Connector Reliability: The Role of Contact Spring Alloys
By Frank Dunlevey, Brush Wellman Inc.

Although conventional, separable connectors perform both electrical and mechanical roles; their functional reliability is usually defined by the capacity to maintain both low and stable contact resistance over the expected service life. High resistance at the contact interface affects signal integrity. In power connectors, resistance may also lead to elevated temperature problems. Thus, predicting, measuring, and controlling contact resistance has been intensely studied since the late nineteenth century, and has culminated in Ragnar Holm’s theory and mathematical model, to predict contact resistance based on the force applied to the contact surfaces. Holm, building on the original hypotheses of Rayleigh and others, proposed in the 1940s that contact resistance stability is controlled by having sufficient mechanical force in the connector spring members to insure adequate metal-to-metal contact area for electrical conduction across the numerous constrictions on the microscopically-rough contact interfaces. A minimum contact force is required to insure low and stable contact resistance, and the minimum contact force varies with the metals that comprise the contact interface.

Very low levels of contact force result in high contact resistance because of the effects of nonconductive surface films such as oxides, corrosion residue, oil, and dust. As the contact force increases, the surface film is penetrated and metal-to-metal contact areas increase in size, as the microscopic points of contact (asperities or a-spots) grow larger from plastic deformation of the interface metals. Eventually, the contact resistance decreases to a stable plateau with increasing contact force. The minimum resistance at the plateau is determined by the bulk resistivity of the contact material. Figure 1 schematically depicts Holm’s principle and it defines the concepts of a stable range for contact force and a minimum contact force required to maintain low resistance.

Figure 1: Contact Force and Contact Resistance Relationship

The minimum required contact force varies depending on the metals at the contact interface, as shown in Figure 2. Both the contact design and the application environment can also influence the minimum required contact force. Because of gold’s resistance to corrosion and ease of deformation, it has the lowest minimum contact force. At the other extreme, tin (although it is softer than electroplated gold) requires a high minimum contact force because tin forms stable oxides on the surface. The tin oxide layer must be forcibly displaced to expose clean metal when the tin-plated connector is mated. After disruption of the tin oxide film during mechanical wiping, the contact force for tin must remain high to prevent subsequent penetration of tin oxides into the contact interface during connector service. For all metals, the minimum contact force depends on the corrosion characteristics and hardness of the interface metal surfaces.

 Figure 2: Minimum Contact Force for Reliability

The contact metallurgy (usually an electroplated layer at the contact interface) determines the amount of contact force that is required to maintain stable contact resistance. However, the mechanical and physical properties and the design of the contact spring alloy determine the amount of contact force that is actually available. Of course, the available force should exceed the required contact force. Figure 3 lists the composition of some commonly available, high performance copper spring alloys for electrical connector applications.

 Figure 3: High Performance Copper Spring Alloys

The spring properties also influence the stability of the applied contact force over the life of the connector. If the contact force changes during connector service, it is possible for the contact resistance to increase to an unacceptable level. Connector performance stability requires that changes in contact force be predicted and accounted for in connector design and in materials selection. The spring alloy properties that control the initial contact force are the elastic (Young’s) modulus and strength. While strength is a mechanical property that can be modified by cold work or heat treatment, elastic modulus or stiffness is fairly insensitive to the alloy processing that defines the temper of the spring material. An alloy with high elastic modulus generates more contact force with less deflection, but it also produces higher stress in the spring. Thus, it is important that the spring alloy possess sufficient strength so that the deflection of the beam required to generate the minimum contact force does not result in permanent distortion of the beam. Strength also provides protection from excessive beam deformation from over-sized pins or obliquely inserted pins.

The general forms of the calculations of contact force and stress in a deflected beam are two equations where d is beam deflection, L is beam length, t is beam thickness, I is the moment of inertia of the beam (a function of the geometric cross section), and E is the elastic modulus of the spring alloy. Both contact force and beam stress are linearly dependent on beam deflection.

Force = 3 d E I/L³

 Stress = 1.5 d E t/L²

While reliability requires a minimum contact force, there are some practical limits to defining the maximum value of contact force. Excessive contact forces result in high insertion force, plating wear, connector housing distortion, as well as the possibility of over-stressing (and permanent set) in the contact beam. In using these equations for connector design, remember that the maximum allowable stress is the spring alloy yield strength. Unless the initial design contact force is sufficiently high, it is possible that a decrease in contact force over the life of the connector will result in an unacceptable increase in contact resistance, as shown in Figure 1. There are two major reasons that cause decreases in contact force over time: permanent set from insertion strain and stress relaxation.

Resilience is a measure of the amount of deformation that a spring beam will accept without significant permanent set. Resilience is defined as yield strength divided by elastic modulus. Highly resilient springs provide better resistance to permanent set from large deflections and better resistance to multiple deflections while maintaining the desired contact force. The calculated elastic resilience for contact spring alloys is provided with other material properties in Figure 6. Stress relaxation is the loss in contact force resulting from a long time, elevated temperature exposure of a beam at a constant strain. All connector materials are subject to some amount of stress relaxation. Plastics often exhibit relaxation at low temperatures, while spring steels offer stress relaxation resistance to 500º C. Copper alloys have varying resistance to stress relaxation in the 75 - 200º C range, where many connectors are required to operate.

