Pan Pacific Symposium Conference Proceedings


SMT ASSEMBLY PROCESS COMPARISON OF PB-FREE ALLOY SYSTEMS - PART I

Authors: Shafi Saiyed and Daryl Santos, Ph.D.
Company: State University of New York
Date Published: 2/18/2003   Conference: Pan Pacific Symposium


Abstract: The impact of the transition from Sn/Pb eutectic solder to Pb-free alloys is being felt by the entire electronics manufacturing supply chain, from component suppliers to equipment manufacturers. While industry has identified possible alternatives to the Sn/Pb solder, there is no drop-in replacement, and much work remains to be done to get our understanding of Pb-free on par with Sn/Pb eutectic. This is especially true in the area of component thermal tolerances, assembly process development and board level reliability.

A key factor in selecting the replacement for Sn/Pb solder is ensuring the compatibility of the new alloy with the current breed of manufacturing equipment and currently available Pb-free board and component lead finishes without compromising board level reliability. While initial research and development efforts have centered on feasibility studies using test vehicles specifically designed for evaluation purposes only, data is still lacking for their processing ability.

This paper seeks to fill the gap by providing an assembly process comparison of Pb-free alloys with Sn/Pb alloy using a test vehicle designed to resemble real world manufacturing applications. This test vehicle has been described earlier [12]. Four different Pb-free alloy systems including two compositions in the Sn/Ag/Cu alloy system are investigated, and their board level assembly processing characteristics are compared to Sn/Pb eutectic alloy for a variety of surface mount components and packages and through-hole components.

The components were assembled on printed wiring boards having five different Pb-free surface finishes. The major processing issues involving Pb-free implementation hinge on two process steps, namely, solder paste stencil printing and reflow soldering. The sequential approach has been adopted in this study that involves the design of the test vehicle and component selection, comparison of printing characteristics for Pb-free pastes, and comparison of processing defects.

This paper discusses the design of the test vehicle, stencil design, and comparison of printing characteristics for Pb-free pastes. The results of the comparison of processing defects, based on visual and x-ray inspection of the assembled boards, will be presented later (tentatively at APEX 2003) as Part II.

Key words: Pb-free, surface finish, printing process characteristics, GR&R.



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