Thermal Conductivity Performance Modeling and Characterization of a Novel Anisotropic Conductive Adhesive Used in Lead-free Electronics Packaging

The continued desire to utilize an alternative to leadbased solder materials for electrical interconnections has led to significant research interest in Anisotropic Conductive Adhesives (ACAs). The use of ACAs in electrical connections creates bonds using a combination of metal particles and epoxies to replace solder. The novel ACA discussed in this paper allows for bonds to be created through aligning columns of conductive particles along the Z-axis. These columns are formed by the application of a magnetic field, during the curing process. The benefit of this novel ACA is that it does not require precise printing of the adhesive on pads and also enables the mass curing without creating shorts in the circuitry. This paper will present the findings of the thermal conductivity performance tests using the novel ACA and its applicability as a thermal interface material and for assembling bottom termination components, power devices, etc. The columns that act as electrical conduction paths also contribute towards the thermal conductivity. The thermal conductivity of the novel ACA was measured utilizing a system that is similar to that in ASTM (American Society of Testing Materials) D5470 standard. The goal was to examine the influence of Bond Line Thickness (BLT), particle loading densities, particle diameters and adhesive matrix curing conditions on the electrical and thermal performance of the novel ACA. This paper will also present a numerical model to describe the thermal behavior of the novel ACA. The novel ACA’s applicability for PCB-level assembly has also been successfully demonstrated by RIT, including base material characterization, effect of process parameters, failures, and long-term reliability. Reliability testing included the investigation of the assembly performance in temperature and humidity aging, thermal aging, air-to-air thermal cycling, and drop testing. INTRODUCTION The use of conductive adhesives as a thermal interface material has been in existence for many years in electronics packaging. Generally speaking most conductive adhesive solutions use some sort of conductive filler particles that are then cured to create the electrical and thermal bonds. The most common type of adhesive is the Isotropic Conductive Adhesive (ICA) [1]. This type of adhesive has become quite popular due to improvements in the reliability of bonds and a refined understanding of its curing process [2]. The general concern with ICAs is its conductivity in all directions, requiring it to be precisely printed, especially for fine pitch applications. Anisotropic Conductive Adhesives (ACA), on the other hand, offers the ability to conduct in one direction, which is crucial for electrical and thermal conductivity. Therefore, ACAs become more useful when dealing with fine pitches [3]. In this paper, the performance of a novel ACA will be examined as a possible thermal interface material. This ACA is unique, as it uses no pressure during assembly, to capture the monolayer of particles and create the bonds, as with traditional ACAs. Instead, the fillers for this ACA are silver coated ferromagnetic materials, which allow for the magnetic alignment of the fillers into conductive z-axis columns, during cure, and provide the anisotropy (Figure 1.). A complete cure schedule and assembly methodology is provided in a previously published paper [4]. The three ACA formulations investigated as part of this experiment include silver coated ferrite particles, which are 3μm, 8μm and a combination of 3 and 8μm diameters, in an epoxy matrix. Copyright © 2012 by ASME Proceedings of the ASME 2012 International Mechanical Engineering Congress & Exposition IMECE2012 November 9-15, 2012, Houston, Texas, USA