Optimization of solder height and shape to improve the thermo-mechanical reliability of wafer-level chip scale packages

This study focuses on the development of a highly reliable solder bumping solution by the optimization of bump height and shape for flip-chip mounting of large die directly on PCB without the use of underfill. To achieve the goal, an elongated solder joint with an optimized shape has been developed that demonstrated far superior board-level thermo-mechanical reliability to that of the conventional BGA bumps. When tested at the 7.2×7.2 mm² die size, the first thermal cycling failure of the improved joint exceeded 800 cycles with the characteristic lifetime reaching 989 cycles without the use of underfill. Experimental result was correlated with FEM analyses to optimize the bump height and shape in order to identify the lowest maximum strain along with improved strain distribution that could lead to the longest lifetime. Moreover, unlike the standard BGA joint where a single crack propagated near UBM is the norm, the fatigue behavior of the optimized joint clearly demonstrated a superior ability to deform along the whole length of the joint. Multiple cracks were constantly observed in the elongated joint indicating the newly designed joint has better capability to absorb stresses and, as a consequence, deforming instead of cracking, that ultimately lead to significantly improved fatigue lifetime.