The Thermal Chamber
18KBObjective
The objective of a Thermal Stress Screening chamber is to provide an enclosure that heats and cools products in order to expose hidden defects. Usually it is more economical to screen several products at once, in which case as far as possible, the chamber needs to subject all of the products being tested in it to the same changes in temperature.
Airflow Distribution
The even distribution of airflow determines how evenly the temperature is distributed. Products in an area of weak airflow are subjected to slow rates of temperature change and may never reach the extremes of temperature experienced by neighbouring products that are in an area of strong airflow. This is clearly undesirable, since the level of screening then depends upon where the product is placed and some products may receive an inadequate screen, leaving their hidden defects unexposed.
Loading
Although it may be desirable to screen as many units together as possible, it is important to ensure that they are spaced to allow air to circulate between, above and beneath them, as it is the air which heats or cools the units. It is also important to take account of the direction of the chamber airflow and to ensure that the units being tested do not block it off. Air does not flow so readily near the walls, roof, floor or corners of a chamber so it is best to site the units away from these positions.
The total mass of the units being tested together with associated racking, can also affect the performance of the chamber and so limit the maximum number of units that can be tested together.
Airflow Velocity
High airflow velocity throughout the chamber helps to force air into the more inaccessible regions, making more of the chamber usable.
Simply heating and cooling the airflow does not in itself guarantee that the product reaches the same temperature as the air. There is always a lag between air and product temperatures and dwell periods are used to allow time for the product to catch up. If the dwell period is too short, the product does not catch up so dwells are usually extended as a precaution.
By far the most significant parameter in Thermal Stress Screening is the rate at which the product temperature is changed, generally the faster the better. Dwell periods in thermal stress screening are therefore usually wasteful of both time and energy. Screens can often be made much shorter and more cost effective by monitoring the product temperature and cutting short the dwell once the product is at the required temperature.
The low thermal capacity of air means that it can only carry a small amount of heat, so for the rapid temperature changes required in Thermal Stress Screening, the air is circulated at a high rate, enabling the heat that has been transferred to the product to be replenished several times per second.
High velocity airflow is also good at creating turbulent flow around a product, breaking up the boundary layer of still air that normally cocoons a product and insulates it from rapid changes in temperature.
Liquid Nitrogen
Cooling by refrigeration compressors constrains air-cooling rates to a maximum of between 20°C/minute to 10°C/minute, with a dramatic rate reduction at low temperatures. For this reason, Liquid Nitrogen cooling is used instead to achieve the very rapid cooling demanded by Stress Screening, even at low temperatures.
Size Matters
A Thermal Stress Screening chamber needs to change temperature rapidly, so its own internal mass is kep t to a minimum and this usually limits the internal volume to less than 1000 litres.