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Cleaning of printed boards is conducted on boards when paste printing is not considered satisfactory, when it doesn't meet the inspection criteria. There is no international criteria for paste, but a document is available to download at www.ppm-monitoring.com

Figure 1 shows paste residues in through holes, a common issue if the cleaning process is not capable of removing the paste. If paste is to be removed using immersion cleaning, initial wiping of the board should be avoided as it makes the process more difficult. Here we see paste in a hole, but the paste could also be pushed into vias, which are more difficult to clean. Paste at the edge of pads trapped in the solder mask undercut is another difficult-to-clean spot.

Figure 1: Post-cleaning paste residues visible.

 

In Figure 2, paste residues are visible after the paste prints have been cleaned using a manual technique.

If a board needs to be cleaned, it is better not to wipe the board surface as it makes subsequent immersion cleaning more difficult. Clearly in this example no provision has been made in the process for poorly printed boards.

Figure 2: Smudged or flattened solder paste.

 

Paste residues are present on the surface of the pads after a cleaning operation in Figure 3. The board has been manually cleaned by wiping the board surface, removing 90% of the paste but leaving unacceptable residues.

If a board needs to be cleaned, it is better not to wipe the board surface as it makes subsequent immersion cleaning more difficult. Immersion cleaning with ultrasonics, agitation or spay is necessary to remove both surface and hole fill. It only takes one or two balls on a gold board to give solder spots!

Figure 3: This board has been wiped, leaving unacceptable residues.

 

Damaged Stencils	Stencil Blockage	Paste Misalignment	Smudged Paste	Poor Definition
Incomplete Prints	Residues	Bleeds	Ears	Scooping
Shorts	Slump	PIHR Printing