Key failure modes of solder joints on ENIG PCBs and root cause analysis

The plating system of electroless nickel immersion gold (ENIG) has been widely used to finish solder pads of printed circuit boards (PCBs). The most attractive advantage of ENIG over other finishes is that it can be applied to fine-pitch assemblies such as BGAs, for superior surface mount component placement and solderability because of its coplanarity. ENIG is used by many board shops and is an established process. But recently many problems occur in ENIG solder joints. The worst cases are BGA package solder joint failures during the surface mount assembly process, or in the product´s final use by a consumer. Based on the authors´ long-term experience of solder joints failure analysis, two kinds of key failure modes for ENIG solder joints were found. One was poor wetting (non-wetting or de-wetting) of the solder to the ENIG surface finish on PCB pads during soldering process, and the other one was solder joint crack or separation from the nickel layer surface, causing an electrical open. At the same time, several typical solder joint failure cases were used to show how the two kinds of failure modes occurred. SEM was used to reveal more details of the microstructure of ENIG plated pad surface and the fracture at the interface between solder and metal pad. The plating composition and phosphorous (P) content were determined by EDX analysis. Additionally the failure mechanisms were investigated. The root cause for poor wetting was the poor solderability of corroded Ni layer which was the result of galvanic hyper-corrosion of the plated electroless Ni by the Au plating bath. For solder joint crack, the reason was the weak bonding due to P-rich layer resulted from Ni depletion at the surface from IMC formation. Finally, in order to prevent the two kinds of key failure modes, several suggestions were given.