Changes

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

=== Brazed Joints===Despite optimized brazing parameters non-wetted defect areas in the brazejoint cannot be avoided completely. These wetting defects can mostly be tracedto voids caused by flux inclusions in the braze joint area. Depending on theshape and size of the joint areas, the portion of the fully wetted joint is between65% and 90%. In its final use in switching devices a joint area of 80% isconsidered good or excellent if the individual void size does not exceed 5% ofthe joint area. Frequently wetted joint areas >90% with voids <3% can beobtained.

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

====Destructive Testing====

*Metallographic Micro-section <br />In a metallographic micro-section perpendicular to the contact surface wetting defects can also be made visible ''<xr id="fig:CdO tip on Cu carrier"/><!--(Fig. 3.14)'' --> which however are only indicative of the brazing temperature and brazing time.<br /><figure id="fig:CdO tip on Cu carrier">Fig. 3[[File:CdO tip on Cu carrier.14:jpg|right|thumb|Mechanical Braze joint with voids.Ag/CdO tip on Cu carrier.]]</figure>

====Non-Destructive Test Methods====Typically the non-destructive testing of braze joints requires more elaborate testequipment. Besides this such test methods have limitations regarding theshape of the contact tips and/or carriers. The prevalent methods are ultrasoundtesting and X-ray analysis. *Ultrasonic testing <br />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.<br />

*Ultrasonic testing <br />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. <xr id="fig:Ultrasound print-out of braze joints"/><!--(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.<br /><figure id="fig:Ultrasound print-out of braze joints">[[File: Ultrasound print-out of braze joints.jpg|right|thumb|Ultrasound print-out of braze joints between Ag/SnO 88/12 tips and Cu carrier with 2different degree of wetting (dark areas = voids)]]</figure>

=== Welded Joints===Since welded contact assemblies are usually produced in rather high quantitiesthe quality of the weld joints is monitored closely. This is especially true becauseof the high mechanical and thermal stresses quite often exerted on the jointareas during switching operations. The quality of the joints is dependent on theprocess control during welding and on the materials used to manufacture thewelded assemblies. Despite the ability to closely monitor the relevant welding parameters such asweld current, voltage and energy, simultaneous testing during and aftermanufacturing are necessary.

A simple and easy Despite the ability to perform quality test is based on closely monitor the shear force.Evaluations of relevant welding assemblies in electrical performance tests have shownhowever that the shear force is only a valid measure if combined with the size of2 the welded area. As rule of thumb the shear force should be > 100 N/mm withthe welded area > 60% of the original wire or profile cross-sectional area. Forcritical applications in power engineeringparameters such as weld current, for example for high currents voltage and/orhigh switching frequencyenergy, a higher percentage of the joint area is simultaneous testing during and after manufacturing are necessary.

During series production every weld A simple and easy to perform quality test is usually probed based on the shear force. Evaluations of welding assemblies in a testing stationelectrical performance tests have shownintegrated in however that the manufacturing line shear force is only a valid measure if combined with a defined the size of 2 the welded area. As rule of thumb the shear force – mostly 20should be > 100 N/mm with the welded area > 60% ofthe maximum achievable forceoriginal wire or profile cross-sectional area. In this way defective welds For critical applications in power engineering, for example for high currents and missingcontacts can be found and sorted out. The monitoring /or high switching frequency, a higher percentage of the actual shear forceand size joint area is performed during production runs based on a sampling plannecessary.

FigDuring 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. 3In 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.16<figure id="fig: Ultrasonic picture of a weld">[[File:Ultrasonic picture of a weld.jpg|right|thumb|Ultrasonic picture of a weld joint, Ag/C tip on Cu carrier(ABB-STOTZ-KONTAKT)]]</figure>

Besides destructive testing for shear force and weld area the non-destructiveultrasound testing of the joint quality is also utilized for welded contactassemblies ''<xr id="fig:Ultrasonic picture of a weld"/><!--(Fig. 3.16)''-->.

=== Selection of Attachment Methods===In the preceding sections a multitude of possibilities for the attachment ofcontact materials to their carriers was described. A correlation of thesemethods to the switching current of electromechanical devices is illustrated inTable <xr id="fig:Correlation between Contact Joining Methods and Switching Currents"/><!--(Tab. 3.2)-->. it It shows that for the same switching load multiple attachmentmethods can be applied. Which method to chose depends on a variety ofparameters such as contact material, material combination of contact andcarrier, shape of the contact, required number of switching operations and lastbut 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 underextremely low electrical loads. In the lower and medium load range contactrivets and welded contacts are used. For high switching loads brazing,especially resistance and induction methods, are utilized. For extremely highloads, for example in high voltage switchgear, percussion welding, electronbeam welding, and copper cast-on processes are preferred.<figure id="fig:Correlation between Contact Joining Methods and Switching Currents">[[File:Correlation between Contact Joining Methods and Switching Currents.jpg|right|thumb|Correlation between Contact Joining Methods and Switching Currents]]</figure>