with their high operating temperatures and crowded packaging conditions, electronic components often face threats from excess heat. potential problems include part and system malfunctions, shortened component life, and even fire-related issues. to assess a given device's thermal conditions and avoid overheating, the temperature of every component must be measured, as well as the heat distribution over the entire pcb. thermal imaging systems can be used for performing this failure detection and analysis.

knowing the temperature distribution of an electronic component is vital to characterizing its thermal and electrical performance. liquid crystal-based thermography provides engineers with a relatively inexpensive, non-invasive system for measuring surface temperatures, which are central to the overall characterization process.

unlike conventional materials, liquid crystals (lcs) do not alter abruptly from a solid to a liquid state when heated. instead they transgress through an intermediate liquid crystal phase. the temperature at which the lc moves into and out of the crystal state is sharply defined and depends on the material's chemical crystal composition.

changes in a liquid crystal;'s state are accompanied by changes in optical behavior, such as their becoming translucent or opaque. in the liquid crystal phase these materials exhibit dual properties of anisotropy (having a different value when measured in different directions, and bireflectance (splitting a light beam into two components). [1]

thermochromic liquid crystals (tlcs) change their molecular structure and their optical properties change in dynamic temperatures. lc thermography employs tlcs, along with solid-state cameras, image digitizers, and higher-speed pcs to provide a system that makes fast, accurate, high resolution surface temperature measurements. liquid crystal thermography can be used in electronic design for locating hot spots and defects on components, microcircuits, modules and pcbs. (see figure 1)

figure 1 liquid crystal thermography reveals hot spots on a component die.

thermochromic liquid crystals

 liquid crystals are in a thermodynamic region between pure solids and pure liquids. tlc based temperature visualization is based on the property of some liquid crystal materials to reflect definite colors at specific temperatures and viewing angles. when a tlc is at its event temperature it will reflect visible light as colors under controlled optical conditions. the reflected colors coming from the tlc will change as temperature rises or falls. when the temperature exceeds the tlc™ clearing point temperature, the material changes to a liquid and becomes transparent. [2]

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. red corresponds to longer wavelengths; blue corresponds to shorter wavelengths. within the color-play interval, the colors change smoothly from red to blue as a function of rising temperature. blue light corresponds to the clearing point temperature. [3]

tlcs also transmit significant amounts of the incident light with no modification. to help prevent this transmitted light from affecting the correct interpretation of the selective reflections, tlcs should be viewed against a non-reflecting background, such as flat black. this is commonly achieved by simply applying black paint to the target components.

working with tlcs

pure liquid crystal materials are thick, viscous liquids. due to their inherently oily form, pure tlcs can be difficult to work with, and their thermal performance degrades rapidly due to chemical contamination and exposure to ultra-violet radiation. methods to help protect the stability of tlcs include microencapsulation and dispersing of the material in a polymer-based matrix.

the chemical make-up of a tlc material fixes its color-temperature response profile. narrow-band tlc formulations have bandwidths below 1 or 2°c, while wide-band formulations range between 5 and 20°c.

procedures for liquid crystal thermography

qualitative temperature visualization techniques for tlcs are easy to implement. they can provide high spatial resolution when properly used in applications that provide optical access to the tlc coated surface. a typical use can reveal an electronic component’s temperature response to changes in thermal conditions such as the rate of air flow or the orientation of a pcb component. [4]

the proper use of lc thermography usually involves these steps:

1. choosing a tlc formula that will cover the temperature range of interest.
2. setting the light source intensity and optical arrangement.
3. applying the liquid crystal material to a calibration surface and to the device being evaluated.
4. calibrating the color-temperature response of the liquid crystal color response.
5.  applying enough power to the device to induce a tlc color reflectance.
6.  electronically capturing the color image.
7.  processing this image through the color-temperature response calibration to reveal the temperature distribution of the device. (see figure 2)

figure 2 temperature contour plot of a hot spot on a thin film resistor.

selecting a liquid crystal formula

the activation temperature and bandwidth of a tlc formula are determined by its chemical composition at the time of manufacture. the tlc activation temperature is that at which a tlc begins to reflect visible light. the temperature bandwidth defines the relative color response range for the tlc formula. formulas with activation temperatures ranging from -30 to 120°c and bandwidths ranging from 0l5 to 30°c are commercially available.

