Desoldering components from multilayered PCBs - Page 1
Forum section: electronics / beginners

author: soldar
author_rank: Super Contributor
timestamp: May 17, 2023, 05:28:16 pm
content: I need to replace several capacitors on a computer mobo. My experience when trying to remove (desolder) components from multilayer PCBs is that they sink a lot of heat, solder does not melt and I often end up making a mess and breaking what I am trying to repair.It is something I have not tried in some time and this time I would like to minimize the chances of doing any damage. I would like to learn the best technique for this.The through-hole wire and the pads are tiny and it is very difficult to transfer a lot of heat into them. A very fine point is needed but that is the worst for heat transfer and the PCB seems to sink all the heat with ease.I am thinking it might help if I heat the general area of the PCB but I would need to be very careful to not overheat and damage the board.Not only do I have problems removing the old component but then the hole is sealed with solder and, again, difficult to get the new lead through.How do professionals do it? I want to learn and apply a technique that is safe and works reliably every time. I do not want to be fumbling around like I have done in the past. Mainly I do not want to damage the board I am trying to repair.

author: ataradov
author_rank: Super Contributor
timestamp: May 17, 2023, 05:53:49 pm
content: One way to safely remove though-hole capacitors from challenging boards is to mechanically cut the capacitor and remove the leads one by one.Sometimes alternating heating of each pin and "rocking" the capacitor back and forth works.If capacitor is relatively small and you can reasonably cover both pins with the soldering iron tip, then using a bigger tip and a solder blob would let you heat both pins at the same time. And the solder blob provides the best thermal interface you can get.For clearing out the holes I usually use a sharp wooden toothpick. Heat the hole, poke the toothpick though. I never could get the results as good with mechanical solder suckers.

author: Faranight
author_rank: Unknown
timestamp: May 17, 2023, 06:42:11 pm
content: In order to desolder components from a multilayer PCB the board has to reach the melting temperature of solder before components can be removed. You could try using a hot air station and apply heat from the back of the board. Never use hot air directly or in close vicinity of electrolytic caps as they may pop. Another option is to use low-melt solder and have it mix with whatever solder the board is using while using a thick iron tip to help with heat transfer. Using low-melt with proper flux also helps wicking the solder off from the holes and pads. Do not use low-melt to solder as it is too brittle. Use it only to desolder, and use leaded to solder new stuff.

author: aduinstat
author_rank: Contributor
timestamp: May 17, 2023, 09:26:31 pm
content: It kinda sounds like your iron is either underpowered or not temperature controlled. A powerful, temperature controlled iron makes a world of difference. Also, how are you removing the solder? The spring loaded plungers are OK if you have a great big blob to remove, but most of the time you'll want to use solder wick and flux. You should use the biggest tip you can fit in there.The professional way to desolder thru-hole parts is a to use a desolding iron that melts and sucks the solder up. Those are a little expensive, but it's the best way.

author: jmelson
author_rank: Super Contributor
timestamp: May 18, 2023, 02:24:20 am
content: All modern PCBs have lead-free solder, whihc has a higher melting point than leaded solder. You can heat the joint with the iron and apply leaded solder to the joint to dilute the lead-free stuff. Then, maybe suck or wick some of the solder away and reapply the leaded solder. Then, the rework will be much easier. Cutting away the top of the capacitor and applying heat from BOTH sides with two irons, while plucking the remnant of the component lead away with tweezers can work, but it requires a lot of hands!Jon

author: TimNJ
author_rank: Super Contributor
timestamp: May 18, 2023, 03:42:34 am
content: Not sure if it’s always a good first approach, but the “rocking back and forth” method mentioned, until you wiggle it out has worked pretty well for me, for these types of difficult situations. With the wiggle method, you don’t need to apply a huge amount of heat for a long time, as you might need to actually suck or wick the solder out of the holes with the component still there. Then once it’s removed, you can try to clear the solder by wicking or solder sucker, with the advantage of bottom side and top side access. Invariably, you’ll be able to get one hole clean, but not the other stubborn one which is apparently connected to some mega 6 layer ground plane. For this situation, I do the reverse wiggle, where I heat up the (uncleared) hole enough so that I can poke one of the new leads through it while it’s molten.All of this is in the interest of saving the board or yes, those oddly tiny pads.

author: TimNJ
author_rank: Super Contributor
timestamp: May 18, 2023, 03:45:43 am
content: Quote from: ataradov on May 17, 2023, 05:53:49 pmToothpicks! I’ll have to try that!

author: tooki
author_rank: Super Contributor
timestamp: May 18, 2023, 09:17:50 am
content: To answer what professionals do, it’s very simple: preheat the board.The challenge in multilayer boards is that they’re basically gigantic heatsinks, especially the ground planes. So when you try to desolder one pin, the heat you pump in with the soldering iron gets wicked away by the ground plane before the joint can ever melt.The problem with that huge, cold thermal mass is that the speed at which it can suck away heat is high compared to the low speed at which your iron can pump heat in through a single small joint (and through a small soldering iron tip).By increasing the temperature of the entire board, or at least of a significant area around the joint, the temperature differential between your joint and the rest of the board drops dramatically. Instead of having to use a soldering iron heated to some crazy temperature, meaning that the joint also reaches that temperature (e.g. 25C board, and a 425C iron), preheating allows the iron to stay at a safe temperature (e.g. 120C board and 330C iron). This is especially important when desoldering, because the hotter the pad is, the worse its adhesion to the board. If you have to heat a joint to 450C to keep it molten long enough, the danger of pad damage when pulling out the component leg is much higher.I suggest heating the board to 100-130C. That’s still well below the temperatures that damage most components, but it’s a huge amount of heat energy that’s been added. But even if you have delicate components that can’t handle that, even heating to 60-80C is a huge help.If you don’t have an infrared preheater, you can either use an oven (set to your target temperature), or what I often do, just using my handheld hot air tool (set to about 300C) with a broad nozzle and medium high airflow to preheat the whole board, just by using constant motion across both sides of the board.

