IR inspection finds elusive faults

Defects that baffle other test techniques succumb to IR inspections of powered boards.

John Sexton, ART Advanced Research Technologies, Montreal QC, Canada -- Test & Measurement World, 8/1/2001

A number of contract manufacturers now use infrared (IR) inspection systems to improve the yield and quality of the circuits and assemblies they produce. IR inspection can locate and help identify problems that prove difficult or impossible to detect using traditional physical and functional tests. And IR inspection can locate areas that seem to operate well during production but that could lead to field failures later on.

Electrically probing small PCBs using an in-circuit tester (ICT) has become more and more difficult. Even the most sophisticated ICT can’t overcome the physical limits of getting a large number of probes to make contact with tiny, closely spaced test pads. An ICT also may have difficulty positioning large PCBs so all the electrical probes make proper contact with the board. And although an in-circuit test can identify short circuits, it cannot locate where they occur. IR Inspection systems encounter no physical limits due to component spacing or board geometry, and an IR system can detect heat through opaque materials.

Traditional inspection equipment (visible-light or x-ray) is also useful on production lines, but such systems provide only physical “tests.” They can identify a fracture, a misplaced component, or missing solder, but problems such as a specific electrical short, a bad transistor, or a circuit that runs hot will all remain invisible.

An IR inspection system obtains an image of a powered PCB to obtain information about energy emitted by the PCB’s components. Each IR image could provide over 150,000 “virtual” infrared probes. A basic IR system might simply provide an image of hot spots on a PCB to aid in locating defects such as short circuits. More complex systems include image-processing and image-analysis software that can identify specific defect types and can display temperature differences as small as 0.2°C. Table 1 describes several types of defects and includes IR images of them.

Software compares images

The software compares an acquired IR image with a statistical model computed from IR images obtained from a set of known-good boards. The model includes acceptable variances, so slight temperature differences don’t cause the inspection system to unnecessarily fail a PCB. And although you may think of an IR system as only locating hot spots on a powered PCB, it can also locate areas of a circuit that are running cooler than normal. Thus, an IR system can locate hot short circuits as well as power transistors that are not producing an expected amount of heat.

Of course, a hot or cold spot may not indicate an immediate failure, but abnormal operating temperatures may indicate the possibility of future failures, perhaps after a consumer buys the product. The ability of IR inspection systems to locate defects complements other inspection techniques and in-circuit tests by pinpointing “hidden” problems, and in many cases, an IR system can identify the cause of problems. Unlike an ICT, an IR inspection system can identify the precise location of short circuits, including high-resistance short circuits. The latter type of short circuit—a latent defect—can worsen and cause a product to fail.

IR inspection also plays a role in off-line inspection. Often, manufacturers dump PCBs with unidentified failures in a bone pile. The pile represents wasted time, inventory, and space, as well as lost production capacity and profits. An IR inspection system can locate undiagnosed problems in these boards, leading to repairs that decrease the pile and save you money.

You can also apply IR inspection to help solve design problems. Although some board failures exist as isolated events, an analysis of failures may show a pattern. Once engineers see a pattern of failures emerge, they can isolate the problem to either a problem that occurs during manufacturing or a problem that results from a design specification. Engineers can react quickly to correct manufacturing problems, and they can redesign components, PCB layouts, or circuits to eliminate problems. T&MW

For more information

Holst, Gerald C., Testing and Evaluation of Infrared Imaging Systems, 2nd ed., SPIE, Bellingham, WA, 1998. www.spie.org.

Maldague, Xavier P.V., Theory and Practice of Infrared Technology for Nondestructive Testing, John Wiley & Sons, New York, NY, 2001. www.wiley.com.

John Sexton is senior VP of industrial systems at ART Advanced Research Technologies. He is responsible for research, product development, and manufacturing of IR systems. He has a BEE from the Polytechnic University of New York, and an MBA from Long Island University. E-mail: jsexton@art.ca.  

Table 1. Typical defects identified by IR inspection of powered circuit boards
DEFECT TYPE AND DESCRIPTION	IR IMAGE OF DEFECT (Note 1)
Solder bridge A solder bridge between two components caused overdrive—and excess heat—in surrounding circuits.
Component installed backward A transistor reversed during assembly overheated. Reversal of the transistor also caused problems in other parts of the circuit.
Broken or cut PCB trace A cut trace between a transistor and a capacitor deprived surrounding components of a necessary driving signal. Thus, the surrounding components ran cooler than expected.
Lifted lead A lifted lead removed power from IC U3. Loss of power to this device also affected IC U5.
Missing component This circuit lacks IC U3, which would normally connect to IC U4. Without IC U3, pins 4 and 5 on IC U4 run too warm. Note that the position normally occupied by U3 appears too cool.
Reversed-polarity capacitor Heating at a capacitor shows the capacitor was inserted backward, thus reversing its polarity.
Note 1. Warmer areas appear as red; cooler areas appear as purple.