Investigating SIR of Cu, OSP, iSn and HASL surface finishes by THB test

The aim of the study was to investigate the Surface Insulation Resistance (SIR) change of different surfaces finishes during Thermal Humidity Bias test (THB). The studied surface finishes was bare Copper (bCu), Organic Solderability Preservative (OSP), immersion Tin (iSn) and Sn63Pb Hot Air Solder Leveling (HASL). The test panels were comb patterns with and without solder mask, and on some of the test panels flux were applied to examine the effect of the flux residues on the reliability of the circuits. For the investigations a real-time SIR measurement system was designed with 16 channels to ensure the same circumstance in the same time for the tested circuits. The results were evaluated with the Mean-time-To-Failure (MTTF). The results show that the flux and the solder mask have a strongly influence on SIR. Further more in the case of tin-bearing surface finishes we have observed an interesting phenomenon: huge SIR drop was detected after starting the THB test, which might be caused by the bonded OH groups on the surface.