battery alkali corrosion and a funky repair ... advice sought - Page 1
Forum section: electronics / repair

author: jaunty
author_rank: Regular Contributor
timestamp: September 10, 2018, 03:48:23 am
content: I just posted the other day about this 1970s era digital delay i'm working on and while going through the other boards (contains about 10-12 daughter cards which plug into a backplane connector) to check for seating problems and/or possible shorts or other issues i found this on one of the DAC cards!Two big issues. Someone had 'built up' some thick solder pads on the edge connectors to force it to mate better with the backplane connector presumably due to a contact issue. the second issue is some severe corrosion (either from cap juice - more likely?) or else battery electrolyte which mysteriously migrated across the chassis ... the board resides about as far away from the board containing the battery as you can possibly get .. it seems to resemble battery electrolyte corrosion however.the easier one first. the corrosion. I'm assuming the correct procedure would be to remove all the components in the region - scrub scrub scrub with mild acid (if it's battery alkali) and/or isopropanol etc - then replace any affected components with NEW (in case the existing ones are affected). Correct? And leave no stone unturned?as for this 'pad' business ... i figure the two main options are to 1. leave it alone and hope the interloper who did this had good reason or 2. remove all traces of solder from the connector and then try to pry up or otherwise adjust the pins on the backplane connector to get things connecting better ... or ... option 3??Thanks for your time and help

author: retiredcaps
author_rank: Super Contributor
timestamp: September 10, 2018, 04:07:36 am
content: Since your questions are spread all over, you may benefit from these old Pace, but fantastic (IMHO, the best) soldering and repair videos.They talk about solder joints, fixing lifted pads, corrosion, proper technique, etc.

author: jaunty
author_rank: Regular Contributor
timestamp: September 10, 2018, 04:18:09 am
content: thanks -maybe you could be MORE condescending?or perhaps i wasn't being clear enough - i do understand the problem with using too much bandwidth but i try to come here only occasionally and deal with a few things at once... anyway there's nothing in your suggestion that might address the 'built up' pads and backplane connectors ... i wanted to post separately since it was a completely separate topic ... as for the corrosion - i've done lots of that work before thank you ... but only when i knew what the source of the 'corrosion' was. it doesn't help if i can't diagnose the corrosion source - so i was hoping someone here might be able to confirm or deny the type. (of course i'm quite capable of dealing with it on my own- i was just looking for a yea or nay on that)

author: digsys
author_rank: Unknown
timestamp: September 10, 2018, 04:49:57 am
content: Possibly dripped there from a leaky cap, or a service / inspection once. Just sat there, heated up and slowly corroded. I've seen similar, but for other reasons -1st generation "lead-free" vehicle electronics were a mess. So many metals that didn't get on with each other :-)The only way I found - was to remove the component - clean off all solder / residue with the usual chemicals - first active, then iso, as you said - then we applieda high-flux (brushed on) liquid solder. forgot what it was called - then replaced new components.With the edge connectors - we removed the bodge and again passive plated the entire edge with an active gold / silver solution - enough to thicken it up.Usually also, bent out the edge connector pins as well.

author: jaunty
author_rank: Regular Contributor
timestamp: September 10, 2018, 05:17:06 am
content: Quote from: digsys on September 10, 2018, 04:49:57 amok thank you very much - very helpful ... mostlyor at least gets me running in the right direction

author: tkamiya
author_rank: Super Contributor
timestamp: September 10, 2018, 01:54:27 pm
content: About that pad.... does it really need to be repaired? It looks like it's just a ground plane, and there are at least 6 pads that are doing the same thing AND the back side. If receiving side is doing the same, then you could leave that alone. I'd just clean it well so the corrosion won't spread.

author: jaunty
author_rank: Regular Contributor
timestamp: September 10, 2018, 03:46:18 pm
content: Quote from: tkamiya on September 10, 2018, 01:54:27 pmyes i agree in a way - though it seems 'suboptimal' and is kind of bugging me - i've just never dealt with this kind of thing before... and wonder if refurbishing the edge connectors and backplane might yield a better connection (if done well enough- but therein lies the trick!!)

