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?

  • 2
    \$\begingroup\$ Difficult process this is... Perhaps a photo? \$\endgroup\$ – Dzarda Jan 30 '15 at 16:31
  • \$\begingroup\$ They used to make soldering irons of various shapes that would heat all the pins at once. (I gather you're already using a solder sucker or solder wick to remove the solder.) \$\endgroup\$ – Hot Licks Jan 30 '15 at 19:46
  • \$\begingroup\$ @Dzarda: If you can help me repair my phones voltage regulator... \$\endgroup\$ – PlasmaHH Jan 30 '15 at 22:53

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.

  • \$\begingroup\$ Wow, thats really a cool thing. They state: Removal Alloy Composition: Sn12/Pb18/Bi49/In21 and Alloy Melting Point: 58 Celsius (136 F). This should definetly save the plastic. A quick search around shows that currently price+shipping is kind of prohibitive, but since they give the formula, I might be able to mix a bit on my own for much less. I will definetly set aside this project until I can test this thing. \$\endgroup\$ – PlasmaHH Jan 30 '15 at 23:00
  • \$\begingroup\$ Plain old lead (SnPb) solder also helps a little when added to RoHS joints. Sometimes it's better to remove all the solder and sometimes it's better to try to keep a big blob on and remove the part while the blob is liquid. \$\endgroup\$ – XTL Nov 11 '15 at 9:15

Easiest thing to try is hot air. Using a rework station, apply hot air to the underside and that will hopefully heat all the pins at once so you can ease the transformer out with pliers. I can usually remove anything with hot air.

Basically you want to heat all the pins (or at least all on one side) at once, so they come up as a group. IF hot air doesn't work then there are soldering iron or soldering gun tips that are T shaped so you can heat a wide area at once. This may be the lowest-cost option since even a cheap hot air station or a temperature controlled heat gun is around $100.

Worst case, if you really need to get these things out without damaging the board and the above doesn't work, you can make a tiny solder bath the size of the transformer and push it against the bottom of the board. Kinda like wave soldering in reverse.

  • \$\begingroup\$ I like the "tiny solder bath idea" but it would probably be too hot for the transformer bases plastic. However since this isn't time critical I will give that other answers quick chip stuff a try. \$\endgroup\$ – PlasmaHH Jan 30 '15 at 23:02
  • \$\begingroup\$ Yeah, the tiny solder bath is my favourite so far. If it's hot enough, it'll be quick and possibly save much of the plastic. \$\endgroup\$ – Dzarda Jan 31 '15 at 14:12

It's hard to tell without a photo. Assuming this is through hole technology, I would proceed as follows:

  • arrange the board with transformer underneath and solder on top.
  • make a solder blob across each set of three pins. Add 60/40 solder until it's a big blob.
  • with the biggest tip in the iron, alternately heat the two solder blobs.

Once all six pins are molten at once, the transformer should fall out by its own weight. If that isn't sufficient, add weight somehow. A Vice-Grip pliers, or pliers and rubber-band combo, or woodworking clamp, etc. Sometimes there is glue under components like this, which will require more force to break.

After the transformer is out, clear up the excess with sucker then braid.

The point is to think about adding solder, not removing solder.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.