Changes

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.

Typically the non-destructive testing of braze joints requires more elaborate test equipment. Besides this , such test testing methods have limitations regarding the shape of the contact tips and/or carriers. The prevalent methods are ultrasound testing and X-ray analysis.

*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 />

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.

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.

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 <xr id="fig:Ultrasonic picture of a weld"/><!--(Fig. 3.16)-->.

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 <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 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 underextremely 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.