Dr. Bob on Contact Spring Material Selection
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.
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.
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.
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