Study Of Isothermal Aging And Electromigration Behaviors And The Size Effect Of Lead-free Solder Mirco-interconnections In Electronic Packages

With the electronic products moving towards lead-free, miniaturization and multi-functions, the reliability of electronic packaging interconnects has attracted more and more attentions. In this thesis, the typical copper-wire/Sn-3.0Ag-0.5Cu/copper-wire sandwich structured butt joints with various micro-scale sizes were prepared under the modeling reflow soldering condition. The isothermal aging and electromigration behavior of the micro-scale solder joints as well as the joint size effect were studied sysmatically. The tensile strength change of the micro-scale solder joints of different sizes after undergoing isothermal aging and electromigration were studied by a Dynamic Mechanical Analyzer (DMA) at the testing temperature of 100℃. The microstructure, fractographies and the electromigration induced polarity effect in micro-scale solder joints were characterized by an optical microscope (OM) and a scanning electronic microscope (SEM) equipped with energy dispersive spectroscopy (EDS).The results of the influence of isothermal aging on the micro-scale solder joints show that both the joint thickness and isothermal aging time significantly affect the ultimate tensile strength of the joints under quasi-static micro-tension. Without isothermal aging, the tensile strength of the joints increases with decreasing the thickness of the joints, the average tensile strength of the solder joints with three different thicknesses of 100, 200 and 300μm are 53.75, 46.59 and 44.38MPa respectively; this is probably owing to the increase of mechanical constraint in the solder joints. It has also been shown that the tensile strength of the joint decreases with the prolongation of isothermal aging time which results in the coarsening of the microstructure in the solder joints. After aging for 48 hours, the average values of tensile strength of the above three types of joints decrease to 50.12, 40.87 and 35.26MPa repectively, and further decrease to 44.13, 38.38 and 33.48MPa after aging for 96 hours. However, the thickness of the intermetallic compounds (IMC) layer at the joint interface shows almost no change obviously even after aging for 96 hours.Furthermore, the effects of electromigration on tensile strength of the micro-scale solder joints were also studied. The eletromigration tests were carried out with a constant current density of 1×104A/cm2 at 100℃for 48 hours. The experimental results show that the electromigration has brought about obvious degradation of tensile strength of the joints. The average values of tensile strength of the joints with the thickness of 100, 200 and 300μm decrease to 44.5, 35.8 and 27.8MPa after electromigration, i.e., decreased by 17.3%, 23.2% and 37.4% respectively. The degradation degree of the joint strength decreases with decreasing joint thickness and exhibits a clear dependence on the solder joints'size.Moreover, it has been shown that the electromigration also induces obvious polarity effect in micro-scale solder joints. After undergoing a certain current loading, the IMC layer at the anode interface of the joints becomes thicker compared with that in the cathode. Clearly, the electromigration resulted in weakening of the solder/IMC interface in the joints'cathode and the decomposition of IMC at the cathode interface; subsequently the microviods and microcrack developed at the cathode interface of the joints, and finally the fracture occured at the cathode interface of the joints. Also, it has been seen that the electromigration leads to the transfer of fracture mode of the joints from the ductile to the combination of brittle and ductile.