Experimental research on electrical resistance of microcontacts

Transcription

1 Experimental research on electrical resistance of microcontacts Cristina Iliuţă, Viorel Ciornei Ştefan cel Mare University, Suceava, Romania Abstract: Contact resistance between two surfaces is dependent on conductive properties of the materials, applied load, current and geometry of the contacting bodies. The present paper illustrates some experimental investigations conducted in order to measure electrical contact resistance for gold plated microcontacts in a dynamic loading regime and for a constant current. Keywords: experimental device, microcontact, electrical contact resistance, contact load, current. I. Introduction An electrical contact is established between two conductive electrodes by pressing them against each other, thus creating a resistance, known as contact resistance. This resistance must have low values and most importantly, it must be stable in time so that a fake contact will not occur. Connector quality and its conduction characteristic can be assessed by measuring contact resistance. In contacts from microelectromechanical systems, gold is usually utilized as contacting material, mostly as coatings, because it assures stable and small contact resistances while functioning. The results about electrical resistance of gold-gold contacts vary from a study to another, as the working conditions vary (air, nitrogen atmosphere, or unspecified conditions), [1-4]. In all investigations, the resistance decreases when the load increases. Studies performed by Myers et. al. [3] regarding contact resistance at various normal loads showed a decrease in contact resistance with increasing load. The experimental set-up uses a standard contact configuration, a hemispherical body with a 2.5 mm radius in contact with a flat, in dry atmospheric conditions. Experimentally obtained contact resistance diagrams for gold, allied with cobalt, placed on a Ni substrate, loaded with 1 50 cn, are in agreement with the theoretical model advanced by Leidner et. al. in [5]. Hannoe and Hosaka [4] showed that at low contact forces (1-2 mn), the resistance of gold contacts is about 0.1 Ω. Glovnea [6] investigated electrical resistance for applied loads up to 255 mn and constant current up to 1 A for gold plated microcontacts between a spatial surface pressed against a flat surface. Experimental investigations showed that at least for low currents conductance doesn t vary linearly with the contact area, but with contact perimeter. In the case of higher currents, the dependence becomes linear with the contact area, as shown in [6]. This work attempts to find experimentally the dependence between electrical contact resistance and contact loading force for a constant current through the contact. II. Electrical contact resistance The contact resistance is the electrical resistance that current has to overcome when passing through a closed contact. Whereas ideal contacts would not show any resistance, real contacts do [7]. Electrical contact resistance is determined by current lines constriction and perturbatory surface layers [8]. Therefore, contact resistance is given by the sum of two resistances: R = R + R, (1) c f where R is the contact resistance, R c is the constriction resistance and R f is the film resistance. The constriction resistance is the increase of resistance for metallically clean contacts due to the constriction of the electrical current when being forced through a small, effective contact area [7]. Due to roughness, the contact between

2 surfaces is made only on a few relatively small points. By increasing the load, the actual contact area increases due to microasperities elastic and plastic deformations. The larger the contact area and contact pressure, the lower the constriction resistance is, [7]. The constriction resistance is given by Holm [9]: R s ρ =. (2) 2a where ρ is the contact material resistivity, and a is the radius of the contact circular area, given by Hertz s formula, [10]. III. Experimental set-up In order to accomplish the proposed experimental investigations, a mechatronics device was conceived and built in order to allow loading of a sphere-plane contact and to deliver the current through it. The device consists of: a casing, electronic circuit that includes the loading circuit, voltage-current converter and power supply block, an actuator from a hard disk drive and a cell phone contact, as is shown in Figure 1, [11]. The contacting bodies consist in a flat electrode, inferior, and one hemielllispoidal, superior. The electrodes are gold plated microcontacts from mobile phones. The hemiellipsoidal electrode is attached to the read head of a hard disk drive and the flat electrode is fixed on a support. From each electrode are connected two wires which form a Kelvin bridge for measuring the contact resistance. This configuration eliminates the electric resistance of the connecting wires. Figure 2 illustrates schematically the way contact resistance is measured, based on a bridge method. The contact resistance is found as the ratio between voltage, visualized by aid of an oscilloscope, and contact current. I Figure 2. Contact resistance measuring principle The microtopography of the contacting surfaces was determined by aid of laser profilometry, using a μscan profilometer produced by NanoFocus. Topography measurements aimed to gain information regarding surface roughness and curvature radii. A 3D image of one of the contacting bodies as measured is given in Figure 3, [12]. It was found that the contacting surface isn t spherical, but rather ellipsoidal. The curvature radii in two planes, reciprocal perpendicularly, are 1 μm and 192 μm respectively. F V Figure 1. Electrical parts of the experimental setup (a), hard drive actuator and microcontact (b) [11]. Figure 3. Contact surface microtopography [12]

