Re-balling BGA with gold-plated copper spheres, the need and the SMT challenges

The electronics industry is switching the majority of their mobile roadmaps to Ball Grid Array (BGA) packages in order to enable thinner form factor devices. Original Equipment Manufacturers (OEMs) have to solder down the packages to their production boards since there is no form-factor production BGA socket available in the industry. This raises several challenges for the OEM supply chain and increases overall product costs. In addition to the overall added height of the socket, one technical challenge that faces the OEM BGA socket is the long-term reliability of the interface between the socket's gold-plated contacts and the package's SAC solder balls. This non-noble metal interface creates the risk of intermetallic formation and growth over the product life cycle, which will cause a bond formation and affect the removability of the BGA; it could also increase the overall resistance of the next socketing cycle due to the intermetallic residue left on the pins. This paper presents an attempt to overcome the interface challenge by re-balling the BGA with gold-plated copper spheres. Several design and SMT process parameters affect the overall re-balling quality. In particular, solder wicking is a major challenge. This paper presents an attempt to optimize the design and SMT process parameters for better results. Pad size, pad define, stencil thickness, stencil aperture size, copper sphere size, and reflow oven vs. SRT are some of the parameters that were studied.