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

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.