Contact Carrier Materials

The reliability and electrical life of contact systems in switching devices as well as in electromechanical and electronic components do not only depend on the contact material. The selection of the most suitable carrier material also plays an important role.

The most frequently used ones are copper based materials. Depending on the application also materials based on nickel or multi-layer composite materials, such as thermo bimetals for example, are frequently used. For special applications in the medium and high voltage technology, as well as for springs and snap discs for the information technology, iron or steel based materials are considered. These are however not included for the purpose of this data book.

Various requirements based on the enduse of the contact components have to be met by carrier materials. Copper materials have to exhibit high electrical and thermal conductivity, good mechanical strength even at elevated temperatures, and in addition a sufficient high resistance against corrosion. If used as springs the carrier materials also must have good elastic spring properties. Besides these, the materials must, depending on the manufacturing processes employed, also have good technological properties like ductility to allow warm and cold forming, suitability for cutting and stamping, and be capable to be welded, brazed or coated by electroplating.

5.1 Copper and Copper Alloys

Standards Overview

Copper and copper alloys to be used in electrical and electronic components are usually covered by national and international standards. DIN numbers the materials by a prefix and/or a material number. The newer European standards (EN) refer to the material's usage products and also show a prefix and material number. For reference we also show in Table 1 the material designation according to UNS, the Unified Numbering System (USA). Other internationally used standard and material numbers include, among others, those issued by CDA (Copper Development Association, USA), and GB (Guo Biao – China).

The most important EN as well as the US based and widely used ASTM standards covering the use of flat rolled copper and copper alloys in electrical contacts are:

Standard Designation	Description
DIN EN 1652	Copper and copper alloys in plate, sheet, strip, and discs for general applications
DIN EN 1654	Copper and copper alloys for springs and connectors
DIN EN 1758	Copper and copper alloys in strip form for system component carriers
ASTM B 103/B103M-10	Spec. for Phosphor Bronce Plate, Sheet, Strip, and Rolled Bar
ASTM B 36/B36M-95	Spec. for Brass Plate, Sheet, Strip, and Rolled Bar
ASTM B 122/B122M-08	Spec. for CuNiSn-, CuNiZn-, and CuNi-Alloy
ASTM B 465-09	Spec. for Copper-Iron-Alloy Plate, Sheet, and Strip
ASTM B 194-08	Standard Spec. for CuBe-Alloy Plate, Sheet, Strip and Rolled Bar
ASTM B 534-07	Sec. for CuCoBe-Alloy and CuNiBe-Alloy Plate, Sheet, Strip, and Rolled Bar

The above DIN EN standards replace in part or completely the older DIN standards DIN 1777, DIN 17670, DIN 1751, DIN 1791.

5.1.2 Pure Copper

Copper is used in electrical engineering mostly because of its high electrical conductivity which with 58 MS/m (or m/Ωmm²) is only slightly below that of silver. Other advantages of copper are its high thermal conductivity, corrosion resistance, and its good ductility. The work hardening properties of ETP copper is illustrated in Figure 1. The increase in strength achieved by cold working can be reversed easily by subsequent annealing. The softening properties are strongly dependent on the preceding cold working percentage (Figure 2 and Figure 3 5.3).

The purity of technically pure and un-alloyed copper used for electrical applications depends on the type used and ranges between > 99.90 and 99.95 wt%. The copper types are designated mainly by their oxygen content as oxygen containing, oxygen-free, and de-oxidized with phosphorus as described in DIN EN 1652 (Table 1 and Table 2 5.2). Table 3 Tables 5.3. and Table 4 5.4 show the physical and mechanical properties of these copper materials. According to these, Cu-ETP, Cu-OFE, and Cu-HCP are the types of copper for which minimum values for the electrical conductivity are guaranteed.

Cu-ETP is less suitable for welding or for brazing in reducing atmosphere because of the oxygen content (danger of hydrogen embrittlement).

Cu-HCP, Cu-DLP, and Cu-DHP are oxygen free copper types de-oxidized with different phosphorus contents. With increasing phosphorus content the electrical conductivity decreases. Cu-OFE, also called OFHC copper, is free of oxygen and also free of de-oxidizing compounds.

