller is sensing the ACTUAL board temperature, which apparently is hotter than the surrounding air, due to the IR from the heating elements.
I use a ramp and soak thermocouple controller. I set it to heat at 2 degree C per minute to 180 C, hold there for one minute, then heat to 245 C at 3 C / min, hold at 245 for one minute, and then ramp down at 5 C/min, although the oven doesn't cool that quickly.
Also, of course, I use boards made of FR-406 for the lead-free, as they are made to stand the temperature. Plain FR4 cannot safely handle it. Some batches will take it with slight discoloration, some others will bubble and split.
And, of course, if you are not controlling temperature with a precision sensor inside the oven, all bets are off. The oven thermostat will cook a pizza, but is WAY too inaccurate for reflow soldering.
Jon