Heatsink Design and Selection

Design factors which influence the thermal performance of a heat sink

Fin Arrangements

A pin fin heat sink is a heat sink that has pins that extend from its base. The pins can be cylindrical, elliptical or square. A pin is by far one of the more common heat sink types available on the market. A second type of heat sink fin arrangement is the straight fin. These run the entire length of the heat sink. A variation on the straight fin heat sink is a cross cut heat sink. A straight fin heat sink is cut at regular intervals.fin arrangements

Figure 1 Pin, Straight and Flared fin heat sink types.

In general, the more surface area a heat sink has, the better it works. However, this is not always true. The concept of a pin fin heat sink is to try to pack as much surface area into a given volume as possible. As well, it works well in any orientation. Kordyban has compared the performance of a pin fin and a straight fin heat sink of similar dimensions. Although the pin fin has 194 cm2 surface area while the straight fin has 58 cm2, the temperature difference between the heat sink base and the ambient air for the pin fin is 50 °C. For the straight fin it was 44 °C or 6 °C better than the pin fin. Pin fin heat sink performance is significantly better than straight fins when used in their intended application where the fluid flows axially along the pins (see table below) rather than only tangentially across the pins.

Comparison of a Pin Fin and Straight Fin heat sink of similar dimensions.

Heat sink fin type Width
[cm]
Length
[cm]
Height
[cm]
Surface area
[cm²]
Volume
[cm³]
Temperature difference,
Tcase − Tair [°C]
Straight 2.5 2.5 3.2 58 20 44
Pin 3.8 3.8 1.7 194 24 51

Another configuration is the flared fin heat sink; its fins are not parallel to each other, as shown in figure 1 above. Flaring the fins decreases flow resistance and makes more air go through the heat sink fin channel; otherwise, more air would bypass the fins. Slanting them keeps the overall dimensions the same, but offers longer fins. Forghan, et al. have published data on tests conducted on pin fin, straight fin and flared fin heat sinks. They found that for low approach air velocity, typically around 1 m/s, the thermal performance is at least 20% better than straight fin heat sinks. Lasance and Eggink also found that for the bypass configurations that they tested, the flared heat sink performed better than the other heat sinks tested.