This is described in the J-STD-001. The contamination that remains after soldering is mainly flux residue, but dust and other particles (hair, tin splashes, solder balls, finger spots, etc.) can also remain on the PCB. Is this allowed and how much FOD may be left behind? This depends on the product class. With class 1 products, no more than XXX µg / cm2 flux residues may remain, whereas for classes 2 and 3 this is considerably lower. How can you measure/test this? This is also described in the J-STD-001.