Parameters/Performance II Contact Normal Force
In my last article I commented on specifying a contact plating or contact finish in terms of parameters, such as plating thickness or performance. For example, a typical performance-related requirement would be to evaluate the effect of mixed flowing gas exposures on a contact. The plating thickness requirement is generally specified, and in some cases, a choice of plating thicknesses to address different application environments is required.
This time I’d like to discuss another parameter, arguably the most important parameter in connector design: contact normal force. Despite its importance, it may be surprising that contact force is generally not specified. An anecdotal “minimum contact force” of 100 grams always was, and occasionally still seems to be, a “generally understood requirement.” In this article I’d like to comment on that “requirement” in terms of its history and relevance today.
There are two documents dating from the early 1970s that cite a 100 gram requirement. The earliest is an article by R. H. van Horn of Bell Laboratories, “Connecting Devices for Electronic Systems: Some General Design Considerations” (CDES), that appeared in IEEE Trans. Parts, Hybrids and Packaging, 11.3, 1975. It was the first, as far as I know, general discussion of connector material/design parameters and is still relevant today. (An earlier version of that article appeared in a Bell Laboratories Series on “The Physical Design of Electronic Systems.”) The second was an informational document from AMP Incorporated specific to tin-plated connectors, “The Tin Commandments: Guidelines for the Use of Tin on Connector Contacts” (TTC), issued in the mid ‘70s, but later published in Plating and Surface Finishing magazine in October 1981. Let’s consider the AMP document first.
The Tin Commandments was issued in response to the first major fretting corrosion field failure outbreak. (Connector Basics: Connector Degradation Mechanisms—Corrosion Part I) Yes, there were 10 Tin Commandments, and the second one said that tin connectors required a minimum contact force of 100 grams. That minimum was specified because one of the approaches to reducing the potential for fretting corrosion was to produce a high friction force at the contact interface to enhance the mechanical stability of the interface, to “prevent” fretting motions from occurring. TTC also contained two other “100s.” Commandment number two said that tin was not recommended for use above 100 degrees centigrade, and Commandment number six called for a minimum of 100 microinches (2.5 microns) of tin plating. With these “hundreds” as a mnemonic device and a wide distribution of TTC, I suspect that TTC may have been a significant contributor to the 100 gram contact force requirement mindset.
As mentioned, the van Horn article was broader in scope and included a discussion of the many interacting connector performance characteristics affected by contact force. Degradation Mechanisms: Loss in Contact Normal Force and a companion paper, “The Design of Separable Connectors” (DSC), found in Proc. 20th Electronic Components Conference, 1970, cite connector mating forces, mechanical wear, film/contaminant displacement, and accommodation of manufacturing tolerances as considerations that led to a minimum one Newton (N) (100 grams) end-of-life contact force requirement. Van Horn notes that high reliability relays use much lower forces, as low as .05 N (5 grams), and suggests that lower forces may be feasible in connectors. He also notes that “connector designers have not had the pressure to reduce minimum contact forces that faced relay designers.” Well, that situation has certainly changed as connectors morphed from 1970s connectors with 100 positions on 100 mil centerlines to several hundred positions on 40 mil centerlines today, the motivation of miniaturization. But all of van Horn’s considerations remain relevant for smaller connectors, some of them even more so today, with mating force being a prime example.
So where are we now with respect to a minimum contact force requirement?
Experience has demonstrated, through highly demanding test protocols such as Telcordia 1212, that connectors with contact forces less than 50 grams can provide high performance. But how low can contact force go? It is known that clean gold contact surfaces can provide contact interface resistances of the order of a few milliohms at forces under 10 grams under mechanically stable test conditions. The key words in that carefully weasel-worded sentence are “mechanically stable.” I suggest that the minimum contact force requirement is the force that can provide mechanical stability under the intended application conditions for the connector. The contact force required to create an acceptable contact resistance is less than the force required to provide mechanical stability in typical applications. In other words, it is an unknown and variable parameter. How can such a parameter be specified? Clearly there is no general purpose minimum value. There are at least two reasons for that. First, the minimum force value for mechanical stability varies with the application. Second, the design of the contact interface and the contact spring influence the mechanical stability of a contact for a given contact force. The simplest example is a comparison of a rigid and compliant single beam contact. The rigid contact will transfer all of the “driving force” to the contact interface, while the compliant contact will absorb some of the force within the beam.
The end result is that the problem of specifying contact force is similar to that discussed last time for plating specifications. Experience with a given plating supplier provides insight into the minimum plating thickness requirement. Similarly, experience with a given contact spring compliance/contact force combination provides insight into the mechanical stability characteristics of the contact design. Force matters, but so does how it is applied.