Characterization of electroplated bismuth-tin alloys for electrically conducting materials

Journal of Electronic Materials, Oct 2000 by Kang, S K, Buchwalter, S, Tsang, C

Electrically conducting adhesive materials are promising alternatives for lead (Pb)-containing solders in microelectronic applications. However, most common silver-filled epoxy materials have various limitations to meet the requirements of the solder joints yet. To overcome these limitations, several new formulations have been developed recently. Among them, a new high conductivity Pb-free, conducting adhesive developed for low temperature applications has been previously reported. This conducting adhesive contains a conducting copper filler powder coated with a low melting point metal or alloy, such as Sn or BiSn alloys. The low melting point layer serves as a joining material among the filler particles as well as to the substrate. In this paper, characterization of electroplated BiSn alloys on a Cu substrate is reported for their microstructure, electrical properties, oxidation behavior, and others. The experimental results have provided a better understanding of the joining mechanism of the newly developed Pb-free conductive adhesive materials.

Key words: Electrically conducting adhesives, Pb-free solders, silverfilled epoxy, BiSn plating, differential scanning calorimetry, surface oxidation, microstructural changes, electrical resistivity

INTRODUCTION

Recent advances in microelectronic devices and the continued concern of technology's impact on the environment have driven improvements in current solder connection technology. Solder connection technology plays an important role in the electronic packaging industry. For example, flip-chip connection (C4), solder-ball connection in ball-grid-arrays (BGA), and integrated circuit (IC) package assembly to a printed circuit board (PCB) all contain electrical connections through solder joints.1,2 Recently, government agencies and the general public are concerned with the impact that Pb-containing solders have on the environment. The Montreal Protocol on Substances that Deplete the Ozone Layer, signed in 1987, has already forced changes in the fluxing and cleaning aspects of traditional soldering processes.3 These processes are currently being reevaluated in regards to lead contamination of the environment. Furthermore, the gasoline and paint industries have already been required to remove lead from their products. In preparation, the electronics industry is investigating two different groups of materials to replace Pb-based solders: Pb-free solder alloys4 and electrically conductive pastes (or adhesives).5,6

Electrically conductive adhesives consist of a polymer matrix containing metallic filler particles. In particular, low temperature conductive adhesives utilize low melting point, nontoxic metals that are coated onto conducting filler particles. Filler particles, a few microns in size, are selected from the following group: Au, Cu, Ag, Al, Pd, and Pt.' The coated layer must be able to achieve metallurgical bonds between adjacent particles and are selected from the following: Bi, In, Sn, Sb, Zn, or their alloys.1 The coated particles are then mixed with an environmentally-safe fluxing agent, and loaded into a polymer matrix consisting of polyimide, siloxane, polyimide-siloxane, polyester, epoxy, or others. 13 The cured polymer resin not only strengthens the structure but also acts as the adhesive between contact surfaces. During the curing process, the coated layer melts and forms an interconnected network between adjoining particles and the surface of the substrate. This network formation improves electrical and thermal conduction. It also improves the mechanical strength at the joint compared to traditional adhesive joints.8,9

Conductive adhesives have many advantages over the current Pb-Sn solder process. First, conductive adhesives are capable of achieving the very fine pitch needed to raise package density. In addition, the processing occurs at lower temperatures without extensive pre-cleaning. The reduction of pre-cleaning residues is not only important environmentally, but also results in less surface insulation resistance failures (SIR).? Another environmental advantage of conductive adhesives is their lack of lead and environmentally damaging flux. Lastly, conductive adhesives have sufficient shear strength required for use as a joining material, although it is not equivalent to that of solders.10

This project focused on a low temperature conductive adhesive that was developed with bismuthtin (Bi-Sn) coated copper powders dispersed in a polymer matrix. Specifically, the goal was to characterize the BiSn alloy, since research focused on this alloy is relatively sparse. The objective was threefold: to characterize surface oxidation ofBiSn, to investigate microstructural changes upon heat treatment, and to analyze the conductivity ofBiSn. In order to achieve these purposes, the following analytical techniques were utilized: differential scanning calorimetry (DSC), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and 4point probe resistivity measurement.