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Thermal Imaging:
More Critical Than Ever

Have you ever experienced this?

Your new design has a problem. Power consumption exceeds the power budget; now you've got to find the cause and correct it. Your first step is to "probe" the board with your "calibrated finger", next you pass the back of your hand over the components. This doesn't work so you get serious. You get a temperature probe and begin looking for the culprits, device by device. Probing of some key components doesn't shed any light on the problem so now you "hook up" thermocouples. The results are the same; there isn't an "aha, that's it" to be found.

Problems with Traditional Temperature Measuring Techniques

How often have designers used their finger or back of the hand in an attempt to find thermal problems? Today's smaller components may not contain enough heat to seem hot to the touch when in reality they are running too hot. Another shortcoming surfaces when the problem is more than just one or two devices being too hot. If a large area of a board or design is affected the situation may be too complex for single-point measurements to uncover. Thermocouples, that old stand-by, may introduce their own problems. They can act as heat sinks and lower a component's temperature. In addition, installing and monitoring a large number of thermocouples can be time consuming and costly. Non-contact thermal imaging, on the other hand, provides a comprehensive thermal picture and assures that temperature readings aren't affected by the heat sinking of contact sensors.

What is IR Thermal Imaging?

Thermal imaging is the process of creating a picture of heat. The technique has been used for years by the military, law enforcement and wildland firefighting agencies . It is now finding uses in commercial applications in process control, design evaluation, non-destructive and reliability testing as well as failure analysis.

Infrared thermal imagers provide non-contact, line of sight measurement and display of surface temperatures and temperature variations. The technology is based on the fact that any object whose temperature is above 0 °K radiates infrared energy.

The amount of radiated power is determined by the following equation.

            W = E * B * T4     Watt cm-2

where
W = Spectral radiant exitance (radiation)
E = Emissivity
B = Stefan Boltzmann Constant (5.67x10-12   Watt cm-2 °K-4)
T = Temperature °K

Radiated energy (power) is proportional to the body's temperature, raised to the 4th power. For example, a black body (emissivity of 100%) at 30° C would have a radiation density of 5.4 mW/cm². That same blackbody at a temperature of 150° C would have a radiation density of 139.2mW/cm².

Infrared thermal imagers capture a portion of this radiated energy and are calibrated to indicate temperatures over a specific range. Through a variety of scanning techniques a spatial map of temperatures a thermal image is then created.

Thermal imaging systems are available with a wide range of capabilities, features, form factors and prices. Scan speeds can range from "real time" to seconds per image. Systems sensitive to 3-5 and/or 8-12 micron wavelength bands are available. Detectors range from simple, single element, thermo-electrically cooled to complex, cryogenically cooled focal plane arrays. Thermal sensitivity of less than .1° is available. Image structure ranges from 30k to500k pixels per image; spatial resolution can be as fine as 15 microns. The price range is equally broad running from $10k to $100k.

Greater Circuit Densities Increase Thermal Problems

Smaller and faster are major driving forces for today's designs. Together they translate into high power densities and the potential of thermal problems reducing reliability. Historically, i386 chips could be expected to dissipate 1.5 watts; i486's between 4 and 5 watts. Pentium(i586) chips can be expected to dissipate between 12 and 15 watts. Each generation of iX86 devices has seen about a 3X increase in power consumption-- and heat. Surface mount assemblies with plastic quad flat packs (PQFP) of 132 leads and 25mil pitch, mounted with a placement pitch of 1.5" are commonplace. IC packages with lead pitches of 10mils and pin grid arrays with pin counts of 256 and higher are coming on-line. MCM packages containing 0.3"x 0.3" die on placement pitches of 0.5" are also becoming commonplace. Ball Grid arrays(BGA's) Chip- on-board(COB) present even more challenges.

Thermal Troubleshooting

How do you analyze thermal performance on these designs when component counts may approach 400 on a 12" x 16" board and components are mounted on both sides of the board?. The sheer complexity of designs precludes the use of traditional thermocouple thermal analysis.

Many of these components are fine-pitch or surface mount devices, and their heat capacity is similar to that of thermocouples or contact heat probes. The effect of these sensors on the temperature of some small components can be profound, altering measurements by as much as tens of degrees.

Infrared thermal imaging offers users not only a fast, easy-to-use, non-contact method of measuring temperatures but also a "map" of heat flow patterns. In just a few seconds, a thermal imager can generate a map of temperatures over an entire board. Turning it over allows one to see how heat is conducted in and by the board. If you connect the system to a computer you can store and display thousands of images over spans of time ranging from parts of minutes to hours, days or even weeks of testing. Each of the stored images contains the full temperature record.

Thermal Management From the Start

With semiconductor packages being more compact, dissipating more power, and having lower profiles, it's no longer sufficient to simply add "a bigger fan" as a downstream fix for poor thermal management techniques. Because conduction now plays a greater role and convection a lesser one in carrying heat away from sensitive devices, thermal management is best accomplished when considered at the beginning of the design cycle. Likewise, it's often more important to consider how effectively heat is being removed, and less so to determine which specific component is getting hot. Heat flow paths must be planned and thermal resistances minimized. Even CMOS can have problems, at clock speeds above 35-40Mhz CMOS circuits can consume more power than similar T2L circuits.

Another thermal image dramatizes the issue:

Figure 3Figure 3

This is an image of a circuit board containing two Pentium chips with heat sinks. Forced air cooling is passing across the board from left to right. This image clearly shows the effect of shadowing from heat sinks on the chips mounted on the board, as evidenced by their higher temperatures. One can also see how the heat is more concentrated near lower right quadrant of the board. This type of analysis would allow an engineer to evaluate the efficiency of the cooling and make necessary changes. These problems aren't characteristic of only computers. Peripherals, including displays and printers, may suffer from the same problems. Even communications products can suffer from heat build-up resulting in decreased product reliability.

Conclusion

Your ability to measure temperatures in complex electronics assemblies can be limited by accessibility to points of concern and the amount of data required to be meaningful. You also have to consider effects of the measuring transducers as their inherent thermal mass can alter the temperature of the components.

To maintain data integrity, non-contact measurements are the thermal measurement choice, and infrared thermal imaging is the leading method for making the measurements. Aside from the compelling technical reasons, thermal imaging is highly cost effective when compared with single point temperature measuring techniques.

Compix, Inc.

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