Investigation of Cu-Sn-Cu transient liquid phase bonding for microsystems packaging

Abstract

ABSTRACT Fabrication, characterization, and thermomechanical reliability of electroplated Cu-Sn-Cu-based transient liquid phase (TLP) bonding required in microsystems packaging are reported. Die-level bonding was achieved using 8-μm-thick electroplated copper pillars covered with 2-μm tin. A mechanical test specimen having rectangular ring-shaped structures was fabricated to measure the strength of Cu-Sn joints. An electrical test vehicle having 250 copper pillars connected in a daisy chain series was designed. The effect of bonding force, temperature, and durations on the shear strength and electrical resistance of the Cu-Sn-Cu joints were investigated. The mechanism behind the improved bonding at higher force, temperature, and duration is reported. Samples bonded at a lower bonding force (5 kg) failed, while those bonded at a higher force showed good strength. A complete transformation of Sn into stable Cu3Sn was essential to achieve good bonding. Due to the presence of Cu3Sn IMC, the bonding strength of samples bonded at higher temperatures and longer duration was higher. Thermal shock and humidity tests were conducted to check the long-term reliability of Cu–Sn–Cu bonded joints. The percentage change in the electrical resistance of a complete daisy chain remained <5% even after 1000 thermal shock cycles.