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Modeling the failure mechanism of electrical vias manufactured in thick-film technology
Keywords: vias, thick film, high currents
Hybrid-thick-film circuits consist of many different components, like screen-printed passive elements (conductors, resistors, and electrical vias), SMDs, and active elements like transistors or ICs [1, 2]. Whereas most of passive components are well investigated and described, the electrical vias remain unattended. Resistive heating caused by high current pulses might lead to the destruction of the vias. In previous work, we set up a 3D FEM model and investigated the influence of non-radial-symmetric contacting and geometric irregularities of the vias on the occurring maximum temperatures [3]. The present contribution deals with the failure mechanism of an electrical via caused by a high current pulse. When the local temperature exceeds a defined melting temperature, the metallization layer will melt and will not be available for conduction any more. The current density will rise as a consequence of the decreased cross section area of the vias and will lead to a higher heat production in a smaller area. This will conduct a further melting of the metallization layer and will result in a positive feedback accelerating via destruction. The approach of this contribution is to model the described failure mechanism in a 3D FEM model. The melting of one volume element is simulated by setting the resistivity to a value that runs against infinity when the melting temperature is reached. The resistivity is calculated in dependency of temperature, using a linear function rho(T) below the melting point and an exponential function above the melting point. The challenge was to deal with very high gradients of physical parameters. The modeling results were validated using high current measurements of electrical vias. Modeling and measurement of the voltage drop during a constant current pulse agree very well, from very low current density pulses up to pulses that lead to the destruction of the vias.
Dominique Ortolino,
Lehrstuhl für Funktionsmaterialien, Universität Bayreuth
Bayreuth, Bayern

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