Robert D. Malucci

Vibration thresholds for fretting corrosion in electrical connectors

Single frequency vibration tests were used to induce fretting corrosion in tin alloy plated contacts. The samples used in this study were connectors consisting of 25 pairs of mated pin and socket contacts. Experimental results for a variety of vibration levels, frequencies, and wiring tie-off lengths are presented. The experiments consisted of running a series of vibration tests at each frequency where the excitation level was stepped through a range of g-levels. During each test run contact resistance was monitored as a performance characteristic. The results exhibit threshold behavior at each frequency for the onset of fretting degradation. Typically a plateau region was observed where similar g-levels produced similar fretting rates. It was also found that outside the plateau region the g-levels varied according to the dynamic behavior of the mechanical system. In addition, a transfer matrix model was used to analyze these results. An empirical fit of the data correlated well with the model when damping was used. This analysis revealed the importance of the bending moment induced at the contact interface as a result of excitation levels and tie-off configurations. Consequently, it is concluded that dynamic response of the mechanical system under various g-levels and tie off configurations can greatly impact the performance of a connector system subjected to vibration stresses.

Dynamic model of stationary contacts based on random variations of surface features

A statistical model was developed to characterize surface profiles and calculate the density of contact spots produced when rough surfaces are pressed together. This model incorporates contact force, microhardness, and real and apparent contact area as parameters which can be varied. The effects these variables have on the number of contact spots and asperity deformation were calculated. These results were subsequently used to evaluate the impact on electrical performance of aged contacts. This was accomplished using a previously developed degradation model due to the author (1990) that utilizes the concept of a third level of constriction to simulate aging. The results indicate that both contact force and geometry play important roles in electrical performance of aging contacts. The connection between geometry and apparent pressure was estimated using the Hertz theory and indicates that one can view pressure as an important vehicle as well. >

Multispot model of contacts based on surface features

A model to account for long-term degradation of stationary contacts is proposed. The model is based on introducing a third level of multispot constrictions on the surface of each asperity. The results of simulating fretting corrosion provided good agreement with existing test data. In the latter case it was found that variation of the basic parameter which represents film breakdown indicates that asperity deformation and film growth are important variables in delaying fretting degradation. This model was also used to provide statistical data on change in resistance, and appears to agree with the often observed increase in variability as contacts degrade.<<ETX>>

Impact of fretting parameters on contact degradation

Two previously developed models, dealing with contact resistance and oxide build-up under fretting conditions, were refined and combined into a single analytical model. This model was used to predict the average effects of contact force and fretting amplitude on contact degradation. The results from fretting tests on specially prepared tin plated contacts were explained within the framework of this model. It was found that decreasing contact force or increasing fretting amplitude produced increased degradation; This is explained in terms of an increase in oxide build-up due to either less asperity deformation (lower forces) or an increase in the number of asperity deformations per cycle (longer fretting amplitude). In addition, thermal shock experiments were conducted on tin plated connector contacts for two widely different temperature swings (i.e. /spl Delta/T=-40 to 105/spl deg/C and 25 to 70/spl deg/C). This data was analyzed to determine the relation between /spl Delta/T and acceleration factor. It was found that the assumed connection between /spl Delta/T and fretting amplitude was consistent with both the fretting probe data and the thermal shock tests. These results provide a basis to model degradation rate in terms of fretting cycles and /spl Delta/T. Consequently, a model was developed to estimate the acceleration factor for a given set of laboratory test parameters to simulate field degradation.

Characteristics of films developed in fretting experiments on tin plated contacts

A series of experiments were conducted to characterize oxide films that develop during fretting degradation. It was found that while the rate of change in resistance depends on contact force, the oxide film characteristics depend strongly on the number of fretting cycles. Moreover, after about 1000 fretting cycles, the oxygen content reaches a saturation level which corresponds to tin volume fractions below the percolation limit for metallic conduction (<0.4). Consequently, it is concluded that conduction is primarily due to the semiconductor properties of tin oxide (SnO). In addition, sub-micron size particles, composed of tin and tin oxide, were found dispersed over the surface. These particles had tin fractions near the percolation limit and may play a role in the mechanism that causes short-term discontinuities in contact resistance.

