CFD Analysis of joining processes using reactive multilayers on native surface morphologies of microelectronic substrates
The interest in using reactive multilayers in manufacturing microelectronics has increased due to the growing need to assimilate diverse components on PCB’s with conflicting coefficients of thermal expansion. Owing to these disparate components conventional methods of manufacturing can lead to unwanted loads being induced on surrounding structures. Computational Fluid Dynamics (CFD) can be used to show the relative reduction in thermal loading, and therefore induced stresses and potential manufacturing defects, through the appropriate deployment of reactive foils compared to the thermal loading incurred during reflow soldering, for example.
The CFD model presented here is effectively a shoebox model created in ANSYS Fluent housed within a large volume of air to incorporate the heat exchange with the surroundings. The shoebox model contains several layers, namely the ceramic substrate with embedded thermocouples, a reactive multi-layer foil, solder and a silicon chip. The idea of incorporating the thermocouples into the model is to later use the temperature-time history measurements for direct comparison with experimental measurements.
The reaction, and the exothermic heat release thereof, was approximated through a user-defined function (UDF) which was used to specify the distribution of heat source terms within the reactive foil and how this propagated with respect to time. This function was effectively a probability density function (PDF), which propagated in the direction of the reaction, where the use of the PDF allowed for a reliable prescription of the peak energy intensity, the reaction width, as well as the reaction speed.
The results are able to show the solder both melting as a result of the reaction, in addition to the solidification after the reaction has taken place. Furthermore, the temperature-time profiles of the entire domain, with respect to each time step, have been ascertained and these can later be compared with experimental measurements at the respective thermocouple locations. The quantity of energy release required during the reaction, in order to melt the entire solder, was determined and this is demonstrates how the CFD approach can be a very useful design tool in determining the number of layers required to melt a certain quantity of solder, as opposed to a more expensive trial and error approach in the field.