Viscosity vs. Flow Rate - Which Is Best in Thermal Interface Materials?
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.
Viscosity is a measurement of a fluid’s resistance to flow. Many materials are Newtonian fluids, meaning that their viscosities do not change depending on the applied stress. The viscosity of Newtonian fluids can be measured quickly and simply using a rotational viscometer, and a single viscosity value can be reported for those test conditions. This value can be entered into simulations, or they can be used to compare products at a glance.
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 on 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.
These complications do not mean that viscosity isn’t a meaningful characteristic. It is a fundamental material property, but it is can’t be reported as a single value in non-Newtonian fluids. A curve of shear stress versus shear rate is a useful representation of viscosity is such materials, because it provides an indication of the product’s shear sensitivity and relaxation time. Other test conditions, such as test temperature and rheometer geometry, should also be reported to ensure that a measurement can be reproduced by a customer.
Parker Chomerics reports flow rate measurements for THERM-A-GAP GELs, because these measurements provide a framework with which to compare different gels based on their dispensability. After all, automated dispensing is one of the primary advantages of dispensable thermal pastes, such as Parker Chomerics THERM-A-GAP GELs. A flow rate reading can be a useful tool to estimate throughput of manufacturing lines.
Measuring flow rate controls some of 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
Viscosity and flow rate are both measurements of how a material flows. Each of these values can be meaningful: viscosity can be useful to enter into a model or simulation, and flow rate can provide a representation of end-use dispensing situations.
