Manufacturing Technologies for Contact Parts

Besides the selection of the most suitable contact materials the design and type of attachment is critical for the reliability and electrical life of contact components for electromechanical switching devices. The materials saving use of high cost precious metals and the most economic manufacturing method for contact parts are most important factors.

There are two basic manufacturing solutions available: One can start with single contact parts such as contact rivets or tips which then are attached mechanically or by brazing or welding resp. to carrier parts. In the second case a base material coated or clad with the precious contact metal - for special applications also clad with another non-precious material – in the form of strips or profiles is manufactured as a semi-finished pre-material from which the contact components are then stamped and formed. Besides mechanical cladding other processes such as electroplating and deposition from the gas phase are utilized. Which of the following manufacturing processes is finally chosen depends on the final application of the contact components in their respective switching devices or electromechanical components. Other considerations such as the required number of electrical operations, the most economical use of precious metals and the anticipated volumes of parts are also important for the process selection.

3.1 Manufacturing of Single Contact Parts

The group of single contacts includes contact rivets, contact tips, and formed parts such as weld buttons. Contact spheres (or balls) are today rarely used because of economical considerations.

3.1.1.1 Solid Contact Rivets

Solid contact rivets are the oldest utilized contact parts. Their manufacturing requires a ductile contact material and is done without scrap on fully automated special cold heading machines. A wire slug is cut off and the rivet head is formed by pressing and hammering. This way contact rivets with various head configurations such as flat, domed, spherical, or pointed can be manufactured depending on the final application and switch or relay design.

• Typical Contact Shapes of Solid Contact Rivets

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• Contact Materials

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• Dimensional Ranges

Bild The respective parameters cannot be chosen independently of each other. They mainly depend on the ductility of the required contact material. Before a final decision on the dimensions we recommend to consult with the contact manufacturer.

• Qualitätsmerkmale und Toleranzen

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3.1.1.2 Composite Contact Rivets

Clad rivets for which only a part of the head (composite or bimetal rivets) or also the shank end (tri-metal rivets) are composed of contact material – with the balance of the body mostly being copper – have replaced for many applications solid rivet versions because of economical considerations. The cost savings depend on the contact material and its required volume for a specific application. These composite rivets are also produced scrap-less from wire material on special machinery with two process variations utilized.

During cold bonding and heading the bond between the contact material and the copper is achieved without external heat energy by high plastic deformation at the face surfaces of the two wire segments (Fig. 3.1). The bonding pressure must be high enough to move the lattice components of the two metals within a few atom radii so that the adhesion forces between atoms become effective. Therefore the head to shank diameter ratio of 2:1 must be closely met for a strong bond between the two metals.

Fig. 3.1: Cold bonding of bimetall rivets (schematic)

During hot bonding the required heat energy is applied by a short term electrical current pulse (Fig. 3.2). In the case of Ag and Cu a molten eutectic alloy of silver and copper is formed in the constriction area between the two wire ends. When using metal oxide containing contact materials the non-soluble oxide particles tend to coagulate and the bonding strength between the component materials is greatly reduced. Therefore the cold bonding technology is preferred for these contact materials. The during cold bonding required high surface deformation ratio can be reduced for the hot bonding process which allows the head to shank diameter ratio to be reduced below 2:1.

For composite rivets with AgPd alloys as well as alloys on the basis of Au, Pd, and Pt the above methods cannot be used because of the very different work hardening of these materials compared to the base material copper. The starting material for such composite rivets is clad strip material from which the contact rivets are formed in multiple steps of press-forming and stamping. Similar processes are used for larger contact rivets with head diameters > 8 mm and Ag-based contact materials.

Fig 3.2. Hot bonding of bimetal rivets (schematic)

• Typical contact shapes for composite rivets

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• Contact materials

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• Base materials

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• Dimensional ranges

bild These parameters cannot be chosen independently of each other. They depend mainly on the mechanical properties of the contact material. Before specifying the final dimensions we recommend to consult with the contact manufacturer.

