Designers must be very careful when selecting high power surface mount resistors because the burden is on them to maintain thermal control. Convection is an important factor to consider, especially if a board is enclosed. The heat that is radiated can quickly increase the ambient temperature to unsafe and damaging levels. If a board is encapsulated, it can be detrimental to the high-power component and surrounding components as it prevents transfer of heat by means of convection. It is highly recommended that high-power resistors not be encapsulated. A similar, more robust technique is to employ a thermal backplane. This technique uses a larger copper plate on the back side of the board to transfer the heat away from the circuit. Submersing a board into oil is an effective method to carry heat away from the board and attached components. Since the oil is in direct contact with every surface of the board, the conduction of heat is very efficient. This technique is limited at steady state if active thermal management is not employed. Actively cooling a board can be very effective but at a much higher cost. One common technique is to employ a chiller, which circulates coolant. This can be used with a thermal backplane, wherein the coolant circulates through channels in the plane or tubing that is thermally connected to the surface of the plane. Another technique is to employ forced air, whether conditioned or unconditioned. Applying forced air to a board is relatively easy and can be achieved at a reasonable cost.
