What consumes more power—a Pentium processor or a 25-W incandescent light bulb? The answer may surprise you. Current popular processors may dissipate 15 W to more than 40 W, depending on the type and speed. Add to that the heat generated by all the other high-density components on the board and you can see why today’s designers have a severe heat-management problem on their hands.
To assess the magnitude of the problem and alleviate potential overheating, the temperature rise of every component as well as the heat distribution over the entire PCB must be measured. Smart designers who wish to avoid unpleasant surprises when these measurements are made now opt for applying thermal modeling techniques before prototype construction.
In the past, most temperature measurements were made by attaching thermocouples to a few components, sometimes selected by limited modeling results. But this is no longer sufficient for highly populated dense assemblies.
The reasons are twofold:
Space limitations may not permit attaching thermocouples to the many potentially critical devices on the board.
Although being innately accurate, thermocouples may not provide true temperature readings.
“Thermocouples tend to read low because they can be improperly attached or may act like a heat sink and absorb heat from the device,” commented Andrew Teich, Vice President of Sales and Marketing at Inframetrics. “The phenomenon of heat sinking is particularly severe for plastic packages and small chips. The problem can be minimized by using 1-mil thermocouples, but their small size also makes them difficult to work with.”
Thermal Profiling With IR Imagers
You can avoid these problems by using infrared (IR) thermal imagers, which enable noncontact line-of-sight measurement and display of surface temperatures. Any object whose temperature is above 0° K radiates IR energy. The amount of radiated energy is a function of the object’s temperature and its relative efficiency of radiation, known as emissivity.1,2
Various scanning techniques can be used to generate a thermal profile of the object of interest. Resolution may be <0.1° C and the image may be composed of 30,000 to 250,000 pixels. While some IR imagers only generate a thermal map for coarse qualitative comparisons, others feature high-precision radiometric capabilities.
The latter type of system is needed to perform design and thermal management assessments. Most systems include extensive calibration and software aids, making their use as simple as taking pictures with an ordinary camera. Typical systems provide completely quantified thermal images with numerical temperature readouts for any point of interest.
IR imaging systems may be used to verify thermal performance of individual components, such as heat sinks, or entire boards and subsystems. For instance, Aavid Thermal Technologies uses IR imaging equipment to evaluate the performance of various heat-sink designs under diverse operating conditions.
“IR imaging equipment helps visualize heat-spreading resistance variations due to changes in power levels,” said Patrick Riley of Aavid Thermal Technologies. “As power levels change, so do the internal isotherms, and thermal-imaging information helps characterize the sink for different power levels and airflow regimes.”
Since IR thermal imaging can gather comprehensive temperature data from many points quickly and relatively accurately, it is well suited for PCB evaluations. “Verifying the thermal performance of a large circuit board containing hundreds of active devices would take many hours and a lot of labor if thermocouples were used,” said Jim Walcutt, Vice President of Sales/Marketing at Compix. “On the other hand, thermal imaging shows all details in a matter of seconds.
“The efficacy of various cooling schemes can be readily assessed since all the nuances of temperature and heat flow show up in images. Even small anomalies which might go undetected with thermocouples are readily visible,” Mr. Walcutt concluded.
Modeling and Verification
The classical method of building a series of prototypes and applying consecutive fixes until satisfactory performance is achieved is no longer an acceptable mode of operation in today’s time-to-market arena. Modeling and performance simulation prior to prototype construction are becoming more commonplace.
Finite element analysis, a computation-intensive technique, forms the foundation of several thermal modeling programs now available on PCs. These programs can be used to provide thermal distribution maps based on given component placements and power consumption or to predict junction temperature within devices as a function of an assumed external environment.
The programs include extensive libraries. “For instance, the SAUNA library not only contains typical component data but also thermal properties of conductors, surface finishes, insulators and IC packages,” said Fred Ebert, President of Tatum Labs.
Unacceptably high temperatures are usually encountered when too many high-dissipative components are too close to each other or when airflow is inadequate. Modeling allows you to move components to alternate locations, evaluate the effectiveness of different heat sinks, examine the effects of airflow changes, and perform other what-if analyses—all before the prototype is built.
Typical situations requiring remedy include inadequate airflow to a critical component due to up-stream blockage by taller components or downstream overheating due to flow stagnation. For these, as well as most other often encountered situations, you must view the heat-distribution map in three dimensions and from all sides.
