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

Bild

• Contact Materials

Bild

• 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

Bild

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

bild

• Contact materials

bild

• Base materials

bild

• 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

bild

• Typical contact shapes of tri-metal rivets

bild

• Contact materials

bild

• Base materials

bild

• Dimensional ranges

bild

• Standard values for rivet dimension

bild

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

bild

• Typical contact shapes of tips and formed contact parts

bild

• 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

bild

• Contact materials

bild

• Carrier materials

bild

• Dimensional Ranges

bild

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

• Quality criteria of standard weld buttons

bilder

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

bild

• Contact materials

bild

• Carrier materials

bild?

• 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

bild

• Contact materials

bild

• Carrier materials

bild

• Quality criteria, dimensions and tolerances

bild

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

bild

• Carrier materials

bild

• Dimensions

bild

• 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

bild

• Contact materials

bild

• Carrier materials

bild

• Brazing alloy

bild

• 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

bild

• 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.

3.3.3 Welding Processes

Welding of contact assemblies has both technological and economic importance. Because of the short heating times during welding the carrier materials retain their hardness except for a very small heat affected area. Of the methods described below, resistance welding is the most widely utilized process.

Because of miniaturization of electromechanical components laser welding has gained some application more recently. Friction welding is mainly used for bonding (see Chapter 9). Other welding methods such as ball (spheres) welding and ultrasonic welding are today used in only limited volume and therefore not covered in detail here. Special methods such as electron beam welding and cast-on attachment of contact materials to carrier components are mainly used for contact assemblies for medium and high voltage switchgear.

3.3.3.1 Resistance Welding

Resistance welding is the process of electrically joining work pieces by creating the required welding energy through current flow directly through the components without additional intermediate materials. For contact applications the most frequently used method is that of projection welding. Differently shaped weld projections are used on one of the two components to be joined (usually the contact). They reduce the area in which the two touch creating a high electrical resistance and high current density which heats the constriction area to the melting point of the projections. Simultaneously exerted pressure from the electrodes further spreads out the liquefied metal over the weld joints area. The welding current and electrode force are controlling parameters for the resulting weld joint quality. The electrodes themselves are carefully designed and selected for material composition to best suit the weld requirements.

The waveform of the weld current has a significant influence on the weld quality. Besides 50 or 60 Hz AC current with phase angle control, also DC (6-phase from 3-phase rectified AC) and medium frequency (MF) weld generators are used for contact welding. In the latter the regular AC supply voltage is first rectified and then supplied back through a controlled DC/AC inverter as pulsed DC fed to a weld transformer. Medium frequency welding equipment usually works at frequencies between 1kHz to 10kHz. The critical parameters of current, voltage, and weld energy are electronically monitored and allow through closed loop controls to monitor and adjust the weld quality continuously. The very short welding times needed with these MF welding machines result in very limited thermal stresses on the base material and also allow the reliable joining of otherwise difficult material combinations.

3.3.3.1.1 Vertical Wire Welding

During vertical wire welding the contact material is vertically fed in wire form through a clamp which at the same time acts as one of the weld electrodes (Fig. 3.9). With one or more weld pulses the roof shaped wire end – from the previous cut-off operation – is welded to the base material strip while exerting pressure by the clamp-electrode. Under optimum weld conditions the welded area can reach up to 120% of the original cross-sectional area of the contact wire. After welding the wire is cut off by wedge shaped knives forming again a roof shaped weld projection. The welded wire segment is subsequently formed into the desired contact shape by stamping or orbital forming. This welding process can easily be integrated into automated production lines. The contact material must however be directly weldable, meaning that it cannot contain graphite or metal oxides.

