Microstructural Evolution and Cracking of Pb-free Ball Grid Array Assemblies under Thermal Cycling

Two kinds of CBGA （ceramic ball grid array） assemblies were made by reflow soldering process using two different Pb-free solders. Microstructural evolution and cracks induced by thermal cycling in CBGA assemblies were examined by scanning electron microscopy （SEM） and finite element method （FEM）. Before thermal cycling, intermetallic compounds （IMCs） Cu6Sn5 and Ag3Sn were observed at the solder interface between Cu and Ag metallizations, respectively. After thermal cycling, another IMC Cu3Sn was observed near the Cu pad in both two assemblies and the layers of Cu6Sn5 and Ag3Sn became thicker. As a result of thermal cycling, cyclic stress and strain were accumulated in the solder joint leading to fatigue cracking. Both experiments and FEM revealed that cracks preferred to initiate at the corner of each solder joint. Multi-modes of the crack propagation were found in the two assemblies. Based on Coffin-Manson equation, the thermal fatigue life was calculated and the predicted life showed good agreement with the experimental results.

Investigation on Aging-induced Softening of Eutectic Microstructure in SnBi/Cu Interconnect by Nanoindentation

Nanoindentation was used to probe softening of the eutectic microstructure in a SnBi/Cu interconnect following thermal aging. Tests were conducted at constant loading rates and under constant loads to characterize the flow stress and creep behavior of the eutectic microstructure. Aging was found to reduce the flow stress but to increase strain-rate sensitivity of the eutectic SnBi microstructure. While the reduction in the flow stress was related to the coarsening of the eutectic microstructure, the increased creep rate sensitivity was resulted from additional contribution of grain boundary movement to indentation creep.

Application of Electroless Fe-42Ni(P) Film for Under-bump Metallization on Solder Joint

Because Fe has a more negative standard reduction potential than Ni, the simultaneous electroless deposition of Fe and Ni is difficult. In this study, Fe-42Ni（P） electroless deposit was prepared by using disodium ethylene diarnine tetraacetate （ETDA-2Na） as complexing agent to reduce the difference in the electrode potential between Ni2＋ and Fe2＋o The solderability and the interfacial reaction between Fe-42Ni（P） alloy and Sn were investigated. It was found that the electroless Fe-42Ni（P） alloy has excellent wettability with Sn. Moreover, the interfacial reaction rate between Fe-42Ni（P） and Sn is very slow. These results suggest that Fe-42Ni（P） alloy may become an attractive under-bump metallization （UBM）.

Thermal Performance of Low-Melting-Temperature Alloy Thermal Interface Materials

Thermal resistance of low-melting-temperature alloy （LMTA） thermal interface materials （TIMs） was measured by laser flash method before and after different stages of heating. The results showed that the thermal performance of the LMTA TIMs was degraded during the heating process. It is suggested that the degradation may mainly be attributed to the interfacial reaction between the Cu and the molten LMTAs. Due to the fast growth rate of intermetallic compound （IMC） at the solid-liquid interface, a thick brittle IMC is layer formed at the interface, which makes cracks easy to initiate and expand. Otherwise, the losses of indium and tin contents in the LMTA during the interfacial reaction will make the melting point of the TIM layer increase, and so, the TIM layer will not melt at the operating temperature.

Hydrous cerium oxides coated glass fiber for efficient and long-lasting arsenic removal from drinking water

A novel arsenic adsorbent with hydrous cerium oxides coated on glass fiber cloth(HCO/GFC)was synthesized.The HCO/GFC adsorbents were rolled into a cartridge for arsenic removal test.Due to the large pores between the glass fibers,the arsenic polluted water can flow through easily.The arsenic removal performance was evaluated by testing the equilibrium adsorption isotherm,adsorption kinetics,and packed-bed operation.The pH effects on arsenic removal were conducted.The test results show that HCO/GFC filter has high As(Ⅴ)and As(Ⅲ)removal capacity even at low equilibrium concentration.The more toxic As(Ⅲ)in water can be easily removed within a wide range of solution p H without pre-treatment.Arsenic contaminated ground-water from Yangzong Lake(China)was used in the column test.At typical breakthrough conditions(the empty bed contact time,EBCT=2 min),arsenic researched breakthrough at over 24,000 bed volumes(World Health Organization(WHO)suggested that the maximum contaminant level(MCL)for arsenic in drinking water is 10 mg/L).The Ce content in the treated water was lower than 5 ppb during the column test,which showed that cerium did not leach from the HCO/GFC material into the treated water.The relationship between dosage of adsorbents and the adsorption kinetic model was also clarified,which suggested that the pseudo second order model could fit the kinetic experimental data better when the adsorbent loading was relatively low,and the pseudo first order model could fit the kinetic experimental data better when the adsorbent loading amount was relatively high.