Impact of material properties and assembly geometry on bond-line stress when bonding PCBs to heat sinks with an isotropic conductive adhesive. Part II

Engineers bonding substrates with dissimilar thermal coefficients of expansion with conductive adhesives are faced with several challenges. While attempting to minimize the throughput cost of assembly, engineers must manage the stress induced by high temperature processing and produce assemblies with high yield. In order to manage these tasks, engineers need to consider various adhesive properties to arrive at a solution that can be produced quickly and reliably. Typical properties that are considered include adhesion strength, cure schedule, electrical and thermal properties. Because miniaturization and higher frequency are continued trends in microwave electronics, new materials are required to meet the demands of these smaller, thinner, and hotter applications. This paper will evaluate the material properties necessary to minimize stress in packages with different geometries. Factors such as modulus, Tg, CTE, and thickness of an adhesive bond-line will be evaluated, and the relative importance of each material property will be determined by evaluating various developmental adhesives. Also, the geometry of the package will be considered from an L/D and substrate thickness perspective in an attempt to learn the importance of package geometry as it relates to stress levels. Lastly, the impact of process temperature will be considered to learn the ultimate impact on package stress.

Introduction

Large area assemblies intended for various microwave applications typically employ electrically conductive (isotropic) film or paste adhesives when bonding a circuit board (FR4 or ceramic) to a metal substrate (Al or Cu core). Conductive adhesives offer improved performance over traditional mechanical fastening technology. Their electrical integrity, bond-line consistency, and improved thermal dissipation insures compliance with regulatory requirements, minimizes losses, minimizes distortion of high frequency digital signals, and maintains low impedance to the ground plane. [1] Additionally, they offer processing advantages over conventional sweat soldering technology. The primary processing advantage offered by conductive adhesives is lower bonding temperatures. This minimizes the stress in the package, and it also allows the use of different board materials.