Fig. 3.9: Vertical wire welding (schematic)

3.3.3.1.2 Horizontal Wire or Profile Welding

During horizontal welding the wire or profile contact material is fed at a shallow angle to the carrier strip material (Fig. 3.10). The cut-off from the wire or profile is performed either directly by the electrode or in a separate cutting station. This horizontal feeding is suitable for welding single or multiple layer weld profiles. The profile construction allows to custom tailor the contact layer shape and thickness to the electrical load and required number of electrical switching operations. By choosing a two-layer contact configuration multiple switching duty ranges can be satisfied. The following triple-layer profile is a good example for such a development: The top 5.0 μm AuAg8 layer is suitable to switch dry circuit electronic signals, the second or middle layer of 100 μm Ag/Ni 90/10 is used to switch relative high electrical loads and the bottom layer consists of an easily weldable alloy such as CuNi44 or CuNi9Sn2. The configuration of the bottom weld projections, i.e. size, shape, and number of welding nibs or weld rails are critically important for the final weld quality.

Because of high production speeds (approx. 700 welds per min) and the possibility to closely match the amount of precious contact material to the required need for specific switching applications, this joining process has gained great economical importance.

Fig. 3.10: Horizontal profile cut-off welding (schematic)

3.3.3.1.3 Tip Welding

Contact tips or formed contact parts produced by processes as described in chapter 3.1.2 are mainly attached by tip welding to their respective contact supports. In this process smaller contact parts such as Ag/C or Ag/W tips with good weldable backings are welded directly to the carrier parts. To improve the welding process and quality the bottom side of these tips may have serrations (Ag/C) or shaped projections (Ag/W). These welding aids can also be formed on the carrier parts. Larger contact tips usually have an additional brazing alloy layer bonded to the bottom weld surface.

Tip welding is also used for the attachment of weld buttons (see chapter 3.1.3). The welding is performed mostly semi or fully automated with the buttons oriented a specific way and fed into a welding station by suitably designed feeding mechanisms.

3.3.3.2 Percussion Welding

This process of high current arc discharge welding required the contact material and carrier to have two flat surfaces with one having a protruding nib. This nib acts as the igniter point for the high current arc (Fig. 3.11). The electric arc produces a molten layer of metallic material in the interface zone of the contact tip and carrier. Immediately afterwards the two components are pushed together with substantial impact and speed causing the liquid metal to form a strong joint across the whole interface area. Because of the very short duration of the whole melt and bonding process the two components, contact tip and carrier, retain their mechanical hardness and strength almost completely except for the immediate thin joint area. The unavoidable weld splatter around the periphery of the joint must be mechanically removed in a secondary operation. The percussion welding process is mainly applied in the production of rod assemblies for high voltage switchgear.

Fig. 3.11: Percussion welding (schematic)

3.3.3.3 Laser Welding

This contact attachment process is also one of the liquid phase welding methods. Solid phase lasers are predominantly used for welding and brazing. The exact guiding and focusing of the laser beam from the source to the joint location is most important to ensure the most efficient energy absorption in the joint where the light energy is converted to heat. Advantages of the method are the touch-less energy transport which avoids any possible contamination of contact surfaces, the very well defined weld effected zone, the exact positioning of the weld spot and the precise control of weld energy.

Laser welding is mostly applied for rather small contact parts to thin carrier materials. To avoid any defects in the contact portion, the welding is usually performed through the carrier material. Using a higher power laser and beam splitting allows high production speeds with weld joints created at multiple spots at the same time.

3.3.3.4 Special Welding and Attachment Processes

In high voltage switchgear the contact parts are exposed to high mechanical and thermal stresses. This requires mechanically strong and 100% metallurgically bonded joints between the contacts and their carrier supports which cannot be achieved by the traditional attachment methods. The two processes of electron beam welding and the cast-on with copper can however used to solve this problem.

3.3.3.4.1 Electron Beam Welding

The electron beam welding is a joining process which has shown its suitability for high voltage contact assemblies. A sharply focused electron beam has sufficient energy to penetrate the mostly thicker parts and generate a locally defined molten area so that the carrier component is only softened in a narrow zone (1 – 4 mm). This allows the attachment of Cu/W contacts to hard and thermally stable copper alloys as for example CuCrZr for spring hard contact tulips (Fig. 3.12).

