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Parameters/Performance
II: Contact Normal Force
By Dr. Robert S. Mroczkowski,
Bishop & Associates Inc.
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.
Contact Dr. Bob at
connNtextassoc@aol.com.
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Dr. Robert S. Mroczkowski
Director Technology, Bishop and Associates Inc.
In 1998, Dr. Mroczkowski founded connNtext associates, a firm
providing consulting services in connector applications to the
electronics industry. Dr. Mroczkowski has more than 30 years
experience in various aspects of the electronics industry. He
joined AMP Inc. in 1971. While at AMP, his responsibilities
included consulting on connector design, materials, and
reliability concerns within AMP, and providing an interface to
AMP customers on the same issues. In 1990 he joined the AMP
Advanced Development Laboratories, where he was responsible for
the development of microstrip cable connectors and a new
microcoaxial connector for medical ultrasound diagnostic
equipment. Dr. Mroczkowski retired in 1998 as an AMP principal.
He is the author of the McGraw Hill Electronics Connector
Handbook, has contributed chapters on connectors and
interconnections to a number of packaging handbooks, and written
more than 20 technical papers. He holds seven patents. In 1997,
Dr. Mroczkowski received the Lifetime Achievement Award of the
International Institute of Connector and Interconnection
Technology.
He holds a bachelor’s, master’s, and doctorate of science
degrees in physical metallurgy from the Massachusetts Institute
of Technology.
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