Thermal Analysis for
Today's Complex Designs
"Smaller and faster" are major driving forces for today's electronic designs because they generally mean higher performance. However, they also translate into high power densities, potentially higher operating temperatures and reduced reliability.
With component 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 thermal problems. Because conduction now plays a greater role and convection a lesser one in removing heat, thermal management is best accomplished when incorporated at the beginning of the design cycle. Effective heat flow management must be planned and thermal resistances minimized.
Elevated temperatures are a major contributor to lower semiconductor reliability. And if heat isn't removed at a rate equal to or greater than its rate of generation, junction temperatures will rise. Higher junction temperatures shorten time to component end of life. As very large-scale integration (VLSI) reaches ever increasing levels of density, semiconductors become "the product" and device reliability imposes a profound impact on overall system reliability. Removing heat from these devices is a major task facing design engineers today.
Key to successful thermal management is the ability to obtain comprehensive and accurate temperature data under as near to real life operating conditions as possible.
But the commonly used method of gathering this temperature data using thermocouples, is limited by the large number of points to be monitored and the small size of the components being measured. Connecting tens to hundreds of thermocouples is very time consuming. Also, thermocouples can act as heat sinks and act to conduct away heat. This can effect the accuracy of measurements since this heat sinking may lower device temperatures.
Infrared (IR) thermal imaging is a method or technique which addresses these issues by providing comprehensive two-dimensional maps of thousands of temperatures in a matter of seconds. This is accomplished without the need to make contact with the components. Figure 1 provides an example of the detail available with thermal imaging.
Note the flow of heat from the devices into and across the board. The lower the thermal resistance to heat flow the lower the temperature. Conversely, if the resistance is high, the junction temperature will be hotter.
What is IR Thermal Imaging?
Thermal images are pictures of heat rather than light. The technology is based on the fact that any object whose temperature is above 0 °K radiates infrared energy.
An IR thermal imager captures a portion of this radiated energy and provides a calibrated temperature presentation. Through a variety of scanning techniques, a spatial map of temperatures is generated and displayed or saved as a computer file. The computer-compatible file format is available for subsequent manipulation, display or for process control.
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 large format focal plane arrays operating at room temperature. Thermal sensitivity of less than .1° C is available. Image structure ranges from 30K to >250K pixels per image; spatial resolution can be as fine as 25 microns. Prices range from less than $10K to more than $100K USD.
The technique has been used for years by military, law enforcement and firefighting agencies and is increasingly finding commercial uses in process control, reliability and non-destructive testing (NDT).
Applications of IR thermal imaging extend from microelectronic levels to scanning wide areas of the earth from space; from scanning fast moving processes to monitoring steady state conditions in reliability analysis.
Airborne systems can be used to see through smoke from forest fires. Portable, hand-held units can be used for equipment monitoring in predictive maintenance (PM) programs.
In the USA, imaging is being used to test refrigerators and ovens for energy efficiency. Scans can quickly show losses around seals or where insulation has been improperly installed. Image data can be saved to computer disk for later evaluation and documentation.
Imaging can be used to monitor continuous processes where heat and temperature are indicators of process control. In paper processing heat profiles can be an indicator of moisture content. One manufacturer is exploring using thermal imaging to measure the temperature uniformity of moulds used in producing chocolate candies. Temperature variations may result in defective candies. There are applications for thermal imaging in almost all industries. Heat can be used as a measure of a product's condition and with imaging, fast, comprehensive mapping can be quickly accomplished.
These applications are different but they all share one thing in common. They provide a comprehensive map of the thermal factors effecting the device or process being monitored. If an engineer were to attempt to gather similar levels of information using thermocouples or temperature probes, it would be a major undertaking requiring many man-hours of technical labor. Clearly, thermal imaging is a fast, cost effective way to perform detailed thermal analysis.
Understanding the effect of heat on the reliability of electronic products and the integrity of manufacturing processes is critical if problems are to be avoided. This means the need to understand thermal management techniques and the need for comprehensive data has never been greater. The tools used in the past, contact probes, thermocouples, fingers can't provide this thermal data quickly or cost effectively. With the availability of thermal imaging systems the engineer can now perform the required analysis in a timely fashion without delaying product development schedules, or interfering with a manufacturing process.