Board Level Heat Sinks
Board Level heat sinks are so named because they are generally attached both to the device and the PCB. Usually constructed as either a stamping or an extrusion, these heat sinks are designed for common package sizes like T0220, T0247, and D2pak. Stamped heat sink can have features that clip on to the device so that a screw or secondary clip is not required. Stamped heat sinks have bent/twisted fins to improve the thermal performance in either natural or forced convection. Aluminum stampings are anodized for improved performance in natural convection. If the heat sink is to be mounted to the PCB, then a solderable tab or pin can be attached to the heat sink.
BGA heat sinks are so named because they are mounted to BGA devices, but are actually just simple extrusions. BGA heat sinks are usually crosscut to convert the extruded fins into pins which allow them to be used in more diverse applications. The number and size of the crosscuts are dependent upon the environment. Because BGA heat sinks are generally small, the attachment of the heat sink is a challenge. The heat sinks can be either attached to the device or to the PCB depending upon the application and the mechanical requirements. Thermal tape/Epoxy is a common method usually associated with low power devices. While not the best thermal interface material, they serve their purpose.
Pushpins are another common attachment method and require two small holes in the PCB. Because the holes are generally just outside the boundaries of the device, they can make chip routing difficult. The push pin, which can be made from plastic or metal, uses the energy of a small spring to provide enough force to keep the heat sink in place.
In cases where the pushpin’s holes are not practical, anchors and wire clips can be used as an alternative. The anchors are either soldered to the PCB using two small holes per anchor or can be installed in a routed slot in the PCB. In the case of the slot, the anchor is installed from the back side of the PCB and is held in place buy the compression force in the anchor design. The advantage of the anchors is that their location is much more flexible and can be moved out of the way of traces in the PCB. A wire clip is a small diameter wire that is bent into various shapes that when deformed, engages with the anchors and applies a force to the heat sink to hold it in location. The actual shape of the wire clips is dependent upon the location of the anchors, the heat sink design, and the amount of force required. Multiple wire clips can be used with a single heat sink in applications where a higher force is required. Wire clips are removable and allow the heat sink to be reused in the case of a reworked device.
Since the pushpins and wire clips are a mechanical attachment method, it opens up the type of interface material that can be used between the heat sink and the chip which can lead to better performance.