More Than Just Pretty Pictures
"Why do you need thermal imaging? We've always been able to get by with thermocouples or temperature probes." It's surprising how often managers respond this way to capital requests from their engineers looking to upgrade their temperature measuring capability. This may have made sense when electronic designs were simple and all the components were accessible, but times have changed.
Today, devices are smaller, more complex and dissipate more power/cm² than previous generations of technology. This means that traditional point contact measurements may introduce errors due to their thermal mass and heat sinking effects. Also, contact measurements may not be practical for complex mechanical structures or moving materials such as are found in paper, film and steel production.
One alternative finding increasing favor among engineers is infrared (IR) thermal imaging.
This non-contact, line-of-sight measurement technology can measure surface temperatures of virtually any surface. Absolute temperatures can be measured with accuracies better than 3%, while relative temperatures can be measured with accuracies better than 1%.
What are Thermal Imagers?
Thermal imagers are instruments that create pictures of heat rather than light. They measure radiated IR energy and convert the data to corresponding maps of temperatures. Today, instruments provide temperature data at each image pixel and, typically, cursors can be positioned to each point with the corresponding temperature read out on the screen or display. Images may be digitized, stored, manipulated, processed and printed out. Industry-standard image formats, such as the tagged image file format (TIFF), permit files to work with a wide array of commercially available software packages.
Fig 1, below, is typical of what a thermal image might look like.
Early IR detector development was directed towards military applications. As early as World War II work had been undertaken to develop IR detectors for applications in target location, tracking, weapons guidance and intelligence gathering. Application areas expanded to surveillance and intrusion during the Vietnam era and shortly thereafter space-based applications for natural resource, pollution monitoring, and astronomy were developed.
More recently, efforts have been aimed at expanding IR sensing to applications including temperature measurement and mapping, forest fire sensing and suppression, surveillance, and multi-spectral earth imaging, etc.
Many of these applications have been over long distances, through the atmosphere, and absorption of IR energy has a been factor in the performance of these systems. Military and space-based applications, generally, can be addressed by detectors whose operating wavelengths fall between 8.0-15 microns where atmospheric absorption is minimized. Other applications fall in the broader waveband of .09-300 microns.
Since military applications were the early driving force in detector development, it is no surprise that sensors would be optimized so that the effects of atmospheric absorption would be minimized. Sensitivity, signal-to-noise and image acquisition speed were optimized and cost was not a major factor. The net result was that IR imaging technology developed for the military spilled over into commercial markets. Initial applications were in laboratory level R&D, preventative maintenance applications, and airborne surveillance.
How does the technology work?
The basis for IR imaging technology is that any object whose temperature is above 0 °K radiates infrared energy. The amount of radiated energy is a function of the object's temperature and its relative efficiency of thermal radiation, known as emissivity.
Radiated energy (power) is proportional to the body's temperature, raised to the 4th power. For example, a black body (emissivity of 100%) at 30° C would have a radiation density of 5.4 mW/cm² . That same blackbody at a temperature of 150° C would have a radiation density of 139.2mW/cm² . This energy can be measured and an instrument calibrated to indicate the corresponding temperature of the surface it's "looking at." Instruments which "scan" an object and create an image or spatial map of surface temperatures are referred to as thermal imagers.
There are a variety of scanning techniques and IR detectors found in instruments today. Temperature resolution, the ability to measure small temperature differences, can be as fine as 0.1° C. Spatial resolution, the ability to measure temperatures on small areas, can be as fine as 15 microns. Temperature sensitivity and measurement range cover broad ranges. Applications extend from microelectronic levels to scanning wide areas of the earth from space. Airborne systems can be used to see through smoke at forest fires. Portable, hand-held units can be used for equipment monitoring in preventative maintenance (PM) programs.
When considering the purchase or rental of an imaging system it's important to have an understanding of your measurement requirements.
The performance requirements will dictate the type of system needed. In some instances temperatures change rapidly, in others it changes very slowly or not at all. In thermal testing, particularly reliability testing, there is transient analysis and then there is steady state analysis. Steady state analysis is normally used in determining reliability characteristics.
