AN INVESTIGATION OF PWB LAYOUT BY GENETIC ALGORITHMS TO MAXIMIZE FATIGUE LIFE Andrew J. Scholand Robert E. Fulton Advanced Electronic Packaging Laboratory Manufacturing Research Center Georgia Institute of Technology Atlanta, Georgia Bert Bras System Realization Laboratory Georgia Institute of Technology Atlanta, Georgia ABSTRACT Thermal considerations in Printed Wiring Board (PWB) assemblies are becoming increasingly important as packaging constraints shrink and power use escalates. In this paper, we provide a study on the potential for a Genetic Algorithm-driven PWB layout design tool to improve the thermal performance of such assemblies. As a case study, the thermo-mechanical fatigue of surface mounted leadless chip carriers on an FR4 epoxy board is used. We have found that by utilizing appropriate formula- based engineering approximations, the efficiency of Genetic Algorithms in finding near-optimal and optimal results makes this approach effective as an explorative 'scouting' approach to identify promising board configurations for more computationally expensive evaluations such as finite element method. 1. INTRODUCTION One of the tasks a designer of a Printed Wiring Board (PWB) faces is placement of electrical components on the board. There are many constraints on possible locations, primarily determined by the electrical circuit design of the board. However, increasing customer demands for more processing power in smaller packages mean that many PWBs are operating at temperatures significantly above ambient (75 - 90°C). This presents a problem for surface mounted components, as the solder connections between the board and the component are stressed by the thermal expansion differences between these two bodies. Repetitive use (i.e. power on, power off cycles) leads to fatigue failure, a phenomena referred to as solder joint thermo-mechanical fatigue. To maximize the fatigue life of a particular surface mounted (SM) component, then, it is desirable that the component be located in a low temperature region of the board. However, since some SM components are more susceptible to fatigue damage than others, and since the temperature gradients across the board may vary as illustrated below, there exists an ordering of components in available locations which will maximize the fatigue life of the entire PWB. (Note that any component failure anywhere on the board constitutes a PWB failure). Figure 1-1 Electrical and other constraints on the placement of the components are presumed to limit the number of available positions to a finite number. There are obviously only a finite number of components to place in some or all of these available