OVERCOMING HEAD-IN-PILLOW DEFECTS IN HYBRID LGA SOCKET ASSEMBLY

摘要

As the implementation of lead-free solder card assembly processes continues to expand across the server industry, additional challenges will arise. While the soldering defect known as 'head in pillow' (HIP) or 'head on pillow' is not new, avoiding these defects on increasingly large lead-free BGA connectors and sockets will become more difficult. As server board assemblies drive greater technology complexity, they also drive increased difficulty in optimizing the soldering process for all of the required components and connectors. For example, the processor card of a new mid-range server system has a unique large mass SMT connector that requires the use of a vapor phase reflow soldering process. In addition, the board contains four ball grid array connectors, known as hybrid LGA sockets, to accommodate the plugging of LGA processors. The large size of these BGA sockets makes them vulnerable to dynamic warpage and other physical changes during lead-free processing. This tendency, especially when other risk factors are present, may create a situation where the solder joints in the connector array are susceptible to the formation of HIP defects. This paper will discuss the optimization of the assembly process for this complex printed circuit board assembly (PCBA). Several design of experiments were evaluated, including solder paste chemistry, stencil parameters, vapor phase reflow profile settings and reflow fixture design. Additionally, the contribution of incoming connector tolerances and thermal dynamic warpage were considered. There was also implementation of containment actions to prevent any escape of head in pillow defects.