LGA Connectors: An Automated Design Technique for a Shrinking Design Space

Abstract

The ever-increasing demand for higher-density interconnection between a multi-chip module and the printed circuit board has resulted in the emergence of Land-Grid Array (LGA) connectors as an alternative to the traditional pin and socket area-array connectors. The design of high-density land-grid array connectors involves trade-off between conflicting performance requirements on the normal force, wipe, bulk resistance, contact resistance, stress, contact z-dimensional thickness, and z-compression. These stringent design requirements have significantly shrunk the space of viable designs and have necessitated automated search procedures for finding designs that satisfy the design requirements. In this paper, such an automated design procedure based on nonlinear optimization techniques is presented. The design procedure includes a general shape representation scheme based on B-spline curves and a set of programs for carrying out automated nonlinear elastic-plastic-contact finite element analysis (for a given shape) using a commercial finite element code. This automated analysis procedure is coupled with a nonlinear optimization code to carryout optimal design of LGA connectors. The design of LGA connectors is mathematically formulated as an optimization problem and nine different design cases (with representative dimensions and material) are solved to determine the influence of initial design and optimization problem formulation. It is shown that better solutions (with less stress) result if both the width and the thickness of the contacts are allowed to vary. In general, the choice of initial design strongly influences the optimal solution. A triply-curved symmetric contact shape is shown to produce the least stress of three possible common LGA shape designs. [S1043-7398(00)00403-5]