Electronics > Repair
Having trouble getting solder to stick to SMD pads on older PCB
thatawesomeguy:
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.
Berni:
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.
thatawesomeguy:
--- Quote from: Berni on October 25, 2021, 05:41:00 am ---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.
--- End quote ---
The 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.
dl6lr:
--- Quote from: thatawesomeguy on October 25, 2021, 05:49:21 am ---
--- Quote from: Berni on October 25, 2021, 05:41:00 am ---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.
--- End quote ---
The 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.
--- End quote ---
I'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.
thatawesomeguy:
--- Quote from: dl6lr on October 25, 2021, 06:31:03 am ---
--- Quote from: thatawesomeguy on October 25, 2021, 05:49:21 am ---
--- Quote from: Berni on October 25, 2021, 05:41:00 am ---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.
--- End quote ---
The 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.
--- End quote ---
I'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.
--- End quote ---
It'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.
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