Enabling Socketable BGAs with Eutectic-Forming Bi-based Coatings on Sn Spheres: A Closer Look at the Bi Deposition Process

Land grid arrays (LGAs) and ball grid arrays (BGAs) are prevalent technologies for the board-level interconnection of numerous packaged integrated circuits (ICs). These technologies find applications in socketing and surface mount (SMT) scenarios, respectively, but bring about different design cycles, manufacturing, and inventory challenges. Consequently, the semiconductor industry has actively pursued a unified solution for microprocessor package design, focusing on enabling a socketable BGA technology. This paper presents a novel approach to address this significant challenge, involving the surface modification of Sn BGA spheres with eutectic-forming bismuth (Bi)-based coatings. Bi has recently attracted industry attention for its potential to enable lower assembly temperatures, thereby reducing warpage and the environmental impact of assembly processes. However, the interest in Bi is relatively recent, and there are currently no commercially available formulations for the electroless deposition of Bi. Autocatalytic Bi deposition presents notable challenges due to two main factors: 1) Bi exhibits relatively low catalytic activity as a post-transition metal, and 2) Sn, a reactive substrate, adds complexity to the process. In this paper, two bath formulations are investigated utilizing tin chloride and sodium hypophosphite as reducing agents, respectively. Although thick Bi films were obtained, challenges related to bath stability and coating adhesion on the substrate surface were encountered. Competition with galvanic displacement reactions (immersion plating) was also explored, where certain bath conditions resulted in hyperactive corrosion of the substrate and uncontrolled growth of Bi films, deviating from the desired self-limiting reaction. This preliminary investigation is crucial for establishing scalable, autocatalytic deposition processes for Bi on traditional solder spheres, a critical step towards enabling socketable BGAs and novel low-temperature soldering processes.