==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 switching properties of brazed and welded contact assemblies is strongly dependent on the brazejoint cannot be avoided completely. These wetting defects can mostly be tracedto voids caused by flux inclusions in quality of the braze joint area. Depending on between theshape contact and size of the joint areascarrier. The required high quality is evaluated through optical methods, the portion continuous control of the fully wetted joint is between65% relevant process parameters and 90%. In its final use in switching devices a joint area by sampling 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 beobtainedfinished products.
Evaluation === 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 quality 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 performed either by destructive ornon-destructive methodsobtained.
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<br />The contact tip is being removed by slow heating and simultaneous applicationof force perpendicular to the contact surface area. Visual inspection of theseparated components reveals the non-wetted defect areas as discolorationfrom either flux remnants or oxidation of the carrier material. *Milling SampleThe contact tip is milled off in layers to a depth that makes the joint area visiblefor optical evaluation. *Saw-CuttingA crossing pattern is cut with a fine saw into the contact tip. Areas that are notbonded fall off in pieces. *Metallographic Micro-sectionIn a metallographic micro-section perpendicular to the contact surface wettingdefects can also be made visible ''(Fig. 3.14)'' which however are only indicativeof the brazing temperature and brazing time. Fig. 3.14:Braze joint with voids.Ag<br /CdO tip on Cu carrier.>
*Shear testMilling Sample <br />The contact tip is sheared milled off from the carrier with the required shear force beingin layers to a measure for depth that makes the bond quality. This method is especially suitable for hard andbrittle contact tip materials such as joint area visible for example tungstenoptical evaluation.<br />
===3.4.1.2 Non*Saw-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 Cutting <br />A crossing pattern is cut with a fine saw into the contact tips and/or carrierstip. The prevalent methods Areas that are ultrasoundtesting and X-ray analysisnot bonded fall off in pieces.<br />
*Ultrasonic testingMetallographic 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">[[File:CdO tip on Cu carrier.jpg|right|thumb|Mechanical Braze joint with voids. Ag/CdO tip on Cu carrier.]]</figure>
This method *Shear test <br />The contact tip is based on sheared off from the disruption of the propagation of sound waves indifferent media. High resolution modern test systems carrier with graphic print-outcapabilities and analytical software are capable to detect even small (<0.5 mmdiameter) voids in the braze joint. The portion of the wetted areas is calculatedas required shear force being a percentage of measure for the whole joint areabond quality. Fig. 3.15 shows an example of differentbraze qualities This method is especially suitable for a Ag/SnO<sub>2</sub> hard and brittle contact tip brazed to a copper carrier andillustrates the position and size of void areas materials such as well as the final joint qualityfor example tungsten.<br />
Fig. 3.15: Ultrasound print====Non-Destructive Test Methods====Typically the non-out destructive testing of braze joints between Agrequires more elaborate test equipment. Besides this such test methods have limitations regarding the shape of the contact tips and/SnO 88/12 tips or carriers. The prevalent methods are ultrasound testing and Cu carrier with 2different degree of wetting (dark areas = voids)X-ray analysis.
*XUltrasonic 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)--Ray testing> shows an example of different braze qualities for a Ag/SnO<sub>2</sub> 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>
*X-Ray testing <br />X-ray testing is an additional method for evaluating brazed joints. Using finefocusX-ray beams it is possible to achieve a sufficient picture resolution. Thereare however limitations about the thickness of the contact tip compared to thesize of the void area. This expensive test method is rarely used for contactassemblies.<br />
===3.4.2 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 to perform quality test is based on the shear force.Evaluations of 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 engineering, for example for high currents and/orhigh switching frequency, a higher percentage of the joint area is necessary.
During series production every weld is usually probed in a testing stationintegrated in the manufacturing line with a defined shear force – mostly 20% ofthe maximum achievable force. In this way defective welds and missingcontacts can be found and sorted out. The monitoring of the actual shear forceand size is performed during production runs based on a sampling plan.<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>
Fig. 3.16Besides destructive testing for shear force and weld area the non-destructive ultrasound testing of the joint quality is also utilized for welded contact assemblies <xr id="fig: Ultrasonic picture of a weldjoint, Ag"/C tip on Cu carrier><!--(ABBFig. 3.16)-STOTZ-KONTAKT)>.
Besides destructive testing === Selection of Attachment Methods===In the preceding sections a multitude of possibilities for shear force and weld area the non-destructiveultrasound testing attachment of contact materials to their carriers was described. A correlation of these methods to the joint quality switching current of electromechanical devices is also utilized for welded contactassemblies ''illustrated in <xr id="fig:Correlation between Contact Joining Methods and Switching Currents"/><!--(FigTab. 3.162)''-->. 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.
===3.4.3 Selection of Attachment Methods===In Based on the preceding sections a multitude of possibilities for end application the attachment offollowing can be stated as general rules: Electroplated contact materials surfaces are limited to their carriers was described. A correlation of thesemethods to the switching current of electromechanical devices is illustrated inwithout or underTable 3extremely low electrical loads.2In the lower and medium load range contact rivets and welded contacts are used. it shows that for the same For high switching load multiple attachmentloads brazing, especially resistance and induction methods can be applied, are utilized. Which method to chose depends For extremely high loads, for example in high voltage switchgear, percussion welding, electron beam welding, and copper cast-on a variety ofprocesses are preferred.parameters such as contact material, material combination of contact <figure id="fig:Correlation between Contact Joining Methods andSwitching Currents">carrier, shape of the contact, required number of switching operations [[File:Correlation between Contact Joining Methods and Switching Currents.jpg|right|thumb|Correlation between Contact Joining Methods and lastSwitching Currents]]but not least the required volume of parts to be manufactured.</figure>
Based on the end application the following can be stated as general rules==References==[[: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, Manufacturing Technologies for example in high voltage switchgear, percussion welding, electronbeam welding, and copper cast-on processes are preferred.Contact Parts#References|References]]
Table 3.2[[de: Correlation between Contact Joining Methods and Switching CurrentsPrüfung_von_Löt-_und_Schweißverbindungen]]
