As the trend of miniaturization, portability and high performance in electronic products goes forward, the assembly density is continuously increasing, which in turn increases the current density in the solder joints. Under high current density, the changes in the microstructure at the solder/substrate interface or near the solder/substrate interface become significant and crucial to the stability of the solder structure. The influence of high current density （>104A/cm2）on the intrerfacial microstructure evolution and reliability of Sn-Bi solder/substrate interconnects was investigated. Segregation of Bi to the anode interface caused a potential mechanical reliability for the interconnect because of Bi’s brittleness, and the following inhibition of Bi migration to the anode side was studied. Besides of Bi migration under high density current stressing, the growth and microstructure of intermetallic compound at the anode and cathode interface was greatly affected in Sn/Ni-P interconnects.
Under current stressing, Bi migration process was in-situ observed in eutectic Sn-Bi sloder and analyzed in Sn-3.8Bi alloy. The dissolution and precipitation of Bi phase occurred at the same time in the eutectic Sn-Bi alloy, which determined the formation of the continuous Bi layer in front of IMC (intermetallic compounds) layer at the anode side and Sn-rich phase formed at the cathode side. It was found that fracture took place in the continuous Bi layer when eutectic Sn-Bi/substrate interconnects after current stressing experienced mechanical strike. By the relationship of Bi migration rate in eutectic Sn-Bi alloy and current density, the Blech product and the value of in eutectic Sn-Bi alloy at the experimental temperature are 393A/cm and -3.5×10-11cm2/s, respectively.
In exploring measures to inhibit Bi migration and postpone the formation of continuous Bi layer in front of IMC at the anode interface, Pb and Sb addition on Bi migration in Sn-Bi alloys were investigated under current stressing. 3wt% of Pb was added into Sn-3Bi, and the Bi segregation at the anode side was retarded. It’s supposed that the reaction of Pb and Bi atoms to form Pb-Bi IMC inhibited Bi migration in Sn phase. Different contents of Sb (1wt%，3wt%，5wt%) were alloyed with eutectic Sn-Bi solder, and the migration rate of Bi to the anode interface decreased with Sb content until it reached 3wt%, at which Bi migration rate was 57% of that in eutectic Sn-Bi solder.
In addition of solder microstructure was affected by current stressing at the interface, the microstructure solder/substrate interface was also affected by the current stressing. The growth of Ni-Sn IMC and P-rich layer at the anode and cathode interface was investigated in the Sn/Ni-P interconnects under current stressing. The thickness of Ni-Sn IMC increased at the anode side and decreased at the cathode side. For the P-rich layer, the thickness increased at both the cathode and anode sides, but the growth rate was faster at the cathode than that at the anode. Current stressing promoted the growth of columnar Ni12P5 layer at the cathode side, while enhanced the growth of the non-cloumunar Ni12P5 layer at the anode side.
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