Hi all,
I have a circuit board which I need to replace a relayon. One of the pads seems to have over heated and left residue on the exposed copper. When I remove the relay I was looking to clean up the pads to make sure the new one is connected correctly.
What is the best way to do this without further damaging the pad? I have rosin flux but this is solid.
Thanks
Lee.
IPA - Isopropyl alcohol and an old toothbrush
If it's old overheated flux it should come off with IPA but usually when a board is soldered the flux residue is washed off. This makes it more difficult to remove components unless you add more (liquid) flux or re-solder the connection before you try and remove the component.
If the residue is burnt carbon from the board scrubbing with IPA will often clean up the loose bits but will not remove the burn embedded into the PCB material.
If the board has been burnt too much it's unlikely that the copper pad is still anchored to the board beneath. All is not lost in this case as all you do is strengthen the connection with a bit of wire. Fit your new relay through the pad (which may be loose), wrap a bit of wire around the pin of the relay and solder it along the track to a bit where the copper is still firmly attached to board. You may have to scrape off any top layer of solder resist to expose bare copper.
Assuming abuse of the board when removing components, if the PCB is double sided/layered and the pad is plated through you may have to get a multi-meter to check that you still have continuity to both sides of the pad(s).
IPA is generally best solvent with fibreglass brush.
But any abrasive will also clean up the board, if the pad is still properly attached to the subtsrate
Domestic meths would be a reasonable substitute if you don't have access to IPA. I cleaned up my router board with that after spilling milk on it.
cellulose thinners and acetone also work, but are more aggressive and may attack components
Water would also work for milk. Ideally de-ionised/distilled water but cold water from the tap will do in some cases. If after using this method it's wise to leave the board for hours in a warm place for any water under components to dry out.
Some assembled PCB boards are also washed in water if a water soluble flux has been used.
usually quicker to scrape a blade over it
I used to use water with a small amount of detergent to remove leaked electrolyte from failed electrolytic capacitors, then clean water to rinse and then leave to dry. Then examine for partly-eaten tracks...
Thanks very much all. I used some meths in the end as I had some and it worked a treat. Thanks very much all for the suggestions
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