VLSI package designs continue to utilize relatively large dies in smaller packages. Elevated thermal stresses are likely to occur in such configurations during fabrication, testing, and operation of these packages. This paper presents a parametric study of thermal stresses in plastic packages that are induced during the die attach and encapsulation fabrication steps. Finite element models (FEMs) of plastic VLSI packages were used to vary package design parameters (die thickness, die bond materials and thickness, lead frame thickness, and package thickness and width/length) over typical ranges. Areas of complexity were considered in more detail using locat finite element analysis. The loss of adhesion between the mold and the die and/or lead frame in the package was simulated using gap finite elements in order to consider the extreme cases of no slip or no adhesion at these material interfaces. Changes in die thickness, die bond thickness, and package thickness caused significant changes in package thermal stresses; whereas changes in lead frame thickness, package width/length and die bond elastic modulus corresponded to minor changes in package stresses. The loss of adhesion at the mold material interface resulted in significantly altered thermal stress fields, e.g., the stresses were concentrated near the corners of the chip and lead frame where cracking has been observed in DIP plastic molding materials.

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A designer often has to deal with complex and ill-structured situations during specification synthesis and preliminary engineering design. To assist in the development of computer-aided design systems, it is desirable to capture the designers decision-making process during these design states. The research presented in this paper is towards this direction. Based on the conceptual understanding of the process, three postulates are presented. The following two postulates; (1) the decisions are neither optimum nor just satisfying but retain certain characteristics of both, (2) the design is driven by the important objective(s) among all the specified objectives, at the preliminary design, although the remaining objectives do have a weak influence on the preliminary design; are used to develop a compensatory and a non-compensatory model of the decision-making. These models are formulated with the help of fuzzy set theory and they implicitly or explicitly follow the two postulates. These models are suitable for discrete decision situations where the above mentioned postulates apply. Examples of material selection during a preliminary structural design are used to illustrate the effectiveness of these models.

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