As a consulting service and test laboratory we get this question regularly. We have use a specific saponifier at 5% concentration to remove the surface oxides to improve solderability or we use a gas plasma treatment if the oxidation is not to thick. If you want to discuss details or our service to do the cleaning please contact me off line.

63/37 HASL finish degrades with the growth over time of an intermetallic layer Cu6SN5. Usually the thickness of the HASL coating is enough to give good solderability for an extended period. If this intermetallic layer has broken through the surface then these boards will be difficult to solder.

As for cleaning, this will help in removing any surface contaminants along with the oxide coating which will help but won't solve the solderability issue if again the intermetallic layer has grown to the surface.

You can test this coating for solder and intermetallic thickness with a Sequential Electrochemical Reduction Analysis Instrument (SERA). Good luck.