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SMTA International Conference Proceedings

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

Authors: Keith Sweatman, et al.
Date Published: 10/14/2012   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 created by the high Ag content of near-eutectic alloys, such as 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 variety 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 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 the International Electronics Manufacturing Initiative (iNEMI) is conducting an extensive investigation using accelerated thermal cycling (ATC) to evaluate BGA thermal fatigue performance of 12 commercial or developmental Pb-free solder alloys. The test program also includes eutectic Sn-Pb solder samples to complete the reliability comparisons. Most of the Pb-free alloys contain less Ag than the near-eutectic SAC alloys (3-4%Ag).

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

Note: This paper was presented as part of the Lead Free Soldering Technology Symposium during SMTA International.
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Key Words: 

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

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