CHALLENGES OF SMT ASSEMBLY OF POP PACKAGES

摘要

The handheld market segment for mobile phones and tablets is driven by increasing multifunctional capabilities while continuously reducing form factors. To enable these next generation technologies, microelectronic packaging has evolved to include package-on-package (PoP) architectures where a memory package is stacked on top of an integrated architecture logic package. Today, one of most popular PoP designs for enabling small form factors involves attaching a memory to a logic device via through mold interconnects (TMI). The TMI stack-up brings a new set of challenges for surface mount (SMT) assembly in comparison to traditional interposer-based PoP configurations. This paper describes unique assembly challenges for TMI-based PoP packages and offers solution paths for improving SMT yield for memory attach. It discusses the impact of interactions between warpage shapes of logic and memory devices and effect of package geometric factor such as mold thickness on TMI failure modes. In addition to PoP geometry, SMT process parameters and assembly materials play a significant role in solder joint failure rates. Optimizing flux dip and transfer efficiency through SMT process can minimize defect rates, but without proper selection of material candidates successful SMT assembly is still in jeopardy. To improve memory assembly yield, flux materials with high tackiness and activity toward Sn/SnO need to be utilized. This study will review key materials characterization methods and materials properties to facilitate proper selection of solder flux materials benefitting the overall assembly yield of TMI PoP components. The disadvantages of choosing a dippable paste for memory package attach process will be discussed. Further, the explanation for why TMI PoP design requires a greater quantity of flux than interposer-based PoP architecture design for formation of defect-free TMI joints will be expounded and the industry's first TMI balls pre-coat technology as the solution for this need will be provided.