SMTA International Conference Proceedings

iNEMI Pb-Free Alloy Characterization Project Report: Part VII - Thermal Fatigue Results for Low-Ag Alloys

Authors: Keith Sweatman, Richard Coyle, Richard Parker, Keith Howell, Elizabeth Benedetto, Joseph Smetana, Aileen Allen, Weiping Lui, Julie Silk
Company: Nihon Superior Co., Alcatel-Lucent, iNEMI, Hewlett-Packard Co., Alcatel-Lucent, Hewlett-Packard Co., Indium Corp., Keysight Inc.
Date Published: 9/28/2014   Conference: SMTA International

Abstract: The electronics industry continues to experience an expansion of Pb-free solder alloy choices beyond the common near-eutectic Sn-Ag-Cu (SAC) alloys, most of which have reduced Ag content. The increased availability of additional Pb-free alloys provides end users opportunities to address acknowledged deficiencies of the higher Ag content of near-eutectic alloys, such as relatively poor mechanical performance under drop, shock, and bend loading, among others. Conversely, the availability of new alloys often precedes a comprehensive performance assessment of those alloys, which introduces a different set of assembly quality and reliability risks.

The full impact of most new alloys on overall printed circuit assembly (PCA) reliability has yet to be determined, particularly with respect to thermal fatigue, a well-known electronic wear out mechanism. Thermal fatigue combined with solder creep is considered a major source of failure of surface mount (SMT) components. Specifically, there is concern that the low Ag alloys will not have fatigue resistance adequate to support critical, high reliability applications. The Pb-Free Alloy Characterization Program sponsored by International Electronics Manufacturing Initiative (iNEMI) is conducting an extensive investigation using accelerated temperature cycling (ATC) to evaluate BGA thermal fatigue performance of 12 commercial or developmental Pb-free solder alloys. The test program also includes eutectic SnPb solder samples to complete the reliability comparisons. Most of the Pb-free alloys contain less Ag than the near-eutectic SAC alloys.

This paper presents the findings from a subset of the iNEMI temperature cycling test matrix derived from only the lower Ag alloys, containing 1 wt % or less Ag. Temperature cycling data are presented for two different BGA components and comparisons are made with multiple temperature cycles. Temperature cycling variables include different cyclic temperature ranges (ΔT), temperature extremes, and dwell times. The failure data are reported as characteristic life η (number of cycles to 63.2% failure) and slope β from a two-parameter Weibull analysis. The ATC data and failure analysis are discussed in terms of the alloy composition and the initial as well as evolving microstructures that result from temperature cycling. The relative performance of the various alloys under multiple ATC conditions, and how this may impact acceleration behaviour, is discussed.

Key Words: 

Pb-free solder, low-Ag, accelerated thermal cycling, characteristic life

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