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.

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.

*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 />

*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 />

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.