light source intensity and optical arrangement

a sufficiently bright and stable white light source is needed to obtain accurate and reliable reflected light intensity measurements from the tlc coated surface. white light sources that remove infrared (ir) and ultra-violet (uv) radiation from their output spectrum are preferred. 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. consistent light source settings and lighting-viewing arrangements between calibration and actual testing of a device are essential to minimize color-temperature interpretation errors. lighting-viewing arrangements can be especially acute because of the complex light reflecting properties of the tlc surface.

preparing a surface for tlc treatment

liquid crystal thermography is based on assigning temperatures to colors reflected from the tlc coated surface. proper surface preparation is important for obtaining high accuracy. proper surface preparation begins with applying a thin layer of inert black paint, followed by a thin coating of the liquid crystal material. the consistency and uniformity of these layers are essential for accurate calibration and specimen measurement. if there are significant differences in the consistency of the test and calibration surfaces, the reflected visible light will be adversely affected and measurement errors will result. the best results are obtained when the test and calibration surfaces are prepared at the same time.

color-temperature response calibration

the first step in quantitative liquid crystal thermography is to calibrate the color-temperature response of the lc material. this is similar to calibrating the voltage-temperature response of a thermocouple. color-temperature response calibration of the tlc is done by subjecting the tlc to known temperature levels and then recording the response of the tlc. for accurate quantitative measurements, a color sensitive camera should be used. the camera improves on the limited color sensitivity of the eye by recording color response while it is subjected to successively higher levels of temperature on a test surface. the system then analyzes the color-temperature responses and builds the calibration data used to interpret the color response of the tlc when it is applied to the device under test.

lighting and viewing

the perceived color of a pure tlc at a fixed temperature is dependent on the lighting-viewing arrangement and on the amount of background and/or secondary light present. using a co-aligned primary lighting-viewing system in the absence of any background or secondary light can minimize this dependency.  to obtain temperature data, tlcs with suitable threshold temperature levels are applied to a heat-emitting object, such as a pcb component. a reflected color image is captured by a video camera, compared to a standard temperature scale and processed in a pc. a multi-color map is produced, and since there is a direct correlation from color to temperature, the resulting map provides a true indication of component temperatures.

image capture and interpretation

liquid crystal thermography allows a trained eye to make effective preliminary observations about an electronic component under test. however, more accurate and repeatable measurements should be made with instrumentation, in this case, using a solid-state color camera and a computer. this approach uses the processing speed and power of modern cameras and data processing to quickly and accurately capture and interpret the color images of tlc coated surfaces. (see figure 3)

figure 3 temperature distribution on an operating disk drive voltage regulator.

most measurement errors are attributable to improper preparation of the surface or application of the tlc material; improper calibration or interpretation of the tlc color-temperature; or errors associated with the system’s measurement equipment.

a typical tlc system features a temperature controlled calibration surface, computer, and color video camera with frame grabber capability to measure coated devices. liquid crystal application tools include vials of lc and black paint, heat guns, and various spray and clean-up materials. (see figure 4)

figure 4  liquid crystal application kit and analysis hardware.

analysis software tools provide users with dynamic data probing capabilities. users can calibrate the physical-to-screen coordinate system for images being analyzed. this feature provides a simple mechanism to make spatial measurements of the thermal phenomena in their thermographs.

the highest spatial resolution available with lc thermography is limited by the formulation of the tlc and the resolution capability of the camera system. tlc formulas that are not encapsulated, targeted by higher performance microscope optics, can provide sub-micron spatial resolution over reasonably sized fields of view.

processing software tools let users extract a color or temperature image plane by region of interest, determine valid regions of the image, and apply spatial filters.

conclusions

 liquid crystal thermography is a viable method for measuring the temperatures of electronic components. because tlcs operate using visible light, common optics systems and color cameras can be used to create or purchase reasonably priced lc thermography systems. the benefits are accurate, high resolution temperature measurements in applications ranging from the ultra high resolution, sub-micron level devices to larger devices and complete circuit boards.

references

[1] jacob, gerald; thermographic imaging essential to avoid thermal calamities; evaluation engineering, august 1996.

[2] farino, dino; making surface temperature measurements using liquid crystal thermography; electronics cooling, october 1995.

[3] stasiek, j.a. and kowalewski, t.a.; thermochromic liquid crystals applied for heat transfer research; opto-electronics review, 2002.

[4] azar, kaveh and farino, dino; measuring chip temperatures with thermochromic liquid crystals; electronics cooling, january 1997.