author: Psi
author_rank: Super Contributor
timestamp: May 18, 2023, 09:28:16 am
content: Assuming TH capacitor..There are a few approachesThe correct way, use a pre-heaterWithout a preheater use dual irons, one for each leg.The hacky way is to rock the cap to work it out a bit at a time while moving the iron between each leg. Once the cap is removed clean the holes out with solder wick. However this will likely only work for 1 of the holes as the other will be ground plane and very hard for the solder wick to work on. Thankfully that is ok, trim the legs so the one going through the cleaned out hole is longer. Now poke it in and align so the other leg is pressed against the blocked hole. (you can make a divot with the iron) Now heat that hole up from the other side with the iron while applying a little force until it goes through. Now tidy up and your done.The dirty dirty hack is to pull the capacitor apart so it leaves its internal legs attached to the pcb. Now bodge the new capacitor in and solder its legs to those floating leg ends

author: soldar
author_rank: Super Contributor
timestamp: May 18, 2023, 04:38:59 pm
content: Quote from: Psi on May 18, 2023, 09:28:16 amThanks to all for the comments. I realize I am woefully underequipped for the task. I have a couple of very small and underpowered irons. I have used before the technique of gradually rocking back and forth between the two terminals but often it just ends damaging the delicate pads.Getting the right desoldering tool for this one job is out of the question for me and I am thinking it might be worth it to take the job to someone with the right tools and save me a lot of aggravation and risk.Failing that I am thinking I should hold the board securely, vertically, in a vise and preheat it without damaging it (easy to say). I do not have a heat gun but I think a hair dryer might be sufficient or maybe a propane torch used with great care, at a safe distance. With the board preheated I think the job will be easier.Then I should use a very hot iron but I have what I have. I have a big 150W soldering iron and I could grind the tip down to what I need for this job but that would ruin it for normal use. I might look into the possibility of buying a more powerful and temperature controlled iron. I need to think about this.

author: Gyro
author_rank: Super Contributor
timestamp: May 18, 2023, 06:34:59 pm
content: If at all possible, I would lose the capacitor bodies and take each lead out in one action.The problem with walking or rocking components out (apart from the danger of losing top side pads as you mentioned), is that with each 'walk', you have less and less lead length available on the bottom side of the board. It is the solid leg that transfers the heat into the solder in the plated through hole, once you run out of that, you can't get much heat into the joint through just the bottom side pad. That's the point where you can get terminally stuck without good equipment.

author: tooki
author_rank: Super Contributor
timestamp: May 18, 2023, 07:15:31 pm
content: Quote from: soldar on May 18, 2023, 04:38:59 pmDo you have a kitchen? Do you have an oven in that kitchen? Then you can do as I already suggested: preheat in the oven at ~120C.A hair dryer doesn’t get hot enough. A blow torch is a terrible idea.

author: soldar
author_rank: Super Contributor
timestamp: May 18, 2023, 07:47:39 pm
content: Quote from: Gyro on May 18, 2023, 06:34:59 pmYes, I have had bad experience with rocking them out.I still have time before I get the new caps but, since I really do not have a powerful enough iron, I am thinking of taking some solid and hefty bar stock and grinding the tip flat to just the right size that it can reach both leads at the same time. The I can heat it up with a blowtorch and it should have plenty of heat stored.I might take some old board and experiment a bit.

author: wraper
author_rank: Unknown
timestamp: May 18, 2023, 07:58:46 pm
content: For electrolytic capacitors it's preheating and wide tip, like 8mm+ so it can desolder both terminals at once. Dual wielding soldering irons does not work for this because you need another hand to pull the capacitor out. Unless you're creative and make some spring actuated contraption which pulls the cap out once solder melts.

author: soldar
author_rank: Super Contributor
timestamp: May 20, 2023, 09:54:42 am
content: Update. In the past I had used my low-powered fine point irons and, as I explained, it was a mess using the alternative lead rocking system.I decided I was going to try massively heating both leads at the same time and for that I needed a suitable, more powerful, iron with a flat tip, which I did not have. I was about to grind a flat, angled tip on a piece of brass round bar stock when I remembered I had thrown in the trash a 60 W iron of the "hammer handle" type. I found it and ground the tip as I wanted it and then proceeded to heat it over the gas kitchen stove.With the tip well tinned and hot and making sure the tin wetted both component leads at the same time, after a few seconds so the heat could transfer, the capacitors would pull out quite easily. Maybe needing a bit of rocking motion but no significant force.I have pulled all ten that need to be replaced. I have left in place a few more that do not show any signs of damage.Now the problem will be cleaning up and clearing the holes because they are clogged with solder and quite messy around the edges.When doin this kind of work, pulling components out of boards, I find very useful those locking plyers that surgeons use. I can lock them onto the component I want to pul and it makes it much easier.https://en.wikipedia.org/wiki/Hemostat