• Quality criteria and tolerances

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• Typical contact shapes of tri-metal rivets

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• Contact materials

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• Base materials

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• Dimensional ranges

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• Standard values for rivet dimension

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3.1.1.3 Braze Alloy Clad Contact Rivets

For special cases, especially high surrounding temperatures with high thermal and mechanical stresses during switching operations, a full metallurgical bond between the contact rivet and the contact carrier may be required to prevent a loosening of the connection and early failures of the device. To accomplish this superior bond a thin layer of brazing alloy is added to the underside of the head and the rivet shank. During assembly a thermal treatment is added after the mechanical staking.

3.1.1.4 Contact Rivets with Brazed Contact Material Layers

For certain applications contact rivets with non-ductile or brittle materials such as tungsten, silver–tungsten, or silver–graphite are required. Rivets with these contact materials can only be fabricated by brazing. Small round tips are brazed to pre-fabricated copper or steel bases using special brazing alloys in a reducing atmosphere.

3.1.2 Contact Tips

Flat or formed contact tips, welded or brazed to contact carriers, are frequently used in switching devices for higher power technology. Depending on the contact material and specified shapes these tips are produced by various manufacturing processes. The most frequently used ones are:

• Stamping from strips and profiles
• Cutting from extruded rods
• Pressing, Sintering, and Infiltrating
• Pressing, Sintering, and Re-Pressing
• Pressing and Sintering

For stamping sufficiently ductile semi-finished materials are needed. These are mainly silver, silver–alloys, silver–nickel, silver–metal oxide, and silver–graphite (with graphite particle orientation parallel to the switching surface). silver–metal oxides and silver–graphite need an additional well brazable or weldable silver layer on the underside which can be bonded to the bulk of the contact material by various processes. To further facilitate the final attachment process strips and profiles are often coated on the brazing underside with an additional thin layer of brazing alloy such as L-Ag 15P (CP 102 or BCuP-5). For Ag/C with the graphite orientation perpendicular to the switching surface the brazable underside is produced by cutting tips from extruded rods and burning out graphite in a defined thickness.

The press-sinter-infiltrate process (PSI) is used mainly for Ag/W and Cu/W material tips with tungsten contents of > 50 wt%. A silver or copper surplus on the underside of the tip later facilitates the brazing or welding during final assembly.

The press–sinter–re-press method (PSR) allows the economic manufacturing of shaped contact parts with silver or copper contents > 70 wt%. This process also alloys parts pressed in two layers, with the upper being the contact material and the bottom side consisting of pure Ag or Cu to support easy attachment.

Press–sinter processes are limited to smaller Ag/W contact tips with a Ag content of approximately 65 wt%.

• Contact materials

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• Typical contact shapes of tips and formed contact parts

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• Dimensional ranges

bild Because of the wide variety of shapes of contact tips and formed contact parts the user and manufacturer usually develop special parts specific agreements on quality and tolerances.

3.1.3 Weld Buttons

For contacts used at higher temperatures, such as for example in controls for stove tops, the use of contact rivets or the direct welding of silver based contact materials on steel or thermo-bimetal carriers is usually not feasible. For such applications weld buttons are suitable contact components.

Weld buttons are round or rectangular tips manufactured from clad contact bimetal or in some cases tri-metal semi-finished materials. The surface layer is produced from the specified contact material, the bottom weldable layer from a material with higher electrical resistivity such as steel, nickel, or for example a copper-nickel alloy. For precious metal savings a third high conductive layer of copper may be inserted between the contact material and weld backing. To improve the welding process the underside often has an embossed pattern with one or more weld projections.

The manufacturing of weld buttons from bi– or tri–metal strip requires a ductile contact material. Weld buttons with tungsten contact layers are therefore produced by brazing of tungsten discs to a weldable pre-formed base.

• Typical contact forms of weld buttons

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• Contact materials

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• Carrier materials

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• Dimensional Ranges

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Equipment for the Production of Wires, Rivets and Miniature - Profiles bild

• Quality criteria of standard weld buttons

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3.2 Manufacturing of Semi-Finished Materials

Semi-finished contact pre-materials can be manufactured from solid precious metals, precious metal alloys, or precious metal containing composite materials. They are made in wire, strip, and profile form by known processing technologies such as extrusion and subsequent annealing and drawing or roll-forming. They are supplied following the manufacturer's internal standards usually related to DIN EN specifications for copper based materials. The most important materials are two – or multiple material layered semi-finished materials with the contact material bonded in its solid phase to non-precious carriers by cladding, brazing, or welding. The contact material can also be deposited on the carrier from the liquid or vapor phase.