Table 5.1: Material Designations of Some Copper Types Table 1: Material Designations of Some Copper Types

WerkstMaterialEN-Designation	EN-Number	DIN-Designation	DIN-Number	UNS
Cu-ETP	CW004A	E-Cu 58	2.0065	C11000
Cu-OF	CW008A	OF-Cu	2.0040	C10200
Cu-HCP	CW021A	SE-Cu	2.0070	C10300
Cu-DLP	CW023A	SW-Cu	2.0076	C12000
Cu-DHP	CW024A	F-Cu	2.0090	C12200

Cu- ETP: electrolytic tough-pitch copper

Cu-OFE: Oxygen Free Electronic Copper

Cu-HCP: High Conductivity Phosphorus Deoxidized Copper

Cu-DLP: phosphorous-deoxidized copper

Cu-DHP: Phosphorous Deoxidized High Conductivity Copper

Table 5.2: Composition of Some Pure Copper Types

Material	Composition (wt%)
EN Designation	Cu	Bi	O	P	Pb	Others
Cu-ETP	>99.90	bis 0.0005	bis 0.040		up to 0.005	up to 0.03
Cu-OF	>99.95	bis 0.0005			up to 0.005	up to 0.03
Cu-HCP	>99.90			ca. 0.003
Cu-DLP	>99.90	bis 0.005		0.005-0.013	up to 0.005	up to 0.03
Cu-DHP	>99.90			0.015-0.040

Table 5.3: Physical Properties of Some Copper Types

Material EN- Desig- nation	Density [g/cm³]	Electrical Conductivityt	Electrical Conduc- tivity [µS · cm]	Thermal Conduc- tivity [W/(m.K)]	Coeff. of Linear Thermal Expan- sion [10-6/K]	Modulus of Elasticity [GPa]	Softening Tempera- tur (approx. 10% loss in strength) [°C]	Melting Tempera- ture [°C]
Material EN- Desig- nation	Density [g/cm³]	[MS/m]	[% IACS]	Electrical Conduc- tivity [µS · cm]	Thermal Conduc- tivity [W/(m.K)]	Coeff. of Linear Thermal Expan- sion [10-6/K]	Modulus of Elasticity [GPa]	Softening Tempera- tur (approx. 10% loss in strength) [°C]	Melting Tempera- ture [°C]
Cu-ETP	8.94	≥58	100	1.72	390	17.7	127	ca. 220	1083
Cu-OF	8.94	≥58	100	1.72	394	17.7	127	ca. 220	1083
Cu-HCP	8.94	≥54	93	1.85	380	17.7	127	ca. 220	1083
Cu-DLP	8.94	52	90	1.92	350	17.7	132	ca. 220	1083
Cu-DHP	8.94	≥46	80	2.17	310	17.6	132	ca. 220	1083

Table 5.4: Mechanical Properties of Some Copper Types

Material	Condition	Tensile Strength Rm [MPa]	0,2% Yield Strength Rp0,2 [MPa]	Elongation A50 [ %]	Hardness HV
Cu-ETP Cu-OF Cu-HCP Cu-DLP Cu-DHP	R220	220 - 260	≤140	≥33	40 - 65
	R240	240 - 300	≥180	≥8	65 - 95
	R290	290 - 360	≥250	≥4	90 - 110
	R360	≥360	≥320	≥2	≥110

Figure 1 Fig. 5.1: Strain hardening of Cu-ETP by cold working

Figure 2 Fig. 5.2: Softening of Cu-ETP after annealing for 3hrs after 25% cold working

Figure 3 Fig. 5.3: Softening of Cu-ETP after annealing for 3hrs after 50% cold working

Figure 1: Strain hardening of Cu-ETP by cold working

Figure 2: Softening of Cu-ETP after annealing for 3hrs after 25% cold working

Figure 3: Softening of Cu-ETP after annealing for 3hrs after 50% cold working

5.1.3 High Cu Content Copper Alloys

The high Cu content alloy materials are closest in their properties to pure copper materials. By defined addition of small amounts of alloying elements it is possible to increase the mechanical strength and especially the softening temperature of copper and at the same time decrease the electrical conductivity only insignificantly Figure 4 (Fig. 5.4). Silver, iron, tin, zinc, nickel, chromium, zirconium, silicon, and titanium are used. Usually the additive amounts are significantly below 3 wt%. This group of materials consists of mixed crystal as well as precipitation hardening alloys. The precipiytion hardening copper-beryllium and copper-chromium-zirconium materials are decribed later in a separate section.

Influence of small additions on the electrical conductivity of copper

From the large number of high-Cu alloys only the properties of selected ones are covered here (Tables 5.5 and 5.6). Some of these materials are not included in the EN standards system.

The low alloyed materials CuAg0.1 and CuCd1 are mostly used as overhead drive cables where they have to meet sustained loads at elevated temperatures without softening.

