Role of FBEOL Al pads and hard dielectric for improved mechanical performance in lead-free C4 products

One of the major reliability concerns of current and next generation integrated circuits is mechanical failure due to stresses induced by the chip-package interactions (CPI). The packaged parts are subjected to thermal-mechanical stresses due to a mismatch of the coefficient of thermal expansion of the Si, lead-free C4 bumps, and the organic flip-chip substrate leading to mechanical delamination or cracking in the weaker low-k/ultra-low K films within the chip. This work discusses the role of Aluminum (Al) pads in the far-back-end-of-line (FBEOL) levels of the chip in CPI stress mitigation of the weak low-k and ultra-low k (ULK) BEOL levels. The affect of the Al pad thickness, size and shape on the CPI stresses have been studied by means of 3D mechanical finite element analysis. "White C4" bump data showing the benefits of increasing the thickness of the Al pads and growing the Al pad size to be larger than the under bump metallurgy (UBM) diameter in alleviating detrimental stresses from the weak BEOL levels is also been discussed in the paper. This paper also outlines through mechanical modeling and "white C4" bump data the reduction in CPI stresses in the weaker BEOL levels with increasing thickness of the FBEOL hard dielectric.