Cleaning PCB's before wave soldering is certainly not a normal procedure.

The only reason I can think of that would require cleaning is if the boards had been contaminated with some material that required IPA to remove.

Boards should not be handled with bare hands as the oils in the skin can contaminate the solder pads and interfere with the solder process.

This would only be required if there were heavy deposits of grease or other contaminates on the PCB. Otherwise the normal cleaning action of the flux will provide the cleaning necessary for the soldering operation.

In my 20 years of experience in PCB assembly, the only time I have seen companies clean before soldering was in very high-end aerospace and military requirements. And there is some logic to this….

The most corrosive or damaging residues on a PCB are the salts (also called “halites” or “activators”) that are in the fluxes. They make the flux “wetter” and help the soldering materials flow into each other, creating stronger solder joints. But heat increases the rate of corrosion development, and a warm circuit board in a working electronic device can develop dendrites and other corrosive conditions in just weeks or months. So cleaning AFTER soldering should, in theory, remove these residues and provide PCBs with longer operational lives.

But over the years a few companies cleaned before soldering. Their logic was that cleaning was a cheap way to ensure that (a) no contamination was trapped in the solder joints, which would create a weak joint, and (b) no contamination blocked the flow of solder and produced incomplete wetting. After all, there are many other contaminates on circuit boards besides just fluxes: fingerprints contain salts and oils; but also dust, marking inks, Kapton tape residues, and even humidity all are found on the boards and can cause problems. By removing all those residues they increase the chances for perfect soldering and decrease the opportunities for problems.

There’s no doubt that pre-cleaning adds expense to the manufacturing process. So unless you’re building a space probe that’s engineered for cruising through the solar system, in most cases it is an expense that’s hard to justify. My suggestion would be to do a systematic test of 200 boards; clean 100 and don’t clean the others, and see if your QC people can detect any differences.

The first objective especially if the PCB assembly is being manufactured using No clean technology is to avoid handling and contamination where ever possible.

Handling contamination will have the potential for two effects

1. it may cause soldering problems. Finger grease on its own is not likely to cause this if an alcohol based flux is being used since the solvent will dilute the effect of the contamination and there would be no advantage from a pre-clean in solvent like IPA. However, finger marks also carry salts which may affect soldering if contact with the metallisation is long enough and in a humid atmosphere. If this does cause corrosion and affect solderability then pre-cleaning in solvent just before soldering will not reverse the effect anyway.
2. if there is contamination, it may also be present in areas not contacted by the flux and its solvent. In this case, the marks with their salt content remain available on the PCB and could initiate future electrical reliability problems in service or reduce adhesion and effectiveness of any post-soldering protective coating. A pre-wash in solvent would eliminate this potential hazard.

The user should therefore assess the extent and nature of handling contamination, the probability flux will wet the affected areas and the service environment for the finished assembly. These are of course all subjective criteria which is why avoiding contamination in the first place is the right way to go.