Scorching on PCBs with no solder mask

Community PCB Design
1

This is probably a very dumb question, but: should I expect to be able to reflow PCBs (FR4 laminate) that have no solder mask, using lead-free temps, without scorching them? Everything I read online about toaster-oven reflow soldering makes it look totally magical, but my boards either come out with a bad sunburn (at 230 C – at 245 C they turn black), or else they have bad solder joints. So I wonder if (a) I’m just doing it wrong, or (b) solder mask gives the boards just enough extra protection that they don’t burn (or if they do it isn’t visible), or (c) most people aren’t using this annoying lead-free solder.

Background: I use Ameritronics ZeroLoad solder paste (96.5 Sn / 3 Ag / 0.5 Cu), with a melting point of 217-219 C. When I assemble my boards with a hot air pencil (set to 325 C but I don’t believe it) and preheater everything works fine (except that I often get shorts underneath a small LED with a QFN-like package – which is why I want to try reflow).

I’m using a B&D Infrawave toaster oven, being controlled by an Arduino with a SparkFun thermocouple and a MAX6675 chip and my own code. I’m fairly confident that I’m measuring the temp accurately (at least, it more or less agrees with my cheapy Harbor Freight IR thermometer doodad, and a regular kitchen oven thermometer too after a long lag), and after the preheat and soak steps (during which no scorching happens), the reflow step swinging above 217 C (topping out at 230 C) and back down takes under a minute, just as it should. And yet my PCBs go off like popcorn, or at least get visibly scorched.

Thanks!!

John Wilson

D Bit

2

Are they just scorching in the areas closest to the heating element?

I’ve use proper IR reflow ovens, skillets and hot air to reflow boards, and never seen any scorching, although I always use solder masks, and lead / tin solder…

3

Try the Homebrew PCB Yahoo group. People there have a lot of experience with PCB assembly using various techniques.

4

jmbw, when using a hot air reflow gun I find the non-solder mask guys are a bit more sensitive. I wouldn’t use the lead free paste in this case. The soldermask seems to make a huge difference in temperature capability.

5

The scorching happens uniformly across the (small) PCB (except where the board is touching the grill, which evidently is slightly cooler so I’ve been aligning the boards to keep the components away from there).

If a soldermask is the answer, then I guess I’ll have to include it when I order new boards. I’m just surprised that googling turns up nothing about being unable to use the toaster on unmasked boards (e.g. homemade PCBs), so I figure I’m probably doing something dumb.

But maybe the lack of googlable supporting evidence is really because most people are using leaded solder, which isn’t an option for me because I sell these boards (whatever percentage doesn’t have shorts, that is) so they need to be legal in Europe.

Thanks!

John Wilson

D Bit