How to Select a Heat Sink for a Computer Chip

As transistors get smaller and smaller, the density of processing capability in computer chips rises. Semiconductor manufacturers take advantage of this by making their chips more powerful, or making the chips smaller so that they can be used in mobile applications. Likewise the heat dissipated by these chips is increasing, which requires the need for bigger and better fans, more of them, and of course heat sinks that can aid in the dissipation. If your chip surpasses a certain temperature, it will fail and cease to operate. Finding a heat sink will be vital to keeping the chip humming and performing as it was meant to.

Difficulty: Challenging

Instructions

1. 
   Simple Thermal Model of heat transfer from a computer chip through a heat sink to the surrounding air. (click to view)

   First, we're going to create a simple thermal model of the dissipation of heat from a computer chip, through a heat sink and to the surrounding air. We can model this as a three resistors in series, where each potential is represented by a temperature. Heat flows from the center of the chip, to the interface of the chip and the heat sink, through the heat sink, and then to the ambient air.

2. The junction temperature of the die is the highest temperature of the inner part of the computer chip. If the junction temperature of the die exceeds a maximum temperature, Tj, then the chip will fail. In order to reach the surface of the computer chip, the heat must travel through the chip package, which has a thermal resistance, Rjc. These two values can be found in the specification for the chip. To find the spec, simply search google with the part number.

3. Once the heat reaches the outer surface of the chip package it must travel to the heat sink. An interface material is usually placed between the chip and the heat sink to maximize conductivity of the heat. The reason is that although the heat sink and chip surfaces appear to be smooth to the human eye, if looked at through a very fine microscope the surfaces are actually quite jagged, making it more difficult for the heat to reach the heat sink. Adding an interface material reduces the resistance to heat transfer. Different types of interface materials, like double-sided adhesive, or phase change material exist depending on the performance that you need. Determine the interface material and its thermal resistance, Ri.

4. Now that the heat has made it to the heat sink, the heat sink must actually dissipate this energy to the ambient air. The ability of the heat sink to accomplish this is dependent on how fast the heat conducts to the fins, the actual surface area available on the heat sink, and how fast the air is moving through the heat sink fins. In order to find the right heat sink, you have to determine the maximum heat sink resistance the chip can handle without exceeding its junction temperature. Using the thermal resistance model, the maximum heat sink resistance, Rhs, allowable is Rhs = (Tj-Ta)/Q - Rjc - Ri, where Tj is the maximum junction temperature of the chip, Ta is the local ambient temperature of the air, Q is the power consumption of the chip. Use the worst case temperature for the air that you can foresee.

5. Now that you know the maximum allowable heat sink resistance, you must find a heat sink that meets those requirements. First determine how fast the air is moving local to this chip. This can be calculated very accurately using computational fluid dynamic software. You can also experimentally determine this. Or you can just plain estimate the worst case. Heat sink manufacturers will provide a thermal resistance versus flow rate curve for each of their heat sinks. Find a heat sink that meets or beats the thermal resistance requirement at the given flow rate.

6. Done! Now all you need to do is find a way to attach the heat sink. If you aren't too worried about earthquakes and vibration, then you can simply use a double-sided adhesive thermal interface material. Otherwise you'll have to use special heat sink attachment clips or screws.

Tips & Warnings

• Use CFD if you can to simulate the flow of air over the board and the particular chip you are trying to spec the heat sink for.
• Try and stick with extruded heat sinks if possible, as those are the cheapest.

[edieness]

on 6/7/2009 Nice article great tips 5* and REC