Burned PCB - Repair Advice Needed - Page 1
Forum section: electronics / repair

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 29, 2015, 06:58:31 am
content: I realized I missed something from the Keithley 193 teardown: this big burned area on the PCB around one of the bridge rectifiers.The charring is visible on the top side as well, and there are some faint smoke darkened areas on the top metal cover near this area.So I decided to replace it with a new one, and this is what happened:My soldering iron is unable to melt the solder on the leads and pads, but did end up melting the partially burned solder mask. Some cleaning with isopropanol was able to remove some of the residue, and now I'm left with what you see. By the way, if you're working on a repair like this, make sure you have good ventilation and/or a gas mask - don't breathe in carcinogenic fumes like I did.At this point, I'm seeking some advice on how to continue the repair. Should I try to remove the surrounding solder mask before continuing with my desoldering? Do I need to turn the iron's temperature up further from 750 F? (Currently using an FX-888D with the default chisel tip) Should I just cut out that entire section of PCB with a rotary tool? Any insights will be appreciated!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2015, 07:30:30 am
content: FWIW, if it's not melting, it may be stuck for good. Copper and solder form a bond with a brittle layer inbetween, called an intermetallic. Normally this is thin, so it isn't very weak. Higher temperatures cause it to grow in thickness, and form more advanced intermetallics. If it grows so thick that the hole plating bonds to the lead, you get, well, basically, metallic cement between them, and you have to rip it out and start over.You may be better off cutting the component off its leads, and tacking the replacement onto the stubs.Tim

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 29, 2015, 08:09:25 am
content: Quote from: T3sl4co1l on October 29, 2015, 07:30:30 amThanks for the info. I had that thought, but the rectifier has zero clearance between it and the PCB. From what I remember from my EDC 522 adventure, that is possibly a cause of the failure in the first place. I suppose I could try and break through the rectifier's packaging.

author: tec5c
author_rank: Frequent Contributor
timestamp: October 29, 2015, 09:47:01 am
content: Just to be sure, did you use fresh solder in your attempt to get the old stuff to melt as opposed to just heating up the old solder?The picture makes it look like you just applied the iron...BTW, 750F should be more than enough heat to melt those pins.

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 29, 2015, 09:55:02 am
content: Quote from: tec5c on October 29, 2015, 09:47:01 amI did, but I only have lead-free solder. Probably not the best practice to mix the two, but it didn't seem to help. I need to try it again with all of the melted soldermask removed, that stuff melts onto the pad and burns, which will clearly prevent the solder from melting.

author: VK5RC
author_rank: Unknown
timestamp: October 29, 2015, 10:12:49 am
content: Some good old leaded solder I find really useful in scrounging parts off unlead soldered boards or maybe even some of that super low melting point stuff you can use to get off multi pin devices with a straight soldering iron?

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 29, 2015, 11:20:59 am
content: Quote from: VK5RC on October 29, 2015, 10:12:49 amYes, I think it would be a good idea to pickup some leaded solder considering how much non-RoHS equipment I pick up.

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2015, 11:42:14 am
content: There's a fair chance the board is carbonised enough to be conductive. I suspect that for long term successful repair, the damaged area is going to have to be cut away and either some extreme board repair done, or the replacement bridge rectifier glued down nearby and bodge-wired.It would be useful to see a top-side photo. I've drilled holes further along tracks, splayed component legs and patch wired in similar situations.

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 29, 2015, 12:48:33 pm
content: Quote from: Ian.M on October 29, 2015, 11:42:14 amYes, that is what I'm prepared to do in the worst case. I can cut a square section of the PCB clean off and glue the new rectifier onto a nearby area with bodge wires soldered onto exposed tracks. I've seen some videos on (proper) extreme PCB repair, but I don't think I'm willing to spend the money on the necessary repair materials.

author: macboy
author_rank: Super Contributor
timestamp: October 29, 2015, 02:05:22 pm
content: I use a big (45 Watt) non-temp-controlled Weller iron with a big wedge tip to do tough desoldering (and less commonly, soldering) jobs like this. The high thermal capacity and heat transfer of that tip make all the difference.If you don't have such a beast, then you can do quite well with pre-heating. Every degree hotter to make the joint/component prior to applying the iron is one degree less that the iron needs to heat it. Try heating the board with a hot air gun, even the paint-stripping hardware store kind, prior to desoldering. Or here's another idea: take a rectangular ceramic power resistor like 10 W, 10 ohm, strap it directly to the rectifier, and crank the power. After a while, the component will be very hot, meaning that it won't draw nearly as much heat away from the joint you are trying to melt.I also agree with others that you will need some good quality lead based solder. Always generously apply fresh solder when melting old nasty joints. A eutectic 63/37 solder is a good choice as it has a lower melting point and solidifies instantly. Get a solder withlotsofgoodflux.Lotsmeans a "66 core" ... this means that the flux core is 66% of the diameter of the solder wire, which is the most you can get. The only time you would want a lesser core is in production where you know for fact you can always get away with less, and you want to have less residue to deal with. On the bench, and especially if you are new-ish to soldering, more flux == better joints, and you can just deal with residue if necessary. Kester 44 is agoodone, the flux is a highly active rosin ("RA") so it can cut through all kinds of old corrosion and wets easily to almost anything, but is still generally considered a no-clean type. When desired it cleans very easily with isopropyl. Of course there aremanyopinions about flux and solder but it's really hard to argue against having a roll of Kester 44 in 63/37 on any bench. The only valid argument is that's it's a strictly RoHS compliant bench.

