Thermal and moisture induced failure analysis in the microelectronics packaging

With the dramatic advances that have taken place in microelectronics over the past decade, flip chip microelectronics packages are increasingly being used in microelectronics system applications such as cellular phones, pagers, laptops, PDAs, and watches. Unfortunately, there is an increasing concern about reliability of flip chip packages due to failures caused by swelling of underfill by moisture absorption and thermal loading caused by unavoidable mismatch of the coefficients of thermal expansion between silicon die and substrate during processing or operating conditions. It has been observed that the major accelerators of failure mechanisms in flip chip packages are moisture and temperature. Both moisture and temperature play a significant role in influencing the mechanical behavior, and therefore, the long-term durability of an electronic package.;The present work focuses on thermomechanical and hygromechanical reliability analysis of the flip chip electronic package. 2D and 3D finite element analyses have been carried out using MSC/NASTRAN FEA software to study the stress and strain distribution in the flip chip package subjected to thermal and moisture loading. The main aim of this paper is to study the effect of material properties of underfill, such as coefficient of thermal expansion (CTE) and coefficient of moisture expansion (CME), on the reliability of the flip chip packages. Six different underfill materials have been studied and compared for reliability performance of the flip chip assembly. Flip chip package without underfill has also been evaluated for the reliability performance. Underfill material increases the thermomechanical reliability of microelectronics package by an order but at the same time adds to hygromechanically-induced stresses reducing hygromechanical reliability. Thus, the use of the underfill considerably reduces the CTE mismatch stresses between substrate and die, while increases moisture-induced stresses. Overall, it has been found that the underfill increases the reliability of an electronic package by an order. Stresses due to hygroscopic swelling found to be much more significant than the thermally induced stresses under the High Accelerated Stress Test (HAST) condition (120°C, 100%RH). Areas of high stress concentration are found out on the UBM (Under Bump Metallurgy) and ILD (Inter Layer Dielectric), which is coincidence with the experimental observation that the UBM opening and ILD delamination are the most common failure modes caused by temperature and humidity loading. Furthermore, parametric studies are performed to investigate the effect of the die thickness and substrate size on the reliability of the package. It has been observed that the die thickness has some implications on the design of the electronic package. Performance of the flip chip assembly is not sensitive to the substrate size. Ultimately, recommendation guidelines are provided for underfill material selection. Overall, this research work results in a comprehensive understanding of the primary mechanisms responsible for the failure of flip chip package due to moisture and thermal expansion mismatch.