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.
Main Articel: Manufacturing of Single Contact Parts

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.
Main Articel: Manufacturing of Semi-Finished Materials

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.

Main Articel: Attachment of Single 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.

Main Articel: Evaluation of Braze or Weld Joints

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).

3.5.2 Stamping Tools

For the design of stamping tools the latest CAD software systems are used. Modern stamping tools usually employ a modular design with integrated dimensional and functional controls (Fig. 3.21). Depending on the requirements on the parts and the volumes they are built with steel or carbide (-steel) inserts which are coated with a wear resistant material such as for example TiN for longer life.

A special stamping process is precision stamping for contact parts made from thin strip materials with thicknesses in the range of 0.05 – 2.5 mm. With high capacity stamping technology up to 1400 strokes/min can be reached for high volume parts. During the actual stamping operation frequently other processes such as thread-forming, welding of contact segments and insertion and forming of contacts from wire segments are integrated. Depending on the production volumes these operations can also be performed in multiples.

The quality of the tools used for stamping, like progressive dies and stamp-forming tools is important for the final precision and consistency of the parts. During high speed stamping the tools are exposed to extreme mechanical stresses which must be compensated for to ensure the highest precision over long production runs. With such high quality progressive dies parts of high precision with a cutting width of less than the material thickness and with strict quality requirements for the cutting surfaces can be manufactured. To ensure the highest demands on the surface quality of precision contact parts quite often vanishing oils are used as tool lubricants. Cleaning and degreasing operations can also be integrated into the stamping process. Additionally most stamping lines are also equipped with test stations for a 100% dimensional and surface quality control. During the design of stamping tools for electrical contacts minimizing of process scrap and the possibility to separate the precious metal containing scrap must be considered.

Fig. 3.21: Progressive die for stamped contact parts

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