Journal of SMT Article


Authors: John Manock, Richard Coyle, Brian Vaccaro, Heather
Company: Alcatel-Lucent, LSI Corp., and Celestica Inc.
Date Published: 7/30/2008   Volume: 21-3

Abstract: The results from a study of Pb-free solder joint microstructure and attachment reliability of a 680 I/O plastic ball grid array (PBGA) package are presented in this paper. This work is an extension of a previously published evaluation comparing the temperature cycling performance of SnAgCu (SAC) and SnPb solder assemblies using typical IPC-9701 test conditions for accelerated temperature cycling (ATC) [1]. In the previous study, a temperature range of 0°C to 100°C with 10 minute dwell times was used. Under those test conditions, the SAC alloy assemblies outperformed the SnPb, consistent with other industry findings for similar test vehicles.

In the current study, multiple temperature cycling conditions are used to extend the evaluation of SAC solder joint reliability and aid in the development of fatigue life modeling capability necessary for the determination of test acceleration factors. To address the ongoing controversy of accumulated creep damage during temperature cycling, the 0°C to 100°C test was conducted using both 30 minute and 60 minute dwell times and the results compared to the previous study that used 10 minute dwells. An additional 30 minute dwell test cell was performed with samples isothermally preconditioned for 500 hours at 125°C to assess potential microstructural effects due to storage prior to use. To explore the effect of different temperature extremes and temperature range, another test was conducted using -40°C to 125°C test conditions. All cells were tested to 63.2% or greater failure rate with a test duration exceeding 4000 cycles. Complete analysis of the test data for each cell is presented.

Post-cycling failure analysis was conducted on representative test samples from each cell. Failure analyses include dye-and-pry analysis to determine the location of the failed joints within a test net, metallographic analyses using optical and polarized light microscopy (PLM), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) to verify the failure mode and evaluate the microstructure. The ATC test data and failure analysis results are discussed in terms of the relationship to the evolving microstructure and mechanical behavior that results from temperature cycling and preconditioning.

Key words: Lead Free, PBGA, Temperature Cycling, Reliability, dwell time, Pb free, microstructure

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