Improvement of Electrochemical Migration Resistance by Cu/Sn Intermetallic Compound Barrier on Cu in Printed Circuit Board

A thin Sn coating was coated on the Cu electrodes on a printed circuit board (PCB) and annealed to form Cu/Sn intermetallic compounds (IMCs). Electrochemical migration (ECM) characteristics were investigated with respect to coating thickness, conductor spacing, and potential bias. From the anodic polarization test, the corrosion current of Cu/Sn IMCs was reduced to nearly two orders of magnitude at 0.217 V (versus saturated calomel electrode) compared to pristine Cu. The corrosion resistance of Cu/Sn IMCs was improved. Water drop test (WDT) results revealed that mean time to failure also increased over 50 times on the sample with the Sn coating (160 nm) at 50-mu m spacing compared to the pristine Cu and a thick Sn coating (160 nm) was more effective than thin Sn coating (90 nm) due to the formation rate of IMCs. Passivity behavior and pitting corrosion were observed in both the anodic polarization test and the WDT. The graph of current-density potential in the anodic polarization test and the TTF potential in the WDT showed a quite similar curve shape. Therefore, dissolution step is the rate-determining step in ECM and governs the entire ECM process. Cu/Sn IMCs can be a new alternative surface barrier to improve the ECM resistance of Cu electrodes on PCBs.