Interfacial fracture toughness of Pb-free solders

Increasing environmental concerns and pending government regulations have pressured microelectronic manufacturers to find suitable alternatives to Pb-bearing solders traditionally used in electronics packaging. Over recent years, Sn-rich solders have received significant attention as suitable replacements for Pb-bearing solders. Understanding the behavior of intermetallics in Sn-rich solders is of particular concern as the microelectronics industry progresses towards Pb-free packaging. The formation of intermetallic compounds results from the reaction of the solder with the metallization on the substrate in the electronic package. While the presence of the intermetallic is an indication of good wetting, excessive growth of the intermetallic can have a dramatically adverse effect on the toughness and reliability of the solder joint. Understanding their fracture behavior will lend insight to their reliability under mechanical and thermomechanical strains. We investigated the intermetallic compound growth associated with Sn-0.7Cu and Sn-4.0Ag-0.5Cu solders on Ni-Au, Ni-Pd, and Cu substrates. (Ni,Cu)₃Sn₄ was present at the Ni interface for both solders but was coarser in the case of Ni-Pd. Cu₆Sn₅ and Cu₃Sn were observed for both solder types. The Cu₃Sn layer was similar in thickness and appearance for both solders, but the Cu₆Sn₅ was smoother and rounder in the case of Sn-0.7Cu. Additional time above liquidus resulted in growth of the Cu₆Sn₅ layer and eventual spalling of the IMC grains. The effect of the intermetallic on the toughness (K_Q) of the solder joint was investigated using a modified compact tension specimen. Typical failure modes included bulk solder failure, intergranular separation, and intermetallic fracture, or cleavage. In some cases, additional time above solder liquidus was used to shift the dominant failure mode from that dominated by the bulk solder to interfacial delamination through the intermetallics. Solder joint fracture toughness was different between Ni-Sn and Cu-Sn interfacial intermetallics and was also affected by the relative intermetallic thickness. The relationship between solder and intermetallic microstructure and mechanical properties is discussed.