Influence of Pd(P) thickness on the Pd-free solder reaction between eutectic Sn-Ag alloy and Au/Pd(P)/Ni(P)/Cu multilayer

Autocatalytic plating deposition of a Pd(P) film over a Ni(P) metallization pad, such as Au/Pd(P)/Ni(P) (electroless nickel/electroless palladium/immersion gold, ENEPIG), has been widely used in microelectronic industry for the past decade. However, the data regarding its soderability with eutectic Sn-Ag (Sn-3.5Ag) is still lacking in the literature. We investigated the effects of Pd(P) thickness (delta(Pd(P))) on the interfacial reaction between Sn-3.5Ag and ENEPIG and the mechanical reliability of the joint system. A slight increase in delta(Pd(P)) from 0 mu m to 0.2 mu m caused the nucleation of a pronounced amount of (Pd,Ni)Sn-4 in the solder matrix after reflow, which gradually migrated to the joint interface and formed a (Pd,Ni)Sn-4/Ni3Sn4 dual layer after subsequent solid-state aging. The formation of the (Pd,Ni)Sn-4 dense layer slowed the growth of Ni3Sn4, facilitated the depletion of the Ni(P) film, and seriously deteriorated the mechanical characteristics of the solder joints. This (Pd,Ni)Sn-4-induced embrittlement in solder joints (termed the Pd-embrittlement phenomenon) had never been found previously in the Sn-Ag reaction system. The formation and migration of (Pd,Ni)Sn-4 was dictated by thermodynamics and could be rationalized using a Pd-Ni-Sn isotherm. The above observations indicated that delta(Pd(P)) was an important factor of the ENEPIG solderability and should be strictly controlled to be 0.1 mu m or less to prevent the undesired Pd-embrittlement phenomenon in Sn-Ag micro joints.