Cleaning Solder Flux Residue

Q. I am seeing a white powder around solder joints on PCBs.  We use Detergent 8 for cleaning.  Do you know why this is happening and how to stop it?

A. The most likely source of white powdery residue around solder joints on PCBs is the formation of insoluble metal salts from the solder flux. Lead or other solder metal rosinate, abietate, or other organic acid salt tends to form when soldering temperatures are too hot.  Using any water based cleaner, such as Detergent 8, will result in the formation of a white insoluble residue in the presence of heavy metals such as lead, or even tin, along with burned acids from rosin or other acid activated solder fluxes.

One way to fix this problem is to lower the temperature when soldering.  If this is wave soldering, using a different temperature profile will solve this. Another way to fix this problem is to clean first with an acid cleaner such as Citrajet® Low-Foam Liquid Acid Cleaner/Rinse  for machine cleaning or Citranox® Liquid Acid Cleaner and Detergent for manual cleaning.

If it is a rosin based solder flux, you will likely have to do the Detergent 8 cleaning afterwards, which will remove any conductive residues. If it is not, you may be able to do an extremely thorough deionized water rinse, to completely remove the Citranox/Citrajet residue, and skip the Detergent 8 step.

A leading aerospace manufacturer studied another solution to this problem — putting potassium silicate (10 mL in 10 L) into the Detergent 8 bath to act as an inhibitor to stop the formation of these white salts.  They claim, in an article, Additives for Aqueous Cleaning, that “use of potassium silicate additives” led to desired cleanliness and “lower ionic contamination levels.” Alconox, however, has not tested this procedure.

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