What is a test?
A test is a combination of an exposure and a measurement.
What is an exposure?
An exposure subjects a connector to a test environment intended to simulate an application environment for a time intended to simulate an application “lifetime”. Tests are usually referenced by the exposure. For example, a corrosion test subjects a connector to a laboratory environment intended to drive a particular corrosion degradation mechanism. Example environments include steady state or cyclic humidity and Mixed Flowing Gas (MFG). Humidity exposures are used for tin plated connectors and MFG for noble metal plated connectors. The two major concerns about exposures are the accuracy of simulation of the intended environment and the relationship between the duration of the exposure and lifetime in the field as will be discussed.
What is a measurement?
A measurement is a quantitative evaluation of a performance parameter. For example, electrical measurements include contact resistance and dielectric withstanding voltage and mechanical measurements include mating force and retention force. A resistance measurement is made prior to and after any exposure(s) and changes in the resistance, commonly referenced as DR (R), are taken as indicative of performance.
Consider now the following simple example of a test sequence:
Contact resistance, R1(measurement)
Unmate/mate (x cycles, conditioning)
Contact resistance, R2(measurement)
This sequence is a corrosion test, MFG exposure, and change in contact resistance is the measurement. The initial resistance measurement R1 is the base measurement. The unmate/mate cycles are a conditioning process because durability cycling can lead to wear through of the plating increasing the susceptibility of the contact interface to corrosion. The number of unmating/mating cycles specified depends on the application requirements. The conditioned samples are now exposed to the appropriate application dependent MFG environment to drive the corrosion degradation mechanism. The final step is the second contact resistance measurement, R2 and the difference between R2 and R1 is the performance metric, ΔR (R) to be compared to an application or specification requirement.
A few more words on the difference between conditioning and exposure may be appropriate. Durability cycling is a conditioning step if it is followed by an exposure which is intended to validate the integrity of the contact plating, the current example. If the following step is a microscope evaluation to check for plating wear through the durability cycling is an exposure because it has caused the degradation process being monitored. Similarly, heat aging (Temperature Life), exposure to elevated temperatures, is a conditioning step if it is intended to drive stress relaxation which reduces the contact force and makes the connector more susceptible to motion and, therefore to corrosion effects. Heat aging is an exposure if it is intended to accelerate the rate of corrosion, that is, to drive the degradation directly.
Now comes the most important question:
What is the relationship between testing and performance?
Some of our previous discussion has indirectly addressed that question. Now we must be more specific. There are two major topics to be addressed. Simulation and acceleration factors.
Does the test exposure replicate the selected field degradation mechanism? In other words, after the connector has been subjected to the test environment, the exposure, does the connector experience the same degradation mechanism it experiences in the field.
Is the relationship between test severity/duration and lifetime in the field known? In other words, after the connector has been subjected to the test environment for a specified duration does the connector experience the same degradation as it would in the field after a known application lifetime?
Some examples of exposure environments and duration will clarify these statements.
The exposure for durability cycling is mating and unmating the connector. This, of course, is a direct simulation of the field as long as the test samples are fixtured appropriately. The acceleration factor is unity with respect to individual mating cycles. But the mating cycles can be performed repetitively in the tests to simulate the expected number of cycles in the application in a much shorter time period than would be experienced in the field so the exposure has a known acceleration factor. If, for example, a product is specified to have a lifetime of 50,000 hours and is rated for 500 mating cycles, the 500 mating cycles can be performed in the laboratory in, say, 5 hours giving an acceleration factor of 10,000. For completeness, it should be noted that the measurement in this test would be an evaluation for wear through of the contact finish.
Temperature Life (Heat Aging)
As noted previously, temperature life can be a conditioning step or an exposure. With respect to simulation, the simulation is direct, 105 °C is the same whether in the test chamber or in the application environment. The significant issue is acceleration factor. In this regard it is important to note that heat aging can drive a number of different degradation mechanisms. The three most relevant to connector performance are stress relaxation, corrosion and InterMetallic Compound (IMC) growth. These degradation mechanisms are all diffusion controlled and, therefore, vary exponentially with temperature.
A “rule of thumb” for acceleration factors for diffusion controlled reactions is that the reaction rate doubles for every 10 °C increment in temperature.
That is, the stress relaxation rate at 115 °C is double that at 105 °C. With respect to stress relaxation, data for many materials is available from suppliers to allow calculation of stress relaxation for given time/temperature relationships.
Both simulation and acceleration factors are more problematic in corrosion exposures. The issue with simulation is whether or not the exposure to the laboratory environment produces the same corrosion mechanisms and corrosion products that are experienced in the field. As noted in Chapter II/184.108.40.206 Noble Metal Finish Degradation Mechanisms, a number of copper alloy corrosion products, oxides, sulfides and chlorides, occur in the field. An appropriate corrosion test environment will produce the same mix of corrosion products as is experienced in the field. Acceleration factors for corrosion exposures are more complex because they include contributions due to chemical concentrations, humidity and temperature. The MFG environments developed over years of testing meet the simulation requirement and acceleration factors have been determined empirically for selected environments as will be discussed.
Mechanical Exposures: Shock and Vibration
For mechanical exposures, in particular shock and vibration, there are significant issues for both simulation and acceleration factor. The issues arise from the fact that mechanical disturbances can drive fretting motions which, in turn, can lead to the various fretting related degradation mechanisms discussed in Chapter II/220.127.116.11 Non-noble Finish Degradation Mechanisms. Simulation issues are application dependent. What sorts of excitations are active in the field; shock, short time high magnitude forces, or vibration, repetitive forces of lower and varying magnitude for example. Acceleration factors are even more problematic given that the frequencies and magnitudes of these events are variable with the application environment. In essence shock and vibration exposures are intended to assess whether a given mechanical disturbance, derived from application conditions, does or does not cause motion of the contact interface.
Let us turn now from this discussion of testing fundamentals to documentation of testing results and requirements, in other words, specifications and standards.
Specifications/standards are developed for different purposes by different organizations. For the purposes of this discussion, specifications are generally narrow in scope, being intended to define a specific product or product family. Standards are broader in scope to address market or industry needs. Some of the more important specification/ standard types are: product, customer, industry/consortium and national/international. These specifications/standards are developed by the product manufacturer, a product user, a group of product users in a certain industry and, by national or international standards organizations respectively. For example, connector manufacturers develop and test to their own product specifications to document the basic performance capabilities of their products and product families. Customers such as Dell, IBM and Intel define product specifications to address the specific needs of their applications. Industry organizations such as the Electronic Industry Association (EIA) develop standards to ensure compatibility among multiple sources to meet their market or industry needs. The next step in standards extends the concern for compatibility to national and international levels through Standards Organizations (SO) such as, nationally, Electronic Industries Alliance (EIA), Verband der Elektrotechnik (VDE), British Standards Institution (BSI), and, internationally, through International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO). Naturally, there is a commonality in philosophy and practice across these specification/standards documents as indicated through the use of common specification sequences.