The materials CuFe0.1 and CuSn0.15 have a high electrical conductivity. The mechanical strength of both is relatively low but stays almost constant at temperatures up to 400°C. The are used as substrates for power semiconductors and also as carriers for stationary contacts in higher energy switchgear.

CuFe2 is a material exhibiting high electrical conductivity and good formability. During an annealing process Fe-rich precipitations are formed in the " -Cu matrix which change the mechanical properties very little but increase the electrical conductivity significantly. Besides being used as a contact carrier material in switching devices, this material has broader applications in automotive connectors and as a substrate in the semiconductor technology.

CuNi2Si has high mechanical strength, good formability, and at the same time high electrical conductivity. This combination of advantageous properties is achieved by a defined finely dispersed precipitation of nickel silicides. CuNi2Si is used mainly in the form of stamped and formed parts in thermally stressed electromechanical components for automotive applications.

CuSn1CrNiTi and CuCrSiTi are advanced developments of the Cu-Cr-Ti precipitation materials with fine intermetallic dispersions. The material CuNi1Co1Si also belongs into this family and has properties similar to the low alloyed CuBe materials.

Tab. 5.5 Physical Properties of Selected High Cu Content Copper Alloys

2 teile!

Table 5.6; Mechanical Properties of Selected High Cu Content Copper Alloys

2 teile!

These newer copper based materials optimize properties such as electrical conductivity, mechanical strength, and relaxation, which are custom tailored to specific applications. Typical uses include contact springs for relays, switches, and connectors.

5.1.4 Naturally Hard Copper Alloys

Alloys like brasses (CuZn), tin bronzes (CuSN), and German silver (CuNiZn), for which the required hardness is achieved by cold working are defined as naturally hard alloys. Included in this group are also the silver bronzes (CuAg) with 2 – 6 wt% of Ag.

Main Articel: Naturally Hard Copper Alloys

5.1.5 Other Naturally Hard Copper Alloys

Main Articel: Other Naturally Hard Copper Alloys

5.1.6 Precipitation Hardening Copper Alloys

Besides the naturally hard copper materials precipitation hardening copper alloys play also an important role as carrier materials for electrical contacts. By means of a suitable heat treatment finely dispersed precipitations of a second phase can be achieved which increase the mechanical strength of these copper alloys significantly.

Main Articel: Precipitation Hardening Copper Alloys

5.1.7 Application Properties for the Selection of Copper Alloys

Important for the usage as spring contact components are besides mechanical strength and electrical conductivity mainly the typical spring properties such as the maximum spring bending limit and the fatigue strength as well as the bendability. During severe thermal stressing the behavior of spring materials is determined by their softening and relaxation. The following briefly describes these material properties.

Main Articel: Application Properties for the Selection of Copper Alloys

5.1.8 Selection Criteria for Copper-Based Materials

The selection of copper-based materials from the broad spectrum of available materials must be based on the requirements of the application. First an application profile should be established which can be used to define the material properties. Usually there is however no single material that can fulfill all requirements to the same degree. A compromise must be found as for example between electrical conductivity and spring properties.

If current carrying capability is the key requirement, mechanical strength may have to be sacrificed as for example in carrier parts for stationary contacts. In this case, depending on the current level, pure copper or low alloyed copper materials such as CuSn0.15, or for economic reasons CuZn30, may be suitable.

For spring contact components the interdependent relations between electrical conductivity and fatigue strength, or electrical conductivity and relaxation behavior are of main importance. The first case is critical for higher load relay springs. CuAg2 plays an important role for these applications. The latter is critical for components that are exposed to continuing high mechanical stresses like for example in connectors. The spring force must stay close to constant over the expected life time of the parts even at elevated temperatures from the environment or current carrying. In this case the relaxation behavior of the copper materials which may cause a decrease in spring force over time must be considered. Besides this easy forming during manufacturing must be possible; this means that bending operations can also be performed at high mechanical strength values.

The increasing requirements on spring components in connectors, especially for use in automotive applications, such as higher surrounding temperatures, increased reliability, and the trend towards miniaturization led to a change of materials from traditionally CuZn30 and CuSn4 to CuNiSi alloys, for example. These CuNiSi alloys and the newer heavy duty copper alloys like CuNi1Co1 are significantly improved with regards to mechanical strength, relaxation behavior, and electrical conductivity.

