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====
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.<br />
*Milling Sample<br />The contact tip is milled off in layers to a depth that makes the joint area visiblefor optical evaluation.<br />
*Saw-Cutting<br />A crossing pattern is cut with a fine saw into the contact tip. Areas that are notbonded fall off in pieces.<br />
*Metallographic Micro-section<br />In a metallographic micro-section perpendicular to the contact surface wettingdefects can also be made visible ''<xr id="fig:CdO tip on Cu carrier"/><!--(Fig. 3.14)'' --> which however are only indicativeof 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>
Fig. 3.14:Braze joint *Shear test <br />The contact tip is sheared off from the carrier with voidsthe required shear force being a measure for the bond quality.Ag/CdO This method is especially suitable for hard and brittle contact tip on Cu carriermaterials such as for example tungsten.<br />
*Shear test====Non-Destructive Test Methods====The contact tip is sheared off from Typically the carrier with non-destructive testing of braze joints requires more elaborate test equipment. Besides this such test methods have limitations regarding the required shear force beinga measure for shape of the bond qualitycontact tips and/or carriers. This method is especially suitable for hard The prevalent methods are ultrasound testing andbrittle contact tip materials such as for example tungstenX-ray analysis.
===Non*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-Destructive Test Methods==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=Typically the non"fig:Ultrasound print-destructive testing out of braze joints requires more elaborate testequipment"/><!--(Fig. 3. Besides this such test methods have limitations regarding 15)--> shows an example of different braze qualities for a Ag/SnO<sub>2</sub> contact tip brazed to a copper carrier and illustrates theshape position and size of void areas as well as the contact tips andfinal joint quality.<br /or carriers. The prevalent methods are ultrasound><figure id="fig:Ultrasound print-out of braze joints">testing and X[[File:Ultrasound print-ray analysisout 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>
*Ultrasonic X-Ray testing<br />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.<br />
=== Welded Joints===Since welded contact assemblies are usually produced in rather high quantities the quality of the weld joints is monitored closely. This method is based on the disruption especially true because of the propagation of sound waves indifferent media. High resolution modern test systems with graphic print-outcapabilities high mechanical and analytical software are capable to detect even small (<0.5 mmdiameter) voids in thermal stresses quite often exerted on the braze jointareas during switching operations. The portion quality of the wetted areas joints is calculatedas a percentage of dependent on the whole joint area. Fig. 3.15 shows an example of differentbraze qualities for a Ag/SnO<sub>2</sub> contact tip brazed to a copper carrier process control during welding andillustrates on the position and size of void areas as well as materials used to manufacture the final joint qualitywelded assemblies.
Fig. 3Despite the ability to closely monitor the relevant welding parameters such as weld current, voltage and energy, simultaneous testing during and after manufacturing are necessary.15: Ultrasound print-out of braze joints between Ag/SnO 88/12 tips and Cu carrier with 2different degree of wetting (dark areas = voids)
*XA 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 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-Ray testingsectional 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.
X-ray During series production every weld is usually probed in a testing is an additional method for evaluating brazed jointsstation 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. Using finefocusX-ray beams it The monitoring of the actual shear force and size is possible to achieve performed during production runs based on a sufficient picture resolutionsampling plan. Thereare however limitations about the thickness <figure id="fig:Ultrasonic picture of the contact tip compared to thea weld">size [[File:Ultrasonic picture of the void areaa weld. This expensive test method is rarely used for contactjpg|right|thumb|Ultrasonic picture of a weld joint, Ag/C tip on Cu carrier (ABB-STOTZ-KONTAKT)]]assemblies.</figure>
== Welded Joints==Since welded contact assemblies are usually produced in rather high quantitiesthe quality of Besides destructive testing for shear force and weld area the weld joints is monitored closely. This is especially true becausenon-destructive ultrasound testing of 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 thealso utilized for welded contact assemblies<xr id="fig:Ultrasonic picture of a weld"/><!--(Fig. 3.16)-->.
Despite === Selection of Attachment Methods===In the preceding sections a multitude of possibilities for the ability attachment of contact materials to their carriers was described. A correlation of these methods to closely monitor the relevant welding switching current of electromechanical devices is illustrated in <xr id="fig:Correlation between Contact Joining Methods and Switching Currents"/><!--(Tab. 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 asweld currentcontact material, voltage material combination of contact and energycarrier, shape of the contact, simultaneous testing during required number of switching operations and aftermanufacturing are necessarylast but not least the required volume of parts to be manufactured.
A simple and easy to perform quality test is based Based on the shear force.end application the following can be stated as general rules: Electroplated contact surfaces are limited to switching without or underEvaluations of welding assemblies in extremely low electrical performance tests have shownhowever that the shear force is only a valid measure if combined with the size of2 loads. In the lower and medium load range contact rivets and welded areacontacts are used. As rule of thumb the shear force should be > 100 N/mm withthe welded area > 60% of the original wire or profile cross-sectional areaFor high switching loads brazing, especially resistance and induction methods, are utilized. Forcritical applications in power engineeringextremely high loads, for example for in high currents voltage switchgear, percussion welding, electron beam welding, and/orcopper cast-on processes are preferred.<figure id="fig:Correlation between Contact Joining Methods and Switching Currents">high switching frequency, a higher percentage of the joint area is necessary[[File:Correlation between Contact Joining Methods and Switching Currents.jpg|right|thumb|Correlation between Contact Joining Methods and Switching Currents]]</figure>
During series production every weld is usually probed in a testing station==References==integrated 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.[[:Manufacturing Technologies for Contact Parts#References|References]]
Fig. 3.16[[de: Ultrasonic picture of a weldjoint, Ag/C tip on Cu carrier(ABBPrüfung_von_Löt-STOTZ-KONTAKT) Besides destructive testing for shear force and weld area the non-destructiveultrasound testing of the joint quality is also utilized for welded contactassemblies ''(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 3.2. 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. Table 3.2: Correlation between Contact Joining Methods and Switching Currents_und_Schweißverbindungen]]
