Eutectic Sn/Pb solder bump and under bump metallurgy: interfacial reactions and adhesion

Abstract

In flip‐chip interconnection on organic substrates using eutectic tin/lead solder bumps, a highly reliable under bump metallurgy (UBM) is required to maintain adhesion and solder wettability. Various UBM systems such as 1μm Al/0.2μm Ti/5μm Cu, 1μm Al/02μm Ti/1μm Cu, 1μm Al/0.2μm Ni/1μm Cu and 1μm Al/0.2μm Pd/1μm Cu, applied under eutectic tin/lead solder bumps, have been investigated with regard to their interfacial reactions and adhesion properties. The effects of the number of solder reflow cycles and the aging time on the growth of intermetallic compounds (IMCs) and on the solder ball shear strength were investigated. Good ball shear strength was obtained with 1μm Al/0.2μm Ti5μm Cu and 1μm Al/0.2μm Ni/1μm Cu even after four solder reflows or seven‐day aging at 150∞C. In contrast, 1μm Al/0.2μm Ti/1μm Cu and 1μm Al/0.2μm Pd/1μm Cu showed poor ball shear strength. The decrease of the shear strength was mainly due to the direct contact between solder and non‐wettable metals such as Ti and AL, resulting in a delamination. In this case, thin 1μm Cu and 0.2μm Pd diffusion barrier layers were completely consumed by Cu‐Sn and Pd‐Sn reaction.