3 Using the measured curvature radii in a Hertz [10] model of the contact, the half axes of the contact area were determined for different loading levels. The obtained values are typical to microcontacts. IV. Results and discussions In order to achieve variable loading of the contact (triangular loading), the input of the amplifier from the loading circuit is linked to a function generator. The amplifier output is connected to the hard drive s head motion mechanism. The applied load is determined by measuring the voltage drop at the terminals of the resistor placed at amplifier s output, which is in a series connection with the hard drive s coil. This voltage is then visualized by aid of a digital oscilloscope. Diagrams of the voltage drops on the contact were raised for constant currents of 0.25 A and 0.75 A, and for loads varying in triangular impulses at frequencies of 3, 5 and, with a positive minimum value. The contact load varied from 0.11 N up to 0.41 N. The diagrams stored by the oscilloscope were then transferred to a computer using WaveStar for Oscilloscopes software. Typical results of the visualized waveforms stored by the oscilloscope are illustrated in Figure 4. Using the applied signal diagrams the contact load is computed, and contact resistance is then determined as the ratio of voltage drop on the contact to the applied current. Each contact resistance value is determined as an average of five measurements. To check if at the microscale the electrical contact resistance depends on the area or circumference of the contact, these dependencies resulting experimentally were plotted graphically. It is known that contact area radius is directly proportional to the applied load at a subunit power. In the elastic deformations domain, the radius is proportional to the load at power 1/3. Also, it is known that resistance is linearly dependent on contact radius. As a result, contact resistance will have a linear dependence on de 1/3 power of the applied load. For a better visualization of this dependency, graphic representations of conductance are preferred, that is the inverse of the contact resistance by report to one the contact area half-axes. In consequence it is preferred to graphically trace experimentally determined conductance as a function of the 1/3 power of applied load. Figure 4. Typical results of load force and contact voltage for constant current of 0.25 A (a) and 0.75 A (b). Figure 5 illustrates such a dependence of conductance to the 1/3 power of force, in a variable loading regime at various frequencies and constant currents A Contact force [N] ^1/3

4 0.75 A 0.75 A Contact force [N] ^1/3 Contact force [N] ^2/3 Figure 5. Conductance - contact radius dependence diagrams for currents of: 0.25 A (a), 0.75 A (b) It can be noticed that the variation is linear over the entire loading domain at low currents and linear on some regions of loads range at high currents. Contact loading frequency has an important influence on the conductance-load diagram, in that at same load and current, contact resistance is inversely proportional to the frequency. In order to visualize contact resistance dependence on contact area, Figure 6 graphically illustrates conductance as a function of 2/3 power of load. In this case the variation is no longer linear over the entire loading domain regardless to currents A Figure 6. Conductance versus contact area for currents of: 0.25 A (a), 0.75 A (b). V. CONCLUSIONS In order to investigate experimentally the dependence between contact resistance and contact parameter, an electronic device was employed, that applies loads and currents on the studied contact. Various diagrams were traced for the variation of voltage drops on the contact for constant current and triangularly variable loads at different frequencies, with a positive minimum. Since the contact resistance is inversely proportional to contact circumference, which depends on a subunit power of the load, electrical conductance curves were raised as functions of subunit powers of load. Contact conductance increases in a linear fashion with a subunit power of the load, either over the whole loading domain, or over portions of it. This aspect results from the fact that contact circumference increases with applied load. It was confirmed that for the studied contact dimensions, contact resistance is inversely proportional to contact circumference. Also, it was shown that contact resistance varies inversely proportional to frequency of load. VI. References Contact force [N] ^2/3 [1] A. Malczewski, et.al., 1999, X-Band RF MEMS Phase Shifters for Phased Array Applications, IEEE Microwave Guided Wave Lett. 9, pp

5 [2] Z. Li et. al., 2000, Bulk micromachined relay with lateral contact, J. Micromech. Microeng. 10, pp [3] M. Myers, M. Leidner, H. Schmidt, H. Schlaak, 2008, Extension and Experimental Verification of a New First Contact Method to Model Performance of Multilayer Contact Interfaces, Proc. of the 54th IEEE HOLM 2008 Conference on Electrical Contacts. [4] S. Hannoe, H. Hosaka, 1996, Electrical characteristics of micro mechanical contacts, Microsystem Technologies, vol. 3, pp [5] M. Leidner, M. Myers, H. Schmidt, H. Schlaak, 2008, A New Simulation Approach to Characterizing the Mechanical and Electrical Qualities of a Connector Contact, ICEC 2008 Proceedings, June 2008, pp [6] Glonvea, M., 2006, Investigations upon micro and nanocontacts with MEMS applications (in Romanian), Research Report, Grant CNCSIS, [7] df. [8] Hortopan, G., 19, Electrical Apparatus. Principles and applications (in Romanian), 3th edition, Editura Didactică şi Pedagogică, Bucharest. [9] Holm, R., 2000, Electrical Contacts Theory and Application, 4th edn, Springer Verlag, Berlin, Heidelbrg, New York. [10] H. Hertz, Über die Berührung fester elastischer Körper (On the contact of elastic solids). J. Reine und angewandte Mathematik 1882; 92: [11] Ciornei, C., Diaconescu, E., Pintilie, D., 2008, Preliminary experimental research on electric contact resistance, VAREHD 14, Suceav [12] Ciornei, C., 2010, Cercetări experimentale preliminarii privind rezistenţa electrică a contatelor şi micro-/nanocontactelor, Referat în cadrul doctoratului, Suceav Cristina CIORNEI (ILIUŢĂ) PhD student, Ştefan cel Mare University, Faculty of Mechanical Engineering, Mechatronics and Management, PhD domain/thesis Mechanical Engineering/ Research on improving electrical resistance of contacts and micro-/nanocontacts, PhD superviser: Prof. eng. Ioan MIHAI, PhD.