Numerical Analysis of Residual Warpage of FBGA Package during EMC Curing Process

In a semiconductor packaging process, the warpage greatly has influenced the reliability of the package as well as the workability. The strip warpage in FBGA package result from the structure of constitutes and the thermal mismatch by the thermo-mechanical properties such as CTE (Coefficient of Thermal Expansion) and Modulus of EMC, substrate, chip and adhesive materials. Therefore, the optimization of material properties and the package structure design has been needed by the numerical analysis. In this research, temperature dependent nonlinear thermal analysis was performed with the high temperature material properties and the result was compared with the actual measurement value. Optimization analysis of the strip warpage was also conducted. The main factors of the strip warpage were selected by DOE(Design of Experiment) process, and then the optimum conditions of design variables were obtained. Lastly, through the reliability analysis, the tolerances of design variables which can satisfy the limited conditions were proposed. Introduction With the packaging product becomes thinner and smaller, the package warpage becomes more important. Because FBGA packages are molded by EMC, they are susceptible to thermo-mechanical stresses that can cause the strip warpage. Mismatches in thermal expansion coefficients of the package constituents create the internal stresses that cause the entire strip to warp[1]. Asymmetries in the internal structure also contribute to strip warpage. Warpage is especially generated during cooling process from mold temperature to room temperature. The molding temperature is usually about 1751, which is over Tg (glass transition temperature) of EMC, therefore the mechanical properties of the EMC change severely in this step. Analysis using elastic model was used for its simplest reasonable modeling and its ease of calculation. This elastic modeling, however, has some limitations in not considering the temperature dependency of material properties[2-3]. Recently, viscoelastic model that can express actual dependency of temperature has been used. This modeling requires much more computing times and has difficulty in measuring the material properties[4]. The ANSYS PDS (Probabilistic Design System) analyzes a model involving uncertain input parameters such as geometry, material properties, boundary conditions, etc. The variations of these input parameters are defined as random input variables and are characterized by their distribution type and by their distribution parameters. During a probabilistic analysis, the random output parameters are computed as a function of the set of random input variables[5]. In this research, the strip warpage analysis process of FBGA package was studied considering nonlinear characteristic of EMC. Also, optimization and reliability analysis were conducted in series. Thermo-mechanical Characteristics FBGA package usually consists of the following constituents; substrate, adhesive, chip and EMC. For the strip warpage of FBGA, it is known that EMC is a most dominant constituent[6]. In this study, to Key Engineering Materials Online: 2007-03-15 ISSN: 1662-9795, Vols. 334-335, pp 385-388 doi:10.4028/www.scientific. net/KEM.334-335.385 © 2007 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-12/03/20,16:27:52) improve the limitation of elastic model for time-dependency of properties, when the mechanical properties were measured, the degree of cure in EMC molding process was considered. The degree of cure at molding temperature, 1751, was calculated from thermal analysis by DSC (Different Scanning Calorimeter). The heating rate 10 1/min was used from 251 to 2501, and the n-th order model of cure kinetics was obtained with experiment data from Eq. (1) as following: n A n RT E Z k dt d ) 1 )( exp( ) 1 ( 1 α α α − − = − = (1) whereα is the degree of cure, Z is the Arrhenious frequency factor, A E is the activation energy and R is the universal gas constant. By integrating n-th order cure kinetics equation, it is known that the degree of cure is 85 % at the cure time 80 sec. Therefore, the thermo-mechanical properties of 85 % cured EMC were used as the data of strip warpage analysis. The elastic modulus, in case of EMC, was measured by 3-point bending test which has been performed by a universal testing machine (UTM; LLOYD, LR-10K). And the Tg and CTE were measured by a thermal mechanical analysis (TMA) machine increasing the temperature at the rate of 5 /min from 20 to 300 . The substrate is the composite composed of three or four materials. The two layer structure is usually made up of lamination the SR (Solder Resist), copper and core. Table 1 shows the thermo-mechanical properties of the SR, Copper and core. Applying the all layers to analysis causes the creation of many elements and the increase of analysis time. Therefore, the effective elastic modulus and CTE can be approximated as following[8];