Once you have your PCB fully loaded, it’s off to the cooker. This is called reflow. There’s several different methods, but they all involve heating up the board to around 190 to 210 degrees Celsius.
You can use an infrared reflow oven, a ReflowR, commercial Nitrogen reflowers, expensive vapour phase ovens, or just crack out your electric frypan.A fry pan with a thermistor attached is good enough for the job. If you saw my ReflowR review video, you’ll have heard me and my mate, Kean, talk about temperature profiles. You can’t just slap heat on to a PCB and expect it to solder. There’s a thing called thermal shock – and it can damage components and cause badly soldered joints.So you need to reflow according to a correct temperature profile. It turns out that the humble fry pan has a mostly accurate temperature profile, just by turning it on until solder melts and then turn off when done.The only downside is that not all the frypan heats up evenly. So using a stick to prod it occasionally helps. Note that as soon as I push down in one area, the PCB makes contact with the frypan and then the solder melts. There’s several things happening here… You’ll see here that too much solder was placed on the PCB, but as the solder starts to melt it is drawn into itself.
This is due, thankfully, to surface tension. Without it the whole electronics industry would be very different.
However, sometimes it doesn’t always work like that and incorrectly placed components, (like the MCU), can cause solder bridges.
And some of the larger components didn’t ever reflow, (like the buzzer).
Using a frypan us useful because sometimes components “tombstone”. This is where the surface tension of the solder lifts up a component vertically. Sadly, it never happened for me this time, so I couldn’t show you.
It’s important not to leave it there for too long. So turn it off and keep the PCB in there until it cools down. This is where the thermistor comes in.