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.
Contents
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
13.3 Test Procedures for the Communications Technology
Testing of the contact behavior in the communications technology is usually performed on the actual devices such as for example in relays. Experience has shown that the interaction between all design and functional parameters such as contact forces, relative movement, and electrical loads, are determining the failure mode. Therefore only statistical performance tests on a larger number of switching devices lead to meaningful results.
One must differentiate between static tests (for ex. contact resistance) and dynamic ones (for ex. electrical life). In certain electromechanical components and switching devices the contacts can be exposed to both, static and dynamic stresses (for ex. connectors, relays, switches, pushbuttons, circuit breakers). For statically stressed components the life expectancy is usually expressed as a time period, i.e. hours, while for dynamically stressed ones the expected functional life is defined as numbers of operations or switching cycles.
Main Articel: Test Procedures for the Communications Technology
13.4 Testing Procedures for Power Engineering
The testing of electrical contacts for power engineering applications serves on the one hand the continuous quality assurance, on the other one the new and improvement development efforts for contact materials. To optimize the contact and switching performance contact materials and device designs have to complement each other. The success of such optimizing is proven through switching tests.
The assessment of contact materials is performed using metallurgical test methods as well as switching tests in model test set-ups and in commercial switching devices. While physical properties such as melting and boiling point, electrical conductivity, etc. are fundamental for the selection of the base metals and the additional components of the materials, they cannot provide a clear indication of the contact and switching behavior. Metallurgical evaluations and tests are used primarily for determining material and working defects. The actual contact and switching behavior can however only be determined through switching tests in a model switch or preferably in the final electromechanical device.
Model testing devices offer the possibility of quick ratings of the make and break behavior and give a preliminary classification of potential contact materials. Since such tests are performed under ideal conditions they cannot replace switching tests in actual devices.
The electrical testing of commercially produced switching devices should follow DIN EN or IEC standards and rules. Special test standards exist for each type of switching device which are differentiated by:
- Make capacity
- Break capacity
- Electrical life
- Temperature rise
The following chapters are limited to metallurgical analysis and the testing of the most important properties of switching devices such as electrical life, temperature rise, and switching capacity.
Main Articel: Testing Procedures for Power Engineering
13.5 Corrosion Testing
13.5.1 Definition of “Corrosion”
The definition of corrosion can be found in DIN 500900 Part 1 as follows: Reaction of a metallic material with its environment, which produces a measurable change in the material and can lead to corrosion damage. This reaction is in most cases an electrochemical one. It can however also be based on chemical and metal-physics effects.
During corrosive influences metal is dissolved. This metal loss can be uniformly spread out over a certain area or be limited to locally smaller spots. This process usually proceeds with constant speed up to the total material loss, or after certain reaction times a natural corrosion limiting surface layer can be formed (i.e. on aluminum).
13.5.2 Special Types of Corrosion
- Contact corrosion:
Corrosion of a metal object after coming into physical contact with another metallic body. This can occur also on metallic impurities in alloys, on chemically and physically heterogeneous surfaces and on heterogeneous solutions on homogeneous surfaces, as well as through contacting a metal object by non-metallic materials through formation of corrosion compounds.
- Hole corrosion (pitting corrosion):
Local narrowly limited corrosion growing by dissolving material in small pin holes or craters to a depth that can lead to holes all the way through the material.
- Inter-crystalline corrosion:
Corrosion along the grain boundaries with the danger for the material to lose all its mechanical strength by decomposition (for example: at weld seams in austenitic stainless steels).
- Selective corrosion:
Preferred corrosion in specific microstructure areas (for example: loss of zinc in brasses with formation of copper enrichments).
- Air access corrosion:
Through differences in the amount of exposure to air or oxygen surface areas of a metal are becoming cathodes at the more exposed spots and therefore corrode less than those protected (for example: gap corrosion in screw or press connections).
- Tension stress corrosion:
Crack formation of stress corrosion sensitive materials which are under mechanical pull stresses while exposed to corrosive media. Especially affected are zinc containing copper alloys (brasses) under the influence of ammonia or nitrates.
- Oxygen corrosion:
Cathodic reaction in aqueous solutions forms reduced molecular, in water dissolved oxygen. Corrosion occurs when the electrochemical potential of the metal is below that of oxygen.
- Hydrogen corrosion:
Cathodic reduction of H to H2 (in acidic solutions) under conditions where the potential of the metal is less precious.
- Fretting (frictional) corrosion:
Enrichment of oxide particles of non-precious metal (especially tinned) surfaces during relative movements at small ampliyudes (< 100 μm). They occur through transfer of oscillation ot thermal displacement energy because of the difference in thermal expansion of the two contacting metals. This effect can be especially detrimental in connectors with tin plated surfaces, such as for example in automotive applications.
- Fatigue corrosion:
Fatigue fracture during repeated mechanical stresses in corrosive environments. This effect is often observed on brittle electroplated surface coatings that are exposed to repeated cycling between mechanical stresses and corrosive chemicals. Air access corrosion: Through differences in the amount of exposure to air or oxygen surface areas of a metal are becoming cathodes at the more exposed spots and therefore corrode less than those protected (for example: gap corrosion in screw or press connections).
13.5.3 Electrochemical Potentials
Corrosion effects are mainly governed by the electrode potential of the respective metals. The electrochemical potential table provides a measure for corrosion resistance. Non-precious (corrosion prone) metals are characterized by a negative, precious (corrosion resistant) metals by a positive normal potential against hydrogen.
Table 13.3: Electrode Potential of Metals
13.5.4 Corrosion Testing
The following pages describe test methods and procedures which are mainly related to the effects of environmental exposure of electrical contacts which are used in contact components for the telecommunication and information technology. Corrosion products on the surface of electrical contacts can reduce the reliability of contact making significantly by, for example, higher contact resistance, which will negatively affect the transmission of current and data signals. This can cause major problems in electromechanical contact components used in the information processing technology. Causes for the formation of tarnish film on electrical contacts include for example the presence of corrosive gases such as H2S, SO2, NOx, O3, Cl2, and NH3 (Table 13.4) in industrial environments.
