*4.1.1 Forced convection heat sinks*

Forced convection heat sinks are typically heat exchangers formed of finned surfaces similar to those presented in **Table 1**, where cooling air is forced through the fins using *fans*.

**Figure 8** shows such a heat exchanger fixed on a CPU board. The fan is mounted on top of the heatsink to draw air through the fin surfaces.

Depending on the type of fins configuration, the fan can also be mounted in the middle of the finned heatsink as shown in **Figure 9**.

**Figure 8.** *A fan on a heatsink mounted on a CPU board. Source [22].*

**Figure 9.** *Fan in the middle of a finned heatsink. Source [23].*

**Figure 10.** *Miniaturized forced convection heatsink module. Source [24].*

Very often, the assembly fins-fan is miniaturized in a cooling module similar to the one presented in **Figure 10**.

Forced convection heat exchangers become necessary when *Qmax* exceeds 70 W/cm<sup>2</sup> [25]: Using a fan to force air through the finned heatsink increases

#### *Heat Exchangers for Electronic Equipment Cooling DOI: http://dx.doi.org/10.5772/intechopen.100732*

significantly heat transfer to air; *convective heat transfer coefficients* can reach values as high as 3000 Wm<sup>2</sup> °C<sup>1</sup> [26]. For comparison, this coefficient is between 25 and 500 Wm<sup>2</sup> °C<sup>1</sup> for natural convection heat sinks [1].

We should underline, however, that including a fan in an electronic system will require an additional power supply. This will certainly impact the final size of the system.
