Acceptance Threshold
Statistical quality criteria establish the maximum allowable nonconformances within a production lot during receiving inspection. Manufacturers enforce batch acceptance limits to ensure that delivered bare boards or assembled printed circuit boards meet contractual yield targets. Rejection of the entire lot occurs when defect counts exceed this established threshold during sampling under IPC standards.
Defect Distribution
Attribute sampling plans divide nonconformances into critical and major categories based on functional severity. Critical defects like conductive bridge shorts or delamination trigger immediate lot rejection with zero tolerance. Major defects such as solder balling or out-of-spec hole plating allow a tightly controlled percentage of occurrences across the sample size before rejecting the batch.
Solderability testing and microsectioning provide empirical counts that determine whether defect levels remain within allowable statistical bounds. High-density interconnect designs require stricter sampling frequency due to increased routing density and finer trace pitch.
Verification Metric
Vendor quality audits rely on historical lot disposition records to track supplier process stability over consecutive fabrications. Downstream assembly failures drop when incoming bare board lots meet tight threshold limits consistently. Uncontrollable variance in plating thickness or solder mask registration forces adjustments to incoming sample sizes.
Acceptance boundaries define the legal liability for rework costs between the fabricator and the assembly house.
