Precipitation of large Ag3Sn intermetallic compounds in SnAg2.5 microbumps after multiple reflows in 3D-IC packaging

Microbumps have been adopted as interconnects between Si chips in 3D integrated-circuit packaging. The solder volume of a microbump decreases dramatically due to fine-pitch requirement and it is approximately two orders smaller in magnitude than that of a traditional flip-chip solder joint. The metallurgical reactions in the microbumps may behave quite differently to those in flip-ship bumps. Liquid-state metallurgical reactions were examined in SnAg2.5 microbumps with Ni metallization. The results indicate that large particles of Ag3Sn intermetallic compounds (IMCs) precipitate after a 10-min reflow on microbumps with 4.0-μm-thick solder, which does not occur with flip-chip solder bumps. It is proposed that the Ag concentration in the remaining solder may increase as Sn reacts with Ni. The increase in the Ag concentration is mainly responsible for the occurrence of the large Ag3Sn precipitates. The formation of these Ag3Sn IMCs would be detrimental to the mechanical properties of the microbumps.