The influence of oxide formation on the solder acceptance of thick-film copper conductors

Results of an examination of the nature of oxide formation on the surface of copper conductors are reported and a method to remove oxides from them to improve solderability is presented. The oxide formed is Cu/sub 2/O, and it takes the form of small grains, approximately 2 to 10 micrometers in diameter, centered around the copper grain boundaries. As the amount of Cu/sub 2/O increases, the solderability decreases accordingly, with an increase in the number of firing cycles. The Cu/sub 2/O can be removed completely and solderability can be improved by soaking the copper conductors in a phosphoric acid solution and using an appropriate type of flux for soldering. Further tests were conducted which showed that any change in the formation of copper grains and Cu/sub 2/O did not affect conductor resistance and adhesion. These findings pave the way for wider use of thick-film, multilayer copper conductors.<>