Interfacial microstructure and mechanical reliability of the Sn-Ag-Cu/Au/Pd(xP)/Ni(P) reactive system: P content effects

Electroless deposition of a palladium film, e.g., pure Pd or Pd(P), between Au and Ni(P) films has received a great deal of attention from the microelectronic industry over the past decade. We investigated the effect of P on the solderability and the corresponding mechanical reliability of the Sn-3Ag-0.5Cu/Au/Pd(xP)/Ni(P)/Cu joint system, where x = 0, 3, and 4.5 wt%. Analyses of electron probe X-ray microanalysis and field-emission transmission electron microscopy showed that a trilayer of (Cu,Ni)6Sn5/(Ni,Cu)3Sn4/Ni3P intermetallic structure grew at the interface at x = 0 wt%, and the growth of (Ni,Cu)3Sn4 was replaced by that of Ni2SnP at x = 3 and 4.5 wt%. In high-speed ball shear testing, the microstructure transition induced by P addition would cause a fracture mode change from brittle to ductile, and increased the shear resistance of solder joints. These findings indicated that the P content in the Pd(P) film is a crucial factor in interfacial strength, and an appropriate amount of P is beneficial to the solderability of the Au/Pd(xP)/Ni(P) surface finish.