“To meet these needs, we have implemented 3-D solid viewing and rotation capability in our BETAsoft-Board thermal-analysis program,” said Dr. S. C. Yao, Director at Dynamic Soft Analysis. “The solid modeling exhibits the predicted temperature distribution on the board and the temperatures of the component casings and junctions within a 10% accuracy. At the same time, the relative heights of all components are clearly displayed (Figure 1).
“The validity of thermal modeling depends to a large degree on complex thermal characteristics and the interrelationships of the individual parts, their heat sinks and the properties of the mounting material itself,” said Pat Finney, Senior Applications Engineer at FLIR Systems. “Thermographic analysis performed by IR imagers complements the modeling process to verify its effectiveness and accuracy.”
“Providing inputs for the confirmation and fine-tuning computer-generated models are major applications for our Thermovision 900,” said Bo Wallin, Technical Director at AGEMA Infrared Systems. “The engineer determines where and why the thermographic measurements are different from the computer model’s predicted characterization, and then re-examines the assumptions on which the model is based. “The IR test data allows the engineer to identify erroneous assumptions and determine what phenomena had not been taken into account,” he continued. Repeating the modeling and test process leads to improvements in modeling techniques and near-perfect results.
Compensating for Emissivity Variations
While many thermal characteristics must be taken into account when modeling components and assemblies, only one device-related factor must be considered for performing accurate temperature measurements, namely emissivity. Emissivity is defined as the ratio of emissive power of a body compared to that of a perfect emitter (black body) having the same area and same temperature.
“Because of the different materials used to manufacture a packaged IC, a hybrid circuit, a PCB or any electronic circuit, its emissivity can span a wide range,” stated Mr. Wallin. It extends from 0.1 or lower to 0.95 or higher, possibly even on the same component.”
Variations in emissivity must be corrected or eliminated to obtain true temperature measurements. Corrections may be performed by first determining the object’s emissivity on a pixel-by-pixel basis, storing this information and using it to recompute each pixel value of the image of interest.
“The emissive characteristics of a target can be quantified by evaluating its relative emittance at two known wavelengths or at two known temperatures,” explained Mr. Finney. “The majority of commercial infrared systems utilize the latter method.”
The FLIR systems, for instance, employ a precision thermal stage to set the target to two known stable temperatures. A relative emittance image is generated at each temperature as well as an image that depicts background radiation conditions. The system then automatically merges this information into a single emissivity correction file.
Most thermometric systems use PC power and proprietary software to perform emissivity compensation automatically—and quickly. For example, the Thermovision 900 equalizes emissivity on a component in near real time, said Mr. Wallin. Software compares the acquired image with the previously stored emissivity map of the component, treating each point individually. It then recalculates and converts the initially acquired apparent temperatures into true temperatures.
While emittance-correction software provides results that correlate well with carefully collected thermocouple data, its use is not always mandatory. “It is often more expedient to raise the emittance of low-emittance devices to known values than to use pixel-by-pixel emittance software,” said Mr. Teich. “This is typically accomplished by coating the components with a high-emittance coating like black paint or spray-on foot powder.”
Caution is necessary when applying a uniform coating to eliminate emissivity variations since it may alter the electrical performance of the components or the board. When properly applied, sprinkling foot powder on a board can provide a uniform emissivity of about 0.95, according to Dr. Yao.
Alternate Techniques, Other Applications
Thermal imaging systems are useful not only for design confirmation but also for a host of other functions, especially failure detection and analysis. Common applications include short detection in bare and loaded multilayer PCBs, void detection in die bonds, and board troubleshooting based on a comparison of the thermal signature of a defective board to that of a known-good board, commented Mr. Teich.
But limitations are encountered when analysis is to be performed at the chip or wafer level. Most IR systems provide a resolution of 20 to 30 microns, and some extend into the 10 to 15 micron range, but even this is insufficient for present-generation micron or submicron ICs. Micron-resolution thermal analysis is possible, however, by using liquid crystals (LCs).
Unlike conventional materials, LCs do not transmute abruptly from a solid to a liquid state upon heating, but transgress through an intermediate liquid crystal phase (nematic) state. The temperature at which the LC progresses into and out of the crystal phase state is sharply defined and depends on the chemical crystal composition.