3.2.1 Clad Semi-Finished Pre-Materials (Contact-Bimetals)

Clad materials consist of two or more layers of different materials, the contact material and the carrier, which are firmly bonded to each other. Depending on the electrical requirements the contact material is mainly an alloy of gold, palladium, or silver based while the carrier material are mainly copper alloys. To bond these materials various technologies are utilized, the two most important ones being described in more detail below.

During hot cladding, the classic process, the materials to be clad are assembled into a cladding package in block or plate form, heated to about 800°C and clad (or “welded”) together under high pressure (Fig. 3.3). At the interface between the two materials a non-separable bond is formed by either diffusion of the reaction partners or in liquid phase by forming a AgCu eutectic alloy when an additional brazing alloy foil is placed between the two materials. Further processing is done by rolling with required annealing steps between subsequent thickness reductions. The disadvantage of this process is the usually limited short length of final material strips.

Fig. 3.3: Hot cladding of pre-materials (schematic)

In the Cold Roll-Cladding process the bond between the contact and carrier material is achieved by cold deformation of > 50% in one rolling pass (Fig. 3.4). The high plastic deformation causes cold welding in the boundary layer between the two materials. To increase the quality and strength of the bond a subsequent diffusion annealing is performed in most cases. This process is most suitable for clad semi-finished strips with thin contact material layers (> 2 μm) and large strip length (> 100 m).

Fig. 3.4: Cold roll-cladding of semi-finished strips (schematic)

• Typical configurations of clad contact strips

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• Contact materials

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• Carrier materials

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• Dimensions

bild When specifying the contact material layer thickness it is recommended to use the minimum required thickness.

• Quality criteria and tolerances

Strength properties and dimensional tolerances of clad contact bi-metals are derived from the standards DIN EN 1652 and DIN EN 1654 for Cu alloys. When specifying the width of the contact material layer it is recommended to use the minimum required value. All dimensions should be specified originating from one strip edge.

3.2.2 Brazed Semi-Finished Contact Materials (Toplay–Profiles)

The toplay process starts with a flat or profile – shaped contact material strip which is fed together with the wider non-precious carrier material and in most cases an intermediate thin foil of brazing alloy into a induction brazing machine (Fig. 3.5). An evenly distributed and reliable braze joint can be achieved this way between contact and carrier materials. The combined material strip is rather soft after the brazing process and re-hardened during a subsequent profile rolling step. In this way different shapes and configurations can easily be achieved.

Fig. 3.5: Toplay brazing with an inductive heating inline equipment (schematic)

• Typical configurations of toplay contact profiles

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• Contact materials

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• Carrier materials

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• Quality criteria, dimensions and tolerances

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Strength properties and dimensional tolerances of toplay profiles are derived from the standards DIN EN 1652 and DIN EN 1654 for Cu alloys.

3.2.3 Seam–Welded Contact Strip Materials (FDR–Profiles)

Seam–welding is the process by which the contact material in the form of a solid wire, narrow clad strip, or profile is attached to the carrier strip by overlapping or continuous weld pulses between rolling electrodes (Fig. 3.6). The weld joint is created by simultaneous effects of heat and pressure. Except for the very small actual weld joint area the original hardness of the carrier strip is maintained because of the limited short time of the heat supply. Therefore also spring-hard base materials can be used without loss of their mechanical strength. The use of clad contact pre-materials and profiles allows to minimize the use of the costly precious metal component tailored to the need for optimum reliability over the expected electrical life of the contact components.

• Typical configurations of seam–welded contact strips

and stamped parts bild Fig. 3.6: Seam-welding process (schematic)

• Contact materials

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• Carrier materials

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• Dimensions

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• Quality criteria and tolerances

Strength properties and dimensional tolerances of toplay profiles are derived from the standards DIN EN 1652 and DIN EN 1654 for Cu alloys..

3.2.4 Contact Profiles (Contact Weld Tapes)

Contact profiles span a broad range of dimensions. Width and thickness are typically between 0.8 – 8.0 mm and 0.2 – 3.0 mm resp. Special configurations, often defined as miniature-profiles or even micro–profiles can have a width < 2.0 mm.

