Fluxless Soldering Technology for MEMS and Photonics

Chin C. Lee

Electrical and Computer Engineering

Materials Science and Engineering

University of California, Irvine, CA 92697-2625

cclee@uci.edu tel: 949-824-7462 fax: 949-824-3732

Abstract

In electronic packaging, conventional soldering methods use reflow solder, solder preforms, or paste as the bonding medium. During the assembly process, the oxidation of solder produces a solid oxide film adhering on the molten solder, thus preventing the liquid solder from achieving chemical bonds with parts to be joined. To remove the oxides, flux is commonly used in the soldering process. In some applications, flux simply cannot be used.

In this presentation, we begin with the fundamental reasons of why materials can join together. Soldering principle is reviewed. Flux action to remove oxides during the soldering process is explained. A fluxless soldering technology is then reported. It is based on the concept of removing the origin of the problem, i.e., the formation of oxides. Oxides and oxidation are prevented right in the beginning by depositing the solder materials in one high vacuum cycle and by further protecting the materials against oxidation with a stable intermetallic layer that is formed in situ in high vacuum. A process based on Au-Sn system will be presented to illustrate the fluxless principle. High quality joints are achieved without the use of flux. The quality is examined using a scanning acoustic microscope (SAM). Compositions and microstructure are studied using an SEM and EDX. The research also enhances scientific knowledge on intermetallic compound growth, microstructure, and interdiffusion in alloys that are important in microelectronics.

Potential advantages of the technology include: (1) fluxless, (2) inherently oxidation-free, (3) control of joint thickness and composition, (4) production of high temperature joints at low process temperature, (5) possible control on the strength and plastic strain of the joint by material composition, (6) lithographic patterning of the composite for selective bonding.