Investigation of intermetallic compound and voids growth in fine-pitch Sn-3.5Ag/Ni/Cu microbumps

Micron level Cu-pillar microbumps, which appear as the size of three-dimensional packaging shrinks down, have to introduce many unexpected reliability problems. The fast growth of intermetallic compounds (IMC) and voids tend to be a serious one. In this paper, the growth behaviors of IMC and voids were investigated in Sn-3.5Ag/Ni/Cu bumps, which were in the diameter ranging from 6 to 11 mu m and fabricated under same reflow process. The consequence manifested that the growth rate of interfacial IMC increased from 0.448 to 0.578 mu m/min as the bump diameter decreased from 11 to 6 mu m and the acquired IMC could be divided into two layers: the (Cu, Ni)(6)Sn-5 layer and Ni3Sn4 layer. As a result of the migration of Ni atoms, many voids were left in the interface between (Cu, Ni)(6)Sn-5 and Ni3Sn4. In the edge of Cu pillar, side wall wetting reaction was detected, which was confirmed by the formation of voids and (Cu, Ni)(6)Sn-5 phase. Further, as the bump diameter decreased, the side wall wetting reaction aggravated, which partly contributed to the acceleration of IMC growth.