Solder joint reliability of wafer level chip scale packages (WLCSP): A time-temperature-dependent creep analysis

A novel and reliable wafer level chip scale package (WLCSP) is investigated in this paper. It consists of a copper conductor layer and two low cost di electric layers. The bump geometry consists of the eutectic solder, the cop per core, and the under bump metallurgy. Nonlinear time-temperature-depende nt finite element analyses are performed to determine the shear stress, she ar creep strain, shear stress and shear creep strain hysteresis loops, and creep strain energy density of the corner solder joint. The thermal-fatigue life of the corner solder joint is then predicted by the averaged creep st rain energy density range per cycle and a linear fatigue crack growth rate theory. The WLCSP solder bumps are also subjected to shear test. Finally, t he WLCSP solder joints are subjected to both mechanical shear and thermal c ycling tests.