AbstractThis research project has developed a computer-assisted methodology whereby the temporal and spatial distribution of temperature in thick film circuits fabricated on ceramic substrates may be predicted. The analogy between thermal and electrical systems is used to define a thermal structure in electrical format which is then simulated using ASTEC3 electronic analysis package.
Procedures have been developed whereby the three heat transfer mechanisms namely conduction, convection and radiation may be modelled. Models have also been proposed which allow the more important sections of a thermal structure to be analysed in finer detail. These procedures have been used hi the solution of some standard heat flow problems whose solutions have also been obtained by other more conventional techniques for comparison. Programs have been developed which facilitate the presentation of the results in the form of contour-maps or 3-D temperature distribution plots. Software has also been developed which can generate the electrical equivalent description of a device in ASTEC3 syntax. Estimates of the computing times required to carry out electro-thermal simulations of hybrid and VLSI devices have been made. The predicted computation times are feasible.
Confirmatory experiments have been carried out in large scale using partially heated samples prepared from printed circuit boards. These were heated electrically and temperature measurements were made using an infrared thermometer. These structures were modelled and simulated using ASTEC3 for comparison. It was found that for an accurate thermal analysis there was a need for reliable data for the thermal conductivity of the glass-fibre laminate and the heat transfer coefficients of convection. Experiments were designed to measure the thermal conductivity of the laminates tangential to the plane of the boards. A standard Lees' disc apparatus was also used to measure this parameter in a direction normal to the boards. A Schlieren optics apparatus was used to study the convection plumes over the surface of the plates in a horizontal position with the heated side facing upwards which provided a significant insight into the flow regime over such surfaces. Values were subsequently determined for the convection coefficients from the boards. Using the measured thermal conductivities of FR4 boards and the estimated convection coefficients, excellent agreement was achieved between the measured and simulated results.
Temperature measurements were also conducted at reduced dimensional scale on especially designed thick film resistor samples. The samples were fabricated by R.S.R.E and temperature measurements were carried out using a thermal imaging equipment manufactured by AGEMA. Again the Schlieren apparatus was used to observe the convection plumes forming over the devices which led to a better understanding of the heat transfer mechanism from such devices. These observations were then used to estimate the natural convection coefficients from the surface of horizontally positioned resistor samples which were then included in the ASTEC3 model of the devices. The subsequent ASTEC3 thermal simulation showed an excellent agreement with the measured temperature profile.
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