Desoldering advice - Page 1
Forum section: electronics / repair

author: antenna
author_rank: Frequent Contributor
timestamp: April 24, 2024, 03:57:41 am
content: I recently took on a repair and I am dealing with solder I have never encountered. It is a scooter that quit working, error code states low motor voltage, even though the battery is good and fully charged. I removed the controller, the mosfets appear fine, so I suspect the relay contacts. Unfortunately, the solder that was used barely melts at the max setting on my soldering iron (480°C) and there are surface mount components including electrolytics all around the relay and underneath it. To make things worse, the relay is soldered into plated vias. So far, I have used an entire stick of ChipQuik with a solder sucker and managed to get the coil leads free and most of the solder off the contact legs, but I struggle getting the solder inside the vias to come out or even melt. The relay won't budge. I plan to keep going with the chipquik alloy, but I wonder how many rounds of getting them hot it can handle before I destroy other things. Does anyone have advice for a situation like this? I would try hot air, but there are SMD electrolytic caps and other chips right next to the relay and I fear hitting it that hard will do damage to them as well.edit: another concern I have is whether or not the chipquik is getting in the via and that the relay is simply glued down, but given how many rounds of chipquik it took to get the bulk of the solder off the legs would suggest its just that high of MP. I want to try to get under the relay and pry, but there are SMD resistors under it and I don't know what might peel off with the glue that may be there. They built this thing to not be repaired. I am tempted to decap the relay to check the contacts and cut it out if bad.Any suggestions would be appreciated, thanks!

author: indeterminate
author_rank: Regular Contributor
timestamp: April 24, 2024, 07:12:31 am
content: You need to yous a very large iron300w 1/2" diameter tipsumthing like thishttps://www.ebay.com.au/itm/295097210355get it up to temperature and then turn it offmake a heavy copper wire bridge between all the pins that need to be desoldered at least 2.5mm2that way you can heat everything at once and pull the relay out.The tip of the soldering iron should be at least 1/2" diameter and 2 > 3 inches long , including the bit inside the element.

author: nvmR
author_rank: Regular Contributor
timestamp: April 24, 2024, 07:55:20 am
content: For TH devices, use a solder wick or solder pump to remove the solder from each and every leg individually.You can also try to place a heat plate beneath the pcb, and give it a preheat.A large blower can also give a good preheat.

author: shapirus
author_rank: Super Contributor
timestamp: April 24, 2024, 08:05:42 am
content: Another method (or rather an additional step) is to use low melting point solder, or simply Rose's alloy, by adding it to the joints and letting it mix with the old solder. The resulting mixture will melt much easier.But after the job is done you will need to remove all the remains of this low melt mixture as thoroughly as possible: you don't want it to compromise the new solder joints you make when you reassemble the thing.

author: antenna
author_rank: Frequent Contributor
timestamp: April 24, 2024, 12:25:00 pm
content: The chipquik is low melting alloy (around 80°C), but it does not like going in my solder wick (MG Chemicals SuperWick #453). I do have a propane torch soldering iron tip that is a half inch diameter, I will try that route. The problem with heating the whole thing to the necessary temp is the surface mount parts under and near the relay. I also worry about lifting the foil off the PCB. There are many SMD parts around and under the relay and I cannot risk them all falling off.

author: Psi
author_rank: Super Contributor
timestamp: April 24, 2024, 12:49:31 pm
content: What soldering iron and iron tip are you using?It sounds like one or both are not up to the task.They make more of a difference than most people think.

author: shapirus
author_rank: Super Contributor
timestamp: April 24, 2024, 12:57:12 pm
content: Photos of the board would be nice btw, to see what we're talking about.

author: donlisms
author_rank: Frequent Contributor
timestamp: April 24, 2024, 04:11:27 pm
content: The description screams "Need more power!" to me, also. Just because you set your iron to 480 doesn't mean that's what you're getting!

