Your yield report shows contamination.
Your particle counter showed nothing.

Electronics assembly is where contamination hides most effectively. A particle counter positioned at the ISO 14644-1 sampling point measures the air at rest. It does not measure what settles onto the PCB during reflow, onto the camera sensor during module assembly, or onto the connector contact during handling. Yield loss attributed to “particle contamination” in electronics manufacturing is frequently untraceable because the measurement that could have detected it was never made at the right location, at the right time, in the right form.

The critical particle size in electronics depends on the component: for high-density PCB assemblies it is defined by the minimum conductor spacing; for image sensors it is the pixel pitch; for precision connectors it is the contact gap. In each case, the contamination event is a macro-particle depositing on a vulnerable surface during the seconds or minutes the product is exposed — during soldering, during inspection, during transfer between stations. The particle deposition rate at those locations during those operations is the variable that determines yield. ISO 14644-17 provides the framework to calculate the acceptable limit and measure the actual value.

Contact surfaces add a second pathway that airborne monitoring cannot address. Pick-and-place nozzles, conveyor belts, handling fixtures, and inspection platforms accumulate particle loads between cleaning cycles. A nozzle that has not been measured for surface cleanliness transfers contamination to every component it handles. SUMON measures the Particle Fall Out level on these surfaces in ten seconds, per IEST-STD-CC1246E, at the critical location, before the production run begins. Combined with continuous APMON monitoring of the assembly environment, this gives electronics manufacturers the complete contamination picture: what is arriving from the air, and what is already present on the surfaces the product touches.