In Part 1 of our 3 part series, “Heat Sink Testing Methods and Common Oversights (Part 1 of 3)” we wrote about the heat transfer path, here in part 2 we’ll cover different types of heat sink experimental set ups.
Heat Sink Set up for Testing
A wind tunnel, air temperature and velocity sensor, thermocouples and a power supply are needed to test heat sinks. A resistor is used to dissipate electrical power in the form of heat energy. The resistor is typically attached to the heat sink using thermally conductive double-sided tape. Such tape also attaches the resistor to a board, which is typically a low thermal conductivity printed circuit board or FR4.
As with a heat sink mounted on a component, the heat dissipated in the resistor is transferred to the environment by two paths. However, the thermal resistance from the resistor to the heat sink and to the board is only the interface resistance, 
Heat Transfer to the air via the heat sink 
will not equal the energy dissipated in the resistor. If this value is used, there will be an error in determining the thermal resistance of the heat sink because the energy dissipated,
is not the heat transfer to the environment via the heat sink.
Thermal resistance from the board to the air increases with decreasing board thickness. Resistance decreases with increasing board thermal conductivity and increased air flow velocity across the board. Though, as previously stated, a low thermal conductivity printed circuit board is used to minimize the loss through the board.
Figure 2. Heat Sink Experimental Set Up
Heat Sink Experimental Set Ups
The first method of heat sink testing is set up in an unducted environment. This is similar to the flow experienced in typical applications. The airflow through the heat sink is affected by its fin density. The higher the fin density, the more airflow bypasses the heat sink. This provides realistic data for the thermal performance of the heat sink.
The test is easy to set up, but requires a higher quality testing facility. One prominent issue is that the heat loss to the board must be taken into account.
Figure 3Â Heat Sink Experimental Set Ups: Unducted (a), Ducted (b)
and Dual Heat Sink Testing Methods [3].
The second method of heat sink testing is set up in a duct. This forces all of the airflow to go through the heat sink. There is no air flow around the heat sink or bypass airflow. It is moderately easy to set up. Vendor supplied thermal resistance data is commonly provided for ducted test results. However, the results are optimistic and can give misleading data when heat sinks are used in an unducted application.
The third method is dual heat sink testing, which uses two identical heat sinks with a heater sandwiched between them. The assembly is suspended on the centerline of a research quality wind tunnel. Dual heat sink testing is a good approach because there are no heat transfer losses to the air, e.g. via a board. But this method is rarely used in industry because it is time consuming to set up and because the approach velocity is difficult to measure without using a quality testing facility.
In part 3 we’ll show the differences in thermal resistance as determined by each testing method. CLICK HERE FOR PART 3
ATSs Thermal Characterization Lab is the perfect partner to help you put this 3 part article series into practice. And the best part is, the first half day is free! Click here to learn more: Thermal Characterization Lab
References
1. Sergent, J. and Krum, A., Thermal Management Handbook for Electronic Assemblies, First Edition, McGraw-Hill, 1998.
2. Advanced Thermal Solutions, Inc., Data Sheet for maxiFLOW ATS-52425P-C2-R0 Heat Sink. http://media.digikey.com/pdf/Data%20Sheets/Advanced%20Thermal%20Solutions%20PDFs/ATS-52425P-C2-R0.pdf
3. Advanced Thermal Solutions, Inc., Heat Sink Design and Characterization, Tutorial.