Thin film cracking in plastic packages-analysis, model and improvements

Describes the physical characteristics of the thin-film cracking (TFC) phenomenon and a model which explains the interactions among the key variables such as die corner layout style, package materials, die size, die thickness, and passivation topology. Data were tabulated on several plastic products to determine if there was any commonality. Comparison was also made between layouts for different products using optical microscopy and SEM/EDX where necessary. It was determined that four main factors (layout style, materials, die thickness, and passivation type) impact thin-film cracking to different degrees. A stress factor versus cycles plot from several configurations were developed which can help predict potential jeopardy as a function of variables identified above. This plot resembles the Coffin-Manson law and depicts safe and unsafe regions for factors that modulate temperature cycling stress. It appears that two possible mechanisms help improve TFC performance. On one hand tiny sacrificial structures help interlock the molding compound and contain the damage, whereas significant decoupling between molding compound and the passivation seems to reduce the overall damaging stress