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 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 hardening copper-beryllium and copper-chromium-zirconium materials are decribed later in a separate section.

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 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 energyswitchgear.

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.

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.

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.

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.

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

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:

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:

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.