Fig. 3.12: Examples of contact tulips with Cu/W contacts electron beam welded to CuCrZr carriers.

3.3.3.4.2 Cast-On of Copper

The cast-on of liquid copper to pre-fabricated W/Cu contact parts is performed in special casting molds. This results in a seamless joint between the W/Cu and the copper carrier. The hardness of the copper is then increased by a secondary forming or deep-drawing operation.

• Examples of Wire Welding

bild

Vertical Wire Welding

• Contact materials

Ag, Ag-Alloys, Au- and Pd-Alloys, Ag/Ni (SINIDUR) als bild?

• Carrier materials

Cu, Cu-Alloys, Cu clad Steel, et.al. als bild?

• Dimensions

bild Functional quality criteria such as bonded area percentage or shear force are usually agreed upon between the supplier and user and defined in delivery specifications.

Horizontal Wire Welding

• Contact materials

Au-Alloys, Pd-Alloys, Ag-Alloys, Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO), Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO), and Ag/C (GRAPHOR D)

• Carrier materials

(weldable backing of multi-layer profiles) Ni, CuNi, CuNiFe, CuNiZn, CuSn, CuNiSn, and others.

• Braze alloy layer

L-Ag 15P (CP 102 or BCUP-5)

• Dimensions

bild

• Quality criteria

Functional quality criteria such as bonded area percentage or shear force are usually agreed upon between the supplier and user and defined in delivery specifications.

Percussion Welding

• Contact materials

W/Cu, W/Ag, others

• Carrier materials

Cu, Cu-Alloys, others

• Dimensions

Weld surface area (flat) 6.0 to 25 mm diameter Rectangular areas with up to 25 mm diagonals

• Quality criteria

Test methods for bond quality are agreed upon between supplier and user

Fig. 3.13: Examples for percussion welded contact parts

3.4 Evaluation of Braze or Weld Joints

The switching properties of brazed and welded contact assemblies is strongly dependent on the quality of the joint between the contact and the carrier. The required high quality is evaluated through optical methods, continuous control of relevant process parameters and by sampling of finished products.

3.4.1 Brazed Joints

Despite optimized brazing parameters non-wetted defect areas in the braze joint cannot be avoided completely. These wetting defects can mostly be traced to voids caused by flux inclusions in the braze joint area. Depending on the shape and size of the joint areas, the portion of the fully wetted joint is between 65% and 90%. In its final use in switching devices a joint area of 80% is considered good or excellent if the individual void size does not exceed 5% of the joint area. Frequently wetted joint areas >90% with voids <3% can be obtained.

Evaluation of the quality of the joint can be performed either by destructive or non-destructive methods.

3.4.1.1 Destructive Testing

Destructive tests can be performed on a sampling basis in rather simple ways:

• De-brazing

The contact tip is being removed by slow heating and simultaneous application of force perpendicular to the contact surface area. Visual inspection of the separated components reveals the non-wetted defect areas as discoloration from either flux remnants or oxidation of the carrier material.

• Milling Sample

The contact tip is milled off in layers to a depth that makes the joint area visible for optical evaluation.

• Saw-Cutting

A crossing pattern is cut with a fine saw into the contact tip. Areas that are not bonded fall off in pieces.

• Metallographic Micro-section

In a metallographic micro-section perpendicular to the contact surface wetting defects can also be made visible (Fig. 3.14) which however are only indicative of the brazing temperature and brazing time.

Fig. 3.14: Braze joint with voids. Ag/CdO tip on Cu carrier.

• Shear test

The contact tip is sheared off from the carrier with the required shear force being a measure for the bond quality. This method is especially suitable for hard and brittle contact tip materials such as for example tungsten.

3.4.1.2 Non-Destructive Test Methods

Typically the non-destructive testing of braze joints requires more elaborate test equipment. Besides this such test methods have limitations regarding the shape of the contact tips and/or carriers. The prevalent methods are ultrasound testing and X-ray analysis.

