I’ve been working on some all SMD PCB designs that include TQFP chips recently and although I’ve hand soldered a few SMD capacitors and resistors before I’ve never soldered any SMT ICs so I thought I should get some practice in. I know people say you can solder even TQFP chips by hand but I don’t have the dexterity, eyesight or patience for that so some sort of reflow method was required. The common hobbysist method you see for reflow soldering is using a toaster oven but an alternative method is to use a hot plate, in fact a lot of people prefer the hotplate method to the toaster oven citing better control of the end result as you can see when the reflow occurs and it avoids problems where plastic connectors are involved where the all round indirect heat of an oven can start to melt the plastic before the solder flows properly. There are actually hot plates purpose designed for this but I saw that lots of people had used ordinary domestic hotplates and you can even do this on your ordinary electric hob if you have one, mine is gas though so I ordered one of these cheap single plate electric hobs along with an IR temperature gun to keep an eye on the temperature, total cost under £22, not bad at all. To practice on I ordered 8 pin and 16 pin SOIC chips and a 44 pin TQFP ATmega32U4 along with some breakout boards.
The first thing you need to do is apply some solder paste to the pads for the components. In the case of resistors, capacitors etc you just need to dab a bit on each pad and I found a toothpick was a good application tool. For ICs you probably won’t be able to apply paste for each individual pin but it is OK to apply it across them all together as when the the solder reflows the capillary action will draw it towards each pin so hopefully you won’t be left with any solder bridges. If you do get some solder bridges they are easily dealt with later using some desoldering braid.
I was surprised how little solder paste you need, for the 8 pin SOIC IC I tried first I really overdid it but it was still easy to clean up with some solder braid, the 16 pin SOIC I did next was much better and by the time I did the 44 pin TQFP I had it pretty much sorted and there were only a couple of solder bridges to clean up afterwards. Once the paste is applied it is just a case of carefully placing the components in place, exact alignment isn’t too important as the capillary action will draw them into place as the solder flows.
Now for the actual reflow. I started with the plate already warmed to 180 °C but then turned off, I placed the populated board on and ramped the temperature up and at around 210-220 °C you will see the solder paste liquify and become shiny while drawing the component into place along with a little smoke. Once this has happened across all the pins you can turn the heat off and remove the board with some tweezers and allow it to cool.
Once cooled it is time to inspect the board for any solder bridges which can then be fixed with some solder braid and a soldering iron. Just heat the braid and drag it across the pins and it will absorb the excess solder.
This whole process was really a lot easier than I expected and I can see I will be doing a lot more SMT designs in the future, the hardest part is getting the correct amount of paste on but I’m sure this will become natural with practice, a stencil would make the job of paste application a bit easier but they are expensive to get made professionally although I have access to a CNC vinyl cutter at work so I’m going to see if I can make a DIY stencil as described here.
Here’s the hot plate with the temperature gun to the left.