Optimal design of an inter-board strain-relief for high-reliability surface mount attachment

Abstract

The long-term reliability of surface mount (SM) interconnections remains an important issue in many electronics packaging technologies. Strain-reliefs with sufficient compliance can minimize the cyclic loads impressed on SM solder connections during operational temperature excursions, appreciably increasing the margin for long-term attachment reliability. Compliance evaluation was performed for an inter-board strain-relief, or bondstrap, in a commercial Low-Noise Amplifier (LNA) used in various AT&T wireless transmission systems. The bond-strap compliance was optimized, within practical forming and assembly constraints, to provide robust SM interconnection during the expected product service life.The bond-strap electrically connects a pair of FR-4 epoxyglass circuit-boards in the LNA. Each LNA module contains two hand-soldered SM bond-straps that bridge the relatively narrow gap between the circuit-boards, which are mounted to a common heat sink. The bond-strap has an approximate bell-shaped configuration, with horizontal feet on both ends for sufficient solder wetted area. One-half of the essentially symmetric bond-strap was structurally modeled as an inclined gull-wing lead commonly used on SM components. Previously published work by the principal author developed a practical analytic formulation for lead compliance based on a family of structural models for commercial lead designs The LNA bond-strap is intended to be sufficiently compliant to accommodate the relative planar movement of the circuit-boards during temperature cycling.The compliance optimization process captures the interaction between the bond-strap design dimensions and the effective stiffness, within practical constraints on the overall height and footprint. Design options for the equal bend radii and intermediate member length are described in terms of the inclination angle and the ratio of the half-footprint to center-line height. Corresponding bond-strap stiffness metrics are provided for the specific copperalloy material and uniform cross-section.For relatively low-profile bond-strap designs, where the halffootprint-to-height ratio exceeds unity, the effective stiffness is minimized with an S-bend contour without an intermediate straight * This paper represents in part work performed by Kevin L. Terre11 in connection with a co-op work program jointly sponsored by New Jersey Institute of Technology and the Product Realization Department in the Network Wireless Systems Business Unit at AT&T Bell Laboratories in Whippany, New Jersey. The principal author served as the project advisor.member. The comparatively small height, however, typically provides a bond-strap with relatively high stiffness and degraded SM attachment reliability margin. The LNA low-profile bond-strap option with 8-mil (0.2-mm) center-line height has an optimized effective stiffness of nominally 440 lbiin (75 Nimm). In relatively high-profile bond-strap designs, where the half-footprint-to-height ratio is less than unity, the effective stiffness is minimized with two equal b...