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Evaluation of Braze or Weld Joints

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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:
*Saw-Cutting <br />A crossing pattern is cut with a fine saw into the contact tip. Areas that are not bonded fall off in pieces.<br />
*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>
*Shear test <br />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.<br />
====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 This method is based on the disruption of the propagation of sound waves indifferent media. High resolution modern test systems 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 a percentage of 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 andillustrates 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 2different degree of wetting (dark areas = voids)
*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 />
=== 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.
== 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:
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
[[Category:Manufacturing Technologies for Contact Parts|Category]]
==References==
[[:Manufacturing Technologies for Contact Parts#References|References]]
 
[[de:Prüfung_von_Löt-_und_Schweißverbindungen]]