• Ultrasonic testing

This method is based on the disruption of the propagation of sound waves in different media. High resolution modern test systems with graphic print-out capabilities and analytical software are capable to detect even small (<0.5 mm diameter) voids in the braze joint. The portion of the wetted areas is calculated as a percentage of the whole joint area. Fig. 3.15 shows an example of different braze qualities for a Ag/SnO₂ contact tip brazed to a copper carrier and illustrates the position and size of void areas as well as the final joint quality.

Fig. 3.15: Ultrasound print-out of braze joints between Ag/SnO 88/12 tips and Cu carrier with 2 different degree of wetting (dark areas = voids)

• X-Ray testing

X-ray testing is an additional method for evaluating brazed joints. Using finefocus X-ray beams it is possible to achieve a sufficient picture resolution. There are however limitations about the thickness of the contact tip compared to the size of the void area. This expensive test method is rarely used for contact assemblies.

3.4.2 Welded Joints

Since welded contact assemblies are usually produced in rather high quantities the quality of the weld joints is monitored closely. This is especially true because of the high mechanical and thermal stresses quite often exerted on the joint areas during switching operations. The quality of the joints is dependent on the process control during welding and on the materials used to manufacture the welded assemblies.

Despite the ability to closely monitor the relevant welding parameters such as weld current, voltage and energy, simultaneous testing during and after manufacturing are necessary.

A simple and easy to perform quality test is based on the shear force. Evaluations of welding assemblies in electrical performance tests have shown however that the shear force is only a valid measure if combined with the size of 2 the welded area. As rule of thumb the shear force should be > 100 N/mm with the welded area > 60% of the original wire or profile cross-sectional area. For critical applications in power engineering, for example for high currents and/or high switching frequency, a higher percentage of the joint area is necessary.

During series production every weld is usually probed in a testing station integrated in the manufacturing line with a defined shear force – mostly 20% of the maximum achievable force. In this way defective welds and missing contacts can be found and sorted out. The monitoring of the actual shear force and size is performed during production runs based on a sampling plan.

Fig. 3.16: Ultrasonic picture of a weld joint, Ag/C tip on Cu carrier (ABB-STOTZ-KONTAKT)

Besides destructive testing for shear force and weld area the non-destructive ultrasound testing of the joint quality is also utilized for welded contact assemblies (Fig. 3.16).

3.4.3 Selection of Attachment Methods

In the preceding sections a multitude of possibilities for the attachment of contact materials to their carriers was described. A correlation of these methods to the switching current of electromechanical devices is illustrated in Table 3.2. it shows that for the same switching load multiple attachment methods can be applied. Which method to chose depends on a variety of parameters such as contact material, material combination of contact and carrier, shape of the contact, required number of switching operations and last but not least the required volume of parts to be manufactured.

Based on the end application the following can be stated as general rules: Electroplated contact surfaces are limited to switching without or under extremely low electrical loads. In the lower and medium load range contact rivets and welded contacts are used. For high switching loads brazing, especially resistance and induction methods, are utilized. For extremely high loads, for example in high voltage switchgear, percussion welding, electron beam welding, and copper cast-on processes are preferred.

Table 3.2: Correlation between Contact Joining Methods and Switching Currents

3.5 Stamped Contact Parts

Stamped electrical contact parts typically consist of a base carrier material to which a contact material is attached by various methods (Fig. 3.17). They serve as the important functional components in many switching and electromechanical devices for a broad range of electrical and electronic applications. On the one hand they perform the mostly loss-free electrical current transfer and the closing and opening of electrical circuits. In addition the contact carriers are important mechanical design components selected to meet the requirements on electrical, thermal, mechanical and magnetic properties.

The increasing miniaturization of electromechanical components requires ever smaller stamped parts with low dimensional tolerances. Such precision stamped parts are needed in the automotive technology for highly reliable switching and connector performance. In the information and data processing technology they transfer signals and control impulses with high reliability and serve as the interface between electronic and electrical components.

