23rd International Colloquium Tribology - January 2022 219 Mounting Positions of Electrical Connectors and the Wear of Coatings under Vibration Loads Kevin Krüger Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, 32657 Lemgo, Germany Dirk Hilmert Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, 32657 Lemgo, Germany Jian Song Precision Engineering Laboratory, Ostwestfalen-Lippe University of Applied Sciences and Arts, 32657 Lemgo, Germany Corresponding author: jian.song@th-owl.de 1. Introduction During operation electrical connectors are exposed to various kinds of environmental conditions, e.g. mechanical loads and thermal cycling. Especially in automotive applications, mechanical loads impact on devices and their connectors for example by vibrations. These vibrations induce micro-motions in the contact zone between the mating parts inside the connectors and subsequently cause wear of the protective coatings. Depending on the mounting position of the electrical connectors the vibrations affect the contacts and their protective coating in different ways. Vibration of the connector-cable assembly in the mating direction induces a relative sliding movement between the mating partners. In contrast, rocking motions are induced by vibration in the orthogonal directions. The aim of this study is to analyse the influence of the direction of vibration on the degree of wear of silver coated electrical contacts. Automotive connectors are mounted on test setups in three different orthogonal directions, to replicate the vibrations along three mutually perpendicular axes. Furthermore, the connectors are stressed in vibration tests according to the TLF 0214 [1]. After performing the vibration tests, the wear of contacts in tested connectors is investigated. 2. Theory Silver coatings are particularly used in applications where connectors need to maintain low and stable contact resistance, as in the case of safety systems. These coatings inhibit fretting corrosion and enhance the performance of the contacts [2, 3]. Nevertheless, silver coatings are subjected to fretting wear which can eventually lead to a wear through of the protective coatings resulting in a failure of the contact. Thereby fretting wear is caused by relative motions between the contact partners. Vibrations in field and likewise in test conditions generate a mixed type of movement. On one hand, this is caused by relative motions between the vehicle or the excitation device (shaker) and the connector as well as between the connector housing parts and the contacts. On the other hand, micro-motions of the contacts inside the connectors are significantly influenced by movements of the attached wirings [4-6]. This study investigates the influence of the vibration direction on the degree of wear of silver coatings. Retrieved from the TLF 0214 technical guideline for validating low voltage automotive connectors the vibration directions are shown in Fig. 1. The X-direction corresponds to the mating direction whereas the Yand Z-direction are normal to the mating direction. Fig. 1: Vibration directions according to [1] 3. Experimental Setup The chosen specimens are high performance automotive connectors which are approved for the highest vibration class V6 of the TLF. The contacts have a 3 µm silver coating with 1 µm nickel underlayer and copper as base material. For the tests the specimens are mounted on three different test setups which cover the mentioned directions. The vibration tests are conducted according to V6 which defines a swept sine test in the frequency domain of 100 to 2,000 Hz with the acceleration ampli- 220 23rd International Colloquium Tribology - January 2022 Mounting Positions of Electrical Connectors and the Wear of Coatings under Vibration Loads tude rising from 15 g up to 82 g. Additionally, the specimens are subjected to a thermal cycling test following the DIN EN 60068-2-14. During testing the contact resistance is measured continuously by using the four-terminal sensing method. After completion of tests, the specimens are analysed in regard to wear of the coating. Firstly, the contact areas are optically inspected. Further, the coating thicknesses are determined using the X-ray fluorescence spectroscopy (XRF) method and the shape of the wear area is geometrically measured via the confocal microscopy (CLSM). Accordingly, the XRF is used to analyse the coating thickness in the contact zone whereas the CLSM measures the surface topology. In case of a potential wear through the contact areas are also examined by the energy-dispersive X-ray spectroscopy (EDS). The correlation between the wear depth and the vibration direction is then determined using these results. 4. Results and Discussion In test the specimens neither show an electrical failure nor a considerable rise in contact resistance according to the TLF 0214. After conducting the tests, randomly chosen contacts are removed from the housings and analysed. The optically most worn contact areas for each vibration direction are contrasted in Fig. 2. While the X-direction does only show a slight amount of wear and scratch marks from mating and unmating, the Y-direction shows a significant amount of wear. On the left side of the contact area base material is visible which indicates a wear through of the coating. In the Z-direction a certain degree of wear is visible and on the right side of the contact area a slight wear through exists. Overall, these figures represent the general tendency. Fig. 2: Most worn contact areas of each direction In Tab. 1 the coating thickness reduction for all three directions are compared to the original condition. The X-direction shows less change in the coating thickness with the maximum reduction being 0.9 µm and the range 1.3 µm. In contrast the Z-direction shows a significant reduction as the maximum is 2.6 µm and the range is 3.5 µm. The coating thickness reduction of the Y-direction is even severe with the maximum being 2.8 µm and the range 4.1 µm. Tab. 1: XRF comparison with new parts Coating thickness reduction [µm] Mean Max Range SD σ X-direction 0.0 0.9 1.3 0.3 Y-direction 0.7 2.8 4.1 0.9 Z-direction 0.5 2.6 3.5 0.6 As the X-direction neither shows potential wear through nor a substantial thickness reduction, it is considered noncritical and no further investigation is made. Specimens of the Yand Z-directions with a significant coating thickness reduction are geometrically measured via the CLSM. The depths of the most worn out pits in the contact areas are listed in Tab. 2. The Z-direction shows a maximum depth of 10.0 µm and a range of 8.9 µm. By contrast the Y-direction shows an even higher maximum depth of 24.8 µm and a range of 22.3 µm. The difference between the XRF and the CLSM is due to the better measurement resolution of the CLSM. Tab. 2: CLSM of the wear scars Depth of most worn out pits [µm] Mean Max Range SD σ Y-direction 5.8 24.8 22.3 4.3 Z-direction 3.5 10.0 8.9 1.6 Though it might be assumed from fretting corrosion tests that the X-direction is the critical direction for wear of the coatings, this does not apply to the results of the vibration tests. In fact, the snap-in locking mechanism almost entirely inhibits relative motions between the mating partners in the mating direction. However, in the Yand Z-direction the attached wires cause significant micro-motions of the contacts. This is due to the fact, that the rotation of the tab is not sufficiently damped. In both directions the wear is well-advanced and some specimens are even worn through. Compared to each other the Y-direction shows a higher influence on the wear of the protective silver coatings for the chosen connector. Nevertheless, both excitation directions are considered critical directions in general. 5. Conclusion A very good correlation between the mounting position and the degree of wear of the coatings can be ascertained. Whereas the mating direction is noncritical in regard to the wear of protective silver coatings at vibration loads, 23rd International Colloquium Tribology - January 2022 221 Mounting Positions of Electrical Connectors and the Wear of Coatings under Vibration Loads the directions normal to the mating direction show a significant amount of wear after the vibration tests. The results provide valuable information for the design of wiring harnesses and the test design for connectors. References [1] ZVEI - Zentralverband Elektrotechnik- und Elektronikindustrie e.V.: Technischer Leitfaden - TLF 0214: Validierung von Automotive-Niedervolt-Steckverbindern, 2021. [2] Fouvry, S., Jedrzejczyk, P., Perrinet, O. et al.: Introduction of a „Modified Archard Wear Law“ to Predict the Electrical Contact Endurance of Thin Plated Silver Coatings Subjected to Fretting Wear, 2012. doi: 10.1109/ HOLM.2012.6336604. [3] Yuan, H., Song, J., Schinow, V.: A Modification of the Calculation Model for the Prediction of the Wear of Silver-Coated Electrical Contacts with Consideration of Third Bodies, 2018. doi: 10.1109/ HOLM.2018.8611633. [4] Zhang, F., Flowers, G. T., Dean, R. N. et al.: A study on axial vibration-induced fretting corrosion in electrical connector pair, 2016. doi: 10.1109/ HOLM.2016.7780023. [5] Sawada, S., Saitoh, Y., Iida, K.: Deterioration Mechanism of Connectors Used in Long Driven Vehicles, 2015. doi: 10.1109/ HOLM.2015.7355087. [6] Zhang, F., Flowers, G. T., Dean, R. N. et al.: A study on axial vibration-induced fretting corrosion in electrical connector pair, 2013.doi: 10.1109/ HOLM.2016.7780023.