Stress Relaxation – Test Group 5

By Dr. Bob Mroczkowski | June 17, 2008

Dr. Bob on Test Group 5: Stress Relaxation

EIA364D Test Group 5 is a “temperature life” test. The complete generic test sequence for EIA 364D was shown in the first article in this series.

Stress-Relaxation-1“Temperature life” refers to an exposure of mated connectors to an elevated temperature for a specific amount of time. The intention of such an exposure is to evaluate the effect of time and temperature on the materials of manufacture of the connector, and to further evaluate how those effects will influence important performance characteristics of the connector. There are some standard time/temperature exposures that provide comparative performance data.

The performance characteristics chosen for evaluation in Test Group 5 are mating/unmating force according to TP7 and contact resistance, either Low Level Contact Resistance (LLCR, TP 23) or Contact Resistance at Rated Current, (CRRC, TP6).  LLCR is the more commonly used contact resistance measurement in Test Group 5.

Both of these measurements are intended to assess the effects of time and temperature on the contact normal force. Mating and unmating forces depend on contact force, though not necessarily in a straightforward fashion. Similarly, contact resistance depends on contact normal force in principle. But in practice, there are some issues to take into consideration, as will be discussed.

There are two major design/materials issues that will affect the response of a connector to temperature life exposures. One involves the connector housing material/design and the other the contact spring material/design.

The effects of the housing material will be of interest only in connector designs where the fixing of the contact spring into the connector housing may impact the contact force. For example, the contact springs in a card edge connector often latch into the connector housing in such a manner that the housing support of the spring may influence the spring deflection, and, therefore, the contact force. In such cases, thermally activated flow, or creep, of the housing may cause a decrease in the contact force over time.

In cases where the contact spring deflection is independent of any housing interactions, for example, a twin beam box-style receptacle contact, temperature life exposure effects will be completely dependent on the contact spring material and design. The effects of time/temperature exposures on contact force in such cases will be due to stress relaxation of the contact spring material. The principles of stress relaxation were discussed in a previous article in the Degradation Mechanisms series. For the purposes of this discussion, it is sufficient to note that the contact normal force due to deflection of a cantilever beam is directly proportional to the stress in the deflected beam. A similar relationship, though more complex, will apply to non-cantilever spring geometries. Given that stress relaxation is a time- and temperature-dependent decrease in stress at constant strain, constant deflection in a connector, stress relaxation will result in a time- and temperature-dependent loss in contact force during temperature life exposures.

Naturally the effects of temperature life exposures on housing materials and contact spring materials will be different. The copper alloys typically used as spring materials in connectors are much more thermally stable than the thermoplastic polymers typically used in connector housings. This difference is very significant in establishing a temperature that is suitable for a temperature life exposure in connector designs, where the housing contributes to the contact normal force. The upper limit to an accelerating temperature will be determined by the polymer selected for the housing.

Given that temperature life exposures will result in a reduction in contact force, how will such a reduction manifest itself in mating/unmating force and contact resistance? The relationship is straightforward with respect to mating/unmating forces in that a decrease in contact force will always decrease both forces. But, it is important to note, the ratio of the decreases may not be straightforward. Mating force is also influenced by frictional and geometric effects and a decrease in contact force may produce a proportionally larger decrease in mating force. Changes in unmating force may more closely reflect the change in contact force, but frictional effects will still be present.

Stress-Relaxation-2The change in contact resistance with a decrease in contact force is not straightforward.  The initial contact resistance will be determined by the initial loading, following the load curve in the figure, as the contact force deforms the contact interface creating the contact area that determines the connector contact resistance. A decrease in contact force due to stress relaxation, however, will follow the unloading curve and may not show a change in contact resistance, because the contact area established on loading will be maintained as the force decreases. If the connector is unmated and remated however, the resistance will follow the loading curve again, and the thermally induced reduction in contact force will result in an increase in contact resistance.

The reduction in contact force due to stress relaxation comes about due to changes in the shape of the contact beam. The kinetics of stress relaxation are such that atomic rearrangements of the crystal lattice occur in a direction so as to reduce the stress locally. These motions change the shape of the contact beam. A simple example is that in a tuning fork-style contact, the gap between the tines will increase due to stress relaxation. This, in turn, means that the deflection on subsequent matings is reduced and, thus, the contact force is reduced. The spring rate of the beam does not change, only the deflection.

This behavior suggests that a second LLCR measurement is advisable after unmating/mating the connector. An alternative is to introduce a disturbance and re-measure contact resistance. This alternative will provide an assessment of the reduction in mechanical stability as a result of loss of contact force. As discussed in the article on contact force in the Degradation Mechanism series, mechanical stability is the primary requirement determining the contact force necessary in a connector system.

In the next article in this series, Max Peel, Senior Fellow at Contech Research, will discuss the practice of temperature life testing.

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