1 Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-9. Key words: solderability, immersion silver, storage environments, Pb-free.">

Journal of SMT Article

EFFECTS OF STORAGE ENVIRONMENTS ON THE SOLDERABILITY OF IMMERSION SILVER BOARD FINISHES WITH Pb-BASED AND Pb-FREE SOLDERS

Author: Edwin Lopez et al.
Company: Sandia National Laboratories
Date Published: 10/1/2005   Volume: 18-4

Abstract: The solderability of an immersion Ag finish was evaluated using the Pb-free solder, 95.5Sn-3.9Ag-0.6Cu (wt.%), and traditional 63Sn-37Pb alloy. The solder temperatures were 245°C and 260°C. The flux was a rosin-based mildly activated (RMA) material. The immersion Ag finish was tested in the as-received condition as well as after the following treatments: (1) simulated storage for 3, 6, 9, 13, 18, 24, 60, and 120 months using a Battelle Class 2 (industrial) environment and (2) steam aging for 8, 16, and 24 hours (88°C, 90% RH).

The solderability metric was the contact angle as determined by the meniscometer/wetting balance technique. The immersion Ag coating improved the solderability of Sn-Ag-Cu versus bare Cu and retained adequate solderability for simulated storage not exceeding 12 months.

Adequate solderability was maintained with Sn-Pb solder for storage times that did not exceed 24 months. The solderability of the immersion Ag coating with the Sn-Ag-Cu alloy was generally insensitive to the steam aging treatments. The Sn-Pb alloy was slightly more sensitive to steam aging, as the 16 hour condition resulted in some solderability degradation. These results confirmed the inapplicability of steam aging to predict the storage life of an immersion Ag finish.

Auger surface analysis established a correlation between the surface concentration of Cu that diffused from the underlying substrate, and the loss of solderability for the immersion Ag finish. The Cu surface concentration appeared to reflect a progression of metastable equilibrium conditions having individual, preferred levels for different aging times.

1 Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-9.

Key words: solderability, immersion silver, storage environments, Pb-free.



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