Engineers warm up to IR vision

Although a thermal, or infrared, vision system can easily inspect printed-circuit boards (PCBs) and their components, many people in the electronics business don’t fully understand what thermal imaging can do for them. In essence, an infrared (IR) camera provides a visual thermal profile that will immediately indicate problems.

The thermal image of a biased computer chip quickly reveals a defect that visible-light imaging would not see. Courtesy of FLIR Systems.

Even when engineers know about IR vision, they can discount its value. “We used an IR camera to show a skeptical researcher that a small IC ran at 80°C,” said Rich Barton, technical director at OptoTherm. “But he used a thermocouple to measure the IC’s temperature as 40°C. On small packages, a sensor acts like a heat sink, so it cannot accurately measure the IC’s normal operating temperature.” After the customer used a smaller sensor, thermal grease, and insulation, sensor measurements climbed close to 80°C. “This fellow then realized all his previous measurements were incorrect.”

A typical IR-vision project starts with a conversation that lets equipment suppliers determine whether an IR-based inspection system can meet key inspection requirements. “We send the inquirer a form with about 30 questions to answer,” said Beck. Based on the answers, a price quotation may follow to quickly separate buyers with a budget from “tire kickers.”

“Mikron manufactures IR cameras and provides a turnkey IR-imaging system, so we can easily add capabilities and answer questions about any problem,” explained Beck. Suppose, though, engineers buy cameras from vendor A, software from vendor B, and computers from vendor C. When their home-built system has problems, who takes responsibility and sorts everything out? “Engineers should find a reputable company and buy a complete inspection system from it for no other reason than the vendor will take full responsibility for the entire system,” said Beck.

IR sensors can employ what seems like gee-whiz technology. FLIR Systems manufactures two types of IR detectors, a heat detector and a photon counter. The heat detector, called a microbolometer, uses small sensors that change resistance as they receive more or less IR energy. The photon-counter detectors, based on indium antimonide (InSb), absorb photons and convert them into electrons that sensors store temporarily as charge.

Hewlett-Package uses IR cameras in its product-safety lab to identify potential hazards. Preliminary IR images help identify components that require further tests. Courtesy of FLIR Systems.

“The InSb detectors cost more and may require liquid-nitrogen cooling at –195°C [78 K],” continued Bursell. “They detect small energy differences and produce a crisper thermal image, though.” People often use InSb-detector cameras in R&D labs when they need high thermal sensitivity. Software provides the key to useful inspection results. “As you apply power to a known-good PCB, you can take a series of images as components warm up,” explained Ross Overstreet, science-segment engineer at FLIR. “Then, you do the same thing for a PCB under test and subtract the known-good and test images to observe any out-of-limits changes that could indicate a problem. The system looks for temperature differences rather than actual temperatures.”

“People might not realize IR-imaging software does not have to deal with the effects of lights and shadows in an image, for example,” said Chris Bainter, FLIR’s senior science segment engineer. “So, that simplifies system requirements, and algorithms can run faster. Thus, software can do more with an image in a given time.”

Some engineers may have overly high expectations about what an IR camera can do. Unlike the IR cameras used in science-fiction movies, real IR cameras cannot see through most objects, so you cannot “see” an overheated chip inside an enclosure. Even with that limitation, though, IR cameras can detect internal problems.

“If you look at electrical panels and one appears hotter than the others, you could infer an overheated device behind the hot door,” noted Beck of Mikron Infrared. “After you open the door, the camera can measure temperatures of individual components.”

A basic IR camera measures radiation, not temperature. Either camera firmware or software on a PC converts radiation measurements to temperatures based on ambient conditions and material characteristics. Camera manufacturers provide calibration information for each camera and sensor type, based on lab measurements. Users adjust emissivity values based on the types of materials they plan to work with.

During power-up, this PCB produces a range of temperatures at its components. Courtesy of OptoTherm.

OptoTherm’s Barton cautioned that if you want to measure a surface temperature accurately, you should have a material with an emissivity of 0.5 or greater. For emissivities below 0.5, software compensation may produce inaccurate results. “At a low emissivity, reflected energy from light bulbs, peoples’ heads, HVAC equipment, and other sources interferes with measurements,” said Barton.

