Brittle interfacial fracture of PBGA packages soldered on electroless nickel immersion gold

Electroless nickel (Ni) / immersion gold (Au), an alternative metal finish on printed circuit board (PCB), has been used successfully for many electronic products. However, brittle fracture of PBGA packages soldered on this metal finish was reported recently by several companies. This paper describes brittle fracture in mechanical testing of thermal enhanced, cavity-down PBGA packages, in the as-reflowed and the aged conditions. In the as-reflowed condition, the fracture is a cleavage between PBGA solder balls and PCB conducting pads. The fracture loads, determined from four-point bending, were typically half of that for peeling off PCB conducting pads. Failure analysis showed that the fracture occurred at the interface between Ni3Sn4 and Ni-P interface. Three possible failure mechanisms were explored: (1) phosphorous segregation at the interface, (2) contamination or oxidation during the Ni-Au plating or after plating via diffusion, and (3) brittle fracture of Ni-P and Ni3Sn4. After aging at 150 degrees C for a few days, the AuSn4 particles, once had resided in the interior of solder joint in the as-reflowed condition, migrated onto the interface. The weak adhesion between Ni3Sn4 and AuSn4 resulted in a more brittle fracture than that before the aging. This Au-relaled brittle fracture occurred although the Au concentration in solder joint was only 0.1 wt %, much less than 3 wt %, the rule of thumb concentration for brittle solder joint. The Au-related interfacial fracture may be eliminated by reflow after the aging which moved the AuSn4 back to the solder joint interior.