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|Filler Dispersion in Epoxy Mold Compound and its Effect on the Reliability of Cu/Low-k Devices in Plastic Ball Grid Array Packages|
|Keywords: Reliability, Epoxy Molding Compounds, Cu/Low-k|
|Epoxy mold compound (EMC) is a crucial component of the plastic ball grid array (PBGA) packaging solutions. Within the complex material system of an EMC, the inorganic fillers are the most important constituent as it greatly affects critical material properties including CTE and modulus. The volume and sizes of the fillers are controlled by material suppliers for the as-manufactured EMC. The uniform filler distribution within the as-manufactured EMC, however, is disrupted and altered during the transfer molding process. Depending on the design of the package, the non-uniformity in filler content distribution across the package can be severe enough to result in significantly higher thermal stresses in some location of the package and thus degrade the reliability of the package. The situation becomes even worse with the usage of low-k interlayer dielectrics (ILD) on the die as many feasible low-k materials are more tenuous and suspectable to damage under high mechanical stress than their predecessors. In this work, filler distribution in PBGA packages with different packaging designs and molding processes for low-k devices were studied. Optical microscopy and digital image processing were adopted to analyze the filler distribution throughout the package. It was found that a region with lower-than-average filler content existed near a specific corner of the die. The level of filler reduction is affected by package structure, mold compound properties and mold process. The mechanical properties change of the EMC in this region were both calculated with theoretical models and evaluated with high-resolution Moiré analysis. The results from these analyses will be discussed. These PBGA packages were tested with AATC and it was found that fracture and delamination failures occurred more frequently at the die corner with reduced filler content. A finite element model was established to assess the impact of filler content reduction on thermal stresses. The results suggest that the localized filler non-uniformity is a significant degrading factor on PBGA package reliability and must be minimized as much as possible.|
|Min Ding, Senior Packaging Engineer
Freescale Semiconductor Inc.