LC state changes are accompanied by changes in optical behavior, such as being translucent or opaque. In the nematic state, the LC exhibits dual properties such as anisotropy and birefringence (splitting a light beam into two components).
LCs have long been used to identify hot spots on wafers. Exact quantitative evaluations, however, could not be performed until it was realized that the temperature of the chip had to be held with high stability and accuracy.
The Liquid Crystal Microthermography System from Temptronic addresses this problem. It combines a temperature chuck and controller, a camera and a frame grabber, a PC and software into an integrated 1-micron, 0.1° C-accuracy system.
To perform an analysis, a few drops of an LC solution are applied to the chip or wafer, which is heated until it turns dark when viewed with polarized light. The temperature is reduced until the device turns clear again and power is applied to the device.4
The heat produced by the applied power will cause darkness to reappear and a picture is taken. The temperature is then reduced, resulting in a reduction of the dark area, and another picture is taken. This is repeated through a series of temperature steps which may be as small as 0.1° C.
Each picture corresponds to a particular temperature and is color coded. When the pictures are superimposed, a temperature map is created and cursors may be used to read the temperature at any point with 1-micron spatial accuracy and 0.1° C temperature precision (Figure 2).
An LC-based system developed by LCImage uses thermochromic liquid crystals (TLCs). When TLCs are in the nematic state, being birefringent, they cause reflected light to become circularly polarized. The color spectrum of the reflected light ranges from the longer wavelength (red) to the shorter wavelength (blue), depending on temperature.
To obtain temperature data, TLCs with suitable threshold temperature levels are applied to the heat-emitting object. A multicolor map is produced; and since there is a direct color-to-temperature correlation, the resultant map is a true temperature-indicating presentation.5
The color image is picked up by a video camera, quantized, compared to a standard temperature scale and processed in a PC. TLC are available in narrowband temperature ranges (1° or 2° C) or wideband covering up to 20° C. It is also provided in a variety of forms, making it suitable for chip, component and PCB applications.
References
1. Walcutt, J., “Thermal Imaging Critical for Today’s Complex Circuits,” EE-Evaluation Engineering, October 1993, pp. 50-53.
2. Walcutt, J., “Reducing Soldering Defects With Thermal Imaging,” EE-Evaluation Engineering, March 1995, pp. 60-62.
3. Finney, P., “The Technology and Application of Infrared Imaging,” EE-Evaluation Engineering, October 1994, pp. 70-73.
4. “Breakthrough: One Micron Defect Detection,” Temptronic, Application Note 1.
5. Farina, D., “Making Surface Temperature Measurements Using Liquid Crystal Thermography,” Electronics Cooling, October 1995, pp. 10-14.
Thermography Products
Thermal Imaging System
Is PC Operated
The Compix PC2000 is a thermal imaging system comprised of an IR camera, a folding stand and a card that plugs into the expansion slot of any IBM-compatible PC. Temperature measurements extend from 17° C to 150° C (5° C to >1,000° C optional) with a resolution of 0.2° C (0.1° C optional). Images contain 47,000 pixels. Thermal evaluation software is provided. The PC2000/e model is portable. It includes an external controller module and connects to the parallel port of a laptop computer. Compix, (503) 639-8496.
System Offered With
Extensive Analysis Software
The IQ 812 Thermal Imaging System for fine geometries includes electronic pan, scroll and zoom capabilities, and has a measurement accuracy of ± 2%. It offers 640 pixels x 480 lines of display resolution and is sensitive to IR radiation in the 8- to 12-micron range. Multiple cross points/color isotherms highlight differences to 0.06° C. The FLIR AnalyzIR+™ software features thermal analysis tools ranging from unlimited line, point and area measurements to trend analysis, image subtraction, alignment, rotation, isotherms and histograms. FLIR Systems, (503) 684-3731.
Modular System Provides
Accuracy, Flexibility
The Thermovision® 900 series of infrared thermal measurement/analysis systems is modular, interfaces with workstations or PCs and is suited for a range of measurement applications. Scanners cover the 3- to 5-micron and the 8- to 12-micron measurement bands and are available in liquid-nitrogen, Stirling or thermoelectricly cooled configurations. Data acquisition is performed with 12-bit resolution for comprehensive image analysis. Temperature-measurement ranges extend from -10 to +2,000° C. AGEMA Infrared Systems, (201) 867-5390.