Miniature–profiles are mostly composed of a contact-bimetal material with the contact material being a precious metal alloy or composite material clad, welded or coated by electroplating or vacuum-deposition (sputtered) onto a weldable base material. Since these profiles are attached to carrier strip materials usually by segment– or seam– welding to the base materials, materials with good welding properties such as nickel, copper-nickel, copper-tin, as well as copper-nickel-zinc alloys are used. The bottom surface of the profiles usually has formed weld rails or similar patterns to ensure a solid continuous metallurgical weld joint between the profile and the contact carrier.

Contact profiles in larger sizes are often used for switching devices in the low voltage technology. For these the contact layer mostly consists of arc erosion resistant materials such as silver–nickel, silver–metal oxides or the weld resistant silver– graphite. The brazable or weldable underside of the metal oxide or silver–graphite materials is usually pure silver with also quite often a thin layer of a phosphorous containing brazing alloy applied to aid the welding process.

• Typical configurations of multi-layer contact profiles

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• Contact materials

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• Carrier materials

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• Brazing alloy

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• Quality criteria

Beause of the variety of configurations of contact profiles usually the quality issues are separately agreed upon between the manufacturer and the user.

• Dimensions and tolerances

bild The thickness of the Au top-layer, which is sputtered for example, is between 0.2 and 5 μm, depending on the requirements. Tolerance of thickness is about + 10%.

3.3 Attachment of Single Contact Parts

The following segments give an overview of the usually applied attachment technologies for contact parts to carrier components. They include mechanical as well as brazing and welding methods used for electrical contact assemblies.

3.3.1 Mechanical Attachment Processes

Rivet staking and the insertion and forming of wire segments into pre-stamped carrier parts or strips with punched holes are the most commonly used methods for the mechanical attachment of contact materials.

Riveting (or staking) for smaller volumes of assemblies is mostly done on mechanical, pneumatic or magnetically operated presses. For larger volumes the staking process is integrated into a progressive die for fully automated assembly. Rivets are fed in the correct orientation through special feeding tracks into the staking station of the tool. To ensure a mechanically secure attachment the rivet shank must be dimensioned correctly. As a general rule the shank length of the rivet should be about 1/3 longer than the thickness of the carrier material. For switch-over contacts part of the rivet shank is formed into the secondary rivet head. To minimize deformation of the contact blade carriers, especially thin ones, this head forming is often performed by orbital riveting.

The insertion and forming of wire segments can be easily integrated into stamp and bending multi-slide tooling (Fig. 3.7). Compared to the use of composite rivets this process uses more precious contact material but for silver based contact materials these costs or often offset by higher and more efficient manufacturing speeds. For the more brittle Ag/SnO₂ materials however close attention must be paid to the danger of crack formation.

Fig. 3.7: Direct press-insertion of wire segments

3.3.2 Brazing Processes

Brazing is a thermal process for the metallurgical bonding of metallic materials in which a third metal component (brazing alloy or solder) is added. In addition a flux or processing in a protective atmosphere is applied to eliminate oxidation of the non-precious carrier. The melting range of the brazing alloy starts at the beginning of the melting (solidus temperature) all the way to complete liquid phase (liquidus temperature). This range always is below the melting points of the two materials to be joined. During the brazing process with solubility of the materials in each other diffusion processes are thermally activated by which elements of the base material diffuse into the brazing alloy and elements of the braze alloy diffuse into brazing alloy. This increases the bond strength and therefore the mechanical stability of the brazed joint.

For attachment of contact parts to carrier base materials only brazing alloys (as opposed to solders) are used. The reason is the higher softening temperature and melting point as well as higher mechanical strength and electrical conductivity of these alloys. The brazing alloys and fluxes used for electrical contact attachment are listed in Chapter 4 in more detail. Following the most frequently used brazing methods are described. References to the bond quality are given according to the test methods described in Chapter 3.4.

3.3.2.1 Flame (or Torch) Brazing

The simplest way to produce braze joints is the use of a gas torch fueled by a burning gas and air or oxygen containing gas mixes. For higher production volumes partial automation is applied. The parts to be assembled are transported after adding the suitable amounts of brazing alloy and flux through a series of fixed gas burners on a turntable or belt driven brazing machine. To limit the amount of flux or gas inclusions it is recommended to slightly move the contact tips forth and back (also known as puddeling) as soon as the brazing alloy is liquefied. The bonded area achieved in torch brazing is typically 65 – 90% of the contact foot print depending on the size and geometry of the contact tip.

