Dr. Bob reviews two recently published papers to draw conclusions on the possible use of nanocrystalline silver alloy contact finishes in electronic applications.
In my previous article, in which I mentioned two recent papers* on the nanocrystalline silver alloys of silver-tungsten (AgW) and silver-palladium (AgPd) and their use as contact finishes, I highlighted the potential benefits these alloys could provide in power contact/connector applications. Power applications typically use high contact forces, several hundred cN, to optimize the magnitude and stability of the contact interface resistance, and require a limited number of mating cycles. Although conventional silver finishes are commonly used in power applications, their mechanical properties, low hardness, and high coefficient of friction (COF) are problematic with respect to mating durability (wear) and mating forces where high contact forces are used.
The nanocrystalline structure of the AgW and AgPd finishes results in significant increases in the hardness and reductions in the COF due to their small grain size, on the order of 20nm. The increased hardness moderates the mating durability issue and the reduced COF does the same for contact mating force concerns. So, in power applications, the nanocrystalline silver alloys provide an obvious improvement. **
Both papers cite the potential for these nanocrystalline finishes in a broader range of electronic applications. The use of silver, however, is more complex in electronic applications where other optimizations must also be considered. In particular, silver tends to form surface films, primarily sulfides or chlorides, depending on the application environment. Such surface films are generally disrupted under the high forces of power applications, but such disruption is more problematic at the lower forces, in the range of 25 to 100cN, of electronic applications. As a result, silver is not commonly used as a finish in electronic applications.
The dominant contact finish system in electronic connectors today is gold (0.30 – 0.75 microns) over nickel (1.25 – 2.00 microns). Any new silver-based finish system will need to offer performance similar to that standard to achieve broad acceptance. Both papers provide some indications that such performance can be realized, and, also, highlight some limitations in validating that position.
The two papers have a similar structure. The first phase compares the respective nanocrystalline finishes to conventional silver to validate their advantages. The second phase compares the nanocrystalline finish to the current standard gold/nickel.
The papers differ in two areas. The AgW paper uses a different nickel underplate, which is also nanocrystalline, and the AgPd paper focuses on a 10% Pd alloy (AgPd10) and includes a gold flash variant.
The comparison of the nanocrystalline silver alloys to conventional silver (CS) leads to similar conclusions in both papers as summarized below.
The AgW grain size is quantified as 15 – 30nm, while the AgPd10 is cited as fine-grained. Both papers stress the thermal stability of grain size after heat aging in the range from 150 – 250°C for up to 1,000 hours. This stability is a critical factor because it is the grain size that provides the improvement in hardness and COF. The boundaries between the grains of a metal are regions where the crystal lattice is distorted and the distortion impedes the deformation process. As grain size decreases, the ratio of grain boundary to grains increases and the metal increases in hardness. The grain size of conventional silvers, in contrast, increases with thermal aging. Nanocrystalline grain size stability translates to stability in hardness and COF in the field.
The AgW paper cites hardness in GPa as 0.8 – 1.4 for CS and 1.8 – 2.2 for AgW. The AgPd paper cites hardness in HV as 70 for CS and 250 for AgPd10. It also cites a value of 160 – 190HV for hard gold. The different hardness scales make a direct comparison between AgW and AgPd10 difficult. It is clear, however, that both nanocrystalline silvers are harder than CS and the comparison to hard gold provides another benchmark validating the increased hardness. The AgW paper also cites a decrease in hardness of the CS samples after thermal aging while the AgW samples remained stable, which confirms the microstructural thermal stability of that system.
The AgW paper cited a limited study of comparative mating durability of CS and AgW platings using dimple, 1.0mm radius, against flat at 50cN/250 cycles and 100cN/250 cycles with and without an organic post treat, before and after heat aging for five days at 105°C. While the results were comparable before heat aging, the CS samples showed significantly increased wear after aging, while the AgW samples were stable, again indicating the thermal stability of the AgW finish.
Coefficient of Friction
The AgW data is at 100cN load and gives values of 1.0 – 1.4 for CS and 0.6 for AgW. The AgPd10 data is at 2N and gives values of 0.8 – 2 for CS and 0.5 for AgPd10. The nanocrystalline silvers clearly have a lower COF than CS. The reduced COF results in reduced mating forces and increases the potential for the nanocrystalline silver in high-pin-count connector systems.
