It is critical for the engineers to accurately determine temperature of a design, whether it is at the chip, component, board, or system level, to ensure that the design will function properly and maintain its optimal performance over its expected lifetime and meet its specified mean time between failures (MTBF). Thermal management of electronics is unarguably a critical component of the design phase.
To optimize thermal management, it is also critical to get an accurate picture of the heat distribution across a device or a board. One of the most precise methods for mapping temperature is liquid crystal thermography (LCT). LCT uses thermochromic liquid crystals (TLC) to give engineers a visual representation of the heat distribution based on the changing colors of the TLC when heated.
Liquid crystal thermography systems give a visual representation of the heat on a device, board, or system using thermochromic liquid crystals, high-resolution cameras, and an LED light source. (Advanced Thermal Solutions, Inc.)
LCT technology has been around since the 1950s and has been used in the electronics industry since the 1980s. In 1975, researchers published a review of LCT and it applications in the study of convective heat transfer that determined the method provided “both qualitative and quantitative heat transfer and fluid flow information to be obtained on heated objects placed in forced convection environments.” [1]
The authors added, “In addition to yielding precise quantitative heat transfer information, the liquid crystal thermographic technique afforded the opportunity to visually observe the effects of flow separation, the separation bubble region, the turbulent boundary layer, and the turbulent wake on the surface temperature of the heated cylinder.”
What are Thermochromic Liquid Crystals?
Thermochromic liquid crystals are the key to the LCT process. Rather than changing from a solid to a liquid when heated, TLC have an intermediate liquid-crystal phase and the temperature in which this phase-change takes place is precisely defined depending on the composition of the crystals. The phase-change causes the TLC, which starts as transparent, to reflect different wavelengths of light, represented visually as different colors.
TLC readings using thermVIEW Lite software from ATS. The TLC shows up as different colors depending on the heat applied, while the software is calibrated to give precise temperature readings. (Advanced Thermal Solutions, Inc.)
As explained by Dr. Bahman Tavassoli in an article for Laser Focus World, “Normally clear or slightly milky in appearance, liquid crystals change in appearance over a narrow range of temperature, called the color-play interval. This is the interval between the first (red) and last (blue) reflection. The displayed color is red at the low-temperature margin of the color-play interval and blue at the high end. Within the color-play interval, the colors change smoothly from red to blue as a function of rising temperature, with blue light corresponding to the clearing-point temperature.” [2]
To visualize the temperature response of the TLC, a bright and stable white light is required to remove the infrared and ultraviolet radiation from the output spectrum. Tavassoli explained, “Any IR energy present in the incident light spectrum will cause unwanted radiant heating of the test surface. Exposure to UV radiation can cause rapid deterioration of the TLC surface, which will result in unreliable color-temperature responses.”
Temperature ranges for the TLC material are established by the manufacturers. Narrow-band TLC have bandwidths below 1-2°C, while wide-band TLC range between 5-20°C. TLC are typically designated by a two-color/temperature system. For instance, R35C5W would indicate that the TLC would show up as red starting at 35°C and that the blue start temperature would be 5°C above the red, allowing engineers to see the estimated bandwith of the TLC.
TLC are inherently oily, and their thermal performance degrades from exposure to chemicals and UV radiation, so manufacturers have developed microencapsulation or polymer dispersion methods to make the materials easier to use in laboratory settings. Microencapsulation provides high resistance to contamination, but the polymer dispersion method provides a more brilliant color response. [3]
How do you use Liquid Crystal Thermography systems?
To obtain the most accurate results from LCT, a smooth and contamination-free surface is important. Test surfaces and calibration tools should be cleaned with alcohol, if possible, and dried before the process begins. A thin and uniform coating of black paint is applied to the test surface and dried using a hot air gun at low temperature. Once the surface is dry, the TLC materials can be applied to the test surface. [4]
A bright, stable, white light source is required to obtain an accurate reflected light intensity from the TLC-coated surface. As explained by Dr. Kaveh Azar in an introduction to LCT techniques, “Consistent light source settings and lighting-viewing arrangements between calibration and actual testing are essential to minimize color-temperature interpretation errors.”
