Repeatability of Material Thermal Impedance Measurements
The Four Factors Controlling Repeatability with ASTM D5470 Testing:
Surface contact resistance: mount/remount variations: Each a time a sample is mounted for testing between the hot and cold test surfaces, the two contact interface resistances created vary from one mounting to the next. The degree of this variation is highly dependent on the material under test and the details of the test surfaces. Since surface contact resistance is caused primarily by the entrainment of air on the contact interfaces, this variation is in essence the consistency with which air is excluded from the contact interfaces each time a sample is mounted. Larger contact resistances generally exhibit larger absolute variations. Typical impedance variations from this phenomena range from near zero to about 0.05 K in2 /W ( 0.32 K cm^2.W) although significantly higher ranges are possible.
Surface contact resistance: pressure, time, temperature history: Surface contact resistances decrease with higher contact pressure due to exclusion of air entrained within the contact interface. As the duration of the pressure application increases, entrained air is progressively excluded as surface "wetting" occurs under the action of the applied pressure over time. Higher temperatures on the interfaces often accelerates this progression. Eventually, the contact resistance reaches a final value. For some liquids and low viscosity greases, "final value" occurs almost instantaneously; for some interfaces, pressure, time, and temperature history can take many minutes to reach a steady final contact resistance. Typical impedance variations from this effect range from zero to about 0.04 K in2 /W ( 0.13 K cm^2.W)
Tester measurement repeatability: Although this is not a specified parameter for the Analysis Tech TIM Tester 1300, it is generally less than one-fifth of the final accuracy estimate that is included with all test data. Most importantly, automatic testing procedures are provided in WinTIM software that allow the user to measure the test repeatability due to each of the four effects listed here. Once variations due to contact resistance and material variations are excluded, the TIM Tester 1300 generally offers repeatability better than +/-0.002 K in^2 /W ( 0.013 K cm^2.W) or +/-0.4%, which ever is larger.
Variation in bulk material properties: Some materials exhibit measurable variation in bulk material properties within the same production lot due to variations in composition-ratio or structural constituency as an unwanted side-effect of the production process. Although worthy of noting, this is not essentially a testing-repeatability issue although can be misconstrued as such if not identified.
Analysis Tech welcomes the opportunity to demonstrate the capabilities of the TIM Tester 1300/1400 by offering no-charge demonstration testing on samples sent to our factory labs. Contact Analysis Tech for more information.