On the performance of epoxy molding compounds for flip chip transfer molding encapsulation

Flip chip assemblies are underfilled to improve their reliability by increasing the solder joint fatigue lifetime. Historically, flip chips have been underfilled with a capillary liquid underfill material. Other approaches have been more recently developed to address increased productivity objectives, such as the no-flow underfill technology. To capitalize on the production efficiency of transfer molding, epoxy molding compounds have been developed which can concurrently underfill and overmold flip chip die during a transfer molding operation. The transfer molding process offers significant productivity enhancements over the traditional liquid underfill process for certain applications. The application of pressure can reduce the occurrence of voids under the die during the molding process. The use of the transfer underfill/overmold process can also eliminate the two sequential encapsulation steps (e.g. underfill and glob top). By the use of the molded flip chip (MFC) technology, the high volume production rates characteristic of conventional transfer molding can be obtained, along with the added advantage of the utilization of the installed capital base. Epoxy molding compounds offer improvements in thermal expansion coefficients (CTEs) and moisture absorption levels relative to liquid underfill materials. Increased filler content offers the ability to adjust the thermal expansion characteristics of the molding compound to match the package requirements. Assemblies encapsulated with the MFC material survive two thousand thermal shock cycles of -55°C to +125°C, equivalent to current liquid underfill encapsulant performance. Molding compound composition parameters also determine JEDEC level performance, with MRTL2 performance at 240°C reflow obtainable. In this paper the performance of several epoxy molding compounds prepared for this application will be reviewed