If steady-state analysis is required and the temperature range to be monitored is 0° C-150° C, acquiring a real-time system with ranges from -40° C to 1500° C may be overkill. It will certainly be costly. As you'll see later, prices for thermal imaging systems can extend over a very broad range and performance requirements will be the determinant of system cost.
Two categories of detectors are commonly found. The first is based on thermal effects and include thermocouples, bolometers, thermopiles and pyroelectric detectors. The second category relies on quantum effects and include photoconductors, and photovoltaic diodes. Today, most commercial thermal imaging / measurement systems are based on quantum effect detectors. These devices can be fabricated into different configurations to address different application areas.
Some are single element while others are fashioned into line arrays and are used for scanners found in applications where materials being tested are moving past the camera. An example of this is process monitoring in paper mills to assure correct moisture content in the paper. Another example of line scanners can be found in "Synthetic Aperture IR Systems". These are flown on aircraft or mounted in satellites. The "forward" motion of the craft forms one axis of the image. These systems are used for resource mapping and airborne forest fire fighting.
A relatively new quantum effect detector technology finding its way into commercial imagers is focal plane or staring arrays (FPAs). As with previous generations of technology, FPAs were first developed for military applications. These sensors are fabricated from materials such as Mercury-Cadmium-Telluride (HgxCdxTe) or Platinum Silicide (PtSi), and are now finding use in top-of-the-line scanning systems. They can provide very high scanning rates and excellent temperature sensitivity.
Quantum detectors may require cryogenic cooling for proper operation. They may employ "solid state coolers" or thermo- electric (TE) coolers, depending on the operating temperature of the sensor. They can be operated in one or both wavebands (3-5 & 8- 12 micron), and they offer great flexibility. These measurement systems typically cost more than $50,000.
Other quantum detectors are capable of operating at relatively high temperatures. Two to six stage TE coolers may be all that is required to achieve required detector performance. The cost for cameras based on these detectors can be significantly lower than for the higher performance solid-state FPA's.
High image resolution, lower cost thermal imaging systems using single element detectors, are also available. The lower cost is achieved through a judicious trade-off in scanning speed. In applications, such as reliability testing, where steady state analysis is required, the slower scan speed is quite satisfactory for obtaining useful results. Systems incorporating these detectors can cost as little as $10,000.
Thermal effect detectors, such as thermocouples and bolometers can be operated at or near room temperature. In one sense, this is a distinct advantage as the inconvenience of liquid gasses, or high cost of closed cycle Sterling coolers is avoided. The limitation is that these types of detectors are frequently available in single point probes and may require direct contact with the object being measured.
The holy grail of IR detectors has been the uncooled detector which can be fabricated into high resolution FPA's. These are currently becoming available in a range of IR viewers.
"But what about emissivity," everyone asks. We saw that the amount of radiated energy was proportional to a factor called emissivity. The practical effect of lower emissivity is for IR instruments to indicate a lower temperature than the true surface temperature. For this reason most systems and instruments provide the ability for the operator to set them to a value which corresponds to the value of the object being measured. Alternative techniques call for changing the emissivity through the use of spray paints, powders and tape or "emissivity dots." Fig. 2 shows an image of an aluminum housing used to shield an RF module.
The image in fig. 2 is of the backside of the aluminum housing. Inside is a circuit board containing some relatively high power devices at one end and low power devices over the balance of the board.
Since aluminum has a low emissivity, "emissivity dots" (e=.95) were placed on the surface in several key locations so that thermal gradients across the surface could be determined. The red and yellow circles (dots) show a distinct warming from the right side of the housing to the left. Also, the aluminum shows up as a cool temperature everywhere but where the dots have been placed. The same correction could have been applied after the image had been gathered by changing the emissivity value for the stored image.