Uncoated metals have a low emissivity, so component legs, exposed PCB traces, solder, stainless-steel tops on ball-grid arrays (BGAs), and similar components can cause measurement problems. Other materials, such as glossy paint and some plastics and ceramics may have low emissivity at high measurement angles. Barton explained that if you position an IR camera within 20° of the axis perpendicular to a component’s surface, you get a relatively high emissivity. But if you move more than 45° away from that axis, reflections can become a problem. “When a surface has an emissivity below 0.5 and people must measure temperatures below 100°C, they can run into difficulties,” he said.

A few tricks can help engineers overcome low emissivity. A thin layer of Kapton polyimide tape or a piece of masking tape has a uniform emissivity of about 0.95. Just apply the material to the component’s surface, and measure the temperature of the taped area. A bit of flat-finish paint or white correction fluid also works well. A small piece of tape or a dab of paint should not affect the thermal properties of the surface.

Some IR-inspection equipment, such as OptoTherm’s Micro thermal imager for semiconductors and its EL system for PCB inspections, operate offline. But Barton claimed that because component densities often preclude the use of inline in-circuit testers and because flying-probe testers take too much time, the electronics industry may start to use more IR-inspection systems on production lines.

When compared with thermal data from a known-good board, the image of a failed board shows specific defects that have higher-than-expected temperatures. Courtesy of OptoTherm.

Often, engineers want to measure component temperatures inside an enclosure that blocks IR radiation. They might need to heat a PCB to, say, 50°C to measure how much heat its components produce under that condition.

“Our cameras operate from 7 to 14 μm, where most materials appear opaque,” explained Barton. “Engineers can cut a hole in an enclosure and cover it with an exotic material such as germanium or an amorphous material transmitting IR [AMTIR], which comes in several formulations. Then, they can measure surface temperatures through the IR-transparent material without disturbing ambient conditions.”

If engineers need a large viewing port, they can cover holes with a thin plastic, but they must adjust their IR camera’s emissivity setting to account for a small radiation attenuation through the film. “Films with a thickness of about 3 mils generally have about a 90% transmittance for long-wave IR radiation. Polyimide is one of the best plastics, and it has a transmittance of about 93 or 94%,” said Barton.

Cameras that use an indium gallium arsenide (InGaAs) sensor extend spectral coverage into near-infrared (NIR) and short-wave IR (SWIR) wavelengths where the sensitivity of standard silicon CMOS- and CCD-sensor cameras tapers off. And unlike thermal IR cameras that may require cryogenic cooling, InGaAs cameras operate at room temperature. NIR wavelengths cover from about 0.75 to 1.4 μm, and SWIR wavelengths cover from 1.4 to 3.0 μm.

SWIR inspections let engineers look below the surface of silicon wafers to spot defects. When viewed in the short-wave IR spectrum, a silicon disk in a flashlight with light output at 1.55 μm appears transparent (left). When viewed in visible light, the silicon appears opaque (right). Courtesy of Sensors Unlimited.

“Silicon becomes transparent at about 1.1 μm, so you can use an InGaAs camera to inspect features and detect flaws below a wafer’s surface,” said Struthers. “You also can verify that production processes have properly laid out a circuit’s sublayers.” This type of inspection can use standard halogen or incandescent lamps that radiate well into the NIR/SWIR band.

Test engineers can bias circuits on a die or wafer and use an emission microscope, which combines an NIR/SWIR camera and an optical microscope, to detect the small numbers of photons emitted by defects. “Those emissions occur at about 1.3 μm, which falls right at the 'sweet spot’ for an InGaAs camera,” said Struthers. “The glass in microscopes transmits radiation down to about 2.5 μm, so it easily passes photons at 1.3 μm. When you select glass lenses for SWIR cameras, though, ensure they do not include IR-blocking filter or coatings.”

Cameras based on InGaAs sensors also play a role in fiber-optic tests and quality control. “The wavelengths used in fiber-optic communications occur between about 1.3 and 1.6 μm, which coincides with the peak sensitivity for InGaAs sensors,” noted Struthers. “Quality-control people can use an SWIR camera to ensure light goes through fiber-optic components properly. And they can inspect laser sources to make sure you don’t have mode hopping and that they produce the expected light pattern and intensity.”