5.2 Nickel and Nickel Alloys

5.2.1 Technical Grade Pure Nickel

Technical grade pure nickel commonly contains 99.0 to 99.8 wt% Ni and up to 1 wt% Co. Other ingredients are iron and manganese (Tables 5.21 and 5.22). Work hardening and softening behavior of nickel are shown in (Figs. 5 – 6) Figs. 5.45 and 5.46.

One of the significant properties of nickel is its modulus of elasticity which is almost twice as high as that of copper. At temperatures up to 345°C nickel is ferro-magnetic. Nickel has a high corrosion resistance, is very ductile, and easy to weld and clad. It is of great importance as a backing material for multiple layer weld profiles. In addition nickel is used as an intermediate layers for thin claddings, acting as an effective diffusion barrier between copper containing carrier materials and goldand palladium-based contact materials.

Because of the always present thin oxide layer on its surface, nickel is not suitable as a contact material for switching contacts.

Figure 5 Fig. 5.45: Strain hardening of technical pure nickel by cold working

Figure 6 Fig. 5.46; Softening of technical grad nickel after annealing for 3 hrs after 50% cold working

Strain hardening of technical pure nickel by cold working

Softening of technical grad nickel after annealing for 3 hrs after 50% cold working

5.2.2 Nickel Alloys

Because of its low electrical conductivity NiCu30Fe is besides pure Ni and CuNi alloys the most widely used backing material for weldable contact components. With 1 – 2 wt% additives of Fe as well as 0.5 – 1 wt% Mn and Co the mechanical strength of the binary alloy NiCu30 can be increased.

The strength values of NiCu30Fe are significantly higher than those of the copper rich CuNi alloys (Figs. 7 – 8) (Figs. 5.47 and 5.48). The good spring properties and thermal stability of NiCu30Fe make it a suitable material for the use as thermally stressed contact springs.

Figure 7 Fig. 5.47: Strain hardening of NiCu30Fe by cold working

Figure 8 Fig. 5.48: Softening of NiCu30Fe after annealing for 0.5 hrs and after 80% cold working

Strain hardening of NiCu30Fe by cold working

Softening of NiCu30Fe after annealing for 0.5 hrs and after 80% cold working

Table 5.21: Physical Properties of Nickel and Nickel Alloys (2 Teile!)

Table 5.22: Mechanical Properties of Nickel and Nickel Alloys (2 Teile!)

5.2.3 Nickel-Beryllium Alloys

Because of decreasing solubility of beryllium in nickel with decreasing temperature NiBe can be precipitation hardened similar to CuBe Figure 9 (Fig. 5.49). The maximum soluble amount of Be in Ni is 2.7 wt% at the eutectic temperature of 1150°C. to achieve a high hardness by precipitation hardening NiBe, similar to CuBe, is annealed at 970 - 1030°C and rapidly quenched to room temperature. Soft annealed material is easily cold formed and after stamping and forming an hardening anneal is performed at 480 to 500°C for 1 to 2 hours.

Phase diagram of nickel-beryllium

Commercial nickel-beryllium alloys contain 2 wt% Be. Compared to CuBe2 the NiBe2 materials have a significantly higher modulus of elasticity but a much lower electrical conductivity. The mechanical strength is higher than that of CuBe2 Figure 10 (Fig. 5.50), the spring bending force limit can exceed values of over 1400 MPa and the fatigue strength reaches approximately 400 MPa.

Precipitation hardening of NiBe2 (soft) at 480°C

A further advantage of NiBe2 is its high temperature stability. Cold worked and subsequently precipitation hardened NiBe2 can withstand sustained temperatures of 400 - 650°C, depending on ist pre-treatment.

Similar to CuBe materials, NiBe alloys are available in mill hardened in various conditions or also already precipitation hardened at the manufacturer.

Nickel-beryllium alloys are recommended for mechanically and thermally highly stressed spring components. For some applications their ferro-magnetic properties can also be advantageous.

5.3 Triple-Layer Carrier Materials

Manufacturing of triple-layer carrier materials is usually performed by cold rollcladding. The three materials cover each other completely. The advantage of this composite material group is that the different mechanical and physical properties of the individual components can be combined with each other.

Depending on the intended application the following layer systems are utilized:

• Conduflex N
  CuSn6 - Cu - CuSn6
  

The high electrical and thermal conductivity as well as the current carrying capacity of copper is combined with the spring properties of the tin bronze. Conduflex N strips are used in a thickness range of 0.1 – 1,5 mm in a maximum width of 140 mm.

