Effect of electroless Ni/immersion Au plating parameters on PBGA solder joint attachment reliability

We reported in ECTC'98 the phenomenon of brittle fracture of PBGA (plastic ball grid array) solder joints attached on electroless Ni / immersion Au (e'less Ni / imm Au) surface finish. In order to identify the root cause for the brittle fracture, a matrix study of e'less Ni / imm Au plating parameters was conducted. The pH value, temperature, and rinsing procedure in the Ni plating bath were varied systematically. The order of putting solder mask before or after Ni / Au plating was compared. Different e'less Ni bath chemistries (low P, high P, boron reducing agent) were also studied. The solder joint attachment strength on the surface finishes was measured by 4-point bending test. The surface finishes were characterized by their surface morphology, phosphorous concentration in Ni, and relative Au layer thickness. The results showed that the solder joint attachment strength depended on the plating conditions, but did not pinpoint a single plating parameter or physical property as a sole factor. The high phosphorous content, in the range of 6 to 12 wt %, was not the root cause for the brittle interfacial fracture. The possible solder mark contamination of Ni and Au plating baths was not the root cause for the interfacial fracture. The results suggest that impurities or damage introduced during immersion Au plating are responsible for the poor interfacial strength and brittle fracture.