A FAILURE ANALYSIS AND REWORK METHOD OF ELECTRONIC ASSEMBLY ON ELECTROLESS NI / IMMERSION AU SURFACE FINISH
Author: Zequn Mei Company: Hewlett-Packard Company Date Published: 9/12/1999
Abstract: Current industrial electroless nickel plus immersion gold used as surface finish on conducting pads on printed circuit boards (PCB) is problematic. Recent reports in the literature described cases of severely reduced joint strength of solder to electroless nickel plus immersion gold. Reported here is another example of extremely weak solder joints of PQFP packages on PCB with electroless nickel plus immersion gold finish. The weak joints were located preferentially on certain package sites and certain lead locations of the packages. After packages with weak joints were removed, the pads underneath were exposed, showing strikingly dark color, similar to the “black pads” or “black nickel” reported in the early literature. ESCA, Auger, and EDX spectroscopes detected extraordinarily high carbon content but normal phosphorus content on the black pads. A rework procedure was developed; new packages could be soldered to the black pads after they were cleaned with flux at high temperature and mechanically rubbed with copper wick. The root cause for the black pads is probably the loose control in plating conditions of electroless nickel and immersion gold. The minor difference in electrical potential among the packages and among the leads caused different types or different degrees of chemical reactions between electroless nickel and immersion gold bath, resulting in the preferential occurrence of black pads.