author: AF6LJ
author_rank: Unknown
timestamp: October 29, 2015, 02:24:43 pm
content: Is this a two sided board or a multi layer board??Trying to desolder this part is a loosing proposition ether way.If it is a two sided board you can live with some of the potential damage and wire in the new part in its current location.

author: Ian.M
author_rank: Super Contributor
timestamp: October 29, 2015, 02:34:28 pm
content: Quote from: nidlaX on October 29, 2015, 12:48:33 pmIf its a double sided board, the repair materials aren't expensive: either bare (scrap) or copperclad FR4 of the same thickness, and dollar store epoxy (any clear one in a two barrel syringe will do).Cut out a rectangular area and carefully file the corners as square as possible. Mark out another rectangle outside it by five times the board thickness and either file an even bevel to the line or grind a step to half the board thickness. Cut out the patch to the outer rectangle size, mark out the inner rectangle on it and bevel or grind to the same profile. The more accurate you are, the better the result.Clean prepared surfaces with acetone and a Q-tip. Epoxy in place. When fully cured, drill new lead holes.If using copperclad, the track pattern should have been replicated by etching or mechanical removal of unwanted copper. If using bare board, replicate the track pattern with copper foil tape or stiff wire, epoxied down.If its multi-layer, you are probably totally FUBAR - or at least it will need a lot of bodge wiring

author: steve207a
author_rank: Contributor
timestamp: October 30, 2015, 03:41:23 pm
content: Hi the centre 2 pins look bad so i would cut them out using a dremmel and get rid of the charing you can probably still use the outside pins ok but first to remove the component easily i would brush on some flux i use powerflow flux sold to plumbers comes in a yellow case really good flux helps to unsolder old joints helps break down the old solder and get it flowingmake sure you wipe off any flux left behind as can corrode if left on boardI find using the flux very good on large joints or where the solder finds it hard to flow over the jointI use a small brush to appy it but do wash it all off afterwardssteve

author: GNU_Ninja
author_rank: Regular Contributor
timestamp: October 30, 2015, 03:59:24 pm
content: Quote from: Ian.M on October 29, 2015, 11:42:14 amWhat he said ^^^Get rid of the carbonised areas and bodge wire in another bridge rectifier.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: October 31, 2015, 01:44:34 am
content: It can be removed, but your going to need to add leaded solder at high temperature with ton of flux, lead free solder has a higher melting point and will just make it more difficulty to desolder if anything and heat at higher temperature. I also recommend cutting the component off, as help eliminate some thermal mass. You going to also have to get rid of the carbon around that area, as can be conductive and may have to run a wire and jump old pads. I dealt with a lot worst that was completely corroded due to bad capacitors. and repair traces to the point of new and remask area. So don't give up on it yet.

author: nidlaX
author_rank: Frequent Contributor
timestamp: October 31, 2015, 06:37:08 am
content: I think I may follow Ian.M's advice and do a repair using copper clad board and epoxy. It will be instructive, and I also need to do a similar repair on my EDC 522.

author: Ian.M
author_rank: Super Contributor
timestamp: October 31, 2015, 09:37:15 am
content: Practice on a scrap board first! FR4 is *extremely* hard on cutting tools, so a set of diamond files is strongly recommended. Also the dust is both bad for you and for any moving parts (switches, pots etc.) so rig effective vacuum dust removal as close to the tool as is practicable.

author: nidlaX
author_rank: Frequent Contributor
timestamp: November 01, 2015, 07:27:03 am
content: Quote from: Ian.M on October 31, 2015, 09:37:15 amThat's the plan! I'm going to try and do this outside near the exit of my garage. With all the scrap I've collected over the years, it should be easy to find something to practice on. When I work on the instrument, I can probably rig up a vacuum setup and/or tape over the rest of the digital board to minimize dust ingress in other parts.A big thank you for all of these tips, it's given me the confidence to go through and attempt this repair.

author: Ian.M
author_rank: Super Contributor
timestamp: November 01, 2015, 10:19:32 am
content: Be aware that you do need to fix the cause of the overheating problem properly (not an issue if its a one-off component failure) as most epoxies soften significantly at high temperatures and will loose much of their strength above 100 deg C. You can get high temperature epoxies, but wont find them in the dollar store!

author: mzacharias
author_rank: Frequent Contributor
timestamp: November 01, 2015, 01:55:26 pm
content: It may well be possible to bend the rectifier back and forth until the device separates from the board, leaving only the stubs remaining. As to removing the stubs, that will be difficult but getting the body of the rectifier out of there will help.