Changes

The most frequently used ones are copper based materials. Depending on the application , also materials based on nickel or multi-layer composite materials,

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.

Copper and copper alloys to be being used in electrical and electronic components are usually covered by national and international standards. DIN numbers thematerials 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 (<xr id="tab:MaterialDesignations"/> ) 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:

Copper is used in electrical engineering , mostly because of its high electrical conductivity<ref>As units for electrical conductivity MS/m and m/Ω.mm² are commonly used. Frequently – and mostly in North America – the % IACS value (International Annealed Copper Standard) is also used, where 100% is equivalent to 58 MS/m or m/Ωmm² .For the description of mechanical strength properties the units of N/mm² or MPa are most commonly used:

1 MPa = 1 N/mm²</ref> 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 (<xr id="fig:Strain hardening of Cu-ETP by cold working" />). 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 (<xr id="fig:Softening of Cu-ETP after annealing for 3hrs after 25% cold working"/> and <xr id="fig:Softening of Cu-ETP after annealing for 3hrs after 50% cold working"/><!--5.3-->).

described in DIN EN 1652 (<xr id="tab:MaterialDesignations"/> and <xr id="tab:Composition of Some Pure Copper Types"/><!--5.2-->). <xr id="tab:Physical Properties of Some Copper Types"/><!--Tables 5.3.--> and <xr id="tab:Mechanical Properties of Some Copper Types"/><!--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-HCP, Cu-DLP, and Cu-DHP are oxygen free copper types , de-oxidized with different phosphorus contents. With increasing phosphorus content , the

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 (<xr id="fig:Influence of small additions on the electrical conductivity of copper"/><!--(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 precipitation hardening copper-beryllium and copper-chromium-zirconium materials are decribed later in a separate section.

[[File:Influence of small additions on the electrical conductivity of copper.jpg|right|thumb|Figure 4: 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 (<xr id="tab:Physical Properties of Selected High Cu Content Copper Alloys"/><!--(Tab. 5.5)--> and <xr id="tab:Mechanical Properties of Selected High Cu Content Copper Alloys"/><!--(Tab. 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 They are used as substrates for power semiconductors and also as carriers for stationary contacts in higher energyswitchgear.

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 changes the mechanical properties very little but increase increases 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.

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.

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.

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 increases the mechanical strength of these copper alloys significantly.

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.

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.

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.

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 (<xr id="tab:Physical Properties of Nickel and Nickel Alloys"/><!--(Tab. 5.21)--> and <xr id="tab:Mechanical Properties of Nickel and Nickel Alloys"/><!--(Tab. 5.22)-->). Work hardening and softening behavior of nickel are shown in [[#figures11|(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 layer for thin claddings, acting as an effective diffusion barrier between copper containing carrier materials and goldand gold or palladium-based contact materials.

[[File:Strain hardening of technical pure nickel by cold working.jpg|right|thumb|Figure 5: Strain hardening of technical pure nickel by cold working]]

[[File:Softening of technical grad nickel after annealing for 3 hrs.jpg|right|thumb|Figure 6: Softening of technical grad nickel after annealing for 3 hrs after 50% cold working]]

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.

[[File:Strain hardening of NiCu30Fe by cold working.jpg|right|thumb|Figure 7: Strain hardening of NiCu30Fe by cold working]]

[[File:Softening of NiCu30Fe after annealing for 0.5 hrs.jpg|right|thumb|Figure 8: Softening of NiCu30Fe after annealing for 0.5 hrs and after 80% cold working]]

Because of decreasing solubility of beryllium in nickel with decreasing temperature , NiBe can be precipitation hardened similar to CuBe (<xr id="fig:Phase diagram of nickel beryllium"/><!--(Fig. 5.49)-->). The maximum soluble amount of Be in Ni is 2.7 wt% at the eutectic temperature of 1150°C. to 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 a hardening anneal is performed at 480 to 500°C for 1 to 2 hours.

[[File:Phase diagram of nickel beryllium.jpg|right|thumb|Figure 9: 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 (<xr id="fig:Precipitation hardening of NiBe2 soft at 480C"/><!--(Fig. 5.50)-->), the spring bending force limit can exceed values of over 1400 MPa and the fatigue strength reaches approximately 400 MPa.

[[File:Precipitation hardening of NiBe2 soft at 480C.jpg|right|thumb|Figure 10: 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

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

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

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 as those for Cu – Invar – Cu system.

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:

The wide variety of thermostatic bimetal types is specified mostly through DIN 1715 and/or applicable ASTM standards (<xr id="tab:Partial Selection from the Wide Range of Available Thermo-Bimetals"/><!--(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.

For the design and calculation of the most important thermostatic-bimetal parts , formulas are given in (<xr id="tab:Design Formulas for Thermostatic Bimetal Components"/><!--Table 5.24-->). The necessary properties can be extracted for the most common materials from (<xr id="tab:Partial Selection from the Wide Range of Available Thermo-Bimetals"/><!--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.

For all calculations according to the formulas in (<xr id="tab:Design Formulas for Thermostatic Bimetal Components"/><!--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):