(A) study on the effect of Zn on the interfacial reactions and impact reliability of lead-free solder joints

Abstract

With legislation put in place by government and industrial bodies, electronic companies are driven to eliminate the use of lead in their products. At present, Sn-Ag, Sn-Cu, Sn-Ag-Cu, Sn-Zn solder alloys have considered as candidates for lead-free solder materials. Especially, lead-free solder bearing Zn is very fascinating in respect that addition of Zn was known to improve the mechanical reliability of solder alloys. Sn-9Zn eutectic alloy has considered as an alternative for lead-free solder alloy due to its low melting point($198\degC$), excellent mechanical properties, and low cost. However, some problems such as poor wettability and easy oxidation still remain. Addition of Ag or Al into Sn-Zn solder could improve the oxidation resistance and wetting behavior. The microstructures of Sn-(Ag)-Zn alloys to UBMs such as Cu and Ni has been known to be completely different interfacial reactions from those of Sn-Pb or Sn-Ag-Cu alloys. In the microelectronics industry, the market of hand-held devices including mobile phone, PDA and notebook computer has been explosively expanded. The mechanical reliability of drop test becomes one of the most important mechanical reliabilities due to the characteristics of hand-held devices. Among Pb-free solders, Zn bearing alloys has been paid great attentions due to the improvement of mechanical reliability. Accordingly, many researchers have conducted drop tests using various Pb-free solders to investigate the effect of microstructure of solder joints on drop reliability. However, relatively less works were conducted on the correlation between the Zn effect and drop reliability. In $\bf{Chapter one}$, varying amounts of Zn (1, 3, and 7 wt%) were added to Sn-3.5Ag solder on a Cu pad, and the resultant solder joint microstructures after a reflow and isothermal aging (150 $\degC$, up to 500 h) were investigated using scanning electron microscopy, energy dispersive x-ray, and x-ray diffraction, which were subsequently correlate...