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A frictional model for copper wire bonding
Keywords: wire bonding, process modeling, frictional model
To ensure a high reliability in IGBT power modules it is important to have good electrical and thermal connections between base plate, DCB, IGBT chips and diodes as well as the frame with its connectors. On the one hand, soldering and in the nearer past sintering processes are understood and show a high process stability. On the other hand, heavy wire bonding as the frequently used interconnection technology in the assembly of power modules is still not fully understood. Although, there have been some approaches on describing the bonding process [Ding2006,Gaul2010(2)], these were mainly limited to a determined tool, wire and substrate [Gaul2010(1)] or were too universal for specific investigations [Hu2006(1),Hu2006(2)]. Therefore, state of the art today are time-consuming DoE (design of experiments) to identify robust bonding parameters. The interactions between these parameters are widely unidentified. This leads to increased problems in the case copper instead of aluminum wire is used, because of the considerable smaller process window. To generate a profound understanding of the wedge/wedge heavy wire bonding process with copper wire, the basic physical procedures have to be described. This interconnection technology is friction activated and therefore the contact characteristics between wire and substrate should be investigated. Demanded is a contact model to predict the first and subsequent arising micro welds between the contact bodies. Here, so called Point Contact Elements [Sextro2007] are used which represent single subareas based on discretization of the contact area between wire and substrate. These Point Contact Elements can be used to describe micro slip, which means some areas inside the contact are sliding while other elements are still sticking. A complete sliding of the wire can easily be described, too. Based on the dissipated energy which results from the model a minimum energy for creating micro joints can be determined by experiments. The output of the model is a prediction of the welded area and therefore a clue of the durability of this bond is given. To calculate the frictional energy, the time invariant variables like contact size and normal pressure must be given as an input for the Point Contact Element Model. Based on this information, the frictional energy and the resulting hysteresis curve for the contact can be calculated. To get accurate results from the model, the pre-deformation phase, the tool geometry, the used bonding parameters and the wire characteristics have be taken into account. The first two parameters can be obtained by finite element modeling. The advantage of this model is that also inclined substrates can be calculated if this geometric boundary is taken into account while doing the FEM calculation as well as all different wires and substrates. A drawback is the adjustment of model parameters for changing substrate and wire, because of the changing contact stiffness as well as a new calculation in case of a tool change. The main advantage of this model is to optimize the system behavior, for instance by choosing the correct excitation of the tool or using the best tool geometry.
Simon Althoff, Research Assistant
University of Paderborn, Chair for Mechatronics and Dynamics
Paderborn, NRW

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