Aylo investigates making of a hot plate using FR4 and aluminum PCB materials.

Both the aluminum and FR4 heaters were able to reach 200c depending on the conditions. Both were capable of melting solder directly on the plate, as you can see. The aluminum heater would benefit from an improved (lower resistance) heater trace.

The remote probe feature allows for the sensing of the temperature of the PCB being soldered. This is good, since that is where the temperature actually matters. This can cause the heater to get much hotter (20c or so) than the top of the target board, which in the photo below, resulted in some discoloration of the solder mask of the heater.

Its clear that hot plate soldering is not the ideal way to solder- something like a toaster seems a lot better and faster, since it can more evenly heat the target PCB. The main issue is that in order to get the top of the PCB hot enough to reflow, the bottom has to be quite a bit hotter. This is worse with traditional lead-free solders, since they need to hit about 220C. Lead-Tin eutectic solders need to hit 185 or so, and bismuth lead-free solders only need to reach 140 C (there is a medium caveat here).

You can see the results of this heating on an adafruit board (below), that I gently reflowed. The left board is new from adafruit, and the right one is reflowed. It didn’t really damage the board, but it certainly would be an unacceptable outcome if a manufacturer overheated a board like this, and the lead free solder (I assume) on top barely reached the liquidus point, and would re-solidify as soon as I moved a component.

Read the full investigation in the article here.