How to resolder this board? - Page 1
Forum section: electronics / beginners

author: owin
author_rank: Newbie
timestamp: August 19, 2023, 05:53:12 am
content: Hello, I'm having a resolder disaster, The pot on the right needs to lok like the left. I needed to break off and replace the right pot but the board is coated in something and has resulted in a burnt mess. What process should I use to somehow fix this? It's not working at the moment I cannot get it to make contact sometimes it works if i wiggle it just the right way.

author: Psi
author_rank: Super Contributor
timestamp: August 19, 2023, 05:59:40 am
content: I assume your issue is trying to get the current pot out of the PCB?Your iron is not powerful enough and when you touch it to the joint it pulls so much heat away that the solder wont melt all the way through. As you leave the iron on the joint for longer and longer to try make it work things start to burn because the board is only designed to handle that kind of heat for a short time.One option is to first preheat the board to 60-120C using an oven, then take it out and quickly try to unsolder the part.Another option is to buy a better iron.What iron are you using? I'm going to guess its one of the older ones where the tips are detachable from the heating element.Try some isopropyl alcohol and a small brush on the burnt area to clear away the carbon and see if anything of the pad is left or not.

author: owin
author_rank: Newbie
timestamp: August 19, 2023, 06:13:32 am
content: It's a 853D soldering station, so I can use the hot air to preheat if needed.So I guess if it IS possible to solder back onto these coated boards, I should break it off and start again by cleaning it all up somehow. Then preheat and try again.I think I done a lot of the damage in the removal process, a burnt black mess that I'm not real sure how to clean.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 19, 2023, 06:16:35 am
content: Solder a thick copper wire on all terminals to be desoldered, use plenty of solder to cover that thick desolder helping wire (more like a bar than a wire) so to heat all terminals at once. Then cover it with solder and heat it until all the solder is well melted. Use the bigest/thickest soldering tip you have, and a powerful soldering iron. Something like in this video (e.g. at minute 7:00+):Different Method To Remove PCB Connectors/Soldering TutorialMr SolderFixOn the same YouTube channel there are some videos about repairing damaged/lifted PCB traces.

author: Microdoser
author_rank: Frequent Contributor
timestamp: August 19, 2023, 12:42:11 pm
content: Quote from: RoGeorge on August 19, 2023, 06:16:35 amI would add : If there's a coating on the terminals (like some sort of conformal coating), scrape that off with a wire brush, craft knife, or maybe sandpaper. You could use the appropriate solvent, but that would be more for if you know exactly what the coating is. You want to get a nice shiny solder look, then follow the method above and you should be golden.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 19, 2023, 01:58:32 pm
content: Another option is, if you have access to the pins of what you want to remove - as in, they aren't under the component - just snip them off with side cutters.When desoldering, if the part you want to remove is broken then you can sacrifice that to make it easier on the PCB. And likewise, if you wish to save the part from a scrap PCB you can sacrifice the PCB.Even with a ground plane I woudl have thought hot air would handle that pot easily.

author: Microdoser
author_rank: Frequent Contributor
timestamp: August 19, 2023, 09:09:29 pm
content: Quote from: Buriedcode on August 19, 2023, 01:58:32 pmGood point on just snipping the problem away. I've seen contacts do this myself when there was a conformal coating on the board, the coating went black and sort of held the solder in place, couldn't add solder, couldn't take it away. Ended up acting like glue.

author: Veketti
author_rank: Regular Contributor
timestamp: August 20, 2023, 05:51:13 am
content: Many ways to skin a cat. I've found it the easiest way to use Chip Quik SMD removal kit. Idea is similar to that video, but you don't need to mess with the wires and create a pool for the solder. That ChipQuik stuff stays liquid for long time so it is easy to remove many pins.

author: luudee
author_rank: Frequent Contributor
timestamp: August 20, 2023, 06:59:39 am
content: A little bit of Acetone on a Q-Tip, will remove the black gunk (most likely burned flux).No Acetone on hand? Use your wife's finger nail color remover!Cheers,luudee

author: Eraldo
author_rank: Regular Contributor
timestamp: August 20, 2023, 08:00:46 am
content: First of all you need to remove the burned flux from the board and assess the problem.If there are damaged contacts then you need to find the pcb traces going to the pot (assuming it being a 2 layer board) and remove the pcb coating to show the bare copper. From there solder a few bare wires directly to the pins (i assilume the pots are high resistance so u dont need very thick wires).This is of course a last resort thing, not because it's very hard but because why damage the pcb when the solution can be more simple and seamless.Also i would recommend a desoldering pump for desoldering. They do a good job. Also desoldering wire (ifk how it's called, whick or something) works fine too but i find it messy. It's usually just to remove the remaining solder after the pump if necessary or to clean the contacts for the next ic to solder.Good luck

author: AndyBeez
author_rank: Frequent Contributor
timestamp: August 20, 2023, 08:45:36 am
content: I agree with @Burriedcode, hack off the potentiometer so you are left with three short dead-legs that can be pulled on with pliers - not fingers. Ramp your iron up to at least 330C and touch thesideof the iron's tip onto the pin - the side stays hotter. I would suggest adding a touch of new solder (preferably Leaded) to assist in heat transfer and to smoke off that dead flux. Pull away the pin downwards - gravity is your friend.Add a desolder pump or 'solder sucker' to your tool kit if you do not already have one - around $10. Desoldering is as much of a skill as is soldering.

author: Ian.M
author_rank: Super Contributor
timestamp: August 20, 2023, 09:22:29 am
content: IMHO Andy is on the right track, but I'd suggest a few small changes:The advice to use the side of the soldering iron tip only really applies to small conical tips. If you are using a bevel or chisel tip use the flat. If you've got a larger bit available use that. In all cases the contact area of the tip *MUST* be tinned with solder. If the side of the tip is oxidised or chromed so it cant take solder, you'll get very poor heat transfer and would be better off using a tinned part of the tip.Cut the *TWO* track end pot legs, as close to the pot body as possible, as the third wiper leg can be pulled out as Andy describes without pliers by pulling on the pot body. That gives you better access for the pliers as the middle pin will be gone before you need to use them.Unfortunately the pliers will heatsink the pin and take a lot of heat out of it making it hard to desolder if you grip the pin while heating it. You'll probably need to heat the pin till the joint is fully melted and its wobbly then quickly grip it and pull it out, keeping the iron on it as you do so. It may be preferable to use cheap side cutters (*NOT* your good ones) to grip the pin to minimise the contact area and thus the heat loss, allowing you to grip it while heating it. Don't squeeze too hard - you want to grip the pin not cut the remnant.Once you've got the pins out, and removed as much solder as possible, (solder sucker then solderwick aka: desoldering braid) you may need to clear the holes. Heating the pad from whichever side has the most copper and pressing through an appropriate diameter lightly oiled mounted steel needle can help. Wiggle it as it cools so its not tight on the hole. Once the hole is open, you should be able to clean the pads with solderwick without closing the hole.Post a new closeup of both sides of the board once the pot is removed and you've cleaned up as much char as possible. I fear there may be serious pad damage.