I’ve been working on my techniques soldering whole boards using hot air reflow. I’ve made many mistakes in the past, many habits I still have. I was very impatient at the start -- cranking the heat way up and toasting the living hell out of any parts I was soldering. Then I learned to use the preheating station, which helped a bit, but I still needed to add too much extra heat with the hot air. Then I realized that the temperature reading on the front of the preheat plate doesn’t reflect the PCB temperature, which is what I really want to know. It’s been a frustrating, iterative process.
My most recent upgrade has been to use an IR thermometer to read the surface temperature of the PCB. This has helped my accuracy quite a bit, but the readings were still unreliable. I was digging around in my parts bin and I came across an old tube of Artic Silver 3 thermal paste, used for building PCs. It was a small Eureka moment -- what if I bonded a temperature probe to the PCB? I tried out a few things today, and this is the workflow that I came up with:
- Screw the board down to the preheating station. This is a prototype board for work, so I can’t yet post any details. This is the bottom side of the board where I’ll be soldering a Samtec QTE connector.
- Next i’ve been experimenting with clamping a Fluke Temperature Probe to the PCB, using a little bit of extra thermal paste to ensure an accurate measurement. I’m using a spare alligator clip from a test cable, which seems to be able to take the heat.
- Next I apply some solder paste to the board. I’ve been using some ChipQuik lead free solder paste and I haven’t been terribly pleased with the results. It’s either come out too runny or too thick for what I need to do. This board I added too much solder and had to wick it away afterwards. The next few boards I used a tissue to wipe away excess solder paste. Not ideal, but it worked.
- My next innovation -- adding a cover to the preheating station! I noticed that the preheating station was not effective at actually heating the board up -- too much heat was allowed to escape around the edges of the PCB. Adding a simple, 2 layer shield of tinfoil has made a dramatic improvement. I’m actually able to get the PCB up to reflow temperature -- I don’t even need to use the hot air gun in many situations.
- You need to be careful heating this board up. There are a variety of factors that make it difficult to regulate temperature reliably. The preheating station has a lot of overshoot when trying to maintain temperature, especially when warming the board from cold. I found it more effective to raise the temperature in 25-50C chunks. Also, the thermal mass of the PCB ground plane causes a lag between the hot air and the PCB actually rising in temperature. To compensate for this, i would watch the PCB temperature and would increase the temperature when I saw the rate of increase slowing down. The rule of thumb I came up with -- increasing the temperature by 25-50C at each point in which the preheat station temperature gauge starts falling.
- Beforehand, I checked the reflow temperature curve for the QTE part, looking in the back of it’s datasheet. It indicates that reflow zone should occur above 217C, and should go no higher than 235-255C. I dramatically removed the cover when the multimeter read 210C. Lo and behold, the solder had already reflowed! I used the hot air gun (set at 260C) to ensure that the part was fully warmed, then I tapped the PCB a little bit to jiggle the part into place. This is another black art of PCB rework -- the surface tension of the solder pads is so great that with the right amount of vibration the part will center itself! It’s not easy to do perfectly -- so often i’d over-tap and the part would slide off the wrong direction…
- Another mistake I made was trying to rework the board while it was at reflow temperatures. At a PCB temperature of 200C, the solder is still molten enough to give way if the part is moved at all. It’s important to remember to cool the board down before trying to wick away excess solder. I used this temperature combo very successfully -- it’s so much easier to work with the board while it’s warm, even with a soldering iron.
- Done! This was a few boards later, when I got a chance to improve. This was the result of using less solder, heating the board to above 210C, and getting better at my PCB tapping skills. I’m really happy it came out so well, and without the need for extra rework! Bonus.