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Technical Note

Completing Your Thermal Project
A macroscopic analysis selects the right fan to keep the average box temperature within the specified limit while pumping out the heat generated inside. Next we have to consider microscopic temperature distribution within the box. A common problem is temperture on high performance chips. They may produce several watts of heat and the concentration on the small chip area can cause the chip to overheat.

INDEK carries a complete set of components from D.C. Pator heat sinks, active coolers, Therm-A-Pipe heat pipes to customized assemblies.

Cooling Component Selection
Determined by the surface material, surrounding temperature, and airflow across the surface, there is a limit to how much heat the chip surface can dissipate. The simplest cooling solution is to increase the chip surface area by attaching a heat sink to the chip.

Each heat sink has its performance specified with a curve relating the thermal resistance to the linear airflow across the heat sink surface, e.g., for a heat sink with the performance curve in, its thermal resistance is 1°C/W when the airflow across it is 4m/s. Knowing the heat sink thermal resistance (Rth) and the surrounding temperature (Tbox), we find the CPU (with Q watts heat dissipation) casing temperature (Tcpu) using the following equation:
Tcpu=Rth(°C/W) x Q(watts) + Tbox (eq 3)

As the chip performance increases, it tends to produce more heat. For high performance chips, passive heat sinks are often too small. We then have two alternatives: i) use an oversized heat sink, or ii) use an active cooler.

  1. In using an oversized heat sink (assume it is rectangular), if one side of the sink exceeds 80mm, we recommend embedding a heat pipe inside the sink. A common place is on the bottom face. This evens out the temperature distribution within the heat sink making effective use of its entire body.

  2. An active cooler usually has its thermal resistance published on a data sheet or specification. The performance includes the interface material resistance on its bottom. Current sizes of the D.C. Pator line-up include: 35x35mm, 45x45mm, 50x50mm, Pentium, Pentium Pro, Pentium II, and Xeon coolers.

In the case of tight spatial requirement, we use heat pipes to bring heat from one spot to another with enough space for a large heat sink or active coolers. Heat pipes are media that rapidly transfer heat from one end to the other. They work in either direction and even with the hot spot in the middle.

INDEK's Therm-A-Pipe are hollow copper tubes with wicks composed of grooves on the inner wall and wire. Wick helps carry condensed liquid from the cold end to the hot end. The pipe's operating fluid is a small amount of pure water injected within the pipe pumped down to near vacuum.

Thermal resistance of a heat pipe depends on the pipe size, length, bends, and orientation. When used within the recommended heat capacity range, a heat pipe assures fast heat transfer with thermal resistance around 1°C/W for its entire length. In many heat pipe applications, we are often more concerned with thermal resistance of the heat sink at the cold end and that of thermal interface material.

interface Material Whether mounting a heat sink directly on a hot chip or a full blown heat plate, heat pipe, and heat sink assembly, we need to estimate the effect of interface material (grease, glue, or double sided tape). Although they add to the overall thermal resistance, these interface material eliminate air holes between metal to metal contact and contribute to better heat transfer. We find the total thermal resistance due to one layer of interface material by:
Rth-Rb1 + Rm + Rb2 (eq 4)

where each term means:
Rb1, 2: Thermal resistance at boundary (°C/W)
K(°C-m2 /W)
Area (m 2)

Rm: Thermal resistance of interface material
Thickness(m)
H(W/m-°C) * Area (m 2)

Constants K and H are listed in interface material catalogs. Typical values are K-3.7x10-4= (°C-m2 /W), and K=0.3(W/m-°C). But remember, analysis never defeats a good repeatable experiment.

Man made devices continue to increase entropy and we continue to lessen the imbalance. The cycle continues as chip producers make them go faster and hotter.