Accelerated testing of tin plated copper alloy contacts

Thermal cycling tests conducted on tin plated contacts were used in conjunction with materials aging data to develop a fretting degradation model. Parameters such as thermal swing, temperature extremes, dwell times and number of cycles are incorporated in the model to account for aging processes such as micro-motion, oxidation rate and stress relaxation. In addition, algorithms were developed to simulate the levels of oxidation, stress relaxation and intermetallic compound formation that occur under various field and accelerated test conditions. These algorithms and the fretting model were used to evaluate various test exposures of tin plated copper base alloy contacts in comparison to typical field conditions. The results indicate three of the four aging processes (fretting corrosion, oxidation and stress relaxation) can be simulated using standard test conditions. Moreover it was found that intermetallic compound formation could not be simulated without excessively accelerating the other processes. These results reveal oxidation rate and stress relaxation per cycle as important thermally activated processes that accelerate the rate of fretting corrosion degradation.

Modeling early stage fretting of electrical connectors subjected to random vibration

Fretting corrosion induced by vibration is a topic of major concern for automotive applications, often leading to increased contact resistance and connector failure. Presently, modeling of the behavior of connectors during fretting corrosion is a difficult matter, requiring many parameters, and is generally highly nonlinear in nature. Experimental testing of sample connectors is currently the only practical method of evaluating connector performance; however, testing can be a time-consuming and inexact task. Prior work by the authors studied the fretting behavior of connectors subjected to single frequency vibration. Correlation of experimental results with simulated behavior showed that, for the primary mode of connector interface motion observed (rocking-type motion), the relative moment at the interface was a good indicator of the observed fretting rate. It was also shown that the moment applied as the result of a given excitation level and frequency could reasonably be predicted via simulation. The current work extends this approach to random vibration profiles, which are a more realistic representation of the connector application environment. A simple model is developed which relates the early stage fretting corrosion rate to the threshold vibration levels for the connector, the dynamic characteristics of the connector/wiring configuration, and the vibration profile. A high degree of consistency between this model and the experimental data was demonstrated. Interestingly, regardless of the excitation profile applied to the overall system, the existence of a characteristic vibration threshold at the connector interface was observed.

Possible mechanism for observed dynamic resistance

A possible mechanism was proposed to explain the observance of motion induced short-term discontinuities in degraded tin plated contacts (less than 1 /spl mu/s). This mechanism requires unique conditions where cold-welded wear particles are stretched and sheared to the fracture limit during sliding. It is speculated that the release of elastic energy during fracture propagates through the surface structure at the speed of sound and causes rapid changes in contact resistance. An analysis of the microstructure was conducted and indicates this mechanism is theoretically possible. Moreover, data are provided that show incremental changes in normal force can cause counter intuitive changes in resistance. This data shows that large changes occur during loading and unloading, and it is believed that these changes are the result of micro rocking that is induced by the step loading system. It is estimated that the distances covered by the rocking range from a few to tens of microns, and the large changes in resistance are thought to result from making and breaking cold welded asperities. In addition, this data suggests that a static contact resistance threshold for discontinuities exist around the 100 m/spl Omega/ level. This is in agreement with other authors and lends credibility to the use of static contact resistance as a measure of contact stability.

The influence of contact interface characteristics on vibration-induced fretting degradation

Vibration induced fretting degradation is a widely recognized failure phenomenon; however, the basic mechanisms that control the onset and progression of such fretting behavior are not well understood and are a topic of considerable interest in the electrical connector community. One specific issue is the need for a more detailed understanding of the mechanisms controlling the fretting degradation. The present study addresses these questions and develops answers using the results from a series of experimental tests of sample connectors which are subjected to single-frequency vibration profiles at room temperature. These test specimens are a series of dual-row 16-circuit automotive connectors in which the plating finish and contact normal force are varied. The results are presented and discussed in light of earlier investigations.

High frequency considerations for multi-point contact interfaces

A statistical model based on random variations of surface features was used to estimate the resistance and capacitance of a typical multi-point contact interface. Values for clean and degraded contacts were calculated and show that, as a contact degrades, the resistance goes up and the capacitance initially rises and then falls as a film grows at the contact interface. Moreover, data were provided that show consistency with predictions from the statistical model. In addition, measurements of the skin effect on series resistance, including contact resistance, were conducted and show a power law frequency dependence of both bulk and contact resistance. While these data appear consistent with the analysis, it is believed that the measurement and analysis techniques can be improved to provide more accurate results. Moreover, the results reveal that high frequency data transmission can be affected by the impedance of a degraded contact interface. While the latter was not fully quantified, this study showed the levels where degradation may impact high frequency signal propagation. It is believed that further refinement of the techniques used in this study will help quantify high frequency effects from the impedance of a multi-point contact interface.