Mechanical properties of intermetallic compounds on lead-free solder by moiré techniques

In this paper, methods for determining elastic moduli and coefficients of thermal expansion (CTE) of intermetallic compounds (IMC) formed at the interfaces between lead-free solder and metal substrates are presented. For the determination of elastic moduli, two kinds of lead-free solder - SnZn and Sn - were used; the metal substrates were copper and nickel. Nanoindentation techniques were adopted to determine the elastic moduli of Cu _33.5Zn_66.6, Cu₃Sn, Cu₆Sn ₅, and Ni₃Sn₄. Results for Cu _33.5Zn_66.6 are new to the literature, and others values are in good agreement with those presented in the literature. On the other hand, for CTE determination, two moiré techniques, namely reflection moiré and shadow moiŕ, were developed to measure the deformation of IMC/metal composite structures subjected to thermal loading. Finite-element analyses using ANSYS were then performed as a convolutional process, and the genetic search algorithm was used to optimally obtain the CTE of IMC. The CTE of Cu_33.5Zn_66.6 was found to be approximate to that of copper, and the CTE of Cu₃Sn was 10% larger. This method is also applicable to on-wafer films and IMC on micrometer order, such as the case of solder/IMC/under bump metallization.