author: modoran
author_rank: Regular Contributor
timestamp: April 24, 2024, 10:57:02 pm
content: Use a preheater or hot air simultaneously to get the whole board arena to at least 80C and the relay will be desoldered much easier.No smd part will fell off even at 230C due to the surface tension. But however aluminium caps will not withstand that .

author: antenna
author_rank: Frequent Contributor
timestamp: April 25, 2024, 02:13:21 am
content: Using an X-tronic 5040-XTS-XR3. Have not tried bigger propane soldering iron yet, just got home, will try tomorrow. I will go out to the shop and get the board and take pics now. I like the idea of preheating, will try that too.

author: antenna
author_rank: Frequent Contributor
timestamp: April 25, 2024, 02:34:28 am
content: I may have damaged one of the SMD resistors under the relay trying to pry as I heated as I didn't notice or expect them under there on my first try, but I think you guys are right and that I need more thermal mass and I will try the bigger soldering iron tomorrow.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: April 25, 2024, 02:58:18 am
content: I would take a hacksaw and start cutting off the relay.While it doesn't look like you can cut it very close to the PCB, it's probably sufficient to turn this into a one pin at a time problem.

author: Smokey
author_rank: Super Contributor
timestamp: April 25, 2024, 03:17:21 am
content: hot plate?

author: djsb
author_rank: Super Contributor
timestamp: April 25, 2024, 08:17:10 pm
content: Have you got a replacement relay? If so, maybe you can remove the plastic casing to get access to some of the pins. If you have access to the pins on the other side of the relay, then you could use another soldering iron at the same time. Also, it also helps to apply liquid flux from a flux pen when using desoldering braid. The best way is a vacuum desoldering iron and preheat.PS Get yourself some GOOT-WICK or Techspray desoldering braid. You can get it in different widths. Good stuff.https://en.goot.jp/products/detail/cp_3015_10

author: antenna
author_rank: Frequent Contributor
timestamp: April 25, 2024, 11:10:38 pm
content: I don't have a replacement yet, I just wanted it out so I could energize it and test it without backfeeding the board or poking at it while the board is live. I will be giving it another try tomorrow using a butane soldering iron. If that don't work, I will get out the heavy copper one for the propane torch. And if that don't work, I will dremel it out.My only desoldering tool besides the wick I mentioned is one of those Engineer SS-02 suckers from mouser. Works ok, some day I will get a legit vacuum one

author: strawberry
author_rank: Super Contributor
timestamp: April 25, 2024, 11:39:13 pm
content: lead solder. 80W should be minimumif relay do not operate and coil is not open then something is wrong with control circuit

author: tooki
author_rank: Super Contributor
timestamp: April 27, 2024, 02:02:58 am
content: You absolutely need auxiliary heat!Think of it like this, assuming the original lead-free solder: you need to raise the joint temperature by over 200°C (ambient 20C, melting point 227C). Right now, ALL of that energy has to flow in through the soldering iron tip, and you’ve got a big board sucking heat away on all sides. Even if your iron can reach 450C at the tip, all this means is that you’re overheating one spot on the board, while still not heating the joint as a whole sufficiently.Now suppose you use a small oven or a hot air gun at a gentle temperature (say, 150C) to heat the entire board to 100C. Now the iron only needs to increase the joint temperature by 130C. Or preheat to 150C and the iron only needs to add 80C to the joint. Those are MUCH more reasonable numbers, and mean you’re less likely to damage the board.And for sure, use a bigger tip. Bigger tip = better thermal conductivity = lower tip temperature needed = less risk of board damage.

author: antenna
author_rank: Frequent Contributor
timestamp: April 27, 2024, 03:03:20 am
content: I thought I would get to it today, but I didn't. I did manage to find my big soldering iron tips for the propane torch, so I will give that a try. My soldering station has an IR preheater, so I will do as suggested and get it warmed up before taking the soldering torch to the leads. I may just replace the electrolytics and not worry about overheating them. Tomorrow is supposed to rain all day and I need a break from what today has been, so hopefully I will have that relay out.Given my next attempt will be with a much larger soldering tip and using preheating, what would be better, adding regular solder to transfer and hold the heat or trying more low MP alloy?

