Bare Board Insulation Resistance Standard Baseline Specifications
Unpopulated printed circuit boards require baseline insulation resistance exceeding ten gigaohms under direct current bias to pass IPC Class 3 delivery criteria.
Comb Pattern Coupon
Test Vehicle
Representative test structures replicate the dense spacing of electrical traces to evaluate manufacturing quality under stressful conditions. A comb pattern coupon features interleaved conductive traces resembling the teeth of a comb, designed to measure surface insulation resistance. The specific geometry allows engineers to monitor the degradation of board materials when exposed to humidity and electrical bias.
Fabricating the coupon alongside the production boards ensures identical process conditions.
Resistance Measurement
Environmental chamber exposure tests the electrical isolation between the parallel conductor lines over extended periods. When the comb pattern coupon is subjected to high temperature and high humidity, a constant voltage is applied to detect leakage current. High resistance values indicate that the laminate and solder mask maintain excellent insulation properties.
If the insulation breaks down, the resistance drops precipitously, signaling a failure in the material or the cleaning process. This continuous monitoring helps to trace the timeline of electrochemical migration and dendritic growth before it causes a failure in field applications.
Contamination Source
Residual chemistry from the fluxing or plating processes often drives the drop in insulation resistance. Ionic residues left on the comb pattern coupon can attract moisture, forming a conductive pathway between the traces. Ensuring clean assembly processes prevents early failure of the circuit under humid conditions.