The Electromigration Investigation on the Newly Developed Pb-free Sn-Zn-Ag-Al-Ga Solder Ball Interconnect

A Sn-Zn based eutectic solder with a relatively lower melting temperature and cost was under investigation for its response to electrical current stressing. The BGA (ball grid array) joint was produced with the Sn-8.5Zn-0.5Ag-0.01Al-0.1 Ga solder ball. The solder joint was supplied from industry and was produced to interconnect PCB and BGA substrate on which the metallization layers are Cu (30-35 um)/Ni (10 um)/Au (0.21 um). The investigation was conducted under air at 120degC in a furnace with a current density in the order of 10^-3 A/cm². Voids were found in th solder near the metallization of the PCB side when the electron flows outward the PCB. Meanwhile, the IMC (AuZn₃ and AgZn₃) formed on the substrate side during reflow was decomposed after current stressing. The decomposition results in the reformation of Zn-Ag-Zu IMC within the solder. Delta-NiZn compound was also detected within the solder, of which the Ni may come from the Cu/Ni/Au metallization layer. The Ni was also found to form NiGa compound within the solder. Hillocks were found for the IMC layer regardless of the direction of electrical current.