Grain modification and formation of tough IMC phase in submicron Sn–Ag microbumps via doping Zn and Ni in Cu substrate

For the advanced 2.5D/3D IC applications, the IMCs suppression and phase stability should be emphasized. With a reduced bonding height of microbumps, the IMCs would occupy a high volume fraction of the joints during the reflow process or operation. A high volume fraction of IMCs and excessive phase transition could induce internal stress and voids, which might be adverse to reliability of interconnection. The novel Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni (by weight) microbumps have been fabricated and the considerable inhibition on the IMCs and void formation was investigated. Four kinds of IMC were found in Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps.: (Cu,Ni)6(Sn,Zn)5, (Cu,Ag)5(Sn,Zn)8, Ag3Sn and (Cu,Ni)Zn. The tough bulk (Cu,Ag)5(Sn,Zn)8 combined with solid solution strengthened and refined (Cu,Ni)6(Sn,Zn)5 grains as well as the void-free microstructure were expected to enhance the reliability of microbumps. The effect of Ag on the formation of tough (Cu,Ag)5(Sn,Zn)8 was also addressed. These findings provided insights into the design of substrate alloy in advanced electronic packaging reliability. •The Cu–26Zn–18Ni substrates slowed down the formation of IMCs significantly and inhibited the Cu3Sn and Kirkendall voids.•The grain size (Cu,Ni)6(Sn,Zn)5 is refined to hundreds of nanometers by co-addition of Ni and Zn in substrates.•Addition of Ag favored the formation of tough bulk (Cu,Ag)5(Sn,Zn)8, which was beneficial to mechanical reliability.