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 the 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.
ZINC MICRO-ALLOYING ADDITIONS IN Sn-0.7Cu LEAD-FREE SOLDER ALLOYS. SHORT REVIEW
Sn-Pb solder alloy has been replaced with lead-free solder alloys due to the negative effect for environment and human health. The additions of Zinc (Zn) micro-alloying element in Sn0.7Cu lead-free solder alloy were reported by several researchers in improving the performance of Sn-0.7Cu solder alloy. This paper reviews the research findings available on the additions of zinc micro-alloying in Sn-0.7Cu solder alloy. It can be concluded that zinc micro-alloying additions were reported to improve the mechanical properties, phase analysis and microstructure of the solder free solder joint.
Measuring Sheet Resistances of Dielectrics Using Co‐Planar Hydrogel Electrochemical Cells with Practical Applications to Characterize the Protective Quality of Paints on Sculptures
Electrical properties of thin dielectric films at the solid-liquid phase boundary are an important performance characteristic of many devices, coatings and sensors. In this paper, co-polymeric hydrogels of polyacrylic acid co-sulfonic acid, swollen with a salt solution to act as the solid electrolyte, were used to assess interfaces using electrochemical impedance spectroscopy (EIS) in a co-planar geometry. Silane-modified glasses were characterized by the co-planar hydrogel EIS cell and found to be distinguishable based on their surface monolayer chemistry. EIS measurements were also made on primed and painted metal substrates, using both test panels and an outdoor sculpture, Tony Smith's Stinger. The panels were then exposed to accelerated and outdoor weathering and showed degradation on the surface paint layers, which was observable electrochemically using EIS and confirmed visually by SEM. Electrochemical spectral features were compared between data from a standard paint-test cell versus this co-planar hydrogel cell; both cell types were able to measure coating capacitance, providing useful information about the condition of the bulk coating. Yet, sheet resistance (Rs) was a spectral feature seen only by the co-planar hydrogel cell. Thus, it can be concluded that the use of such co-planar hydrogel cells can provide an earlier warning sign to coating degradation and such cells provide a new type of spectral information that is not assessable by the standard geometry.