Study of IMC Morphologies and Phase Characteristics Affected by the Reactions of Ni and Cu Metallurgies with Pb-Free Solder Joints

Most Pb-free solders proposed to replace Pb-containing solders for microelectronic applications are Sn-rich solders with a higher melting point than the eutectic Sn-Pb solder. Due to their higher Sn content and higher reflow temperatures, the interfacial reactions between Sn-rich solders and the commonly used pad metallurgies such as Cu or Ni are found to be more aggressive and, therefore, become a concern that may
affect the integrity and reliability of the solder joint. The interfacial microstructures on Ni layer have been investigated in terms of intermetallic compound (IMC) morphology and its growth kinetics. Cu, when plated either as an underlayer or overlayer on Ni, is shown to influence the interfacial reactions, which is also affected by the characteristics of Ni plating and reflow conditions. As a result, an in-depth study has been carried out to understand the interfacial reactions between Pb-free solder and Ni (both electro- and
electroless plated) by comparing the IMC morphology and phase identification. The three variables that have been investigated in this study include types of Ni plating, Ni thickness, and reflow conditions. The effects of Cu under- and over-layer on Ni are also examined by varying both Cu and Ni thickness. Based on this study, it was found that the type of Ni plating affects the IMC morphology significantly. A compact structure of Ni-Sn IMC was identified on electrolytic Ni, and a porous structure on electroless Ni. In addition, Ni thickness as well as reflow time also affect the IMC phases formed at the interface with the presence of a Cu layer. When a thin Cu layer is plated over Ni and subsequently reacted with Sn solder, the interfacial microstructure and IMC phases strongly depend on the Cu thickness. For thin Cu layer, Ni-Sn IMC is the dominant phase, while Cu-Sn IMC is the major phase for thick Cu. The results obtained in this study have provided a more detailed understanding of the interfacial reactions, which leads to the designing of a more reliable metallurgical system for Pb- free solder joints.