Calibration is the key to using LCT. According to Dr. Tavossoli, the system for calibrating TLC is similar to calibrating the voltage-temperature response of a thermocouple. TLC is subjected to known temperature levels and the response is recorded with a color-sensitive camera. The response is recorded at different temperature levels on a test surface and the system develops calibration files that can later be used to interpret the response of TLC on the device being studied.
The following graph shows the relationship between temperature and color of a range of TLC:
While TLC response is visible to the naked eye, the LTC system uses a high-resolution, solid-state color camera and calibrated software to more precisely determine the temperature.
Advanced Thermal Solutions, Inc. (ATS) has recently added to its line of LCT systems to provide a cost-effective tool for temperature mapping studies. tvLYT™ is accurate, easy to assemble, and easy to use. It comes in a portable case containing the arm, high-resolution macroscopic optic camera, LED light source, black paint, and TLC material for the required temperature range. tvLYT™ can be quickly connected to a computer through a USB and uses thermVIEW™ Lite software (downloadable on the ATS website) to calibrate the readings and provide precise results.
The tvLYT™ liquid crystal thermography system from ATS provides highly accurate temperature mapping capability using TLC. (Advanced Thermal Solutions, Inc.)
For more information about tvLYT™, visit https://www.qats.com/Products/Instruments/Surface-Thermography/tvLYT.
What are the benefits of LCT?
There are several techniques for measuring temperature across a system, including the use of thermocouples and resistance thermometers. These are common techniques used in labs across the world, but there are challenges and limitations to that method.
As one report noted, “Traditional techniques employing sensors such as thermocouples and resistance thermometers can measure temperature at individual locations. Hence, a large number of sensors are required for complete mapping of the surface. Since physical sensors occupy space, the measurements are to be interpreted as spatial averages. This route may prove to be disadvantageous in regions of localized peaks and valleys of heat transfer. Liquid crystal thermography proves to be useful under these circumstances.” [5]
The tools for thermal imaging, which visualize IR emissions to show heat patterns, continue to improve, seemingly by the day, and several companies have released cameras that can be attached to a mobile phone to improve the mobility and accessibility of thermal imaging. [6] IR cameras can give a quick visualization of the heat emitted from a device and are excellent tools for finding hot spots at the system level, but they lack the precise temperature readings that LCT captures and accuracy counts in thermal management.
Comparison of LCT and IR systems. [4]
Because precision is the most important benefit to using LCT for temperature mapping studies, ATS has offered four free calibrations during the first year after purchasing tvLYT™ and also lifetime technical support.
For more information about ATS liquid crystal thermography systems, or thermochromic liquid crystal materials, visit https://www.qats.com/Products/Instruments/Surface-Thermography.
If you have questions about any ATS product or its consulting and design surfaces, contact ATS at ats-hq@qats.com.
References
1. http://heattransfer.asmedigitalcollection.asme.org/article.aspx?articleid=1436225
2. https://www.laserfocusworld.com/articles/print/volume-41/issue-12/features/thermal-imaging-liquid-crystal-thermography-characterizes-heat-issues.html
3. https://www.electronics-cooling.com/1995/10/making-surface-temperature-measurements-using-liquid-crystal-thermography/#
4. https://www.ewh.ieee.org/soc/cpmt/presentations/cpmt0201b.pdf
5. http://www.nptel.ac.in/courses/112104039/pdf_version/lecture35.pdf
6. https://www.thermal.com/compact-series.html
7. https://onlinelibrary.wiley.com/doi/pdf/10.1002/pc.20453
8. https://www.sciencedirect.com/science/article/pii/S0924424715300224
9. http://zm8pc.ippt.gov.pl/papers/JCPT_2014012610120554.pdf