3.3.2.2 Furnace Brazing

Furnace brazing is usually defined as brazing in a protective atmosphere or in vacuum. Both processes do not require the use of fluxes.

The protective atmosphere brazing is conducted in batch operation in either muffle or pot furnaces or as a continuous process in belt furnaces using a reducing atmosphere of pure hydrogen (H₂) or dissociated ammonia (H₂,N₂).

A vacuum is a very efficient protective environment for brazing but using vacuum furnaces is more complicated and rather inefficient. Therefore this process is only used for materials and assemblies that are sensitive to oxygen, nitrogen, or hydrogen impurities. Not suitable for vacuum brazing are materials which contain components with a high vapor pressure.

Parts with oxygen containing copper supports should not be brazed in reducing atmosphere because of their susceptibility to hydrogen embrittlement. Similarly contact tips containing silver–metal oxide should not be exposed to protective atmospheres because a reduction of the metal oxide even in a thin contact surface layer changes the contact properties of these materials.

3.3.2.3 Resistance Brazing

In this process the resistive heating under electric currents is the source of thermal energy. For contact applications two methods are used for resistance brazing (Fig. 3.8).

During Direct Resistance Brazing the electric current flows straight through the joint area composed of the contact tip, brazing alloy, flux, and the contact carrier. These components are secured between the electrodes of a resistance brazing machine and heated by an electrical current until the brazing alloy liquefies.

In Indirect Resistance Brazing the current flows only through one of the components to be joined (usually the non-precious contact carrier). This process allows to move the contact tip (“puddeling”) when the brazing alloy is in its liquid stage and this way remove residue bubbles from the heated and boiling flux and increase the percentage of the bonded area. Two different kinds of electrodes are used for resistance brazing:

• Electrodes from poorly conducting carbon containing materials (graphite)

The heat is created in the electrodes and thermally conducted into the joint area

• Electrodes from higher conductive and thermally stable metallic materials

The heat is created by the higher resistance in the joint area which, through selected designs, creates a constriction area for the electrical current in addition to the resistance of the components to be joined.

Graphite electrodes are mainly used for indirect resistance brazing and for joint 2 2 area > 100 mm . For contacts tips with a bottom area < 100 mm which are already coated with a phosphorous containing brazing alloy the heating time can be reduced to a degree that the softening of the contact carrier occurs only very closely to the joint area. For this “short-time brazing” specially designed metal electrodes with compositions selected for the specific assembly component pairings are used.

The bond quality for normal resistance brazing with the application of flux ranges from 70 to 90% of contact size, for short-time welding these values can be exceeded significantly.

Fig. 3.8: Resistance brazing (schematic)

3.3.2.4 Induction Brazing

During induction brazing the heat energy is produced by an induction coil fed by a medium or high frequency generator. This creates an electromagnetic alternating field in the braze joint components which in turn generated eddy currents in the work piece. Because of the skin–effect these currents and their resulting heat are created mainly on the surface of the assembly components. The distance of the inductor must be chosen in a way that the working temperature is generated almost simultaneously in the full joint area. For different contact shapes the geometry of the induction coil can be optimized to obtain short working cycles. One of the advantages of this method is the short heating time which limits the softening of the material components to be joined. Typical bond qualities of > 80% can be reached with this method also for larger contact assemblies. The widely varying working times needed for the different brazing methods are given in Table 3.1.

Table 3.1: Brazing Times for Different Brazing Methods

• Examples of brazed contact assemblies

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• Contact materials

als bild? Ag, Ag-Alloys., Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO), Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO) and Ag/C (GRAPHOR D) with 2 brazable backing, refractory materials on W -, WC- and Mo-basis

• Brazing alloys

L-Ag 15P, L-Ag 55Sn et.al. als Bild?

• Carrier materials

Cu, Cu-Alloys. et al. als Bild?

• Dimensions

Brazing area > 10 mm²

• Quality criteria

The testing of the braze joint quality is specified in agreements between the manufacturer and the user.