Coupling Effects between Two Interfacial Soldering Reactions in Flip-Chip Solder Joint

Autor: Shen-Jie Wang, 王信介
Rok vydání: 2006
Druh dokumentu: 學位論文 ; thesis
Popis: 94
It has been reported that as Ni and Cu bond pads are soldered to form a joint, Sn/Ni and Sn/Cu interfacial reactions would interact mutually, as reviewed in chapter 1. The dissolved Cu atoms from the Cu pad would move toward the Ni pad, then, a Cu-Sn compound layer formed on the Sn/Ni interface. In our previous study, the driving force of the migration of dissolved Cu atoms toward the Ni side has been proposed to attribute to the reduction of the Cu solubility near the Sn/Ni interface. This dissertation discusses the coupling effects between two interfacial reactions using a series of different metal/Sn/Cu sandwich structures. In chapter 2, we further study the Cu diffusion mechanism in Ni/Sn/Cu sandwich structure. Using Ni/Sn/Cu sandwich sample, the mutual interaction between Sn/Ni and Sn/Cu interfacial reactions has been studied. On the Cu side, the major interfacial reaction product is Cu6Sn5, on the other hand, a ternary (Cu,Ni)6Sn5 compound layer formed on the Ni side. We found that the growth kinetics of the interfacial compound layers on the both sides would reach a steady-state in the late stage of reflow. The interfacial compound layer on the Cu side would remain a constant thickness. On the contrary, the interfacial compound layer on the Ni side grew in a relatively fast rate, which was found to be linear with time. Our results indicate that the growth of the ternary (Cu,Ni)6Sn5 compound layer was controlled by the Cu dissolution flux at the solder/Cu6Sn5 compound interface. The dissolution constant of the Cu6Sn5 compound into the molten Sn was determined to be 0.13 (�慆/s). Using Pt/Sn/Cu sandwich structure, the cross-interaction between Sn/Cu and Sn/Pt interfacial reactions was studied. In chapter 3, we found that the interfacial Sn/Pt reaction was greatly influenced by the opposite Sn/Cu reaction. The PtSn4 interfacial compound formation rate is very sluggish, compared to that in the single Sn/Pt reaction case. On the other hand, the growth rate of the Cu6Sn5 compound at the Sn/Cu interface was not affected by the opposite Sn/Pt reaction, which has a similar rate with the single Sn/Cu reaction case. But, the morphology of Cu6Sn5 grains differed from that of the single Sn/Cu reaction case, i.e, conventional scallop-type shape. For the sandwich case, Cu6Sn5 grains have column-like appearance. The column-like morphology of Cu6Sn5 grains is due to small interfacial energy, �莗older/Cu6Sn5, caused by the Pt dissolution in the molten solder. Also, we found that the Pt dissolution would cause a reduction of Cu solubility in the molten solder. The changes of above two parameters are found to diminish the ripening flux among Cu6Sn5 grains. Hence, the smaller Cu6Sn5 grains would not be depleted, then, the separation distance among Cu6Sn5 grains would not be widened. We report the coupling effect between Sn/Au and Sn/Cu interfacial reactions by using the Au/Sn/Cu sandwich structures in chapter 4. By FE-EPMA analysis, the AuSn, AuSn2 and AuSn4 phases were formed at the Sn/Au interface, which dissolved Cu content less than 1 at.% after initial stage of reflow. At the late stage, only AuSn2/AuSn phases were observed at the Sn/Au interface and AuSn2 phase was distributed in the Sn matrix. After only 1 minute, we have demonstrated that the original (Cu,Au)6Sn5 was transformed to Au25Cu25Sn50 at the Sn/Cu interface, which based on the AuSn structure by XRD results. Since Au atoms participated into the opposite Sn/Cu interfacial reaction, so, the sandwich case shows more Au consumption than the single reaction case. Finally, the equilibrated formation between Au25Cu25Sn50 and Cu foil is (Cu,Au)3Sn phase. In chapter 5, the morphology and growth rate of Ag3Sn at Sn/Ag interface shows no significant difference between sandwich Ag/Sn/Cu, Ag/Sn/Ni and single Sn/Ag reaction. It implies the Sn/Ag reaction would not be affected by the Sn/Cu reaction in the sandwich Ag/Sn/Cu and Sn/Ni reaction in the sandwich Ag/Sn/Ni, respectively. For the Ag/Sn/Cu sandwich structure, the chunk-like Ag3Sn plate was preferentially segregated at the Sn/Cu interface after initial reflow and the precipitated Ag3Sn formation is independent with reflow time. On the contrary, we did not see any Ag3Sn precipitation in the vicinity of the Sn/Ni interface in Ag/Sn/Ni sandwich structure. Yet, we can find a large amount of Ag3Sn particles distributed uniformly in the Sn matrix. We observed a microstructure variation along the Ni/Sn/Cu solder joint in chapter 6. This asymmetrical solder microstructure was resulted from the Cu concentration gradient along the Ni/Sn/Cu solder joint. The mechanical test shows that mechanical property of the Ni/Sn/Cu solder joint highly correlated with the asymmetrical solder microstructure. The Ni/Sn/Cu solder joint test samples show the fracture interface occurred in the solder near the Ni interface. Finally, the correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps has been studied in appendix. Upon the solid-state aging, a diffusion-controlled process was observed for the interfacial Ni-Sn compound formation of the Sn/Ni(P) reaction couple and the activation energy is calculated to be 42 KJ/mol. For the Sn0.7Cu/Ni(P), in the initial aging, a needle-shape Ni-Sn compound layer formed on Ni(P). Then, it was gradually covered by a layer of Cu-Sn compound in the later aging process. Hence, a mixture layer of Ni-Sn and Cu-Sn compounds formed at the interface. For the Sn3.0Cu/Ni(P), a thick Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. On the contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu solder bumps is relatively weak and exhibits poor resistance against the aging process. Upon the reflowing process, the gap formation at Ni(P)/Cu interface caused a fast degradation in the interfacial strength for Sn solder bumps. For Sn0.7Cu and Sn3.0Cu solder bumps, Ni3P formation was greatly retarded by the self-formed Cu-Sn compound layer. Therefore, Sn(Cu) solder bumps show better shear strengths over Sn solder bump.
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