Relocating through-hole capacitor - TH pad missing - Page 1
Forum section: electronics / beginners

author: halfmoonmissing
author_rank: Contributor
timestamp: May 06, 2023, 07:14:11 am
content: Hi all,I bought a 2nd hand motherboard from Ebay. When I went to recap it I noticed one capacitor was missing both eyelets (I believe that is what the are called) for positive and negative.I was thinking just glue the capacitor and run a cable from each leg to their respective part. Can I do this, or should I buy eyelets and replaces or are there other options.Am a newbie, declaring that up front.Thanks

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 06, 2023, 09:09:14 am
content: Quote from: halfmoonmissing on May 06, 2023, 07:14:11 amYou mean the copper pad around the through hole on the board? With these capacitors being the larger through-hole kind with wire legs?

author: halfmoonmissing
author_rank: Contributor
timestamp: May 06, 2023, 09:30:48 am
content: Yes, that is it. I did know what it is so had to look it up and found eyelets as a description.There is nothing to solder to and keep the capacitor in place on both sides.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 06, 2023, 10:12:52 am
content: I recommend you edit your first post, changing the thread title to say"Relocating through-hole capacitor - Pads gone"or something.I'm a pure hobbyist myself on the electronics side, but I would consider using silicone caulk/adhesive (platinum curing, safe for PCB use one!) to glue the capacitor to its original place; the same gunk often used to keep capacitors in place anyway. Maybe MG Chemicals 1035 Premium RTV Silicone Adhesive Sealant or Loctite SI 5084?

author: m k
author_rank: Super Contributor
timestamp: May 06, 2023, 11:14:50 am
content: "TH pad missing" would be better, component type is not relevant.Quote from: halfmoonmissing on May 06, 2023, 09:30:48 amI don't understand this.Surely there are holes and bent legs will keep the component in place.Soldered legs can keep components in place.If environment is vibrating much or component is tall or heavy and needs more support it's not a glue between the PCB and component bottom.Wave soldering is moving components when it happens and if component is tall enough its center of gravity can move so much that it becomes disoriented.If pad is missing and no trace is present the pad was more like a cosmetic thing.That pad is then not needed but possible new soldering is better to do from the other side.Other side because there it is easier to see when tin falls in.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 06, 2023, 12:21:49 pm
content: Why are you "recapping" a motherboard? Is it broken? If yes, do you think that particular capacitor is the problem? No, you can't just wire it up somewhere. Changing a design requires as much design competence than designing it in the first place, and layout parasitics are important.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 06, 2023, 12:36:29 pm
content: Note that not all caps marked on the PCB are always populated.Quote from: Siwastaja on May 06, 2023, 12:21:49 pmAlso antenna effects, considering most of the larger caps are used to filter out noise, maybe?I'm assuming something like battery leakage has eaten the pads for a nearby cap. Then, using the same holes, but recreating the traces using wire might work, if one also glues the cap in place using suitable silicone adhesive.Adrian Black does such repairs to old motherboards on Youtube every now and then:video list.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 06, 2023, 12:41:55 pm
content: Quote from: Nominal Animal on May 06, 2023, 12:36:29 pmExactly - changing the layout by adding wires to power decoupling capacitors is an excellent way to destroy the EM compliance of the device. And quite obviously, the capacitor with added inductance will act worse as a filter, leaving more voltage ripple, also stressing the other capacitors on the same DC bus more so they fail sooner.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 07, 2023, 12:07:46 am
content: Quote from: Siwastaja on May 06, 2023, 12:41:55 pmThat is fine mate, the part isn't going into a vital production system or being resold. This is a learning experience for me - I think of it as a can do. "I don't want to say too hard, I've wasted my money I have to throw it out" without trying to learn and do.Appreciate your feedback.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 07, 2023, 12:08:36 am
content: Quote from: Nominal Animal on May 06, 2023, 12:36:29 pmThe board I purchases was without the caps. I assume that is why it was being sold cheap. Whoever had it last ripped of the TH pad themselves. Thanks for the Adrian Black video's. I'll see if he covers it.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 07, 2023, 12:09:24 am
content: Quote from: Siwastaja on May 06, 2023, 12:21:49 pmI bought it from Ebay. I knew from the picture it had no capacitors. The price was cheap and its a great a experience.This isn't a vital part for SpaceX. It's a way for me average Joe to enjoy and learn something. But yeah I don't know how to deal with this one so any help would be appreciated.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 07, 2023, 12:10:25 am
content: Quote from: m k on May 06, 2023, 11:14:50 amThere isn't actually anything there for the solder to hold onto. There is nothing on the other side either. I am effectively trying to solder onto the PCB wafer or whatever it is called.Out of curiosity I got a multimeter out and check there is not connectivity at all.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 08, 2023, 08:57:04 am
content: Could you take a photo so we don't have to guess?In general - we may use long cap legs instead of missing wires.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 09, 2023, 10:21:32 am
content: Quote from: Vovk_Z on May 08, 2023, 08:57:04 amSure, no problem. Thank you. What do you think?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 09, 2023, 11:22:55 am
content: Looks somebody ripped the pads and through hole plating away with excessive temperature and force. It is very difficult to repair as the plating originally connected to mid layers. There's no way to get contact when the holewell plating is lost.

