Thermal imaging finds faults quickly
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 4/1/2009 2:00:00 AM
As chip sizes continue to get smaller, the ability to detect uneven heat generation and thermal dissipation on a chip's surface becomes increasingly difficult but also increasingly important. Engineers in failure-analysis labs and design labs need thermal-imaging tools that can measure temperature distribution over chip surfaces and quickly detect hot spots, which decrease efficiency and frequently lead to early failure.
 Thermal-analysis software can help engineers in failure-analysis labs measure temperature variations on chip surfaces and detect hot spots. Courtesy of OptoTherm. |
“Hot spots reduce chip performance, because current and speed decrease exponentially with temperature,” said Richard Barton, technical director for OptoTherm. “An increase of only 10 to 15°C can decrease chip life by 50%. And large thermal gradients can cause signal-integrity and timing problems, such as when a hot processor circuit is located next to relatively cold RAM.”
Other causes of hot spots on chips are incorrect wiring patterns, shorts or breaks in wiring, incorrect deposition of insulating films, increased transistor leakage, and improper die bonding. If the thermal-imaging system shows that there is indeed a hot spot or spike in temperature on a packaged chip, the system user may need to decapsulate the device in order to look at the bare die and try to isolate the hot spot, said Barton.
Thermal imaging can also make it easier for engineers to locate short circuits. For example, OptoTherm's Micro thermal-imaging microscope system includes a Find Shorts software tool that quickly detects very small increases in temperature as a chip or board is powered. “Using this tool, we've found temperature increases as small as 0.03°C, which translates into an increase of only a few hundred microwatts,” said Barton.
Although thermal imaging is typically used in failure-analysis labs, engineers can also use it to verify new designs. At the design stage, engineers can check for problems like thermal runaway, which is caused by transistor current leakage, which in turn increases as temperature increases.
Failure analysis of small boards, such as identifying defective and marginal components, is also made easier with thermal-microscope systems. Another OptoTherm software tool, called Model Board Comparison, detects very small temperature differences between properly functioning components and defective components on small circuit boards. It captures a sequence of thermal images during functional or diagnostic tests of one or more known-good boards that have an established, correct thermal performance. Those images are then used to create a software model of the board, which is compared with a defective board.
“If you are running a functional test where different areas or components are powering up at different times, it's important to know where hot or cold spots occur, or to identify an area of a module that's not receiving power, or a component that's activating when it shouldn't be,” said Barton.
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