I have to replace a couple of small transformers without destroying the PCB or the transformers.
Unfortunately my phone camera is unusable so I need to describe them and what makes the situation so hard for me.
The transformers are almost cubical with a little more than 1cm in size. They consist of two E shaped parts of ferrite, that are sitting on a plastic base. On opposite corners there are three pins on each side, close together (total of 6, lets call them winding pins). On the other two (opposing) corners, there are on each side one pin that is part of a kind of metal clamp (lets call them clamp pins), holding together the E shape. If you remove those clamps, then the E shape falls off, and you have direct access to the wiring. The whole thing is mounted on the PCB with almost no space under it.
Now for my problems with soldering:
- applying too much force will easily break the plastic that the 6 winding pins are molten into
- applying too much heat will loosen the winding pins (plastic melts), and ultimately dissect them from the plastic bottom, which will rip the winding attached to them of. Or when pushed from below, push the pins into the windings.
- removing the clamp pins first is possible, but then part of the ferrite comes of, and the windings are on some plastic tube kind, which will be dangling and ultimately rip of the winding wires from the pins.
- The pins do fit really tightly into the holes, and even when removing all of the solder I can, they are stuck. I have tried to empty one of the holes as good as I can, and clean one of the pins as good as I can, and I need to apply quite some force to get the pin into the hole again.
What worked ok-ish so far is to apply only a tiny bit of pressure from the backside to the pins with a short amount of heat, and then try to support it from the other side with little force until the solder is solid again.
But this takes like 100 rounds until one gets of, and even then the plastic is a bit molten (btw. varying temperature between 250°C and 350°C didn't seem to make any difference here).
What else could I try that allows me to get off (at least most, I have lowered my expectations) of the transformers in one piece?
Best Answer
It sounds as if one of your problems is the high temperature needed to melt the solder. There is a commercial solution called "Chip Quik" ChipQuik that should help.
Basically, this is a very-low melt point solder that is added to the existing solder on the board. The resulting mix of solder metals has a much lower melt point than regular solder and you can usually manipulate the plastic bobbin without melting the plastic while the solder is still molten.
You have to be careful to remove all of the contaminated solder before resoldering the both the removed part as well as the PC board but that is easy with any of the standard solder-removal techniques (solder wick, vacuum desoldering station, etc).
ChipQuik used to send out free samples, enough to remove a few chips. The company was recently sold and I don't know if they still do that - but it's worth asking.
The free sample I received many years ago convinced me that the product was effective and I now keep it around on both my lab bench as well as our rework stations.