Journal of SMT Article

New Generation Of Pb-Free Solder Alloys: Possible Solution To Solve Current Issues With Main Stream Pb-Free Soldering

Authors: Polina Snugovsky, Simin Bagheri, Marianne Romansky, Doug Perovic, Leonid Snugovsky, John Rutter
Company: Celestica Inc. and University of Toronto
Date Published: 7/30/2012   Volume: 25-3

Abstract: Conversion to RoHS-compliant Pb-free assembly has proved to be a considerable challenge to the electronics assembly industry due to the higher reflow process temperature requirements of main stream Sn-Ag-Cu (SAC) alloys. This higher temperature can thermally damage the PCB and components. High Tg board materials used with the main stream Pb-free solder SAC305 are prone to Pad Cratering. The higher cost of soldering materials has also become a concern as the result of the continuously rising cost of silver (Ag), one of the main constituents of conventional SAC305 alloy.

One possible solution to all the current SAC process shortcomings is to use alternative Pb-free solders with lower process temperatures. The lower process temperatures would allow the use of standard board laminate materials which have a lower propensity to "pad cratering" failures. It will also reduce the risk of damaging temperature-sensitive components (TSC).

This paper is focused on solder alloys selection for an assembly process temperature in a range of 220 to 226°C, which is comparable to SnPb soldering process parameters. The methodology of alloy selection is discussed. Several ternary and quaternary Sn-based alloys containing Ag and/or copper (Cu), and bismuth (Bi) are proposed for further reliability studies of primary attached and reworked leaded, leadless and ball grid array components. The Bi addition reduces the melting temperature and improves thermo mechanical properties. An additional benefit of using Bi-containing solder alloys is the possibility to reduce the propensity of whisker growth. The alloys containing Bi were not used in recent studies due to the formation of the low melting Sn-Pb-Bi eutectic with SnPb components. As the supply chain has transitioned component finishes away from those containing lead in the last decade, the time has come where the use of Bi-containing alloys is no longer a reliability risk, and may indeed provide significant benefits to product field reliability


Pb-free, Bi-containing solder, Melting Temperature

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