author: tkamiya
author_rank: Super Contributor
timestamp: September 10, 2018, 03:59:47 pm
content: Well.... once lifted and damaged, PCB traces will never be right. It's especially hard at card edge connectors. They will be under mechanical stress. If I need that particular pin for a functional reason, I'd actually cut off the bad section, substrate and all, and replace that with parts from donner pieces. Either that or foil tape over and around the edge secured on both side.Because of the location, you'll have problem if the fix moves and shorts out the next pin. I wouldn't do half-baked solution. I really would clean it well and pretend it wasn't even there.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 10, 2018, 04:20:49 pm
content: If there was a NiCd battery that leaked, here is the method I have used to correct the problem. I have had older circuit boards with leaking NiCd batteries that caused similar corrosion on leads and runs. If you think there isn't anything on the board that would be damaged by getting the board wet, you could first brush and wash the immediate area with vinegar to try to neutralize/remove the alkali. I've had to use a fine point probe or needle to remove the crud built up between some IC pins but so far haven't had to remove any ICs. Even a stiff bristle small brush or brass bristle brush for open areas might help.After getting it as clean as possible I use soap and water to further clean then alcohol to remove as much moisture as possible then carefully use a heat gun on the area to dry it off. So far this has worked for me but you can decide if this is something you want to try. I would just reheat the solder on the two end connector pads to wipe the excess solder off and be happy with that as a fix on the connector.If there was no NiCad battery in this unit I'd suspect a leaky wet slug tantalum that uses sulfuric acid as an electrolyte so a different solution (sodium bicarbonate/baking soda) would be needed to neutralize the crud. In either case, check for the part that has cause the problem and replace it.

author: jaunty
author_rank: Regular Contributor
timestamp: September 10, 2018, 07:00:12 pm
content: Thanks Arthur - did Zaphod put you up to this???will take it under advisement ... i just pulled the capacitors to put in OEM tantalums and inspect the bottoms to see if they were the source but apparently not ... doesn't look like any venting of weeping from the bottomsso back to the drawing board - must be the ni-cd electrolyte then from a bad ni-cd many years before - though it's located far across the chassis - about 7 boards over - though i did a bit of a brushing with mild muriatic acid last night (with leftover chipquick on the board that i was playing with) and now that i have the little buggers off here's what i'm left with .... ACK!! what a mess ...

author: jaunty
author_rank: Regular Contributor
timestamp: September 10, 2018, 07:09:14 pm
content: also trying to figure out if that 'staining' on the resist isn't actually electrolyte that's weeped UNDER the resist by capillary action ...update - it's resist that's separated from the plating and has wicked god knows what electrolyte or solvent underneath it - so i guess the best thing to do is peel it off since it's very soft now anyway

author: ArthurDent
author_rank: Super Contributor
timestamp: September 10, 2018, 10:23:30 pm
content: If the instrument was stored on its side the leakage might have seeped from one board to the next lower one. Check the edge connectors and backplane connectors carefully because if this crud did travel it might have contaminated the connectors as well. That happened on the HP 3586 unit I had and I had to clean the backplane connectors, which took some work.On one of the boards I had that had a leaky NiCd, the electrolyte did creep under the resist and made it look discolored. I could (and did) peel the resist off in big chunks so I could neutralize and clean the area under the lifted resist flakes. I left the resist anywhere it looked o.k. and hadn't lifted or stained. I wanted to make sure that any chemical reaction was stopped and no further damage was done. The foil on the board didn't look too bad once cleaned so I didn't bother giving it an overcoat of some clear sealer like fingernail polish.I had consulted Marvin the Paranoid Android about what to do but he was too depressed to give me a direct answer.

author: BradC
author_rank: Super Contributor
timestamp: September 11, 2018, 01:32:50 am
content: Quote from: jaunty on September 10, 2018, 07:00:12 pmI'd suggest you might want to avoid hydrochloric acid in future. Even when properly neutralised any remaining chloride ions really have an affinity with copper and will cause further corrosion. Copper chloride makes a great etchant.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 11, 2018, 03:15:16 am
content: Quote from: jaunty on September 10, 2018, 04:18:09 amSorry, my post was not meant in that way, but when I see multiple posts roughly about the same topic, I don't know what your expertise level is. You could be a complete noob or one of the authors in the pace worldwide video.I also don't read all the responses if I'm generally not interested in your topic. You may have clarified what you were looking or later, but I likely didn't read it.My suggestion to view the videos was only to help you get some ideas on what are good practices in pcb soldering and board repair. You may learn a new technique?

author: jaunty
author_rank: Regular Contributor
timestamp: September 11, 2018, 04:38:48 am
content: Quote from: retiredcaps on September 11, 2018, 03:15:16 ami thought long and hard about posting it under the same thread - but it seemed like a disservice to people in the future looking for this or that ... this one was unique enough that i had to seek 'expert' advice on backplane connector repair ... if you didn't read it then I'm not sure how you can make the judgement as to what it's 'about' (?) and that you aren't 'interested' (is this site all about you and your own interests? ididn't know sorry) ... (joking) ... but hey i get also definitely DON'T want to be wasting bandwidth or forum topic space ... i just happened to be on and responded to other, older posts at the same time.I guess the real topic here is PECKING ORDER and the fact that i am relatively new to the forum ... it's an 'old boy's club' ... hey i'm nearly 53.. cut me some slackanyway - all is good and yes i was posting more topics than i would have liked to inside of one week