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 09, 2023, 03:25:51 pm
content: Quote from: Siwastaja on May 09, 2023, 11:22:55 amI had a similar situation once. Tried to solder it as usual (because there was nothing other left to do) and it soldered somehow and worked fine.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 09, 2023, 10:22:16 pm
content: Quote from: Siwastaja on May 09, 2023, 11:22:55 amI was thinking of placing the cap. Maybe a dab of glue on one side and then run a cable from both legs to where the trace go. Will that work / do you see any problems with that?

author: donlisms
author_rank: Frequent Contributor
timestamp: May 10, 2023, 12:45:37 am
content: My perspective is that you should keep it simple, unless or until some problem shows up that requires you to get more complex. For example, don't worry about connections to internal layers until you first check whether the boar has internal layers, and don't worry about connections there until you discover whether there were any connections there, and so on.These are different rules than you would follow if you were going to get this certified, and make and sell a lot of them.So... fasten it down any way you like, run connections (on both sides, if necessary), and see if you can get the thing working. It's a start. If there are issues, you find them, and fix them.There are lots of tutorials on repairing traces on PC boards; I like Mr SoldefFix on YouTube. (I think he used to be one of the Beatles.)

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2023, 07:13:00 am
content: Quote from: donlisms on May 10, 2023, 12:45:37 amWhat kind of advice is that? All motherboards have internal layers, and just from the picture alone it is obvious the capacitorisconnected to internal layers. You are just wasting time with this kind of "don't worry, first confirm water actually is wet" advice.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2023, 07:21:02 am
content: Quote from: Vovk_Z on May 09, 2023, 03:25:51 pmI think this is also the OP's best bet: just find a capacitor with correct size and lead spacing and try to solder it in, applying flux to the holes and feed in enough solder, hoping it will stick to what's left of the holewell plating, or the edge of the internal layer. Just solder the positive first as it's likely connected to just one layer, so you can physically feel if it's connected as the leg stops moving after the solder cools, but test very gently and apply some glue afterwards to reduce stress to the weakened connection.Another option is to try reverse-engineering the board a bit to confirm in which power net the capacitor is connected and find the same net somewhere else, as close to the original location as possible, and add the capacitor there, using shortest possible leads.Quote from: halfmoonmissing on May 09, 2023, 10:22:16 pmGeneral rule of thumb: minimize the length as much as possible. If the capacitor lead length itself is not enough for your modification (they usually come with almost an inch or so), that sounds a bit worrying. Using glue to hold the capacitor in place is a good idea.

author: Shock
author_rank: Super Contributor
timestamp: May 10, 2023, 08:12:30 am
content: Either solder to what is left or find where it goes and solder there. If you think you have no other option excavate. How much solder area you have determines if a smd needs bonding to the board separately to prevent movement.The negative side of the cap can be common to other components, so one side is often easier for finding suitable places to wire to or check continuity from. The schematic, nearby components and circuits function will help you determine what caps are connected to.Even if it's a tiny trace of copper exposed, finding were goes helps for continuity testing after repairing, ensuring joints are good.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 10, 2023, 09:18:53 am
content: Quote from: donlisms on May 10, 2023, 12:45:37 amI'm keen to learn and you have me intrigued with the Beatles reference. Thank you.

author: halfmoonmissing
author_rank: Contributor
timestamp: May 10, 2023, 09:20:14 am
content: Quote from: Siwastaja on May 10, 2023, 07:21:02 amThanks for the advice. Appreciate it.