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The Use of Cobalt in Plated Copper to Inhibit the Formation of Brittle Copper-Tin Intermetallic Compounds - at the Copper-Solder Interface on Printed Circuit Boards
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Abstract
The formation of copper-tin intermetallic compounds of the Cu6Sn5 and the Cu3Sn type at the interface between copper and solder on printed circuit (PC) boards has been previously documented, as have the circumstances under which these brittle layers can cause peeling of the solder and greatly reduce the solderability of the PC boards shown.
The formation of these intermetallic layers is caused by diffusion, mainly of the copper into the solder, and is a function of time and temperature.
Inhibiting the diffusion will inhibit the formation of the intermetallics. Several methods have been tried, such as the use of a metallic barrier layer. This paper will show that one of the more successful methods is the codepositing of small amounts of cobalt (0.05-0.5%) with the copper.
This process is similar to the use of cobalt in wrought copper alloys to reduce solid-state mobility during thermal processes.
These small amounts of cobalt do not reduce the solderability, increase the difficulty of achieving sound plated through holes, or react differently to standard etching solutions than pure copper.
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Citation
Breen, J., Toledo, E., and White, V., "The Use of Cobalt in Plated Copper to Inhibit the Formation of Brittle Copper-Tin Intermetallic Compounds - at the Copper-Solder Interface on Printed Circuit Boards," SAE Technical Paper 710793, 1971, https://doi.org/10.4271/710793.Also In
References
- Fulton T. Toledo E. Breen J. E. “Printed Circuit Board Solder Adhesion Failure Investigation Final Report.” Raytheon Co. January 28 1970
- Toledo E. Bonis S. Breen J. E. O'Brien F. “The Case of Slippery Solder Joints.” Circuits Manufacturing January 28 1970 64
- Toledo E. Bonis S. “Sequel to Case of Slippery Joints in P.C. Boards.” Circuits Manufacturing July 1970 48
- Henmi Z. Nagai T. “Mechanism of Precipitation Hardening in Beryllium Copper Alloys.” Trans. Japan Institute of Metals 10 3 May 1969
- Mass. Material Research Report