L. J. Ernst Faculty of Design, Engineering and Production, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands G. Q. Zhang Center for Industrial Technology/Philips, P.O. Box 218, 5600 MD Eindhoven, The Netherlands K. M. B. Jansen e-mail: k.m.b.jansen@wbmt.tudelft.nl Faculty of Design, Engineering and Production, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands H. J. L. Bressers Philips Semiconductors, ATO-Innovation, Gerstweg 2, 6534 AE Nijmegen, The Netherlands Time- and Temperature- Dependent Thermo-Mechanical Modeling of a Packaging Molding Compound and its Effect on Packaging Process Stresses For reliable virtual thermo-mechanical prototyping of electronic packages appropriate descriptions of the mechanical behavior of the constituent materials are essential. In many packages molding compounds are used for encapsulation and underfill to provide environmental protection and/or to improve the package thermal mechanical reliability. Therefore, among others, the availability of appropriate constitutive models for various epoxy-molding compounds is one of the requirements for computational prototyping. As there is a large variability of available molding compounds, it is essential to be able to experimentally establish the model parameters in an efficient manner. Because of the implied simplicity, linear visco-elastic models combined with the time-temperature super- position theory are mostly used in thermo-mechanical simulations. Among the various experimental possibilities to efficiently establish the model parameter functions, in the present paper the use of unidirectional creep testing is worked out for a chosen molding compound. Here isothermal one-day creep experiments at different temperatures (ranging below and above the glass transition temperature of the compound) are performed. The tensile creep compliance and the time-dependent Poisson’s ratio of the material at differ- ent temperatures are successfully used to construct visco-elastic master curves. As the Poisson’s ratio shows a significant change during a creep or relaxation test, its effect in partly constraint situations (as in packages) will be evident. Therefore it is not reliable to approximate this variable using a constant value. Further, the visco-elastic model of the material is implemented in a finite element program and verified by means of a shear stress relaxation experiment and a creep experiment both under nonisothermal conditions. Moreover, the effect of the creep behavior of the molding compound on the packaging process stress field and its evolution is investigated. Substantial cost saving was realized by package design optimization based on the reliable prediction of the packaging process stresses. DOI: 10.1115/1.1604156 1 Introduction High residual stresses are induced in the components of an electronic package during thermal processing due to the mismatch in thermal expansion/contraction of the constituents. It has been observed that these stresses may cause various types of failures of the package, i.e., die cracking, solder degradation, passivation cracking. The prediction of the stress fields and the probable failed regions can be true only if the complex time- and temperature- dependent mechanical behaviors of the package constituents are taken into account in the package modeling. Thermosetting resins and molding compounds, like other poly- meric materials, have significantly time-dependent mechanical properties even if they are filled to a high percentage with solid particles of a filler material. The creep and the relaxation of the packaging material during the heating and cooling processes will cause redistribution of the stress and the strain fields. However, in most of the previous works on thermo-mechanical simulation of electronic packages, the visco-elasticity of molding compounds has been totally or partially neglected for the sake of simplicity 1,2. As a consequence, the predicted stress and strain evolution in the package may not be reliable. In Refs. 3–5 it was shown that by using a linear visco-elastic model for molding compound, the predicted stresses and deforma- tions can be reasonably approximated. Xiong and Tay 6 used a linear visco-elastic model as obtained in Ref. 4 to investigate the effect of visco-elasticity on interfacial delamination in integrated circuit IC packages. They showed that if the visco-elastic behav- ior of epoxy molding compound is not taken into account the energy release rate can be severely overestimated. In publications considering linear visco-elastic behavior of molding compounds generally just the shear modulus was mod- eled as time dependent, thus neglecting the time dependency of the Poisson’s ratio. This might seriously affect the results of simu- lations as in packages the molding compound always is partly constraint. This simplification has been made because of the dif- ficulties concerning the measurement and modeling of the lateral contraction/dilatation. The shear modulus and the Poisson’s ratio represent two inde- pendent mechanical properties of the assumed isotropic material, which in principle might show different shift factors when apply- ing time-temperature superposition. Also a shift factor in the form of the William-Landel-Ferry WLF equation as previously as- sumed requires a proper verification. The present work focuses on time- and temperature-dependent thermo-mechanical characterization and modeling of a molding compound, including the Poisson’s ratio effects. First, some basic formulas of the linear theory of visco-elasticity are repeated in Sec. 2. Some of the appropriate measurement techniques are Contributed by the Electronic and Photonic Packaging Division for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received Nov. 2002. Copyright © 2003 by ASME Journal of Electronic Packaging DECEMBER 2003, Vol. 125 Õ 539 Downloaded 12 Dec 2012 to 131.180.49.207. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm