The Effect Of Micro Motion On Electric Contact Failure And Method Discussion

As the portable electric devices such as notebook, mobile and MP3 are popular in daily livies, the reliability becomes more and more concerned. In recent years, complaint about low-grade quality of electric devices is a hotspot, in which failure owing to low reliability is up to much ratio. Primary failure analysis shows the failed connctors are the one of main factors. So in this paper some failed connectors are selected from failed mobiles to investigate the reasons of failure through micro motion tracks and contamination on contact interface. Then, the process which causes electric contact failure is simulated by finite element method (FEM) when contamination particles move along the contact interfaces, some sources causing micro motion such as the connector structure, vibrating and shocking are also carried out and discussed.Some important and new findings are summarized and listed as follows:1) Contamination and micro motion are the main factors of failure of conectors. The effect of micro motion on contact can be summarized as follows: first, it makes the contact metal worn and torn and the basic material exposed, which decreases the performance of electric contact. Secondly, it causes fretting corrosion and oxidation. The exposed substrate or base material is easily oxided in air, especially in a corrosive environment, electro-chemistry reaction may happen owing to the electric potential difference of metals at the contact, the products of corrosion and oxidation are insulated and adhered to the contact surface, when the generated insulated film is thick enough, it increases contact resistance sharply. Finally, it causes the contamination such as dust particles outside contact area access to contact area, because the dust particles are very hard and high insulated, which can easily scrape the contact surface and increse the contact resistance. Composition of contamination at the contact illustrates that not only the plated materials elements but also the corrosion and dust elements are found.2) The different range of micro motion and various tracks at the failed contact interfaces. Wear tracks on these contact areas can be classified as 3 groups depending on their wear morphology. Group 1 is uniform wear tracks, representing about 20% in all the inspected samples, group 2 is clutter wear tracks, which about 22%, group 3 is wear without clear wear tracks. The breadth of micro motion varies with connector structures, even for the same structure, the breadth is much different. It can be concluded that the factors causing micro motion are various: therefore, failure of one connector is the combination effect of several failure reasons.3) The physical model analysis of particle contact system. By simplifying dust particles as spheric rigid, the effect of dust particle size, radius of spring, normal contact force and sliding friction cofficience on contact reliability is studied with static physical method. Result shows that particle will slide along the printed circuit board (PCB) surface when the radius ratio of particle and spring is larger than square of sliding friction cofficience between particle and PCB (that is r/R≥f~2), furthermore, particle will also slide relatively to spring, which would cause contact break and finally connector failure between spring and PCB; if the radius ratio is smaller than the value of equation, whether the particle slides or not depending on the value and acted direction of normal contact force. Those particles staying at the contact surface will cause the decrease of contact force between spring and PCB.4) The dynamic finite element method simulation of particle at the contact. For rigid particle, the value and acted direction of normal contact force and particle morphology are the main factors that influence electric contact reliability. When particle is entirely embedded into contact surface under pressure, it can obtain good contact between spring and PCB; but when partcle is embedded in part, in the condition that relative sliding happen among particle or particle just staying below the spring, which can cause the spring break away from the PCB, as a result, connector failure happens. With respect to soft particle, because of deformation and smash under the pressure of contact force, the debris sticks to the contact surface; spring can pass through during the course of sliding, so the effect on electric contact reliability depends on the thickness of film formed on contact surface.5) Primary research in origin of micro motion. Result illustrates that the fluctuation of normal contact force is the main reason causing micro motion through the analysis of connector structure with FEM, and the breadth of micro motion is also related to connector structure. The simulation result shows that the natural frequency of connector is far away from that of operating condition and environment surround, so little effect is brought on reliability with respect to vibration. Then, an impact model is set up to analyse the effect of shock by use of the relative impact theory and FEM, and some relative experiments are carried out, both simulative and experimental results show that the micro motion tracks formed on the contact surface falling off from 0.2 meter is equal to that on failed contact surface, the breadth of micro motion is proportional to the square root of drop height.