Method for Mitigating the Warpage of Ultra-Thin FC-CSPs by Controlling of EMC Properties

Recent developments in high-speed communication technology have accelerated reductions in the size and thickness of semiconductor packages. Fan-out wafer level packaging (FO-WLP) is an emerging packaging trend. Considering cost and scalability, there are also high expectation for ultra-thin Flip chip-Chip scale package (FC-CSP) technology which has evolved into a structure that is an approaching die-sized dimensions. As the packaging structure changes to Ultra-thin FC-CSP, it is necessary to control package warpage with smaller quantity of epoxy molding compound (EMC.) This study evaluated strip warpage of molded array package (MAP) and singulated unit warpage after dicing to investigate the effect of EMC properties on the package warpage. It has been understood that the mold shrinkage of EMC is the most dominant factor in warpage control of packages. However, the degree of warpage amount is not proportional to various properties in a packaging structures with a high ratio of silicon die to EMC, such as Ultra-thin FC-CSP when modelling low strip and unit warpage. For strip warpage, high mold shrinkage and low 25°C flexural modulus showed a proportional relationship. While high Tg demonstrated some effectiveness for unit warpage, clear correlation was not established. This investigation developed a "molding stress index" which is calculated by integrating the storage modulus and the coefficient of thermal expansion (CTE) multiplier over a temperature range from temperature (35 °C) to molding temperature (175 °C.) The results demonstrate that this parameter can be used to evaluate the changes in properties such as CTE, modulus and Tg in accordance with the change of temperature comprehensively. There is a tendency toward strip and unit warpage when the molding stress index is high. This study suggests that the molding stress index is an effective evaluation method to lower the unit warpage. This evaluation also confirmed that both lower strip and unit warpage for various substrates were realized with EMC having high mold shrinkage and low modulus at normal temperatures while maintaining a high molding stress index. In addition, further warpage reduction is possible by jointly optimizing the properties of EMC and substrate.