Testing Procedures
The procedures and standards for testing electrical contacts described below are mostly concentrated on contact applications in electromechanical devices. Since the range of applications for electrical contacts is very broad, a complete description of all relevant test procedures would extend the scope of this chapter of the Data Book. Therefore we limited the content here to contact coatings and switching contacts for information and power engineering. Because of the ongoing miniaturization of electromechanical devices the testing for effects of corrosive influences by the environment play an important role. Special testing procedures, such as these for brazed, soldered, and welded contact joints are covered already in chapter 3.
13.1 Terms and Definitions
Every technical device has to fulfill a series of requirements. Some of those which are important for agreement between contact manufacturer and user are part of DIN 40042 standard and described here in a summarized version:
- Availability (Ready-for-Use) and Reliabilty
Availability (for use) defines the probability of a system or switching device to be in a functional stage at a given time
Reliability describes the system's ability to fulfill at any time the requirements of an application within pre-defined limitations.
Both, availability and reliability, are guaranteed for a pre-determined time span and/or a specific number of switching operations. This means they warrant the life expectancy of a switching device. At the end of the live span the failure rate exceeds pre-defined limit values.
- Electrical Life
Electrical life is the number of operations reached under a given electrical load under specified operating conditions. Since the criteria which determine the electrical life of switching contacts depend on the type of switching devices they are used in, they are described in more detail under the testing procedures in information and power engineering.
13.2 Testing of Contact Surface Layers
For applications at low switching loads contact layers with thicknesses in the range of just a few micrometers are widely used. For testing such thin layers the actual coating properties must be distinguished from the functional properties. Coating properties include, besides others, porosity, hardness, and ductility. Depending on the application, the most important function properties are for example frictional wear, contact resistance, material transfer, or contact welding behavior. Besides these other technological properties such as adhesion strength, and solderability, maybe of importance for special applications like those for electronic components.
The following descriptions are mainly applicable to electroplated contact coatings which are of the most economical importance in contact applications. They also apply however in similar form to surface layers which have been created by mechanical cladding or by sputtering.
Main Articel: Testing of Contact Surface Layers