EXPERIMENTAL VERIFICATION OF NON-LINEAR FINITE ELEMENT ANALYSIS FOR COMBINED HYGROSCOPIC AND THERMO-MECHANICAL STRESSES Samson Yoon, Bongtae Han and Seungmin Cho, Chang-Soo Jang 1 Department of Mechanical Engineering University of Maryland College Park, MD 20740 2 Semiconductor Materials R&D Center Samsung Techwin Co., Ltd Suwon, Korea ABSTRACT The deformation induced by the combined hygroscopic and thermo-mechanical effect is investigated with a FE based numerical scheme and a moiré interferometry. The scheme employs the conventional moisture diffusion and heat transfer analysis capabilities available in the commercial FEM packages, but a new formulation is introduced to allow the time- dependent non-linear analysis of package deformations induced by hygroscopic as well as thermal expansion mismatches. Validity of modeling scheme is verified using time-dependent displacement fields of a bi-material specimen documented by real-time moiré interferometry. INTRODUCTION The moisture absorption has been known to induce popcorning [1] and delamination [2] in plastic encapsulated microelectronics. When a polymeric material absorbs moisture, water molecules effectively increase the inter-segmental hydrogen bond length and collectively cause the polymeric material to swell [3]. Hygroscopic stresses arise in an electronic package due to the swelling mismatches among mold compound, lead frame, die paddle, and silicon chip. It is reported that the moisture induced deformation can be as large as or even bigger than the deformation induced by temperature change in some plastic packages [3]. In environments, such as automotive and telecommunication applications, where packages are subjected to both temperature excursion and relative humidity change, the hygroscopic swelling mismatch induced deformations must be considered together with the thermal expansion mismatch induced deformations for accurate reliability assessment. Accelerated life testing conditions such as a HAST (Highly Accelerated Stress Test) chamber, where temperature, humidity, and pressure are used, also witness the complications produced by the combined hygroscopic and thermo-mechanical issues. Numerous studies using the finite element analysis have been conducted to determine thermo-mechanical or hygroscopic stresses individually [4-8]. No rigorous attempt to develop non-linear stress analysis capabilities for the combined hygroscopic and thermo-mechanical stresses has yet been reported in the literature. The authors proposed an effective nonlinear stress analysis scheme to analyze the combined effect of thermal and hygroscopic deformation [9]. The scheme employs the conventional moisture and heat diffusion analysis capabilities available in the commercial FEM packages (ABAQUS and ANSYS), which produce the moisture concentration and the temperature distribution within the package as a function of time [10]. In this paper, validity of the scheme is verified experimentally using deformation fields obtained by real-time moiré interferometry after reviewing briefly the modeling scheme. STRESS ANALYSIS FOR THERMO-HYGRO-MECHANICAL LOADING In real environments where packages are subjected to both temperature excursion and relative humidity change, the total deformation is the sum of the mechanical strain(also called as the stress induced strain), thermal strain and hygroscopic swelling strain. mech th sw ε ε ε ε = + + (1) where ε,ε mech ,ε th and ε sw are the total, mechanical, thermal and hygroscopic strains, respectively. The mechanical strain can be represented as the function of stress, temperature and time: (2) ( , ,) mech f Tt ε σ = where σ, T and t denotes stress, temperature and time, respectively. The thermal and hygroscopic strains are defined as the linear relationship with temperature, T, and moisture concentration, C, as