Fluid dynamics and heat transfer considerations for gel thermal interface materials (Part 1) and the underfill process (Part 2)

The assembly by squeezing flow and the thermal resistance of thin layers of one gel composite thermal interface material (TIM) with micron-sized alumina particles dispersed in a heat-curing silicone resin are studied. Thin layers (10 microns) are produced with reasonable squeezing flow pressure (20 psi) in several minutes time at room temperature and exhibit low thermal resistance in comparison to typical gel TIMs. Rheological parameters measured with a commercial viscometer are used in a fluid mechanics model to predict the squeezing kinetics. The model predicts the increase in squeezing speed obtained by increasing temperature. The thermal resistance of cured layers of various thicknesses is measured. The observed linear relationship between thermal resistance and layer thickness is interpreted in terms of the bulk effective thermal conductivity and the wall region thermal resistance. There is significant sample-to-sample variation in both of these parameters, which suggests layer defects. The measured effective bulk thermal conductivity is compared to effective medium theory prediction. The feasibility of predicting layer thermal resistance as a function of the squeezing flow procedure and the measured bulk effective thermal conductivity and rheology is assessed.; An experimental and theoretical study is reported of the capillary flow of a Newtonian liquid (mineral oil) in a Hele-Shaw cell in which the gap varies sinusoidally in one coordinate direction. Flow takes place in the direction of constant channel cross-sectional area. The geometric non-uniformity of the gap is observed to produce interface fingering. Finger length is observed to increase with decreasing spacing between plates of fixed shape, and increasing distance from the channel inlet. In the regime of interest, finger length increases slowly with increasing interface advancement, motivating a quasi-steady model. The gross interface advancement is predicted by a Lucas-Washburn model, while the local detail of interface fingering is predicted by a Hele-Shaw model of steady flow in the vicinity of the interface. The steady interface velocity in the Hele-Shaw model is set equal to the instantaneous interface velocity from the Lucas-Washburn model. The predicted fingering matches the experimentally-observed trends.