Simulation of particle compaction for conductive adhesives using discrete element modeling

One aspect of highly filled composite adhesive systems such as conductive adhesives which is often ignored has been the effect of compaction. Theories such as percolation deal with the description of the particle orientation on conductivity and some treatments also deal with the particle distribution, but they do not describe the dynamics or the pathway that allow the particles to arrive at that configuration. Although laboratory studies with die attach show that both particle distribution and shape will affect the conductivity, the final "settled" configuration which also impacts the conductive chain cannot be predicted using simple experimental extrapolation. In order to gain a more predictive perspective on the particle effects, Johnson-Matthey and Colorado School of Mines have collaborated on the simulation of compaction in conductive die attach adhesives using discrete element modeling (DEM). DEM computationally takes into account each discrete particle as well the interactions existing between the particles. We have found that by using DEM both particle shape and distribution may be simulated to derive an overall picture of the settled or compacted state of the material. The conductive pathways or chains may further be calculated and compared to experimental results. In addition, specific details such as particle contact area and contact force may also be derived to give a better overall picture of the quality of contact that is achieved. Particle distribution as well as particle shape are key variables considered in the simulation. Particle shape is treated by use of either spheres or flattened ellipsoids. Considerations of the simulation treatment and analysis will be discussed which correlate to experimental observations.