In Part 1 and Part 2 of our 3 part series, “Heat Sink Testing Methods and Common Oversights” we wrote about the heat transfer path and summarized different types of heat sink experimental set ups. Here in our final installment, part 3, we’ll show the differences in thermal resistance as determined by each testing method.
Example of Heat Sink Test Results
To show the differences in thermal resistances, as determined by each testing method, we can compare the resistances of a maxiFLOWTM and a straight fin heat sink. The maxiFLOW heat sink has a base size of 42.5 x 42.5 mm, and is 17.5 mm high. However, the straight fin heat sink has a base size of 80 x 76 mm and is 20 mm high. Due to these size differences, the thermal resistance of the smaller maxiFLOW sink will be higher than that of the straight fin heat sink. The straight fin sink is 284% bigger in volume than the maxiFLOW heat sink.
As shown in Table 1, the maxiFLOW heat sink has a thermal resistance of 1.5 K/W when ducted, and 1.9 K/W when unducted at 1 m/s,. This results in a 21% difference between the testing methods. The dense straight fin heat sink was simulated using CFD. It has a ducted thermal resistance of 0.38 K/W, and resistance of 1.31 K/W when unducted as shown in Table 2.
Table 1: Ducted and Unducted Thermal Resistance of an ATS-52425P-C2-R0 at ~1 m/s (200 LFM) (42.5 x 42.5 x 17 mm) [2]
Table 2: Ducted, Unducted, and Dual Thermal Resistance of a Straight Fin Heat Sink at 1 m/s (~200 LFM) (80 x 76 x 20mm)
This results in a difference of 71%. The large difference in thermal resistance between testing methods is due to the dense fins. Airflow goes around the heat sink in an unducted test, as shown by the particle tracks in Figure 5. All of the particle tracks go through the heat sink in the ducted simulation, as shown in Figure 4.
Figure 4: CFD Image of a straight fin heat sink in a ducted environment, dissipating 10 W at an inlet Air Flow Velocity of 1 m/s, (~200 LFM)
CFD Image of a straight fin heat sink in an Unducted Environment, Dissipating 10 W at an Inlet Air Flow Velocity of 1 m/s (~200 LFM)
Table 3: Summary of Heat Sink Testing Methods
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.