Today, the trend is to make electronic products smaller. But making products smaller presents heat-sinking and heat-distribution challenges. As a result, electronics manufacturers are turning to thermography to help solve complex thermal problems.
Thermography is the use of a thermal imaging camera to take a heat picture of a target. Thermographic cameras view infrared (IR) energy, as opposed to visible light energy, and display the resultant temperatures as shades of gray or colors.
Background to Thermography
Early in the 1990s, a new detector technology called focal-plane array (FPA) opened up the applications for thermography. The FPA is an IR detector that incorporates rows and columns of individual sensors. Each IR detector stares at the target and creates its own pixel of information.
An FPA system typically uses a 256 × 256 or 320 × 240 IR detector that has more than 65,000 IR sensors. The new sensor design allows cameras to be significantly smaller so that thermal imaging systems now resemble camcorders.
Previously, all IR systems required some sort of cooling of the detector, typically using liquid nitrogen or gaseous argon. A new technology in IR detectors eliminates the need for any type of cryogenic cooling.
Innovations in IR Technology
Recent innovations in a century-old technology have provided the next generation of thermal-imaging systems: thermal detectors that operate at room temperature. This technology also is referred to as uncooled-detector technology.
Among the most exciting thermal-detector technologies to evolve is commercial uncooled technology, such as microbolometer and ferroelectric or pyroelectric detectors. The resistive microbolometer is the only uncooled thermal-detector technology yielding high response that is DC restored and does not require a mechanical chopper to modulate the detector response.
The linearity of the microbolometer is better than most cooled detectors and translates directly into low spatial noise over a broad temperature range. Isolation from neighboring pixels ensures negligible crosstalk, resulting in high thermal sensitivity. In side-by-side measurement tests, the uncooled microbolometer has proven itself more accurate and repeatable than comparable cooled cameras.
Modern silicon processing and micromachining techniques are used to create an array of microbolometers, each less than 47 µm wide. These highly uniform elements are formed into an array of detector cells, typically in a 320 × 240 format, to match standard display devices. In monolithic designs, each microbolometer element is a thermally isolated microbridge suspended over a silicon substrate containing a readout IC and an analog-to-digital converter.
For example, thermal imaging systems such as FLIR’s ThermaCAMs™ can produce very high-resolution images. This is a result of increased spatial resolution and increased sensitivity. This combination also allows the camera to detect small targets.
In addition to their solid-state design and enhanced image quality, the ThermaCAMs with microbolometer technology are small and lightweight. Also, the capability to operate at room temperature permits the camera to be used in a variety of commercial and industrial applications, from PCB inspection to other forms of predicative maintenance. Optical design advances for commercially available systems provide powerful magnification down to 25-µm targets.
Technical Challenges
Typically, PCBs are comprised of multiple materials; the boards may be a mixture of plastics, gold, copper, lead, aluminum, and epoxies. Each of these materials emits IR energy differently. As a result, a thermographic camera can have difficulties properly identifying true and correct temperatures.
Emittance correction software is used to compensate for the varying materials found on a PCB. The principle is quite simple: Two separate thermal images of the target are taken at two different target ambient temperatures. The software then compares the two images and performs a mathematic calculation. With two equations, it is possible for the software to calculate the one unknown—emissivity.
Additionally, thermal-image processing software for circuit-board analysis includes a feature called image subtraction. Basically, two thermal images can have the temperature of each pixel subtracted from each other. If the temperatures of corresponding pixels in two targets are equal, there will be a 100% subtraction. Otherwise, the resulting image will only show temperature differences between two targets.
Applications
IR imaging systems have been used to provide complete thermal images of PCBs, hybrids, and VLSI ICs. One of the most common applications is the detection of thermal bugs in components. For example, the image displayed in Figure 1 gave a design team early warning of a poor die attachment caused by inconsistent manufacturing practices. Early detection of the problem saved the company thousands of dollars in warranty work and product recall.
Traditionally, thermocouples have been used to monitor component temperatures. This process is labor intensive, and thermocouples tend to yield inaccurate readings since they act as heat sinks and often are poorly attached. In addition, unidentified hot spots often can be overlooked entirely.
In some cases, designers are forced to use failure analysis to identify design and manufacturing problems. This process does not provide any information regarding adjacent components and forces design teams to endure protracted tests in an attempt to induce failure.
IR is a better thermal diagnostic tool since it accurately supplies a real-time thermal map of the unit under test (UUT). In addition, IR is a noncontact measurement device that provides a complete thermal image of thousands of individual temperature measurement points.
Any and all thermal anomalies can indicate a fault or defect. Components running too hot or cold can indicate a short or an open, partial short, a diode placed backwards, or bent IC pins. Elevated temperature operation shortens the life of ICs, while large temperature gradients between components and the PCB increase thermal stress that can cause early failure.
Thermal imaging and measurement systems provide an effective means for identifying and resolving thermal-related problems by giving a direct measurement of the actual component. Modern thermal imaging and measurement systems have evolved into a useful and necessary tool for this high-tech industry.
About the Authors
Chris Alicandro is the business development manager at FLIR Systems. Previously, he was a domestic sales engineer and later international sales manager at Inframetrics until the company was acquired by FLIR earlier this year. Mr. Alicandro holds a B.S.M.E. from Worcester Polytechnic Institute. FLIR Systems, 16 Esquire Rd., North Billerica, MA 01862, (978) 901-8000.
Doug Little is the public relations manager at FLIR Systems. Before joining the company in 1998, he was with Diamond Multimedia’s Communications Division. Mr. Little has a B.S. in communications from Concordia University. FLIR Systems, 16505 SW 72nd Ave., Portland, OR 97224, (503) 684-3731.
Copyright 1999 Nelson Publishing Inc.
August 1999
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