Study of warpage characteristics of molded stacked-die MCP using Shadow Moiré and Micro Moiré techniques

Abstract

As the mobile and consumer devices continue to demand for more functionality and less power within a smaller dimension, integration of IC packages becomes essential for the successful introduction of electronic devices in the market. Stacked-die packages are finding increased usages in SiP (system-in-package) and PoP (package-on-package), while various new packages based on CoC (chip-on-chip) and TSV (through-silicon-via) technologies are being developed. To ensure the reliability of assembled parts, the dimensional stability of stacked-die packages has to been properly controlled. This is to ensure a predictable performance of interconnections can be achieved after SMT/PoP assembly and at various operating conditions. Especially, warpage characteristics caused by thermo-mechanical stresses need to be considered in product design. In this study, a systematic study of single-die, four-die and eight-die land-grid-array (LGA) package warpage characteristics was carried out using both Shadow Moire and Micro Moire techniques. Shadow Moire was used to study the package warpage in a temperature range from room temperature to 260°C to simulate the solder ball reflow conditions; while a Micro Moire instrument, with a resolution of 417 nm without phase shifting, was used to understand localized stress distribution within the package between room temperature and 104°C. Effects of die sizes, die thicknesses, number of die stacked, type of molding compounds and mold cavity heights can thus be correlated. When using single die packages as a control, Shadow Moire measurement results showed molding compound types have a greater impact on package warpage than die thicknesses and mold cavity heights. In an eight-die MCP package, larger die size exhibited higher warpage during thermal cycling, whereas under same conditions choice of molding compound is insignificance to achieve improved performance. It was also found when die sizes and mold cavity heights were kept the same, four-die packages with approximately 3 times the die thickness exhibited slightly less warpage than that of eight-die packages. However, of the two, eight-die package exhibited dimensional changes within a narrower temperature range under reflow conditions. Micro Moire measurement results clearly showed the stresses were concentrated at the die edges between adjacent stacked dice. These results are useful for employing new designs lead to better product reliability.