0-7803-8985-9/05/$20.00 ©2005 IEEE 21st IEEE SEMI-THERM Symposium Thermomechanical Behavior of Organic and Ceramic Flip Chip BGA Packages Under Power Cycling S. B. Park 1 , Rahul Joshi, and Bahgat Sammakia Department of Mechanical Engineering State University of New York at Binghamton Binghamton, NY 13902 Phone: 607-777-3415 1 sbpark@binghamton.edu Abstract Numerical and experimental techniques were employed to assess the thermomechanical behavior of ceramic and organic flip chip packages under power and accelerated thermal cycling (ATC). In power cycling (PC), the non-uniform temperature distribution and different coefficient of thermal expansion (CTE) of each component make the package deform differently than in the case of ATC. Conventionally, reliability assessment is conducted by ATC that assumes uniform temperature throughout the assembly. This is because, ATC is believed to be a worse case condition compared to PC, which is similar to the actual field condition. In this work, for ceramic and organic flip chip ball grid array (FC-BGA) package, numerical simulations of ATC and PC were performed by combination of computational fluid dynamics (CFD) and finite element analyses (FEA). For PC, CFD analysis was used to extract transient heat transfer coefficients while subsequent thermal and structural FEA was performed with heat generation and heat transfer coefficient from CFD as thermal boundary condition. The numerical simulations were compared with in-situ, real-time moiré interferometry experiment. It was found that for certain organic packages, power cycling was more severe condition that causes solder interconnects to fail earlier than ATC while ceramic packages fail earlier in ATC than PC. Accordingly, qualification based on ATC testing may overestimate the life of the package. Keywords Power Cycling, ATC, Flip Chip PBGA, CBGA, FEA, CFD, Moiré Interferometry 1. Introduction Accelerated thermal cycling (ATC) has long been performed to assess reliability of solder interconnects where the entire assembly is subjected to uniform temperature changes in an environment chamber (heating/cooling). However, the actual package is experiencing non-uniform temperature distribution across the assembly with the chip as the only heat generation source. Consequently, it is obvious that ATC does not truly represent real service conditions. In case of ceramic packages, testing under ATC made the reliability projection more conservative than under PC and accordingly, ATC was accepted as standard test method. This is because the substrate has higher flexural rigidity and the second level interconnect failure is mostly due to the shear deformation driven by thermal expansion mismatches between substrate and PCB. However, in case of organic packages depending upon chip, substrate, PCB dimensions and coefficients of thermal expansion, PC could be more severe condition than ATC and the assembly, which passed ATC, may not be safe in the field within its service life [1]. In this regard, detailed analysis of power cycling is required to project field life of organic packages. In previous studies of power cycling simulation, use of integrated thermo-mechanical analysis methods were recommended for solder joint reliability of various ball grid array packages. Darveaux and Mawer presented a comprehensive study on thermal and power cycling limits of a wire bond based 225-ball PBGA assembly with Sn62-Pb6- 2Ag solder joints at a pitch of 1.5mm [2, 3]. Hong et al, presented integrated flow-thermo mechanical and reliability analysis of a flip chip plastic ball grid array (PBGA) package under power cycling [4]. Mawer et al [5] and Subbarayan et al [6] presented work to correlate accelerated thermal cycling and power cycling for FC-PBGAs. Mawer et al [5] reported that though the failures occurred in the solder joints near the die edge, the FC-PBGA solder characteristic life was longer in PC. Rodgers et al showed reverse trend in characteristic solder life behavior [7]. The present study uses flow-thermo-mechanical analysis for power cycling simulation wherein integration of the interrelated quantities such as transient heat transfer coefficients, temperature, and deformation in and around the assembly in a convective environment are required [1]. The analysis incorporates maximum accumulated plastic work per cycle, local transient heat transfer coefficients, time- independent plastic strain and three-dimensional FEA. Moreover, in-situ moiré interferometry experiment has been conducted to document the deformation behavior of the FC- BGA packages under ATC and PC. 2. Experimental Study Moiré interferometry is a full field optical method to measure in-plane deformation. It has a high in-plane displacement measurement sensitivity of 0.417 µm per fringe count [8]. In this method, interaction of the high frequency cross-lined specimen grating with the virtual reference grating of the interferometer creates the fringe patterns. The fringes obtained are contours of U and V displacement that the specimen undergoes during temperature excursions.