Certainty of measurement using an automated infra-red laser inspection instrument for PCB solder joint integrity

An analysis is presented of the basic principles governing the operation of Vanzetti's Li/6000 Laser/Inspect instrument for the automated inspection of solder joints on printed circuit boards (PCBs). The instrument operates by monitoring the infra-red emission from a solder joint which has received a short pulse of heat from a YAG laser. Theoretical expressions are established for the response of the instrument to defects, both in the bulk of the solder fillet and also at the surface, both for populated and unpopulated PCBs. The predictions, supported by measurements on unpopulated PCBs, show that enclosed voids in the solder fillet of a plated-through-hole PCB give an insignificant change in signal, but that blowholes on the inspection side, with vents greater than 0.07 mm in diameter, may be clearly discerned. The important effect enabling discrimination is that the vent acts as a black body absorber and radiator whereas the solder surface largely reflects the radiation.