Evaluation of Ni Pd Au as an alternative metal finish on PCB

Multi-layer surface finish, from the bottom to top, of electroless Ni, electroless Pd, and immersion Au (Ni Pd Au) have been introduced in the printed circuit board (PCB) industry recently. This paper reports an evaluation of this surface finish from the perspective of solder joint attachment reliability, especially to see if the Ni Pd Au could be immune from the brittle interfacial fracture of PBGA on electroless Ni immersion Au, recently observed and reported by us. PCBs with Ni Pd Au finishes, made from two vendors with varied Pd layer thickness were attached with PBGA packages, and tested in four-point bending. When joint strength is strong, bending tests resulted in peeling off the PCB pads; otherwise, brittle fractures occurred at the interface between solder balls and PCB pads. After aging, solder joints on all Ni Pd Au and reference metal finishes failed by the same brittle fracture at the interface between Ni-Sn and Au-Sn intermetallic compounds. It is concluded that the interfacial fracture was controlled by something other than the Pd, and the existence of the Pd did not prevent the interfacial fracture. Also, the presence of Pd could not prevent the Au migration and subsequent fracture.