Study of a Laser Soldering Process for Fine-Pitch Leads (3rd Report)

Development of Fluxless Laser Soldering

Abstract

This paper describes the development of a new laser soldering technique for surface mount assemblies. The new technique, which uses a YAG laser beam, has a superior performance. It can be applied to tape carrier package (TCP) assemblies for fine pitch leads and offers a no-bridge soldering process. This paper describes fluxless laser soldering developed for the no-clean assembly process. In this technique a focused YAG laser is used to remove oxides and other contamination from the soldering surfaces. First, the removing process induced by lasersputtering, is experimentally evaluated. The fluxless laser soldering concept is proved by Auger electron spectroscopy (AES) analysis of the oxide film on laser-sputtered solder. Second, the reducing atmosphere (H₂-N₂ gas) is shown to acheive high solderability. The experimental results are comparable with laser soldering in air using a rosin mildly activated (RMA) flux. From these experiments, this fluxless laser soldering technique offers the ability to assemble high lead count and fine pitch ICs for use in TCPs.