Having trouble getting solder to stick to SMD pads on older PCB - Page 1
Forum section: electronics / repair

author: thatawesomeguy
author_rank: Newbie
timestamp: October 25, 2021, 05:28:57 am
content: I have an older PCB I am trying to repair by replacing various 0603 and SOT23 components that have either dry jointed or gone bad.The trouble is I can't seem to get any solder to stick to the pads, as best as I can tell I think the flux in the solder might be preventing adhesion (I know that's the opposite of its purpose). The reason I say this is when I inspect the pad after attempting to solder there's always a significant layer of flux over the pad but no additional solder.Things I have tried so far:Thoroughly clean the area with isopropenyl/acetone.Manually scraping the pad to remove flux and expose fresh metal.Using higher or lower iron temperature (300-350 degrees C).Use bigger or smaller tips.Using additional flux paste.I am open to any suggestions. I have attached a photo both of the pad pre-cleaning and post-cleaning.

author: Berni
author_rank: Super Contributor
timestamp: October 25, 2021, 05:41:00 am
content: Yeah flux does that sometimes when it dries up in a thick enough layer to prevent you from properly getting a soldering iron in there, its easily cleaned away.However the top pad there instead looks like it was ripped off the board and you are actually looking at the brown PCB core material. The bottom pad looks fine. No worries you can just solder the component to the bottom pad only, scrape off some of the green soldermask of the trace leading into the missing pad to expose the copper, then just put down a larger blob of solder to bridge over to it.

author: thatawesomeguy
author_rank: Newbie
timestamp: October 25, 2021, 05:49:21 am
content: Quote from: Berni on October 25, 2021, 05:41:00 amThe top pad is uncleaned, the dark area is actually just the raised surface of a blob of solder I managed to get to stick, just the lighting angle and low resolution make it appear not shiny.I've considered exposing the trace as a last resort and ultimately might have to do that. It just puzzles me that despite exposing the pad, and confirming it's clean and continuity across the whole pad I can't get anything to stick. This is not unique to this one pad, pretty much any of the smaller 1mm pads are like that, but I have no problem with the slightly larger 2mm pads on the same board.

author: dl6lr
author_rank: Frequent Contributor
timestamp: October 25, 2021, 06:31:03 am
content: Quote from: thatawesomeguy on October 25, 2021, 05:49:21 amI've discovered that with corroded pads from a crapacitors electrolyt leaking out. Usually I clean the pads with alcohol, then brush them with a glassfibre erasure and clean them again with alcohol. That normally gives me very clean pads with most of the old solder removed. I suppose the old tin reacts with the electrolyt and gives an unsolderable mess.

author: thatawesomeguy
author_rank: Newbie
timestamp: October 25, 2021, 06:38:58 am
content: Quote from: dl6lr on October 25, 2021, 06:31:03 amIt's funny you say that because there was some electrolyte leaked around these areas but it didn't seem to damage the traces/pads. I have a glassfibre pen but it's 4mm, I'd be concerned with removing too much material.

author: Shock
author_rank: Super Contributor
timestamp: October 25, 2021, 07:34:01 am
content: Test it on an old pcb and see, my fibre brush doesn't remove material just marks up the surface. You can also scrape a scalpel blade or tweezer tips, but again test it on an old pcb first.You can also try using leaded solder (if you aren't already), ensure your iron tip is clean.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: October 25, 2021, 11:14:00 am
content: no copper/metal pad whatsoever ?seems scrapped, how can work on such

author: thatawesomeguy
author_rank: Newbie
timestamp: October 25, 2021, 12:18:27 pm
content: I did manage to polish it up a bit with the fibreglass pen seemed to help, wasn't sticking amazingly, but enough to tack a small blob on there.

author: Shock
author_rank: Super Contributor
timestamp: October 26, 2021, 07:20:01 am
content: If you continue to have problems you can either try a tacky flux that gives more working time or perhaps a more aggressive rosin flux. To clean pads dip clean braid/wick in flux and see if you get a clean shiny pad first try. Some of the no clean flux and flux pens become plastic like once dry so can prove difficult to use when reworking old boards.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2021, 06:34:23 pm
content: For rework I almost always rely on a more aggressive activated rosin flux like Kester 44.Quote from: dl6lr on October 25, 2021, 06:31:03 amAfter neutralizing the leakage from the electrolytic capacitors with a water solution, my solvent of choice is acetone or lacquer thinner Usually scrubbing with an old toothbrush is sufficient but in stubborn cases I lightly scrub the pads with a Scotch-Brite pad, although acetone or lacquer thinner will eventually dissolve it.