Solder Volume Effects On The Formation And Evolution Of Microstructure And Shear Fracture Behavior Of BGA Structure Cu(Ni)/Sn3.0Ag0.5Cu/Ni(Cu) Interconnects

With increasing miniaturization and multifunction of modern electronic products and devices, the volume of solder interconnects has been scaling down continuously; and accordingly the ratio of the thickness of the interfacial intermetallic compound （IMC） layer to the standoff height of the joint is increasing gradually. It has been recognized that the intrinsic brittleness of IMC and the mechanical constraint effect in the miniature solder joints are more likely to result in brittle fracture failure of solder interconnects in service, thus this has posed a serious challenge to the reliability of electronic products and devices. In this thesis study, a technology based on the geometry-based truncated sphere method and force-balanced analytical solution was used to design ball grid array （BGA） structure solder joints with different standoff heights and then to prepare true BGA structure joints. The influences of substrate materials, standoff height and reflowing assembly sequence on the solidification behavior, formation and evolution of the interfacial IMC and microstructure, and the shear fracture behavior of BGA structure Cu（Ni）/Sn-3.0Ag-0.5Cu/Ni（Cu） interconnects were investigation systematically.The results show that the growth rate constant of the total thickness of interfacial IMC in the single-sided interface Sn-3.0Ag-0.5Cu/Cu joint increases with the concentration of Cu in the solder matrix. The cross-interaction of Cu and Ni in Cu/Sn-3.0Ag-0.5Cu/Ni solder joints has obvious influence on the composition and morphology of the interfacial IMC; and the IMC spalling phenomenon has occurred at the interface of Ni side. The cross-interaction of Cu and Ni is beneficial for suppression of growth of Cu₃Sn and reduce the growth rate of interfacial IMC at the Cu side, while having no significant influence on porosity ratio of Kirkendall voids in the Cu₃Sn layer at the Cu side of the joint. Furthermore, the cross-interaction of Cu and Ni in BGA structure Cu（Ni）/Sn-3.0Ag-0.5Cu/Ni（Cu） joints can increase the consumption of Cu pad and retard the consumption of Ni under bump metallization （UBM）.The influence of standoff height of BGA structure joints on the formation and evolution of interfacial IMC of Cu（Ni）/Sn-3.0Ag-0.5Cu/Ni（Cu） joints is significant, and it has been found that the cross-interaction of Cu-Cu and Ni-Ni make the growth rate constant of interfacial IMC and Cu₃Sn increase with standoff height irrespective of the assembly sequence. Compared with the cross-interaction of Ni-Ni, the influence of cross-interaction of Cu-Ni on the growth kinetic of IMC at the Ni side depends on the reflowing assembly sequence and solder volume. Further, compared with the cross-interaction of Cu-Cu, the cross-interaction of Cu-Ni can notably retard the growth kinetic of interfacial IMC and Cu₃Sn at the Cu side. Moreover, it has been found that the reflowing assembly sequence in Cu（Ni）/Sn-3.0Ag-0.5Cu/Ni（Cu） joints can make a clear difference in the growth rate constant of Cu₃Sn; in particular, the growth rate constant of interfacial IMC and Cu₃Sn at the once-reflowed side increases with standoff height but it is opposite at the twice-reflowed side.Furthermore, the results show that the solder volume, pad size and reflowing assemble sequence have significant influence on the undercooling and solidification behavior of BGA structure joints, and the primary solidification phase and its volume fraction play a key role in determining the undercooling of solders and joints. Generally, the undercooling of solders and joints decreases with increasing the solder volume; and compared with Cu substrate, Ni substrate can effectively reduce the undercooling of Sn-based solder alloy. Moreover, it has been shown that the influential degree of Cu substrate on the undercooling of the Sn-based solder alloy depends very much on the composition of the solder; for example, the undercooling of the hypo-eutectic Sn-3.0Ag-xCu（x=0,0.5）/Cu joints is dominantly determined by the composition of the solder, while for the hyper-eutectic Sn-3.0Ag-xCu（x=1.2,2.0）/Cu joints the Cu substrate size has a stronger effect than the change of composition of the solder.In addition, it has been found that the shear behavior of BGA structure Cu（Ni）/Sn-3.0Ag-0.5Cu/Ni（Cu） joints is mainly influenced by standoff height and there is a parabolic relationship between the shear strength and standoff height. The curves of impact resistance and creep strain rate versus standoff height show the same trend with the shear strength in Cu/Sn-3.0Ag-0.5Cu/Cu joints. The typical fracture path in BGA structure Cu/Sn-3.0Ag-0.5Cu/Cu joints changes from the middle of the solder matrix to the position near the interface when the standoff height increases, and the fracture mode of BGA structure joints shows a clear sensitivity to the volume （standoff height） of joints. Further, it has been shown that the effect of Kirkendall voids at the interface on the facture behavior of Cu/Sn-3.0Ag-0.5Cu/Cu joints becomes stronger with increasing standoff height and aging treatment; however, there is no evidence to show that extensive Kirkendall voids at the interface near the Cu side may influence significantly the shear fracture mechanism of Cu（Ni）/Sn-3.0Ag-0.5Cu/Ni（Cu） joints and affect the creep behavior of Cu/Sn-3.0Ag-0.5Cu/Cu joints.