New developments in thermal-imaging equalization software can provide a real-time, accurate picture of how operating temperature affects your electronic components and circuits. Infrared (IR) thermography has many advantages over other forms of temperature measurement. Unlike contact techniques, in a typical measurement situation, an IR camera pointed at an object-under-test will not inhibit the natural convective flow of air around it, a key requirement in preventing components such as PCBs from overheating. And while contact techniques can only measure the temperature at one point on an object, an infrared scanner will allow other areas of a PCB to be inspected as well, revealing anomalies and heat transfer patterns which might otherwise go undetected.
However, when using an infrared measurement technique, observed temperatures can be affected by the emissivity (and consequent reflectivity) of a component being measured, to an extent that the component might appear much cooler–or warmer–than it actually is. The emissivity of a component depends on the material of which it is made and its surface condition; for example, whether it has a shiny or matte surface. The emissivity value (between 0 and 1) represents the amount of radiation emitted by that object (its apparent temperature) compared to a true blackbody (equal to 1) at the same temperature.
But now, new emissivity and reflected ambient equalization software makes it possible to measure the true temperature of all points on a PCB, hybrid circuit or microchip. Specially adapted extrapolation software will even allow the temperature of inaccessible PCBs mounted in racks to be measured.
PCB Inspection
When measuring the temperature of a PCB, the reflected ambient can be considered the same for all points, providing there are no heat lamps or other strongly radiating objects behind the infrared camera (Figure 1). To compensate for the effects of different component emissivities around the board, a reference image of an unpowered board heated to a known temperature above ambient is first captured under controlled measurement conditions (Figure 1, top right screen). The PCB is then powered up and a second thermogram, or thermal image, is taken under the same measurement conditions.
Using data from these two images, the equalization software is then able to recalculate temperatures, using emissivities measured for every point in the reference image, producing a true thermal picture of the board (Figure 1, bottom screen). The emittance information provided by the software can be used for spot measurements in subsequent testing.
Subcomponent Level
When looking at heat transfer patterns at the subcomponent level with a microscope, the setup is much the same as for PCB inspection, except that software using two reference images is now required to compensate for the effects of reflection. A typical test component, such as a power transistor, incorporates a number of very shiny (i.e., low-emissivity) devices. Between them are much less reflective (high-emissivity) surfaces, leading to vastly differing emissivity values across the measurement surface.
Since the component is being scanned very closely under strong magnification, and the scanner’s lens occupies much of the reflected background, the temperature of the camera’s cold detector, typically -196(degree)C, will also be reflected across the image, further complicating true temperature measurement. This so-called narcissus effect limits the value of measurements obtained from systems not equipped with equalization software.
Rack-Mounted PCBs
For applications which require the temperature measurement of concealed rack-mounted components, use a software subset based on an algorithm which assumes that the temperature of an object will adapt itself to its new or changing ambient temperature along an exponential temperature curve. The program allows you to extrapolate backward to produce a thermal image representative of conditions prior to powering down.
The test requires the PCB first to heat to its normal operating temperature in the rack. The PCB is removed and a thermal image is captured at the designated power-off time. Two more images are captured at intervals of 20 seconds and, using extrapolation software which takes into account the different cooling rates of each component on the board, it is possible to calculate the initial temperature of the PCB. At this point, normal emissivity equalization techniques using a reference image will result in an extrapolated image which displays the true temperature of the PCB in its normal operating environment.
The Future
With the increasing pressure to reduce development time and costs, improve product quality and minimize warranty claims, more and more companies will turn to infrared thermography to provide them with increased confidence in the quality of their products. These new software options can accelerate this process.
About the Author
Greg McIntosh is Manager of Technical Support for AGEMA’s Industrial Products Group in North America. Previously, he was a consultant for Dow Chemical, Public Works Canada and Viewscan Ltd. Mr. McIntosh graduated from Carleton University with a degree in mechanical engineering. AGEMA Infrared Systems, 550 County Ave., Secaucus, NJ 07094-2607, (201) 867-5390.
Copyright 1995 Nelson Publishing Inc.