• Cu - FeNi36 (Invar) - Cu

The high electrical conductivity and ductility of copperis combined with the low coefficient of thermal conductivity of the Invar alloy. The dimensionsional range is 0.2 – 1.8 mm in thickness with a maximum width of 140 mm.

• Cu – Fe or Steel – Cu

The high electrical conductivity and good arc mobility properties of copper are combined with the mechanical strength and magnetic properties of iron or steel. The thickness and width range of material strips are the same of the ones for Cu – Invar – Cu system.

The thickness ratios of the components can be selected according to the application requirements. The two outer layers usually have the same thickness.

5.4 Thermostatic Bimetals

Thermostatic bimetals are composite materials consisting of two or three layers of materials with different coefficients of thermal expansion. They are usually bonded together by cladding. If such a material part is heated either directly through current flow or indirectly through heat conduction or radiation, the different expansion between the active (strong expansion) and passive (low expansion) layer causes bending of the component part.

Directional or force effects on the free end of the thermostatic bimetal part is then used as a trigger or control mechanism in thermostats, protective switches, or in control circuits. Depending on the required function of the thermostatic bimetal component different design shapes are used:

• Straight or U-shaped strips for nearly linear motion
• Circular discs for small linear motions with high force
• Spirals and filament spring shapes for circular motion
• Stamped and formed parts for special designs and applications

The wide variety of thermostatic bimetal types is specified mostly through DIN 1715 and/or applicable ASTM standards Table 5 (Table 5.23). The different types have varying material compositions for the active and passive side of the materials. The mostly used alloys are iron-nickel and manganese-copper-nickel. Mainly used in circuit protection switches (i.e. circuit breakers) some thermo-bimetals include an intermediate layer of copper or nickel which allows to design parts with a closely controlled electrical resistance.

Table 5.23: Partial Selection from the Wide Range of Available Thermo-Bimetals

Designation DIN 1715	Designation ASTM	Specific Thermal Deflection [106/K]	Sprecific Electrical Resistance k [μΩ·m]	Typical Application Range [°C]	Application Limit [°C]	Composition
TB 20110 TB 1577A TB1170A	TM 2 TM 8 TM 1 TM 3 TM 4	21.1 15.3 15.5 14.2 11.7 10.6 8.5	1.12 1.41 0.79 0.78 0.70 0.71 0.66	- 70 – + 260 - 70 – + 260 - 70 – + 370 - 70 – + 370 - 70 – + 425 - 70 – + 480 - 70 – + 425	350 350 450 450 480 540 540	Two components
TB 1517 TB 1511 TB 1303 TB 1109	TM 28 TM 26 TM 25 TM 24	14.9 14.9 14.3 13.9 13.2 13.1 12.3 11.5	0.17 0.11 0.15 0.08 0.03 0.05 0.03 0.09	- 70 – + 260 - 70 – + 260 - 70 – + 315 - 70 – + 315 - 70 – + 260 - 70 – + 315 - 70 – + 315 - 70 – + 380	400 400 350 350 300 350 350 400	Three components with Cu intermediale layer
TB 1555 TB 1435 TB 1425	TM 17 TM 15 TM 13 TM 11 TM 9	15.0 14.8 14.2 14.1 14.0 13.6 12.8 10.7	0.55 0.40 0.66 0.50 0.25 0.33 0.25 0.17	- 70 – + 260 - 70 – + 260 - 70 – + 370 - 70 – + 370 - 70 – + 260 - 70 – + 370 - 70 – + 370 - 70 – + 370	450 450 480 480 450 480 480 480	Three components with Ni intermediale layer

5.4.1 Design Formulas

For the design and calculation of the most important thermostatic-bimetal parts formulas are given in Table 5.24. The necessary properties can be extracted for the most common materials from Table 5 Table 5.23. The values given are valid only for a temperature range up to approximately 150°C. For higher temperatures data can be obtained from the materials manufacturer.

Table 5.24: Design Formulas for Thermostatic Bimetal Components

	Shape of the Thermostatic Bimetal	Deflection	Mechanical Action Force	Thermal Action Force
Cantilevered strip
Dual supported strip
U-shaped element
Spiral
Helical spring
Disc
Reversed strip
Reversed U-shaped element

5.4.2 Stress Force Limitations

For all calculations according to the formulas in Table 5.24 one should check if the thermally or mechanically induced stress forces stay below the allowed bending force limit. The following formulas are applicable for calculating the allowable load (Force P_max or momentum M_max):

Single side fixed strip
Both sides fixed strip
Spiral or filament
Disc

= bending stress

References

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