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Processing and Reliability of Low Silver Alloys
Processing and Reliability of Low Silver Alloys
Paper reviews processing and reliability concerns for low silver solder alloys.
Materials Tech

Authored By:
Mathias Nowottnick and Andrej Novikov
University of Rostock, Rostock, Germany

Joerg Trodler
W.C. Heraeus, Hanau, Germany
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Summary
The change from lead to lead-free solders is underway since several years. After consumer and IT industry, the European automotive industry has also to make efforts to adapt lead-free solders till 2011. First major product runs for lead-free automotive electronics will be expected already for the next year. Besides some other alternative alloys, Sn-Ag-Cu solders became worldwide accepted and are already widely used. But the drawback of lead-free technology is the growing expense of materials and processes. Therefore the attempt has been made to reduce the costs on the material side within the realms of possibility.

The decrease of silver content of solders can have a share in cost reduction. At present especially in Asia some projects are being executed with low-silver alloys for mobile communication and furthermore for automotive electronics. The wetting behaviors of low-silver alloy SnAg1.0Cu0.5 are comparable to a considerable degree with eutectic standard SnAg3.0Cu0.5 and can be used mostly as a "drop in" solution. Certainly a matt surface should be considered for optical inspection. In contrast to SnAg0.3Cu0.7 a wider melting range already influences the wetting behaviors considerably. Therefore the soldering parameters must be especially adapted for this very low-silver alloy. But with matching soldering temperature a good solderability is also possible. Besides the processing conditions, the reliability of test boards for solders with a different silver content after thermo-cycling will also be compared.
Conclusions
The soldering and wetting properties of the silver reduced alloy SnAg1.0Cu0.5 correspond to a considerable degree to the standard solder SnAg3.0Cu0.5. But the more matt surface of the solder joints has to be considered for the optical inspection. However the increased melting range of the SnAg0.3Cu0.7 alloy already influences the wetting behaviors.

Therefore the soldering temperatures should be adapted especially for the use of this very low-silver solder. A satisfactory processing is also expected for a sufficient high soldering temperature. Surprising was the improved reliability especially for this very low-silver alloy, which shows the best results of all four alloys. The influence of copper and the further development for a higher number of temperature cycles should be the subject for a future investigation.
Initially Published in the SMTA Proceedings
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