In solder ball grid array (BGA) technology, solder joint reliability is one of the critical issues in microelectronics manufacturing industries. In this reliability aging study, Sn3.5Ag0.7Cu solder joints were subjected to accelerated temperature cycling (ATC) test in TBGA assembly. Fatigue fracture occurred, very close to the solder/intermetallic compound (IMC) interface, at the TBGA component side due to the larger coefficient of thermal expansion (CTE) mismatch compared to the PCB side. During reflow, needle-type and scallop-type morphologies of $({hbox {Cu,Ni}})_{6}{hbox {Sn}}_{5}$ IMCs were formed at the TBGA component and PCB interfaces. In the process of thermal cycling, a layer of $({hbox {Ni,Cu}})_{3}{hbox {Sn}}_{4}$ IMC grew beneath the $({hbox {Cu,Ni}})_{6}{hbox {Sn}}_{5}$ IMC due to the out diffusion of Ni from the under bump metallization (UBM). After extended thermal cycling aging, Ni驴Sn驴P IMC was found between the $({hbox {Ni,Cu}})_{3}{hbox {Sn}}_{4}$ IMC and the ${hbox {Ni}}_{3}$P layer at the printed circuit board (PCB) interface. Grain ripening and spalling of $({hbox {Cu,Ni}})_{6}{hbox {Sn}}_{5}$ IMC grains into the solder joint was also observed in the process of thermal cycling. The spalling phenomena of $({hbox {Cu,Ni}})_{6}{hbox {Sn}}_{5}$ IMCs was caused by interface structure change and cyclic shear stresses and strains incurred during temperature cycling.
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