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Code: EP112

Description

The increasing packing density of microelectronics is resulting in high power densities and makes it necessary to manage and solve issues of heat removal from compact circuits in restricted space. Solutions resorting to CMOS technology are not necessarily effective in reducing power dissipation. Unwanted temperature excursions are shown to affect reliability, expansion mismatch and also performance of ICs, thick-film resistors and capacitors. Having indicated the need to control temperatures, the various mechanisms and the underlying equations and electrical analogies for heat flow by conduction, convection and radiation are presented. Practical calculations and comparisons of heat flow thermal resistances for typical materials and temperatures are very revealing of the clear advantage of heat loss by conduction. Convection ranges from poor (by laminar flow of gases) to good (using boiling liquids). Radiation is found to be negligible for terrestrial applications.

The video is liberally sprinkled with practical examples and helpful guidance for estimating and improving heat flow in many situations and applications. The analysis of one dimensional heat flow is extended to multilayered solids, aided once again by electrical analogy, and illustrated by practical examples and solutions.

Conductive heat flow analysis is extended to three dimensions both by theoretical equations and by providing simplifying models for heat spreading. Practical examples and calculations are given for thick and thin heat sources and for single and multi chip packages. Calculations and comparisons with measured thermal resistances for ICs in ceramic and plastic chip carriers show how and where effective heat sinking can be achieved. The dramatic improvement of thermal resistance of EPIC Chip Carriers by using thermal vias is shown.

Transient (rapidly time varying) power arising from power-on or power-off pulses can cause isothermal and adiabatic temperature changes. Rigorous solutions and extension of the electrical analogy to analyse heat flow behaviour are given. Thermal equivalent circuits are shown, and the manner in which time constants can be apportioned to the parts of the microelectronics assembly are discussed. An example is given of use of transient analysis to improve the design of transient protection devices.

Practical aspects of convective cooling by the use of fins are presented together with the assessments of the efficiency of fins. The use of the Biot modulus is described and fin effectiveness for different fluids and flows calculated. Finally, the more exotic techniques such as heat pipes, miniature refrigerators and microchannel cooling are described.

The lecture material enables the viewer to consider trade-offs to make his or her choices.