Fig. 3.17: Plated and contact containing pre-stamped strips and stamped parts for different applications

3.5.1 Types of Stamped Parts

Stamped parts are produced as single pieces, in pre-stamped strip and comb configurations. Depending on the requirements and application the contact and base material as well as the coating and attachment technology is carefully selected.

• Coated stamped parts

Stamped parts can be selectively or completely coated with precious metal containing materials based on gold, palladium, and silver as well as non-precious materials such as tin, nickel and copper (Fig. 3.17). For stamped parts in high volumes like those used as electrical components in automobiles the carrier material is mostly coated in a reel-to-reel process starting with either solid or pre-stamped strips (see also chapter 7.1.1.3). Frequently the prestamped strip will be used directly in further automated assembly of the finished functional component. As an alternative finished stamped parts can be electroplated using barrel and rack plating methods.

Very thin coating layers with tight tolerances are deposited by electroplating. For many applications the high mechanical wear resistance is advantageous. Since even very thin layers are mostly pore-free, these coatings also act as an effective corrosion inhibitor. The type of coatings, the sequence of multiple layers, and the coating thickness, for example for connectors, are chosen according to the requirements for the end application.

• Clad stamped parts

For many applications thicker precious metal surfaces or AlSi layers are necessary. These cannot be deposited by electroplating. Besides meltmetallurgically produced materials on the basis of gold, palladium and silver, also powder-metallurgical materials are required frequently. The metallurgical bond between these contact materials and the mostly copper based substrates is achieved through various mechanical cladding methods (see also chapter 3.2.1). In this way also aluminum clad strips are manufactured in which the aluminum layer serves as the bondable surface in the interface between electromechanical connections and electronic circuits. These clad semifinished materials can be further fabricated into pre-stamped strips, in comb form, or single stamped parts (Fig. 3.18).

Fig. 3.18: Examples of clad stamped parts

• Welded stamped parts

Welded stamped parts can be fabricated by various methods (see also Chapter 3.3.3). Single contact pieces can be attached to pre-stamped or finished stamped strips as weld buttons and wire or profile segments by electrical resistance welding. Contact parts can also be stamped from seam-welded semi-finished strip. Fitting the end application contact materials based on gold, palladium and silver. Depending on the contact material and the design of the finished contact component the contact bottom surface may be consist of a weldable backing material.

• Brazed stamped parts

Brazed stamped contact assemblies are manufactured by two joining methods (see also chapter 3.3.2). The contact material is either attached by resistance or induction brazing to base metal carriers as prefabricated contact tip or they are stamped from brazed semi-finished toplay strip. It is typical for brazed contact parts that the contact material consists of silver based contact material and a good conducting copper base material with larger cross-sectional area for the usually higher current carrying capacity.

• Stamped contact parts with rivets

Riveted stamped contact parts are manufactured with the use of contact rivets which are transferred over suitable feed mechanisms correctly oriented into holes punched into the carrier (Fig. 3.19). Frequently also wire or wire segments resp. are used which are subsequently coined and formed into the desired contact shape (see also chapter 3.3.1). Both attachment methods have their distinct advantages. Using composite or tri-metal rivets allows limiting the use of precious metal custom tailored to the volume needed for specific switching requirements. For wire staking the precious metal usage is usually higher but the staking can be performed at significantly higher production rates and the additional rivet making step is eliminated.

Fig. 3.19: Examples of riveted stamped parts

• Pre-mounted component stamped parts

Components stamped parts consist of a minimum of two carrier parts which differ in their material composition and geometrical form and the contact material (Fig. 3.21). The assembly of these components as single pieces or stamping progressions is performed in a stamping die by riveting or coining. To increase the current carrying capacity at the joining area an additional welding step can be added. Depending on the requirements the different properties of the two carrier components can be combined. As an example: the high electrical conductivity of a contact carrier blade is joined with the thermal or mechanical spring properties of a second material to form a functional component. For this process both carrier base materials can also be coated with additional layers of other functional materials.

Fig. 3.20: Examples of pre-mounted stamped component parts

Stamped parts which are insert molded into or combined with plastic parts are used in electromechanical components (see Chapter 10).