To determine reliability many companies use the methodology set forth in MIL-H-217E or Bellcore Technical Reference, TR-NWT-000322, "Reliability Prediction Procedures for Electronic Equipment." Temperatures of the key components can be measured through thermal imaging and the resultant data is fed into the MTBF models and reliability calculated. Tektronix, Inc., for example, claims they've been able to get more reliable predictions as compared with the old method of counting pins, connectors, number of active devices.
In many fields, heat can be a contributor to unreliability. In electronics, a key reliability factor is semiconductor junction temperature. The higher this temperature, the shorter the time to failure. When operating, a semiconductor is dissipating power and generating heat. As long as the device is generating heat, the temperature of the junction will rise until the heat finds a path to flow from the device. At that point, heat flow from the device will be equal to the heat being generated and thermal equilibrium will be reached. Thermal images can provide measurements of semiconductor case temperatures. Assuming one knows the value for thermal resistance, the junction temperatures can be determined. These values can be used in the MIL-H-217X calculations.
A second and less well recognized thermal problem occurs in manufacturing when circuit boards are reflow soldered. As a result of the high temperatures to which they are subjected and their complex structure, some boards become misshapen or warped. Others exhibit mechanical problems with components or solder connections due to thermal stresses.
IR reflow ovens assume a fairly homogeneous object is being heated while in fact the temperatures across the board are frequently non- uniform. In Fig. 1 we saw a circuit board which had just come from an IR reflow machine. Ideally, one would expect this board to look fairly uniform since it had just come from the oven. In this situation, this obviously isn't the case. The significant point to note in this image is the large swing in temperature from minimum to maximum. This board's temperatures showed a delta T of more than 70° C. The traditional method of characterizing this oven profile with thermocouples failed to detect this situation. Since the temperature profile for the machine was based on thermocouple placement, the board stayed cooler in some areas than in others. This could have resulted in bad solder connections. The value of seeing the whole temperature presentation on one display shows the power of thermal imaging.
Fig 3 is an image of a heat sink and a TO-3 style semiconductor mounted to it. This was part of an experiment to determine the efficiency of a new mounting material for semiconductors compared to zinc oxide compounds traditionally used. The image shows some localized hot spots on the case of the device, which indicates that heat is not being evenly carried away. Viewing the surface thermal profile also shows a thermal discontinuity between the transistor and the heat sink.
The conclusion was that the mounting material had a high thermal resistance compared with the paste.
Fig. 4 shows the thermal image of a burn-in board used by a semiconductor manufacturer. Devices are placed in sockets, power is applied and then the test fixture is periodically scanned and recorded. Uniformity of power dissipation and condition of the devices can be quickly determined. If a device should exhibit an early failure, it will be seen as a darker or lighter spot on the "matrix." If the system is connected to a computer, image data and manipulation can be performed and temperature records made. Systems might be used to monitor a production process as part of a closed-loop system. Thermal data would be monitored and variations to the control limits would be used to correct variations. Thermal data could also be used for statistical process control.
These applications are different but they all share one thing in common. They provide a comprehensive map of the thermal factors effecting the device or process being monitored. If an engineer were to try to gather similar levels of information using thermocouples or temperature probes it would be a major undertaking requiring many man hours of technical labor. Clearly, thermal imaging is a fast, cost effective way to perform detailed thermal analysis.
Future Direction in Thermal Imaging
The latest trend in imaging systems is the mating of imaging cameras to the power of the personal computer (PC). The electronics are contained on a card which can plug directly into the computer and take advantage of the high resolution display, processing capability and mass storage. The camera connects directly to a port on the computer. The cost overhead associated with separate displays, separate memory and enclosures is thus avoided.
New focal plane arrays, based on thermal effects, will result in high resolution cameras which can be fabricated in significantly smaller configurations and at much lower cost. As they don't need cryogenic cooling, they should be considerably more reliable and will first find their way into thermal viewers. Versions capable of making thermo-metric measurements will quickly follow. These trends will further spur the growth in thermal imaging. The broad number of applications utilizing imaging attests to its viability and growth in new applications will be dramatic.