Both papers used MFG exposures to assess the environmental performance of the CS and nanocrystalline silver systems with the disclaimer that such environments are intended to assess the performance of gold-over-nickel systems. In particular, the acceleration factors that apply to gold-over-nickel systems are not appropriate for silver finishes. With that said, the tarnish films that formed on the CS and the nanocrystalline silvers after mixed flowing gas exposures consisted of silver sulfides and chlorides as would be expected. But it is important to note that the MFG environments tend to be more severe with respect to the tarnish films formed in the field and, therefore, represent a more severe exposure. Both papers cited contact resistance measurements after MFG, which indicated that the effect of the MFG exposure on the electrical performance was attenuated on the nanocrystalline silvers as compared to the conventional silvers.
Comparisons to the Gold Standard
Different methodologies were used in the two papers so each will be discussed separately.
AgW Comparison with Gold
Two finish systems were evaluated. The AgW system was 1.0 micron AgW over 1.0 micron nanocrystalline nickel alloy (NCNA) over copper alloy. The gold standard system was 0.75 micron nickel-hardened gold over 1.5 micron sulfamate nickel over copper alloy. These platings were applied to both the rider, 1mm hemisphere, and flat. The mechanical system was a reciprocating motion stage with a stroke length of 3mm at 1Hz. The contact forces were 25 – 100cN at 250 – 1000 cycles. Cycling of like and cross-matched finish systems was performed with and without an organic post treatment (OPT).
Exposure of the barrier layer and material transfer between the rider and flat were noted for samples run at 50cN and 1000 cycles with and without an OPT. For like mating, the gold system showed an exposed barrier layer on the rider and flat with gold transfer between them without the OPT and only exposed barrier with the OPT. For the AgW system, the rider showed exposed barrier layer and the flat some thinning. For the mixed systems, the gold rider AgW flat with and without OPT showed exposed barrier on the rider with AgW and gold transfer and AgW transfer to the rider respectively. The AgW rider to gold flat system with and without OPT showed exposed barrier layer on both sides with gold transfer and minor gold transfer onto the rider respectively. The paper cites that the gold half of the mating pair limits the wear performance.
AgPd10 Comparison with Gold
The gold comparison in this paper used a header-receptacle contact system with a nominal 80cN contact normal force. In the test, a “standard” receptacle contact with 0.75 micron gold over 1.25 micron nickel underplate was used as a probe, and the header contacts were the test samples. The plating systems studied are shown in Table 1, which is also Table 1 in the paper.
All test samples use a 1.25 micron nickel underplate and some samples also were lubricated as indicated in Table 1. Note that the “standard” header finish is gold-flashed PdNi. The test schedule was adapted from the product specification and included 100 durability cycles, five days unmated CIIa MFG, five days mated CIIa MFG, and 100 durability cycles.
Figure 1 (Figure 5a and 5b in the paper) provides a summary of the results of the testing. After the initial five-day CIIa MFG exposure, the AgPd10 shows the greatest variation, which is attenuated by the application of HM15 contact lubricant and the gold flash individually, and more so by the combination of HM15 and gold flash. At the end of the test program, the AgPd variability has increased somewhat. Recalling, however, that the MFG environment is more severe for silver finishes than for the standard gold or palladium-nickel finishes, the AgPd finish performs very well in this evaluation.
The preliminary data in these papers demonstrates that the AgW and AgPd nanocrystalline silver finishes evaluated provide significant improvement in performance over conventional silver finishes, making them viable candidates as alternatives to gold finishes. The thermal stability is particularly significant because previous candidates have failed to meet this requirement. Qualification of these finishes, to Telcordia 1217 for example, requires reassessment of MFG exposures and durations due to unknown severities and acceleration factors for these finishes under current test exposures.
*The two papers referenced are:
- “Connector Level Performance Evaluation of a New High-Speed Reel-to-Reel Electroplated Silver Palladium Alloy Contact Finish,” presented at ICEC 2014: The 27th International Conference on Electrical Contacts.
- “Performance Testing and Evaluation of a Ag-W Nano-Crystalline Silver Alloy as a Gold Replacement in Electrical Connectors,” presented at Holm 2014: 60th IEEE Holm Conference on Electrical Contacts.
**Note from Dr. Bob: “In full disclosure this article includes a limited selection of topics from the papers. I found them both very exciting and potential game-changers in connector finish design, and I recommend that those with an interest access the original papers for further insights and details.”
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