July 3, 2013

Design Factors in Heat Sink Turnkey Manufacturing

There are several components to consider when discussing heat sink turnkey manufacturing. One of the most important is the heat sink, especially a finned heat sink. A heat sink is a vital component in an electronic system. It allows heat to exchange passively, thereby cooling the pertinent device by causing heat to dissipate into the air. For example, heat sinks in a computer system cool the CPU (central processing unit) or the graphics processor. Heat sinks work in tandem with highly powered semiconductors like optoelectronic devices – lasers or light emitting diodes (LEDs) for example – or power transistors when the capability of the basic device to dissipate heat is insufficient to control the temperature of the device.

Thermal resistance

The concept of thermal resistance is vital in the simplification of the selection process of heat sinks in a semiconductor component of any number of widely used consumer and industrial electronic devices. This is because the hot temperature flow between the ambient air and the semiconductor die is built to be a series of heat flow resistances.

What this means is that there is an inherent resistance from:

1) The semiconductor die to the device case,

2) The device case to the heat sink, and
3) The heat sink to the ambient. The total sum of this series of resistances is the overall thermal resistance from the semiconductor die to the ambient.

Keep in mind, however, that the concept of semiconductor heat sink thermal resistance is not currently based on an exact science and is, at best, an approximation. There are certain factors that cannot be totally precise. This is because items like non-uniform heat distribution over a heat sink or device are not taken into account. It uses a model only a thermal equilibrium system, factoring in neither the inevitability of temperature change over time nor the non-linearity of convection or radiation in an environment with rising temperatures.

For example, heat sinks made from commercially extruded aluminum have a thermal resistance that range from 0.4 °C/W (large heat sink designed for TO3 devices) all the way to 85 °C/W (small clip-on sink designed for a TO92 plastic case).

Fin efficiency

Finned heat sinks are highly common. There are a number of parameters that stress the importance of high thermal conductivity materials. A sink fin should be considered a flat plate, featuring heat which flows into one end and dissipates to the outer fluid as it flows to the opposite end. As the heat energy flows through the fin, the heat loss resulting from convection in combination with the thermal resistance of the sink blocking the heat flow, will cause the fin temperature and fluid heat transfer to diminish from the base on to the end. Therefore, the efficiency of the fin can be determined to be the total heat transferred by the fin divided by the transfer of heat, if the fin were isothermal.