Software Models, Analyzes
Thermal Characteristics
The BETAsoft 3-D Thermal-Analysis Software evaluates heat conduction convection and radiation to facilitate component, board and system thermal designs. The component-analysis software models thermal aspects of active and passive devices. Heat-sink or conduction-pad models can be added with mouse clicks. The BETAsoft-Board software simulates multilayer boards with up to 2,000 components. The system-analysis package determines cooling requirements for card cages and cabinets. Component placement data may be imported from most CAD programs. Packages operate under Windows 3.1, NT, DOS and UNIX. Dynamic Soft Analysis, (412) 683-0161.
Imaging System Configurable
For Field or Fixed Application
The TH3100 Series Thermal Imaging System consists of a detector unit, interchangeable lens and a control/display module for field applications. A plug-in card is provided for PC-based fixed-location operation. The TH3102 covers the -50° -to-250° C range, the TH3104 extends from 0 to 300° C, and the TH3114 is a 290° -to- 2,000° C version. Resolution is 0.1° C for the TH3102 detector unit and 0.3° C for the TH3104 and TH3114 detector units. Up to 5,000 images may be stored on disk. Mikron Instrument, (201) 891-7330.
Modeling Software Addresses
Thermal Issues During Design
The SAUNA™ Electronic Equipment Thermal Modeling Software is used during the design process to evaluate thermal aspects. It models and performs thermal analysis on components, heat sinks, PCBs, multiboard stacks and enclosures. All heat- transfer modes, conduction, convection and radiation, are accommodated. SAUNA generates thermal distribution maps, including isotherms, to determine a specific temperature at any point. What-if analysis can be performed to assess the effects of dimensional, component placement or airflow changes. Tatum Labs, (313) 663-8810.
Palm-Sized Infrared Camera
Suited for Scientific, Lab Use
The SC1000, a new member of the ThermaCAM™ line of hand-held focal plane array radiometers, features low power consumption, high sensitivity and accuracy, and PCMCIA compatibility for storing up to 256 images on a card. It provides a 12-bit real-time video output and interfaces with the ThermaGRAM PRO 95 digital image- processing system for Windows 95™. Spectral filters facilitate measurements to 2,000° C, and lenses range from a 15-micron resolution microscope to telescopes with field of views from 32° to 2° . Inframetrics, (508) 670-5555.
LC Thermography System
Provides 1m m, 0.1° C Resolution
The Integrated Thermal Imaging System utilizes liquid-crystal technology, a precision temperature-control system, a camera and a PC with frame-grabber and custom software to generate thermal maps featuring 1-micron spatial and 0.1° C temperature resolution. The ThermoChuck® assembly for wafers or the ThermoSocket® assembly for packaged components holds devices at a controlled temperature. The ThermoJogger® hand-held control unit facilitates temperature adjustments in 0.1° C steps. Temperature profiles at cursor-selected locations and direct temperature readouts are provided. Temptronic, (617) 969-2501.
TLC-Based Imaging System Uses
Common Color Video Camera
The LCImage Temperature Measurement System performs direct temperature measurements using any calibrated thermochromic liquid crystal (TLC) formulation. Spatial resolution of <1 micron can be achieved. The system may be used for micro- and macro-scopic applications, such as temperature measurements on IC dies and PCBs, respectively. It includes a solid-state color camera, a stable light source, a color frame grabber, a color/temperature calibration device, a PC or a Macintosh platform, and TLC materials in an easy-to-use kit form. LCImage Partnership, (617) 965-9109.Thermal Video System Provides
High Temperature Resolution
The TVS-8000 Thermal Video System uses indium antimonide 2-D focal plane array technology which detects temperature variations of less than 0.0025° C. The system converts infrared light/heat to 320 x 240 pixel images and has a high-speed frame time of 1/51.4 s, ± 0.4% accuracy, 12-bit resolution and a display speed of 60 frames/s. Other features include auto-ranging modes, real-time subtraction, five movable cursors, frame averaging and 30-frame disk storage. Cincinnati Electronics, (800) 852-5105.
Copyright 1996 Nelson Publishing Inc.
August 1996