Resistance to stress relaxation requires specialized testing and it can not be predicted from the alloy’s chemical composition and other properties. Figure 4 shows the percent of contact force retained as a result of stress relaxation at 150º C and 200º C for 1,000 hours. Both time and temperature increase stress relaxation. High ambient temperature, coupled with high current flow in a power connector, can result in exposure temperatures near or exceeding 200º C. Ideally, the percent of force retained should remain near 100 percent. The effects of stress relaxation are cumulative; a single exposure of 1000 hours is roughly equivalent to 1000 exposures each for one hour duration.

 Figure 4: Stress Relaxation Resistance of Copper Alloys

Both ambient conditions and the spring alloy resistivity contribute to the overall operating temperature of the connector. Spring alloys with high resistivity can generate significant heat at moderate to high current levels, and this resistive heating can be a significant factor in stress relaxation performance. Temperature increases of more than 100º C are easily possible in small connectors. Pure copper has high electrical conductivity (low resistivity) but the high performance alloys of copper exhibit wide variation in conductivity. Connector spring alloys generally range from about 5 percent to 75 percent of the conductivity of pure copper (IACS) as shown in Figure 6.

Permanent deformation of the contact beam that accumulates with multiple insertions can also decrease the contact force over time. Permanent deformation means that subsequent insertions generate less beam deflection. Contact force is proportional to beam deflection; less deflection means a decrease in contact force. A spring alloy with high yield strength will resist permanent deformation. If the designed beam deflection is large enough to cause the stress to exceed the spring’s yield strength, the design can be stable when the spring alloy has sufficient strain hardening capability to allow the permanent set to reach a plateau value. Strain hardening in an alloy is the ability to increase yield strength as a result of plastic deformation or strain. Strain hardening data is not easy to obtain; but strain hardening capability can be inferred from the numerical difference in the alloy’s yield strength and ultimate tensile strength, or from the yield strength as a percentage of tensile strength. A large difference between tensile and yield strengths indicates better strain hardening capability and more resistance to damage from multiple insertion cycles. Figure 6 provides spring alloy tensile/yield ratios.

The integrity of thin plating can be compromised by excess wear from multiple insertion cycles. Wear of the corrosion-resistant plating layer exposes the contact interface to inevitable corrosion of the spring alloy. In addition, wear generates debris that can contaminate electronic systems. Harder plating at the surface can improve plating wear resistance. A hard sub-plating layer, such as nickel, or a harder spring alloy can also contribute to the desired wear resistance.

In determining connector reliability for a given plating system, contact design and spring alloy properties are often interactive. The successful connector engineer recognizes that deficiencies in some material properties can be compensated by design changes. Similarly, the use of a lower performance plating or spring alloy typically requires some design modification to achieve equivalent performance and reliability to a high-performance alloy.

Figure 5 is a summary of important spring alloy material properties and how they are related to connector performance and reliability issues.

 Figure 5: Effect of Properties on Connector Performance

Figure 6 is a summary of material properties for several high-performance copper spring alloys. Each alloy is available in several tempers (strength levels). Alloy tempers were selected to provide sufficient bend formability to allow a minimum 90º bend radius equal to two times the strip thickness; for heat treatable alloys, the appropriate mill hardened (pre-heat treated) tempers were selected for the comparison.

 Figure 6: Material Properties of Copper Spring Alloys

Just as there is no single spring alloy that comes out on top in all of the properties’ categories, there is no universal ranking of the importance of the material properties for all connector designs. Specific connector applications will favor different property combinations. Figure 7 suggests the important material properties for different connector applications.

 Figure 7: Material Property Requirements of Connectors

Conclusions
Connector reliability depends upon maintaining low and stable electrical resistance at the contact interface; interface resistance is determined by contact force. The metallic plating at the contact interface determines the amount of contact force that is required for reliability. However, the contact spring alloys supporting the plating and the contact design dictate how much force is actually available. The spring alloy properties that influence reliability in high performance connector applications include: elastic modulus, strength, resistance to stress relaxation, electrical conductivity, and ductility. Since no single spring alloy possesses the maximum in each of these categories, effective material selection will be based on a ranking of material properties according to the application requirements. Based on the required properties, the optimum spring alloy in a specific connector application may not be the best choice for other applications. Ultimately, material selection for connectors will be based on performance, reliability, and cost.

Frank Dunlevey is the director of applications engineering for Brush Wellman Inc. Additional information is available on how Brush Wellman’s high-performance copper alloys can help you improve the performance and reliability of your connector designs. Visit www.brushwellman.com/designcenter or contact Brush Wellman Technical Service at 1-800-375-4205.