Pad popped off - Page 1
Forum section: electronics / projects

author: Nightowl
author_rank: Newbie
timestamp: February 07, 2020, 12:43:12 am
content: I'm trying to repair a circuit board, and the boards manufacturer and the company that makes the device refuse to offer up any kind of plan or schematic to make this easier. So this laminated board is a big pain. 12v in then most things are 3.3v and 5v. I could not trace anything so I started replacing i.c's.I ended up with a bridge that was too stubborn to get off, so I removed that chip. This chip (3 of them) is very small with 16 leads and I have to work with it under a microscope. Well, the pad popped off. I spent 3 hours last night scraping the laminate off on both ends where the pad was.My question is........... Is it possible to have an i.c with 16 legs/leads where some are not used in the circuit? I ask because I can't seem to see what that pad was attached to? I have a copper path on both ends of the pad that I uncovered, but it looks like nothing was connected? The pad seems shorter than the distance to the copper path. I realize it probably burned away some, but it does not look like it, and the part that would have burned was a ways under the laminate.I am kicking myself in the .... because I did not trace the pad to a point that I could have made a jumper BEFORE it popped offAnd how do I get pics on here? I dropped it in the box but it does not show?

author: wraper
author_rank: Unknown
timestamp: February 07, 2020, 12:58:01 am
content: Of course some pins may not be used. Either because use of particular IC pin is not needed in particular circuit or because it is not used in IC to begin with. The fact IC has particular package does not mean all of the pins must be functional.QuoteDon't get what you mean, if pad is not used, there should be no traces going from it.

author: james_s
author_rank: Super Contributor
timestamp: February 07, 2020, 01:06:49 am
content: It's very common to have ICs with unused pins.It sounds like you are using the shotgun approach, that's a dangerous method as you've found out because it results in removing a lot of parts that may be just fine and doing damage in the process. Don't flail, approach this logically and try to narrow down the fault. What is the device and what is the symptom?

author: tooki
author_rank: Super Contributor
timestamp: February 07, 2020, 01:17:42 am
content: For what it’s worth, I’ve found that pads for unused pins (that is, with no traces leading off from them) tend to be far easier to lift. Probably because there’s no trace to draw away heat, nor provide mechanical support.So if there’s no obvious trace from the lifted pad, there’s a good chance it never had one.

author: Nightowl
author_rank: Newbie
timestamp: February 07, 2020, 02:36:39 am
content: Reply to all.I'm no expert, that's why I tried to explain as best I could. I didn't know whether that leg connected to anything or not, is why I started scraping the laminate. I tried to post a picture that shows exactly what I was talking about, but for some reason it does not show in my post. I was not going to assume it connected to either path because if I reconnected with some form of solder trace or jumper wire, it may very well blow the thing again, or other things would stop working. AND it would be 50 50 at best because there are paths at both ends. So I'm doing the best I can at figuring out a circuit board with nothing to go by. I do know there were 8 i.c's (fets?) that were toast because they were causing the initial issue, along with the 8 diodes beside them. After replacing those, it half way worked again. I had a local electronic guy look it over and he tested those, and suggested I replace them. That much $ per hour to have someone try to diagnose something without schematics was not an option for me, or him from what I could tell by looking at his face when I handed it to him. I could have bought a new truck and would of been cheaper.This is a module that controls the air ride on my truck. Yes, I have no other way than a shotgun approach. After replacing the fets and diodes and finding it still was bad, I noticed when looking through the microscope that 1 of the i.c's had burnt the plastic base on 2 connectors (1 on each side of it) and this i.c is the one I was replacing when the pad popped off. I figured this one was bad for sure if it put out that much heat to melt plastic a quarter inch away and only measuring less than a quarter inch itself.I assumed that pad went nowhere, but need to be sure. If it does, then I'm lost because I can't test or see where it would go, other than copying the path of the other 2 identical i.c's but that's also not a good idea. Each one has a resistor array, and 16 connections going different ways.I have no idea what caused the problem in the first place. The truck was at a body shop getting panels fit and painted. There was a leak in 1 of the air lines, and the shop had to keep airing up to move the truck around. No engine, just cab and frame with the airride installed. I suspect they killed the battery then tried to jump it. I also assume they jumped backwards, or even used a 110vac transformer for their electric fence to jump it. They won't admit to any of it. But it worked just fine when dropped off, and blown when picked up. And this particular module is obsolete.Thank you guys for replying. I'm a bit of a long winded pitbull, and just can't stop trying to get this thing going.Again, how do I get a picture posted here?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 07, 2020, 06:47:23 am
content: On an almost completely unrelated note -- almost every time I see this thread I can't help but remember this:Cheers,Tim

author: mikerj
author_rank: Super Contributor
timestamp: February 07, 2020, 08:43:32 am
content: Quote from: Nightowl on February 07, 2020, 12:43:12 amGet the datasheet of the part and see what the pin function is.

author: Nightowl
author_rank: Newbie
timestamp: February 07, 2020, 07:37:31 pm
content: Quote from: mikerj on February 07, 2020, 08:43:32 amI tried. I can't find it anywhere.

author: james_s
author_rank: Super Contributor
timestamp: February 08, 2020, 03:19:34 am
content: What is the number on the part? It's possible that it's house numbered.Can you find someone local with some experience to help you out? This sounds like something that could be repaired by an amateur but it's really easy to turn a repairable board into a write-off, I've seen a lot of good stuff over the years ruined by ham fisted repair attempts.