RELIABILITY OF TRANSFER MOLD-UNDERFILLED FLIP CHIP DEVICES

It is well known that underfilling a flip chip device with a capillary liquid encapsulant results in a substantial improvement in device reliability. The use of capillary underfills has therefore become commonplace in flip chip technology. However, increasing reliability requirements and the ever-present need to cost reduce the package are driving the industry to consider alternate technologies, such as no -flow underfills or fast -flow, snap cure encapsulants. Recently the development of suitably engineered epoxy molding compounds designed to encapsulate (including underfilling) flip chip devices have been investigated and proven to be technically feasible. This approach takes advantage of the production speeds of transfer molding and can offer significant productivity enhancements over the traditional liquid underfill process for certain applications. The use of the transfer underfill/overmold process eliminates two sequential encapsulation steps (e.g. underfill and glob top). By the use of the molded flip chip 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. In this study the effect of no-clean flux type used in the flip chip assembly process on the interfacial adhesion of molded flip chip devices was examined by scanning acoustic microscopy. Two different perimeter-bumped die with a 2.5 mil offset (either 170 by 215mil or 340 by 430mil in size) were subjected to various argon plasma cleaning times. For the large die assembled with the high residue no-clean flux, short cleaning times (3 min) lead to initial mold compound-substrate interface delamination in areas away from the gap area. The plasma cleaning process may be removing residue from underneath the flip chip die and redepositing it on the substrate nearby. Increasing the plasma cleaning time eliminated this delamination. Molded flip chip test assemblies survive two thousand thermal shock cycles of -55°C to +125°C. JEDEC Level 3 performance with 240°C reflow is also achieved. In this paper the performance of several epoxy transfer molding compounds developed for this application will be discussed.