Dr. Bob on Specmanship – Parameters or Performance?
I recently had an interesting experience discussing contact finish specifications with a working group of a major standardization organization. The subject was basically whether to specify finish parameters or performance requirements. The discussion regarding “parameters or performance” has been ongoing for decades. Of all the connector design/materials decisions, choosing the contact finish is arguably the most complex. This is, of course, because plating practices—and the “quality” of the resulting plating—are the most sensitive of all of the many processes necessary to connector manufacturing.
This claim is not intended to suggest that injection molding of the variety of polymers used in connector manufacture of fine pitch connectors, or the rolling processes of copper alloys to realize consistent properties at decreasing contact spring thicknesses, for example, are easily accomplished. Far from it, high pin-count, fine-pitch connectors and sockets, using contact spring thicknesses approaching one mil, are significant manufacturing accomplishments.
But connector performance, in terms of field failures, is determined primarily by the contact interface, where the contact finish of the plug and receptacle come together. My mentor at AMP Incorporated in the ‘80s, Jim Whitley, used to say that a connector is simply two contact surfaces held together by supporting structures. There is no question that the “supporting structures,” the connector housing and the contact springs, are critical to connector performance. But the interface between the two contact finishes is where the action, or the failure, usually takes place.
While these comments apply to both noble (gold) and non-noble (tin) contact finishes, tin finishes are less interesting because the failure mechanism is predominately fretting corrosion. Fretting degradation has been discussed in a previous article in the Connector Degradation Mechanism Series, Connector Degradation Mechanisms: Corrosion I. Gold interface degradation, however, can occur for multiple reasons, as discussed in Connector Degradation Mechanisms: Corrosion II. The following condensed discussion will focus on gold finishes.
A noble metal contact finish is a system consisting of the topcoat, generally gold or gold-flashed palladium-nickel, a nickel underplate, and the base metal of the contact spring. The contact spring is a part of the system for two reasons. First, it affects the contact resistance of the interface, and second, the stresses of the contact force penetrate into the contact spring because the topcoat and nickel underplate are so thin. That is all that will be said about the contact spring in this discussion.
As mentioned, the topcoat is generally gold in one of three thicknesses—flash, 0.38 microns (15 microinch), or 0.75 microns (30 microinch). Flash thicknesses are variable. Thicknesses range from having only a gold appearance to 0.1 microns or so. Suffice it to say, a gold “appearance” can be realized without complete gold coverage of the surface. Due to this variability, it is advisable to specify a flash thickness. Gold flash is used primarily as a topcoat over a palladium-nickel finish, commonly 0.38 microns. As Max Peel notes in Peel’s Law, “Anyone that specifies a gold flash contact finish deserves all the problems he is going to get.” The nickel underplate thickness is typically in the range of 1.25 to 2.5 microns, and is generally a ductile nickel. The two main functions of this noble metal finish system are corrosion protection and mating durability.
Corrosion protection is necessary because the contact springs are generally copper alloys, and therefore, susceptible to corrosion in typical connector operating environments. Said another way, corrosion in noble metal finished contacts always takes place at sites of exposed copper, such as pore sites, plating defects/scratches, or unplated bare edges in pre-plated contacts. Thus the quality of the finish, both the nickel and the gold, is critical to corrosion protection.
Porosity is the main issue with respect to the gold plating, because gold is inherently corrosion-resistant. Porosity decreases as the gold thickness increases. The gold thickness, of course, influences the cost. It is also important to note that porosity varies with the quality of the plating process, that is, with the supplier of the plating. In other words, a “quality” plating process can produce the same porosity level at a lower thickness than that of a lesser “quality” process. “Quality” is used here as a general comment, reflecting the fact that porosity depends on many plating practices.
The nickel underplate performs two functions with respect to porosity. First, the “quality” of the nickel underplate can reduce the overall porosity level for a given thickness of nickel, and thus, of the gold topcoat. Second, pore sites originating in the gold will have a corrosion-resistant nickel plate at the base of the gold pore site, preventing exposure of the contact spring base metal. It should also be noted that nickel provides a barrier to corrosion migration. This property can reduce the rate of corrosion migration from pore sites that penetrate to the contact spring, as well as reducing the rate of corrosion migration from the exposed copper at bare edges of pre-plated contacts.
With respect to contact durability, the relative hardnesses of gold and nickel come into play because the durability, or wear resistance, of a plating increases with the hardness of the plating. The hardness of the gold is important and dependent on the plating process. Variations in hardness are typically less than variations in porosity. The hardness of a “hard” gold (cobalt- or nickel-hardened, for example) is of the order of 200 Knoop. Nickel hardnesses vary with the plating bath and process, and can range from 300 to 500 Knoop. The nickel underplate, therefore, increases the composite hardness of the finish. This hardness enhancement increases with the nickel thickness.
This brief discussion of the effects of contact finish on performance provides a context for discussing the relative merits of using plating parameters or performance requirements for specifying a contact finish. Consider specifying plating thickness as an indicator of corrosion stability, and durability in terms of performance requirements.
There is a reasonable correlation between corrosion stability, as measured by performance in mixed flowing gas exposures and plating porosity. There is also a correlation, as mentioned, between porosity and plating thickness. Thus, plating thickness is a parameter that can provide an indication of corrosion stability. Given that the correlation coefficients are not known and are expected to vary significantly with the supplier of the plating, a parametric thickness specification, to ensure a given level of performance, would have to be conservative. Field experience is arguably the most reliable source of “appropriate” plating thickness values for a parametric approach to finish specification. Remember, both gold and nickel thicknesses must be specified. The alternative approach—specifying performance—would be based on meeting specific requirements, generally contact resistance stability, and after exposure to an appropriate mixed flowing gas exposure that is representative of the intended/expected connector operating environment. In the performance-based approach, a product that meets the performance requirements from a given supplier would determine the minimum plating thickness requirement for that supplier. If multiple suppliers meet the requirement, the range of minimum thicknesses of gold and nickel required, could lead to a “general” minimum thickness requirement.
A similar approach applies to durability, with the exception being that the nickel may be the more significant parameter due to the composite hardness effect mentioned previously. Again, a “field experience” value, conservatively chosen, could serve as a parametric specification value. Alternatively, a durability requirement appropriate for the product at issue can be specified as a benchmark. In durability testing, however, contact resistance after durability cycling is not an appropriate criterion. Two alternative possibilities are the measurement of plating thickness after cycling, to ensure that the gold remains intact, or an appropriate corrosion exposure after cycling, followed by contact resistance measurement to validate the integrity of the contact interface. The same approach to a “minimum” thickness, in this case nickel, is applicable as described above.
An additional benefit of a performance-based approach is that an accumulation of relevant data for various products and applications may provide indications of parametric values that can be used with enhanced confidence for new product designs and/or qualifications.
A solid field history base, along with experience with your supplier base, may be sufficient to use a parametric approach to plating specifications. This is, in essence, what is often used in practice. But as cost pressures create a driving force to reduce plating thicknesses, a performance-based approach may be more appropriate in the short term, especially with new connector designs that have no field history.
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