Microelectronics Reliability, Vol. 40, No. 7, pp. 1173-1180, July 2000

Guidelines to Select Underfills for Flip Chip on Board Assemblies and Compliant Interposers for Chip Scale Package Assemblies

J.H. Okura, S. Shetty, B. Ramakrishnan, and A. Dasgupta
CALCE Electronic Products and Systems Consortium
University of Maryland, College Park, MD 20742

J. F. J. M. Caers
Philips Centre for Manufacturing Technology,
P. O. Box 218, 5600 MD Eindhoven, The Netherlands

T. Reinikainen
Nokia Research Center, Finland
P. O. Box 407, 00045 Nokia Group, Finland


The effect of thermo-mechanical properties of underfill and compliant interposer materials, such as coefficient of thermal expansion (CTE) and stiffness (Young's modulus), on reliability of Flip Chip on Board (FCOB) and Chip Scale Packages (CSP) under thermal cycling stresses is investigated in this study. Quasi three-dimensional viscoplastic stress analysis using finite element modeling (FEM) is combined with an energy partitioning (EP) model for creep-fatigue damage accumulation, to predict the fatigue durability for a given thermal cycle. Parametric FEM simulations are performed for five different CTEs and five different stiffnesses of the underfill and compliant interposer materials. The creep work dissipation due to thermal cycling is estimated with quasi 3-D model, while 3-D model is used to estimate the hydrostatic stresses. To minimize the computational effort, the 3-D analysis is conducted only for the extreme values of the two parameters (CTE and stiffness) and the results are interpolated for intermediate values. The results show that the stiffness of the underfill material as well as the CTE play important role in influencing the fatigue life of FCOB assemblies. The fatigue durability increases as underfill stiffness and CTE increase. In the case of compliant interposers, the reverse is true and durability increases as interposer stiffness decreases. Furthermore, the interposer CTE affects fatigue durability, with durability increasing as CTE decreases. The eventual goal is to define the optimum design parameters of the FCOB underfill and CSP interposer, in order to maximize the fatigue endurance of the solder joints under cyclic thermal loading environments.

Complete article is available to CALCE Consortium Members.

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