Solid state reaction of Sn3.0Ag0.5Cu solder with Cu(Mn) under bump metallization

In flip chip technology, Cu thin-film is a widely used under bump metallization (UBM). However, the major disadvantages of Cu UBM are fast consumption of copper, rapid growth of IMCs and easy formation of Kirkendall voids [1, 2]. Many efforts have been focused on suppression of Kirkendall voids which are detrimental to solder joints reliability in the microelectronics industry [3, 4]. In this study, a novel Cu(Mn) UBM design was provided by sputtering to improve the conventional Cu metallization. For the higher Mn concentration (10 at.% Mn) in the Cu(Mn) UBM, a new Sn-rich phase formed between Cu
6
Sn
5
and the Cu(Mn) UBM. However, a layer of crack formed after aging. For the lower Mn concentration (2 at.% Mn), the growth of Cu
3
Sn and Kirkendall voids was significantly suppressed during thermal aging. The interfacial reaction was analyzed by a field emission electron probe microanalyzer (FE-EPMA) to evaluate the composition distribution. Kinetic analysis and X-ray color mapping gave the evidence that Mn diffusion into Cu
3
Sn slowed down the diffusivity of Cu in Cu
3
Sn layer. The Mn-enriched Cu
3
Sn might serve as a diffusion barrier to reduce interfacial reaction and Kirkendall voids formation. These results suggest the Cu-Mn UBM with a low Mn concentration is beneficial to retarding the Cu pad consumption in solder joints.