Electrodeposition Complexity and the Root Cause of Interfacial Voiding in Solder Joints with Plated Nickel

Soldering onto nickel (Ni) pads almost invariably results in the formation of submicroscopic voids between intermetallic compound (IMC) Ni3Sn4 and the solder. However, these voids remain virtually undetectable until the IMC reaches a thickness exceeding 5 mu m. At that point, they become a concern for joints' conductivity and reliability, especially upon high-temperature exposure during operation. Previously, this issue was regarded as unavoidable, but our research has revealed a connection to impurities introduced during the plating process. The present study aims to understand the underlying cause of void formation by investigating the influence of various factors, such as Ni electrodeposition parameters, deoxygenation, and deliberate modifications to the electrolyte composition. Our findings indicate that impurities, likely in the form of Ni(OH)(2) or NiOOH, undergo gasification during the reflow process, thus initiating the nucleation of interfacial voids. With the IMC layer development, these voids grow, driven by Kirkendall vacancy formation and possibly amplified by mismatch stresses between the solder and IMC. To assess the impurity content, we utilized void volume as an indicator, and composition analysis was employed to examine the electrodeposited Ni for the presence of oxygen and fractions of ionized Ni species. Confirmation of the presence of oxidized Ni inclusions was obtained through thermogravimetry and X-ray photoelectron spectroscopy.