Reliability comparison of aluminum redistribution based WLCSP designs

Redistribution layer (RDL) WLCSP technology is often used in area array ICs where both high performance and low cost are important considerations. RDL can be implemented in two ways: a) as topmost level metal in the wafer fab (Wafer fab-RDL), and b) as an additional metal layer during bumping operation (Assembly-RDL). Selection of an appropriate RDL technology and its optimization are necessary for building reliable products. In this paper, we describe the structural differences between the two technologies and compare their thermal reliability performance. Aluminum is used as the redistribution layer material for both technologies. To explain the experimental findings, we construct a finite element model (FEM) of the experimental IC which is packaged as CSP area array and mounted on a FR4 board. We investigate the critical role of process parameters such as UBM stack composition, and thicknesses of various PBO layers in determining the overall device reliability. Finally, we also explore the reliability improvements obtained as a result of using an underfill layer during board assembly.