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Contact Spring Material Selection
By Dr. Robert S. Mroczkowski, Bishop & Associates Inc.
In the
first article in this series I identified three contact spring
material parameters of particular importance to electrical connectors.
These parameters are elastic modulus, which affects contact normal
force; yield strength, which also affects contact normal force; and
conductivity, which affects the current carrying capacity of a contact.
I noted that the elastic modulus, E, is a material property that varies
slightly with alloying. Copper and copper alloys have values of E in the
range of 16 to 22 (all E values are in million psi).
In the
second article, I discussed some basics of copper alloy metallurgy,
including alloy composition, strengthening mechanisms/processing
practices, and conductivity. During the discussion of strengthening
mechanisms, in particular relating to precipitation and dispersion (P/D)
hardening, I introduced an additional material performance parameter,
stress relaxation. Unfortunately, I forgot to define stress relaxation.
In terms of connector performance, stress relaxation causes a decrease
in contact normal force as a function of time and temperature. The
article noted that P/D-hardening alloys had better stress relaxation
resistance than solution-hardening alloys. At room temperature, all
copper alloys will retain nearly 100 percent of their initial contact
force after 1,000 hours. Stress relaxation becomes important, however,
at higher temperatures. For example, in generic terms (in other words
the following data are not to be used for design purposes—consult your
material supplier for engineering data) after 1,000 hours at 150 degrees
C, C51000, phosphor bronze (Cu-5%Sn), a solution-hardening alloy, will
retain about 65 percent of its initial contact force, while C19400
(Cu-2.3% Fe), a dispersion-hardened alloy, will retain about 75 percent,
and C17510-HT (Cu-0.4%Be/1.8%Ni), a precipitation-hardened alloy, will
retain about 92 percent, a significant difference. The disparity in
performance between solution- and P/D-hardening alloys continues to
widen as temperature increases.
Conductivity
As noted in the previous article, copper alloys are among the
most conductive of alloys, with performance levels ranging from 15 to
90+ percent of the conductivity of copper. For most connector
applications, conductivity is not a significant parameter. This changes
dramatically for power applications, however. For the purposes of this
discussion, a power application will be defined as an application in
which the temperature of the contact interface at rated current rises by
10 degrees C over the ambient temperature.
Conductivity is
important in two ways. As noted in the first article:
The resistance of a
contact interface depends on the area of the interface and the
conductivity of the materials in contact. A simplified expression of
this relationship, for a circular unplated contact area, is:
RContact
= r(
H / FNormal)1/2
RContact
is the resistance of the contact interface,
r
is the resistivity of the contact material, H is the hardness of
the contact material, and FNormal is the contact
normal force—that is, the force perpendicular to the contact interface.
The general form of this equation applies to contact interfaces of
geometries other than circular.
In the equation, the resistivity,
r,
the reciprocal of conductivity, is the cited parameter. In terms of
conductivity, of course, the contact interface resistance decreases as
conductivity increases.
Similarly, the bulk resistance of the contact decreases as the
conductivity increases. The resistance of a circular wire, Rwire,
is given by:
Rwire =
r (L/A)
L is the length of the wire and A is the
cross-sectional area of the wire. For a contact, the geometric term will
be more complex, but the direct dependence on
r,
and therefore, inversely on conductivity, remains. For power
applications, minimizing both contact interface and bulk resistances is
critical to avoid the negative consequences of Joule, or I2R,
heating.
For the majority of connector applications, the conductivities of C2600,
brass (28%IACS) and C51100, phosphor bronze (20% IACS) are adequate. But
for power applications, it is generally necessary to use higher
conductivity materials, C15100 (Cu – 0.1%Zr, 90% conductivity), C19700
(Cu- 0.6 %Fe, 80% conductivity, C19400 (60% conductivity), and C70250
(Cu- 3.0%Ni, 0.65%Si, 0.15%Mg, 40% conductivity), are often considered.
There are, of course, other alloys that merit consideration, given that
yield strength, formability, and stress relaxation parameters may also
be important.
Another reason to consider higher conductivity alloys is because size
matters. The bulk resistance of a contact spring increases as the size
of the spring decreases. Given that printed wiring boards in
computer/telecom applications are becoming increasingly power hungry,
smaller contacts become necessary to fit the available envelope for a
connector. In these applications, multiple lower current capacity
contacts are increasingly used in parallel, to support high-current
requirements.
Yield Strength/Stress Relaxation
Contact normal force is arguably the most important connector
design parameter for two reasons. First, for a given contact material
and design, it determines the contact resistance using the equation
above. Second, it strongly influences the stability of contact
resistance through the friction force it creates at the contact
interface, which, in turn, provides the mechanical stability of the
contact interface. It is the application-dependent requirement for
mechanical stability that determines the “necessary” contact normal
force.
Yield strength enters the discussion through the following equation,
repeated from the first article in this series:
Fcontact
= f2 [
s,
geom ]
For our purposes, it is sufficient to
note that the contact force is directly dependent on
s,
the stress created in the receptacle contact beam, due to beam
deflection on mating of the connector. The “maximum” elastic contact
force will depend on the yield strength of the contact beam material.
The “size matters” comment also applies to yield strength, in that as
contact dimensions decrease to meet contact density requirements in high
pin count connectors and sockets, meeting the contact normal force
requirements becomes increasingly difficult and higher yield strength
spring materials become necessary. If achieving the necessary contact
force is difficult, the loss of contact force over time due to
application temperatures and lifetimes, which is stress relaxation,
clearly becomes an increasingly important consideration.
Consider a white goods application. Factors in play: room temperature
plus the temperature rise during operation. Stress relaxation is
basically negligible. (It is also generally easier to achieve high
contact forces in such applications). Telecom/computer applications are
a different story. Connector sizes are decreasing, operating
temperatures are increasing, reliability requirements are more
demanding. Stress relaxation becomes a design factor. The same thing
holds true for power applications. Joule heating results in contact
temperatures tens of degrees centigrade above the ambient temperature,
thus increasing stress relaxation rates. Under-hood applications in
automotive are another “hot” application in which stress relaxation is
important.
In such applications, it becomes necessary to consider the use of
P/D-hardened alloys with their improved stress relaxation performance.
An additional decision is the balance of yield strength, stress
relaxation, and conductivity (to reduce Joule heating) necessary for a
given application. An alloy seeing increasing use due to its balance of
these properties is C70250 (Cu, 2.4%Ni, 0.5%Si, 0.1%Mg). C70250 can
provide yield strength in the 120KSI range, with 82 percent contact
force remaining after 1,000 hours at 150 degrees C, and a conductivity
of the order of 40 percent IACS.
Summary
The intent of this
article is to provide a basic overview of the importance of an
appropriate balance of yield strength, stress relaxation resistance, and
conductivity of copper alloys in/for a given connector application. The
article has focused on UNS alloys for convenience. There are competitive
alloys available from European and Japanese suppliers, and such
suppliers are the best source for detailed design data and materials
recommendations for specific applications.
Contact Dr. Bob at
connNtextassoc@aol.com.
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Dr. Robert S. Mroczkowski
Director Technology, Bishop and Associates Inc.
IIn 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 the electronics industry. He joined AMP Inc. in
1971. While at AMP, his responsibilities included consulting on
connector design, materials, and reliability concerns, and he
provided an interface to AMP customers on these issues. In 1990
he joined the AMP Advanced Development Laboratories, where he
developed 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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