Flip chip assembly on organic substrate technology has been developed and implemented as a key packaging technology for cost and performance advantages. When organic substrates are used for the recent high pin count flip chip assembly, the interconnection is made of high-melting solders(97Pb-3Sn, 320℃) on the chip combined with low melting solders(37Pb-63Sn, 183℃) on the organic substrates. But environmental and health concerns over the use of lead have led to investigation of the alternative Pb-free solders to replace commonly used Pb-Sn solders in microelectronic packaging application. In this study 80Au-20Sn / 58Bi-42Sn combined solder structure is proposed as the Pb-free candidate for combination structure solder for flip chip in MCM(Multi-Chip Module) packaging. Eutectic 80Au-20Sn solder has very good melting point(280℃) for high melting solder, and it has been successfully used for highly reliable die attach and fluxless soldering in optoelectronic packaging. Eutectic 58Bi-42Sn solder has also attractive melting point of 139℃, and it has good wettability with Cu and Ni substrate, good mechanical performance and long-term stability for creep.
Three kinds of experiments were conducted. Firstly 80Au-20Sn / 58Bi-42Sn solder combination structure was demonstrated. For good demonstration of the combination structure, Jig soldering and RA-type flux were applied. Secondly, the interfacial reaction in the solder joint of Au-Sn / Bi-Sn solder was studied. Interfacial reaction between Au-Sn solder and Bi-Sn solder with Ni UBM made $(Au,Ni)_3Sn_2$, $Ni_3Sn_4$ and $AuSn_4$ type IMCs, which can be predicted by Au-Ni-Sn isothermal section. Interfacial reaction between Au-Sn solder and Bi-Sn solder made $AuSn_2$ phase with thin inner layer of AuSn. Finally, reflow test for 2nd level BGA package was conducted for once, twice and three times. Among the various IMCs in the combination structure solder joint, $AuSn_2$ layer is much thicker than other IMCs, while the others are less than 3um. Until once and twice reflow test, shear strength of solder joint is improved, but it deceases from three times. After lap shear test, fracture surface was observe at the interface of Bi-Sn solder and Ni UBM, so $Ni_3Sn_4$ is brittle region for combination solder joint, and its thickness is major factor for shear strength.
