A common question often asked by our customers is the reason why flow rate is reported on datasheets of liquid-dispensed thermal interface materials instead of viscosity. And it’s a fair question; viscosity is a fundamental property of fluids such as thermally conductive pastes. But measuring viscosity, however, is more complicated than meets the eye.
Parker Chomerics THERM-A-GAP Thermally Conductive GELs belong to a class of fluids referred to as “thixotropic.” In case you’re not familiar, thixotropy is a time-dependent decrease in viscosity as shear stress is applied.
This shear stress may be introduced during mixing, pumping, or dispensing of the product. The extent of the viscosity decrease depends of the duration and magnitude of the agitation, and viscosity recovers gradually over time after the stress is removed. The act of measuring viscosity introduces shear stress, so gathering repeatable data requires carefully controlling the duration of the test and the relaxation time between trials.
To complicate the measurement further, viscosity is strongly influenced by sample temperature. A warmer dispensable thermal paste flows more quickly than one at room temperature, a characteristic that is particularly relevant in thermal interface materials.
Therefore, a viscosity measurement is only accurate for a given shear stress, duration, and temperature. The resulting value of viscosity is difficult to generalize, so it is more convenient to use flow rate, as it is a measurement that is more representative of actual end-use conditions.
Automated dispensing is one of the primary advantages of dispensable thermal pastes, such as Parker Chomerics THERM-A-GAP GELs. Publishing flow rate data provides a framework with which to compare different gels based on their dispensability.
Additionally, measuring flow rate instead of viscosity controls the critical parameters described above, such as:
- Shear stress, which is controlled by dispensing from a specified container at a constant pressure
- Test duration
- Ambient temperature
Measuring flow rate is repeatable and more representative of actual dispensing situations than a viscosity value.
So, the next time you’re reading a datasheet, you’ll be armed with the knowledge that flow rate is a more accurate representation of real-world conditions.