| 其他摘要 | In electronic packaging, much attention has been paid to the studies of lead-free soldering because Pb element presents potential hazard to the human health and environment. With increasing packaging density, the size of the soldering joint becomes smaller and smaller. As a result, the reliability of the lead-free solder joints becomes an important concern, and therefore it is necessary to further investigate the failure mechanisms of the lead-free solder joint. This study was intended to investigate the dependence of the deformation behavior on the second phase and grain boundary in Sn-Ag-Cu solder and interfacial damage mechanisms on micro-scale between the solder and substrate, so that insights may be gained to effectively improve the reliability of the soldering joint.
As one of the most promising lead-free solders, the Sn-Ag-Cu alloy has been widely studied. Within this alloy, the small amount of Ag and Cu tend to appear in the form of intermetallic compounds. It was found that the rod or needle-like second phase was prone to cracking, resulting in the loss of the tensile elongation, while the small intermetallic nodules provided significant dispersion strengthening effect and were also beneficial to the ductility of the alloy. When the coarse dendritic microstructure was extruded into fine equiaxed grain structure, the alloy exhibited a great elongation but very low strength in the low strain rate range. The strain rate sensitivity of the extruded alloy was notably higher in the low strain rate range than that in the high strain rate range. It was suggested that the fine equiaxed grain microstructure increased the volume percentage of the grain boundary, which promoted the time-dependent grain boundary deformation mechanism. When the Sn-Ag-Cu alloy with a fine equiaxed grain structure was subjected to strain-controlled cyclic loading, the alloy exhibited a rapid cyclic softening behavior in the initial few cycles. At the same time, a large number of intergranular cracks were found after the initial few cycles. The in-situ crack observation in three-point bending fatigue indicated that the crack density had an approximately logarithmic relationship with the cyclic number. Based on the microcrack model, the predicted cyclic softening curves agreed well with the experimental results. Therefore, it was concluded that the rapid cyclic softening had resulted from the intergranular cracks. The grain boundary sliding and increased volume percentage of grain boundary in fine equiaxed grain structure accelerated the intergranular cracking during strain-controlled fatigue.
To examine the interfacial failure mechanisms, solder/copper single crystal joints were subjected to cyclic loading. As the persistent sliding band (PSB) formed in the fatigued copper single crystal, the dislocations moving along the PSBs channels were blocked by the IMC interface, resulting in the dislocation pile-up and stress concentration. With continued impingement of the interface by the PSBs, the reliability of the interface was examined by observing cracking mode on a micro-scale level. It was found that the crack formed readily at the intersection of PSB with the thick and planar interface, and then rapidly propagated across the IMC layer. The failure along the planar IMC/solder interface was faster than that along the scallop-like IMC/solder interface. For the Sn-Bi/copper single crystal interface, the crack appeared initially on the IMC/copper substrate interface after aging at 120oC for 7 days, due to the Bi segregation onto the IMC/Cu interface. After electroplating a thin Ag film on the copper substrate, the Ag3Sn layer was formed during reflowing. Since the Ag3Sn layer was effective in blocking off Bi from segregating onto the IMC/copper substrate, the interfacial embrittlement was successfully avoided.
The interfacial reliability of Sn-Ag-Cu with Fe-Ni alloys was investigated. A very thin FeSn2 IMC layer formed between the Sn-Ag-Cu solder and Cu substrate electroplated Fe-Ni layer during reflowing. Aged at 120oC, the FeSn2 layer grew very slowly; but after high temperature aging at 180oC, a new IMC layer consisting of Cu, Ni and Sn formed and grew at a faster rate. It was suggested that the high temperature aging accelerated the diffusion of Cu atoms across the FeNi layer and onto the interface. The results of the ball shear test demonstrated that the Sn-Ag-Cu/Cu (FeNi) interface had comparable mechanical properties to those of Sn-Ag-Cu/Cu interface, indicating that FeNi layer may be used as a reliable UBM. |
修改评论