author: shapirus
author_rank: Super Contributor
timestamp: April 27, 2024, 06:59:36 am
content: Quote from: antenna on April 27, 2024, 03:03:20 amIt depends if anything in that area of the board is expected to get hot during normal operation. If it is so, then I would only use low-melt solder when there's still not enough heat transfer for the regular solder to melt. The remaining traces of bismuth can potentially lower the melting point of the post-repair joints enough to cause reliability issues if they get hot enough.

author: fmashockie
author_rank: Frequent Contributor
timestamp: April 27, 2024, 02:12:59 pm
content: After seeing the photos, the reason you are having so much trouble desoldering this relay is because look how beefy the traces are that it connects to. You need a lot of heat to get those relay leads removed. When I am working on boards like this (with thick power traces) I use a combination of hot air station and my iron. I hold the iron on the lead to be desoldered, and hover the hot air over it. This probably isn't good for the tip (I haven't seen any issues with my tips and I've done this fairly often), but this works. Of course using a larger iron could possibly work. But this method works fine with a Hakko FX-888 station and a decent hot-air station. When I am dealing with multi-lead components like a relay that you can't just pull out one leg at a time, I also use solder wick to suck up the solder at each leg. Again, you are only going to be able to do this with enough heat. Use flux as well and continue to wet the wick with flux to get the solder to adhere to it and away from the leg to be desoldered.The same goes for getting that relay back on the board. You're going to need a lot of heat to get it soldered back on. Same method works. Add solder with the iron, then heat it up with both iron and hot air to get it to form a nice joint. Obviously do this for as short a time you can to not overheat the component.

author: KrudyZ
author_rank: Frequent Contributor
timestamp: April 27, 2024, 05:00:43 pm
content: I don't get why you are trying to save the relay.You can certainly test its function while soldered in place.If it tests good, then it's not your problem.If it's bad then just get it out by all means, the easiest is to deconstruct (destroy) it from the top by sawing grinding snipping whatever works.Then you can work on each solder joint separately, which should be trivial.

author: antenna
author_rank: Frequent Contributor
timestamp: April 28, 2024, 06:54:47 pm
content: Thanks for the preheating and bigger soldering iron advice, the relay came right out.It tested fine, all components under it tested good too. I used some 63-37 and wicked it off to remove the low MP alloy, then put the relay back in with some Sn Cu Ag stuff that melts at 237°C instead of using the 60-40 188°C stuff I usually use. I found no continuity at the motor, so their "low voltage" error code must have meant low current. It seems that when it detects no current, the relay drops out. Opening the motor and checking the brushes next.I appreciate all the help, thank you!Update: The commutator was dirty and the brushes were stuck. The scooter is fixed!

author: fmashockie
author_rank: Frequent Contributor
timestamp: April 28, 2024, 09:10:34 pm
content: Glad to hear it! And that you got the scooter working!It may not have been the original cause you were thinking, but you got to it eventually. Plus you learned a new skill along the way!

author: tooki
author_rank: Super Contributor
timestamp: April 29, 2024, 03:07:51 pm
content: Quote from: antenna on April 28, 2024, 06:54:47 pmI assume your SnAgCu is actually 227C.Anyhow, in the future, I’d avoid mixing up all these alloys — or at least “rinse” with your final solder multiple times. In particular, what you really want to avoid is soldering with tin-bismuth after using tin-lead: lead lowers the melting point substantially, and can result in an alloy that melts below 100C!

author: antenna
author_rank: Frequent Contributor
timestamp: April 29, 2024, 07:58:33 pm
content: Quote from: tooki on April 29, 2024, 03:07:51 pmLooking at it closer, it is a Sn Cu Bi Ag alloy that I used, and looking it up, the MP is supposed to be between 215°C and 233°C. The surface was cleaned with wick fairly well though and when I applied this stuff, I had to crank it up to get it to flow. I will remember not to mix alloys in the future though. Thanks!