The beneficial effect of Zn additions on the microstructure of SnCu and SnCuNi solder joints to Cu substrates

The commercial success as Pb-free solders of alloys based on a trace addition of Ni to Sn-0.7Cu and the academic recognition of the metallurgical effects that deliver the excellent performance in soldering processes and in service, means that Sn-Cu-Ni is now well established as one of the main alloy systems on which the electronics industry depends for the assembly of reliable circuitry. One important effect of the Ni addition is the stabilization of the hexagonal form of the Cu₆Sn₅ phase that forms at the interface between the solder and the Cu substrate. However, with the limited amount of Ni available in a solder joint, growth of the intermetallic layer by reaction with a copper substrate can result in the Ni level in the Cu₆Sn₅ falling below that required for stabilization. Increasing the Ni in the solder alloy beyond the level required for optimization of properties such as fluidity that are important in soldering processes can result in the appearance of undesirable high melting point Sn-Ni phases, It would therefore be useful if a second stabilizing element could be introduced to the system to support the effect of the Ni. Ni works as a stabilizer by partially substituting for Cu in the Cu₆Sn₅ so potential additives were considered on the basis of reports of their partially replacing either Cu or Ni in that crystal structure. Zn was selected for this study because it can partially substitute for Sn in the Cu₆Sn₅ crystal, potentially complementing the role of Ni. The effect of a Zn addition was studied in the basic Sn-0.7Cu and when there was also an addition of Ni. By itself a Zn addition did not promote a eutectic microstructure in the way that a Ni addition does so powerfully but alone or with Ni it substantially reduced undercooling and in the alloy with both Ni and Zn there was an overall refinement in the near completely eutectic microstructure. In combination with t- e Ni the Zn significantly refined the grain size of the Cu₆Sn₅ that formed at the solder/Cu interface. By itself the Zn did not suppress the formation of Cu₃Sn at the interface between the Cu and the Cu₆Sn₅ in the way that Ni does but it does not interfere with that beneficial effect of the Ni. Diffraction studies with high energy synchrotron-generated X-rays confirm that as well as stabilizing the hexagonal form of the Cu₆Sn₅, Zn reduces the anisotropy of this phase and increases the randomness of the crystal orientation in the intermetallic layer. These effects should result in a stronger intermetallic layer that is less likely to degrade during thermal cycling. The combination of all of these effects mean that an addition of Zn to the widely used Sn-Cu-Ni lead free solders should result in an overall increase in reliability of the solder joint, particularly in harsh environments.