When working with applications that require liquid thermal interface materials such as thermal gel, understanding the properties of that material is essential in a means to expand the longevity of the application at hand. If these technical specifications are overlooked without careful scrutiny, it could have immediate and/or latent implications on your project – and usually for the worst.
Viscosity can be most valuable when entering a model or simulation, while flow rates are merely for dispensing conductive materials. Flow rate and viscosity are two terms an engineer will come across on a daily basis, but we must also ask:
- What is the difference?
- Why are they used in such similar contexts?
- Are the terms interchangeable?
Our R&D engineers at KraFAB help to answer this question.
What are thermal interface materials?
Thermal interface materials (TIM) are used in engineering and manufacturing applications to optimize the thermal exchange between two bonded surfaces. These materials are inserted between two or more heat-sensitive components to fill air space between the two components and protect from heat-related reactions, such as an increase in surface temperature within a non-heat producing component.
Viscosity vs. Flow Rate: What’s the difference?
Viscosity describes the internal friction within a particular object of matter, usually in a liquid state. This can be otherwise described as the stickiness or thickness of a particular substance.
Flow rate, however, pertains specifically to the dispensability of a particular substance – again, usually in a liquid or gel state.
Both of these metrics are considered in very similar circumstances to understand and measure how material flows.
The difference between the two is not necessarily drastic, but crucial to consider: while measuring viscosity warrants consideration of several other factors, such as ambient temperature or shear stress, flow rate assumes that these third-party factors are controlled and focuses exclusively on the rate at which a material is dispensed.
Newtonian fluids describe those that do not change viscosity when pressure is involved (ie: water). When using a Newtonian fluid, one can assume that shear stress will not affect the viscosity of the substance. In this situation, there is far less concern about whether it’s appropriate to measure viscosity or flow-rate.
Non-Newtonian fluids, as their name suggests, are susceptible to pressure changes. This circumstance would warrant greater consideration to measure flow rate as opposed to viscosity.
To measure flow rate assumes that the following metrics are controlled and remain constant: shear stress (the pressure involved in dispensing a substance) ambient temperature (commonly known as room temperature, or the temperature that affects matter from its surrounding environment) test duration.
Thermal interface materials like thermal gels are classified as “thixotropic” materials but are still highly susceptible to factors like temperature and pressure. When in a gel state, these materials can increase or decrease in viscosity over time, making their viscosity particularly challenging to measure. Of course, viscosity is still a valuable and purposeful metric, but it cannot be respectfully reported as one single variable without considering other outlying factors mentioned above.