Designers have to manage airflow carefully when solving the task of cooling heavily populated PCBs or Printed Circuit Boards. For effective cooling, the movement of airflow along the board is important. Two factors play a crucial role when selecting and qualifying heat sinks for dense PCB applications. The first factor is thermal resistance and the other pressure drop.
Thermal resistance is the increase in temperature in degrees Celsius for every watt and it measures how effectively the heat sink transfers heat from the heat-generating device to the ambient environment. A better heat sink has a lower thermal resistance. In other words, a better heat sink offers lower resistance to heat flow, such that it can cool hotter objects faster. Another way to say the same thing is heat sinks with lower thermal resistance cool hot devices before they reach their maximum allowed temperature. For any heat sink, its thermal resistance value depends on the airflow over the heat sink. Faster airflow results in lowering the thermal resistance values.
Pressure drop, the other factor, is the resistance the air faces while moving through the fins of the heat sink. This is the difference in the airflow speed as it enters the array of fins of the heat sink to the airflow speed of the air as it exits the array. If the pressure drop is more, it means the heat sink is taking up more air for its own heat removal, leaving less air for the other devices on the board. Therefore, with heavily populated boards, designers must balance the low-pressure drop and the low thermal resistance for a heat sink. This requires a good understanding of the relationship among pressure drop, airflow, heat sink fin-density and heat sink performance.
The performance of a heat sink is highly dependent on the speed of the air stream approaching the heat sink. Air, being a bad conductor of heat, tends to move slowly while circulating on the surface of the heat sink. A faster air stream pushes the hot air away from the heat sink and lets in cooler air, resulting in better heat removal. For heat sinks that have many fins, air remains trapped within the fins because the low speed of the incoming air does not move the hot air away from the fins. Therefore, to enable a densely populated heat sink perform efficiently, forced air cooling is necessary; this improves thermal resistance more than 100 times in certain heat sinks, compared to natural convection cooling – that is, cooling without airflow.
When the board contains only one heat sink, the selection is simple as the thermal resistance of the heat sink is the predominant factor. For multiple heat sinks on a board, the allocation of available airflow along the board takes predominance. All heat sinks on a densely populated board, apart from affecting the device that each resides on, also affects the neighboring devices. Therefore, for proper selection of a heat sink depends on both its thermal resistance and its pressure drop.