What went wrong with this rework? - Page 1
Forum section: electronics / general

author: tscharf
author_rank: Newbie
timestamp: March 18, 2025, 04:23:33 pm
content: I recently got a set of boards to diagnose with a lot of defects. After some debug it was determined all the faults had to do with rework performed. It looked clean on top but but there is a lot of residue under the components that turned 10K resistors into 400 ohm resistors among other assorted faults.Is this bad solder paste, bad flux, bad rework technique? Any ideas?

author: jpanhalt
author_rank: Super Contributor
timestamp: March 18, 2025, 04:45:55 pm
content: In IMG_4263 is that chip what was removed from U32? The answer may be "all of the above."Have you tried cleaning in an ultrasonic? Kester's water-based flux cleaner 5768 is quite effective (https://www.kester.com/products/product/5768-cleaner).Since you are in the US, I use ZEP Heavy Duty Floor stipper diluted 1 part to 3 or 4 parts total with water in an ultrasonic for 1 to 2 minutes.

author: SMTech
author_rank: Frequent Contributor
timestamp: March 18, 2025, 04:51:39 pm
content: reworked with what, a blow torch and plumbers flux?

author: tscharf
author_rank: Newbie
timestamp: March 18, 2025, 04:55:23 pm
content: Yes, U32 was removed and just flipped upside down for inspection. In this case U32 was dead and needed replaced.It's large board and didn't fit in the ultrasonic cleaner of the people who did the rework.

author: ajb
author_rank: Super Contributor
timestamp: March 18, 2025, 07:40:34 pm
content: All those bits of black gunk looks to me like someone isn't cleaning their iron properly, and likely has it set to too high -- or they're using an unregulated iron. They may have been struggling with the ground plane connections and cooking the flux and components. Plus lack of cleaning -- some flux residues are conductive, and some of that residue still looks a bit goopy, like it may not have all reached activation temperature.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 18, 2025, 08:40:34 pm
content: The water based cleaners I mentioned don't require ultrasonic. That's just quicker. They are cheap and filling a small tray (like a glass baking dish) and letting them soak probably works too, but is not as effective at cleaning under parts. After a soak, I would use compressed air to blow under components. Then repeat with DI water. Kester has emphasized the need of a "saponifier" (alkaline in water). Ethanolamine is added to give saponification. ZEP has that too. Your usual non-aqueous flux removers are not saponifiers.

author: Stray Electron
author_rank: Super Contributor
timestamp: March 18, 2025, 09:06:26 pm
content: Quote from: tscharf on March 18, 2025, 04:55:23 pmThen your company has chosen the wrong company to be reworking the boards! I have to wonder what other short cuts they're taking or what other low quality or incorrect parts (like solder and/or flux) that they're using.

author: thm_w
author_rank: Super Contributor
timestamp: March 18, 2025, 09:31:37 pm
content: Poor quality work.Can't tell if there are solder balls in those photos or if thats just splash? But I'm sure the solder itself used was fine.The flux looks nasty and dirty. If they don't want to clean it they need to use good quality no-clean flux with low residues. Then whats left over will be a bit of very lightly brown and slightly tacky, and in most applications ok to leave without cleaning.But as mentioned here before, need to be sure all of the flux was heat activated:https://iconnect007.com/article/128271/knocking-down-the-bone-pile-cleaning-of-no-clean-fluxes-in-pcb-rework/128274/smt

author: tscharf
author_rank: Newbie
timestamp: March 18, 2025, 10:11:22 pm
content: From the microscope it appears to be solder balls mixed with some dark brown sludgy flux(?).The real problem is it ended up being partially conductive somehow. Resistors weren't simply shorted but had lower resistance of varying degrees. Cleaning all the sludge from under an offending resistor would return it to its nominal value. I haven't seen this sort of thing before.I'm wondering if it might be expired or bad solder paste?

author: ajb
author_rank: Super Contributor
timestamp: March 18, 2025, 11:23:36 pm
content: Fluxes may be conductive until fully activated, and/or may leave residue that is conductive even when used properly -- that's one of the reasons you generally want to clean boards after soldering (the other big reason is that fluxes and their residue may be corrosive). Residue from a proper no-clean flux should not be conductive (or corrosive), or at least so poorly conductive to not matter for most applications. Flux that is as contaminated with gunk as that stuff appears to be... who knows?Quote from: tscharf on March 18, 2025, 10:11:22 pmThere's way too much flux visible for it to have come from the solder paste, and you wouldn't necessarily use paste for this kind of rework anyway, you could do everything I see here with solder wire just as if not more easily. Someone had to have added a bunch of flux during the rework -- that isn't itself a problem, we use flux for good reason, and you would expect to add flux during rework. The problems arise when unactivated flux, or residues from non-no-clean flux is left on the board. So the symptoms you're seeing aren't at all surprising given what we see in those photos. As for the solder balls, those could have come from the rework, or be leftover from the original reflow soldering -- but the board was probably cleaned after the original assembly, since there's no visible residue on the joints that weren't reworked, so that's unlikely but not impossible.

author: tscharf
author_rank: Newbie
timestamp: March 19, 2025, 03:40:19 am
content: Thank you everyone for your valuable input. I need to do some further checking here.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: March 19, 2025, 02:54:23 pm
content: Silly question, but why are the tp locations soldered?It looks like gel flux was used, then the board partially cleaned with wick, then paste smeared over the area, parts put on and soldred with no regard to activating the flux and then the paste. Finally just not cleaned with anything. I would ask them what flux type they used (may be a no clean no wash, but doubtful)

author: tscharf
author_rank: Newbie
timestamp: March 19, 2025, 03:39:43 pm
content: TP's are soldered for design fixes after production. Pull-down resistors to GND where most convenient.The board looked OK on casual inspection but problems hidden under the parts.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 19, 2025, 05:15:38 pm
content: What are the approximate dimensions of the boards?

author: tscharf
author_rank: Newbie
timestamp: March 19, 2025, 05:30:04 pm
content: 11" x 18" board size

author: passedpawn
author_rank: Contributor
timestamp: March 19, 2025, 05:50:11 pm
content: You can clean half at a time if it doesn't fit entirely. Not ideal, but with the correct additive to the cleaning water, and heat, will work great. Not sure if that gunk would come out from under the part.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 19, 2025, 05:51:26 pm
content: My ultrasonic is a very old (1970's) Bransonic. It's too small, but I have done boards where only half can be immersed at a time. Vevor sells a variety of sizes. The largest (30L) is advertised at $229. A 15 liter is $118.Attached is a snipped of the tank sizes. If they work, that does not seem cost prohibitive for anything important.

author: passedpawn
author_rank: Contributor
timestamp: March 19, 2025, 05:55:29 pm
content: Quote from: jpanhalt on March 18, 2025, 08:40:34 pmThis is correct. I highly recommend this product below for a saponifying additive to your ultrasonic cleaner.https://www.sra-solder.com/ultrasonic-cleaners/ultrasonic-solutionsHere's the ultrasonic cleaner I use. I've used it a bunch, works great. Seemed like the best compromise between size and cost and ultrasonic power/transducers.https://a.co/d/djUj39G