The Electrochemicalmigration Study Of Silver Plate On Immersion Silver PCB

Under the stress of saving cost, many manufacturers began to use the metal silver which has high cost performance as plate material of printed circuit board(PCB).As the development of high miniaturization and densification PCB, and the working environment of PCB getting more harsh, the electrochemical migration(ECM) of immersion silver finished PCB caused more concern. This study researched the ECM characteristics, mechanism and influence factors of silver coating on immersion silver finished PCB through the water drop (WD) test and the temperature humidity and bias voltage(THB) test.The WD test is the simulation of water condensation on PCB surface wires, the test system can observe the ECM phenomenon in time and store the surface insulation resistance (SIR) of the PCB. The THB test simulate the real work environment of PCB, the multipass SIR test system can store the SIR of30PCBs under test.This study researched the ECM characteristics of silver coating combined with the time-resistance curve, we can divide the ECM into three periods according to the time-resistance curve. This study also explained the ECM mechanism of silver coat on immersion silver finished PCB, and drew a conclusion that it was copper that migrated on the insulation material. In the humidity environment, when the silver plate of immersion silver PCB cannot cover the base metal well, there will be an galvanic cell reaction between silver and copper, the reaction will cause copper which has low electrode potential be oxidized into copper ions; the copper ions then migrate to the cathode by the effect of a certain electric field strength and get electrons in the cathode, then the dendrites which grow directed to anode form. The influence factors on ECM failure time were also investigated in this study. There is a negative exponential relation between the ECM failure time and voltage; the ECM failure time and the space have a linear relation; the ECM failure time and the initial electric field strength have an egative exponential relation; the voltage has significant influence on ECM failure time. Ensured the critical failure voltage at constant space.The results of the influence of voltage and space on ECM failure time in THB test had consistency with the WD test results. According to this study, we can use the WD test in qualitative investigation of ECM since the WD test has high efficiency. There should be an improvement of immersion silver process to make a better silver coating which can cover the base copper well to increase the reliability of PCB with immersion silver.