Insufficient paste deposits are the leading defect detected by SPI. Some estimates hold that half the boards rejected are for insufficient defects that are repairable with solder paste jetting technology. The jet dispenser is mounted on the SPI inspection head (FIGURE SI). Following the inspection routine, if the only defects found on the PCB are insufficient paste, the jet dispenses enough paste to reach the desired volumes. After dispensing the additional solder paste, the SPI system scans the deposits again to ensure the proper paste volume was achieved. If the print contains defects that cannot be repaired by the jet, such as excessive solder or bridges, the jet does not dispense and the board is rejected. Jetting "top off" solder paste to repair insufficient defects inside the SPI machine eliminates the need to clean and reprint the printed circuit boards. Doing so can reduce SPI reject, repair and scrap rates by as much as half.
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