SMTA International Conference Proceedings

The Effect of Bismuth, Antimony, or Indium on the Thermal Fatigue of High Reliability Pb-Free Solder Alloys

Authors: Richard Coyle, Dave Hillman, Richard Parker, Charmaine Johnson, Michael Osterman, Joe Smetana, Babak Arfaei, Andre Delhaise, Keith Howell, Stuart Longgood, Andre Kleyner, Jasbir Bath, Hongwen Zhang, and Jie Geng
Company: Nokia Bell Laboratories, Rockwell Collins, iNEMI, CALCE, Nokia, SUNY-Binghamton, Celestica, Nihon Superior Co., Ltd., Delphi, Bath & Associates Consultancy and Indium Corp.
Date Published: 10/14/2018   Conference: SMTA International

Abstract: Although SnAgCu (SAC) Pb-free solder alloys typically have better fatigue life than traditional eutectic SnPb solder, their fatigue reliability is limited at higher operating temperatures. In response to the need for higher temperature performance, numerous new commercial Pb-free solder alloys are being developed and introduced. These alloys are based on the SAC system but have significant solute additions to promote solid solution strengthening at higher operating temperatures. This paper presents some of the initial thermal cycling results from a major industrial consortia project established to evaluate the thermal fatigue reliability of multiple SAC-based solder alloys containing various combinations of solid solution strengthening agents.

Daisy chained ball grid array (BGA) test vehicles were fabricated with SAC305 as a performance baseline and three different developmental alloys each employing significant additions of different solution and dispersion strengthening elements, either Bi, Sb, or In. The BGA components were soldered to daisy chained test boards using matching alloy solder paste, and subsequently thermally cycled in accordance with the IPC-9701 attachment reliability guideline. Data are reported for three distinct thermal cycling profiles, 0/100°C, -40/125°C, and -55/125°C, as characteristic life ? (the number of cycles to achieve 63.2% failure) and slope ? from a two-parameter Weibull analysis. A baseline characterization was performed on representative board level assemblies from each of the experimental legs to document the basic microstructures before temperature cycling for comparison to samples removed from the temperature cycling chambers for failure analysis. Microstructural characterization and failure analysis was done using optical metallography (destructive cross-sectional analysis) and scanning electron microscopy (SEM).

Key Words: 

Pb-free alloys, alternative alloys, high reliability solder alloys, thermal fatigue reliability, and solid solution strengthening.

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