As promised, here’s what I’ve been working on today:
The power stage of a forklift motor controller (replacement cost ~2800 $). Bonus points if you can spot the issue!
Easy enough, right? Just solder on that binding post, Bob’s ya auntie, right? Well, yes and no. The question is, how do you solder these solid copper slugs without access to vapor phase soldering equipment? Making matters worse, the substrate of the PCB is alumin(i)um, which soaks away a lot of the heat you put into the solder joint.
This board is actually a returning patient. I previously attempted to solder this binding post back on, but the repair only lasted for a few months. What I did last time, is I put some solder paste onto the pads, heated up the binding post with a blowtorch until it was hot enough to melt solder and pressed it onto the PCB. While this approach kind of worked, there wasn’t enough heat to fully melt the paste (shown in the following two images).
On to the actual repair!
Step 1: Preheating
When in doubt, preheat! I placed the board onto my hot plate and set the temperature to 100 - 120 °C.
Step 2: Cleaning and Pre-tinning
With a bit of preheating on the PCB, I used my soldering station to clean and pre-tin the solder pad and binding post.
Step 3: Soldering
Now for the most important step: the actual soldering.
After the cleaning, I added a generous amount of flux to the solder pad. Using a blowtorch, I heated up the binding post until felt like it was hot enough and I then quickly pressed it down onto the PCB, which was still on the hot plate. The first time I did this, I didn’t use enough heat, so I had take the binding post off again (using the blowtorch, because no mortal soldering iron or hot air station is up to that task) and repeat the process. The second time round, I got a decent squeeze out of solder which tells me, I got a proper joint.
Round 2 - Fight!
For good measure, I checked if the other binding posts were still fully attached and sure enough, one of them was starting to detach, so I took it off (again using the blowtorch) and repeated the entire process, this time achieving a proper solder joint on the first try.
Here is what the completed board looks like after I removed the flux residues: