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 (δPd(P)) on the interfacial reaction between Sn-3.5Ag and ENEPIG and the mechanical reliability of the joint system. A slight increase in δPd(P) from 0 μm to 0.2 μm caused the nucleation of a pronounced amount of (Pd,Ni)Sn4 in the solder matrix after reflow, which gradually migrated to the joint interface and formed a (Pd,Ni)Sn4/Ni3Sn4 dual layer after subsequent solid-state aging. The formation of the (Pd,Ni)Sn4 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)Sn4-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)Sn4 was dictated by thermodynamics and could be rationalized using a Pd-Ni-Sn isotherm. The above observations indicated that δPd(P) was an important factor of the ENEPIG solderability and should be strictly controlled to be 0.1 μm or less to prevent the undesired Pd-embrittlement phenomenon in Sn-Ag micro joints. [Display omitted] •New Pd-embrittlement phenomenon•Strong effect of Pd(P) thickness on the Sn-3.5Ag/Au/Pd(P)/Ni(P)/Cu reaction•Formation and migration of (Pd,Ni)Sn4•Brittle (Pd,Ni)Sn4/Ni3Sn4 interface caused joint mechanical degradation.