Meaningful SPI Production Control Through Automatic Normalization

Automated pre-placement solder paste inspection (SPI) has established itself in SMT assembly as an essential tool for optimizing the process and reducing end of line defects.  It’s been commonly accepted for many years that a majority of assembly defects can be traced back to print defects, and by using raw measurement data from printed PCBs for volume, area, height and offset, SPC histograms and control charts can quite readily be used to optimize the process.

Following optimization, SPI is used to detect print anomalies. However, the multitude of “normal” variables introduced from board characteristics, clamping methodologies, stencil variations and paste type force unacceptably wide tolerance settings.  These board-wide good/bad tolerances can easily mount to +/- 50% or more.  For most, this is unacceptable, because of course, with such wide tolerances production drift can be missed and important defects ignored. Continue reading

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