Thick oxide layer on solder surface, can't get reading on DMM - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: September 28, 2023, 05:49:52 pm
content: I have a dishwasher PCB here I'm debugging, with lots of mains AC and DC traces on it. The solder is nice and shiny, and it melts higher than 60/40 solder, but IDK what kind it is.I'm having a really hard time probing on it with a DMM, I have to put huge pressure on the tips to pierce the oxide layer just to get a resistance or continuity reading. I don't want to bend my probes, or the PCB, or be trying this with power on it, or have to do it every freaking time.I put a bit of 60/40 solder on some joints, but not enough, it wasn't really easier.So besides sharping my probe tips up a bit, what else can I do? Besides resoldering the whole thing with 60/40, or attaching a bunch of wires, (probably what I'll do).What do folks do ?

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 28, 2023, 05:58:16 pm
content: I use TL910 probes with pointy (like a needle) probes. Nice thing is that these needles can be changed.However they are very expensive and not that good, recently I had to cut and resolder the cable as it had broken open inside the silicon cover.Don't know if there are some cheaper copy

author: PartialDischarge
author_rank: Super Contributor
timestamp: September 28, 2023, 06:01:03 pm
content: Maybe these ones...

author: LinuxHata
author_rank: Frequent Contributor
timestamp: September 28, 2023, 07:43:58 pm
content: Is it really an oxide?maybe it is some kind of protective lacquer (conformal coating), which is quite often on PCBs, which might be subject to high humidity?Anyways, try a cotton swab soaked with vinegar on some of them and check whenever it starts to bubble. If yes, then wash with vinegar, let it dry and here you go.

author: MathWizard
author_rank: Super Contributor
timestamp: September 29, 2023, 01:58:55 am
content: I sharpened up the probes, that helped a lot. Yeah there also seems to be a residue or very thin coating on some parts, I can see it gum up the PCB after cleaning with a IPA, and it melts a bit like glue, and there's no glue anywhere. But it's the solder mixuture too.

author: coppercone2
author_rank: Super Contributor
timestamp: September 29, 2023, 04:35:11 am
content: its so god damn annoying I recommend solventing the board and then recoating it later

author: u666sa
author_rank: Frequent Contributor
timestamp: September 29, 2023, 06:54:56 am
content: Cold solder is usuallyTHEproblem you get on these. So you take flux and go around the board retouching as much as you can. Sometimes you can see bad joints, other times you can't, that's why you want to do everything.

author: jonpaul
author_rank: Super Contributor
timestamp: September 29, 2023, 08:28:16 am
content: Needle probes pierce the solder mask and conformal coatings, as well as oxide.Easy to make a set DIYAlternate pointed X-acto knife with insulated handle.J

author: IanB
author_rank: Super Contributor
timestamp: September 29, 2023, 09:14:29 am
content: Quote from: u666sa on September 29, 2023, 06:54:56 amThis is true. When my dishwasher control board failed the problem was with AC high current traces connecting to an onboard relay switching the heater. As soon as there was a weak joint the high heater current caused the joint to heat up and burn out. In the end, not only did I have to reflow the joints, but I also had to reinforce the joints with added copper.

author: jpanhalt
author_rank: Super Contributor
timestamp: September 29, 2023, 10:03:20 am
content: I went through a dishwasher repair a few years ago (2019). It was a 9-year old Maytag (probably very similar to anything made by the Electrolux group). While there were several sensors, there was only one control board and the touch panel. The control board was like the TS describes. The touch panel was sealed.1) There was a service bulletin that applied. Owners were not notified and it was virtually impossible to find out what was in it without spending a relatively high premium to get a "factory" set of parts. I was able to locate one and got the parts at eBay prices. Several relays (3) and some electrolytic capacitors were changed (replaced with slightly different values). In the process, I was curious and opened up a couple of the old relays. Contacts were a mess and well burned. They were all standard footprint sealed relays. I suspect the capacitor charges were intended to decrease that burning, but I don't think those changes were necessary to get it working again.2) Prior to diving into the control board, I went through the Maytag troubleshooting procedures for the touch panel, which are included in the folded up sheet that is stashed inside a panel on American appliances (and probably others). The touch panel passed. However, after the control panel mod, the washer ran one cycle and failed again. Weeks later, it would start and run again. (I live alone and don't use a dishwasher every day.) That made me reconsider a problem in the touch panel, namely moisture sensitive switches. There is also a thermal fuse. That consistently tested OK. On a gamble, I bought a new old stock, touch panel. It was about $200 v. $1000 for a new washer. Replaced it, and the dishwasher ran and continues t run like new.In conclusion, the problem was probably the touch panel and the troubleshooting guide did not detect that, perhaps because it was a moisture-related problem. Second, in hours of searching, I found that there is a "dirty water" sensor located on the bottom. It is difficult to access or test, but if clogged, it shuts the dishwasher down. I pre-rinse all my dishes and never put food through the dishwasher, so I considered that sensor as an unlikely cause. Third, I consider the control board changes, while probably not required, as "preventive maintenance."

author: MathWizard
author_rank: Super Contributor
timestamp: October 04, 2023, 04:23:51 am
content: Yeah there is definitely very thin a water resistant coating on all the solder joints.The problem with the dishwasher is the soap dispenser won't open during operation. But the triac/opto-relay and logic chip for it, works, the triac works in both directions with 12V DC anyways.The wax motor on the dispenser read's as ~2.2k ohms and ~1mH at 1kHz anyways. So unless the main MCU has some problem and it's output pin is shot, what else might go wrong ? I think the DW finishes the full cycle, but just doesn't open the soap door.I can reflow some solder joints of the MCU, it all looks pretty new though, and in good shape.I see some dishwasher's have a dirty water sensor, so do they rise the dishes, then drain the tank, then release the soap ? Maybe that sensor has a problem, and the MCU thinks there's still dirty water and doesn't release the soap ? But the full cycle seems to finish according to the owner.

author: David Hess
author_rank: Super Contributor
timestamp: October 04, 2023, 09:07:22 pm
content: Get sharper probes. Good test probe tips are hardened steel for exactly this reason.