Thermal Simulation of a 34-140 Mbit/s Optical Fibre Terminal - The Brio Rack

This paper describes the thermal analysis performed at the French Telecommunications Research Centre, regarding a new mechanical rack used for an optical fibre terminal: the BR 10 rack. BR 10 is constructed in the form of a beam to which receivers and transmitters are fastened. Each unit is fitted with two printed circuit boards, which are plugged in horizontally. An experiment was first conducted to analyse the main physical heat transfer paths. Measurements made with thermocouples at different locations in the rack showed that the air temperature inside the units was independant of the height at which the unit was inserted in the rack. The thermal simulation was therefore restricted to a single set of receiver-transmitter. The mathematical model which was set up is based on nodal analysis. Analysing experimental results permitted to validate the assumptions made to obtain a simple thermal resistive network. Such a thermal model was of a particular interest to extrapolate the temperature map inside the units, which different physical configurations. Several modifications were studied, among which were - increase in heat dissipation - addition of fins on the front side of the unit - change of the type of unit coating - addition of a thermal link between the unit and the most dissipating chips. Besides, using a finite element computer code dedicated to the thermal analysis of integrated circuits (computer code ATCOMP), it was possible to test the system reliability by evaluating the junction temperature of the most critical elements.