To meet the inspection needs of today’s increasingly complex ICs and PCBs, design engineers are searching for inspection systems that let them attain better thermal design efficiency and defect detection. They want thermal modeling to predict performance and thermal measurement systems to locate hot spots on powered devices.
Modeling is used before prototype construction and typically is a computation-intensive process to predict heat distribution. It allows you to place components, evaluate the effectiveness, and perform what-if analyses before the prototype is built. Modeling also offers thermal maps to show how components are affected by their placement on a PCB.
Thermal modeling is used mainly in the design of PCBs to help avoid overheating, said Greg Lazzaro, product manager at Dynamic Soft Analysis. Accuracy for some modeling packages, such as the company’s BETAsoft thermal analysis program, is within 10% of the actual temperature rise, which is acceptable for most designers.
A thermal-analysis package usually is linked to a computer-aided design software program that allows the transfer of board-placement information and component libraries. Other links include connections between electronic simulation and modeling software to help provide accurate power dissipation of component information and an interface to reliability programs that share junction temperatures of components used for reliability analysis.
A link to a mechanical design package also will guarantee the mechanical compatibility for a thermal design model, said Mr. Lazzaro. For example, it would ensure there is no board spacing conflict when a heat sink is added to a component.
High-resolution modeling of the temperature distribution across an IC enables semiconductor designers to locate hot spots or faults in the IC early in the design, said Ed Lowerre from Temptronic. In addition, a thermal-mapping accuracy of 1 micron and 0.1°C enables you to view the heat distribution across the chip as a range of colors. This allows you to see how the different elements of the design are performing. Detecting faults and hot spots early in the design phase expedites development of ICs that operate effectively and saves the expense of a redesign.
Thermal modeling is a theoretical calculation of the expected heat generation in a product, and it can be used to monitor or inspect the thermal activity on a PCB, said Mike Smith, senior applications engineer at FLIR Systems. Thermal modeling, however, is only as good as the data on which it is based. You also need to verify the thermal models and capture measurement data on the actual product.
Temperature Considerations
To get the most information from temperature measurements on a product with an infrared (IR) camera, you must understand emissivity and background temperature, said Mr. Smith. Emissivity is a property of every object. It describes the efficiency with which an object emits thermal IR energy. The emissivity value is a ratio of the quantity emitted compared to the theoretical amount given off by a perfect emitter, such as a black body, at the same temperature.
Basically, emissivity affects temperature measurements made with IR cameras, said Jim Walcutt, vice president of sales and marketing at Compix. Low emissivity objects, metal in particular, indicate temperatures that are lower than the actual temperature. IR instruments can compensate for emissivities less than 1, which is the value for an ideal radiator or black body. Metals are particularly low in emissivity, typically 0.01 to 0.30, while ceramics are better radiators, ranging from 0.5 to 0.7.
A common technique for minimizing the effects of different emissivity values is to paint the circuit board a uniform matte black, said Mr. Walcutt. This normalizes the surface to an emissivity of approximately 0.95 to 0.97.
Accounting for background temperature also is important for accurate measurements because most surfaces reflect some energy from their surroundings, said FLIR’s Mr. Smith. The magnitude of the reflected energy depends on the emissivity of the surface and the temperature of the background environment reflected by the surface. To accurately measure the temperature of an object, the background temperature must be known and properly calculated into the object’s temperature. This compensation process generally is performed automatically by most IR systems, including portable units. The more sophisticated research and laboratory systems, however, provide a larger array of tools that allows you to deal with the complex object emissivities found on small PCBs.
Thermal precision is a key factor when performing thermography and hot-spot detection using nematic liquid crystals or thermochromic liquid crystals, said Temptronic’s Mr. Lowerre. When using thermochromic liquid crystals, bring the liquid crystals to a desired temperature so the response can be captured by the camera and processed to create a color thermographic image. With nematic crystals, precise temperature control to 0.1°C is needed to bring the liquid crystals to the appropriate temperature so they will change from a clear to an opaque state.
Two other factors that influence measurements are the distance to the target and the atomospheric humidity, said FLIR’s Mr. Smith. These factors do not have a significant effect until you get 15’ or more from the object. The atmosphere attenuates the IR and increases as the distance extends and the humidity goes up. Under controlled manufacturing conditions, however, these factors typically are insignificant for PCB applications.
Some cameras, such as the ones from FLIR, can automatically calculate the temperature based on the user’s settings. State-of-the-art atmosphere transmission models are used in the calculations. All of the calculations are done in the background mode so that the camera still can record images at 60 frames/s with the proper temperatures associated with each pixel.
Trends
The higher level of complexity of today’s ICs requires a more efficient thermal design and better defect detection. To keep up, engineers need thermal measurement systems that enable them to quickly locate the thermal hot spots with high-resolution thermal mapping to show the heat distribution across the device. IC designers must locate hot spots and faults in this initial design phase in a timely, affordable manner.
The most significant trend, according to Compix’s Mr. Walcutt, is toward lower cost for thermal analysis. The effort by the Department of Defense to get lower cost sensors is bearing fruit. The microbolometer and ferroelectric detectors as well as the DSP chip are fueling the move toward significantly less expensive IR viewers and, with time, thermal imaging systems with full metric capability.
Thermography Products
Thermal Imaging System
Measures to 150°C
The PC2000 is a thermal imaging system comprised of an IR camera, a folding stand, and a card that plugs into the expansion slot of a PC. Temperature measurements can be made from 17°C to 150°C with a resolution of 0.2°C. Images contain 47,000 pixels. Thermal evaluation software is provided. The portable PC2000/e includes an external controller module and connects to the parallel port of a laptop computer. Compix, (503) 639-8496.
Software Outputs
Board Temperature Maps
BETAsoft-Board is thermal analysis software used for board design. It outputs board temperature and gradient maps, component and junction temperatures, and the amount the temperatures exceed the designated limits. The program is accurate within 10%, uses 3-D modeling techniques, and has a computation speed of 20 s on a 200-MHz PC for a PCB with 200 components. It models the effects of gravity, pressure, airflow direction, heat sinks, chip fans, and conduction pads. Dynamic Soft Analysis, (412) 683-0161.
Hand-Held Imaging Unit
Features Instant-On Operation
The AGEMA 570 is an uncooled, hand-held imaging and measurement system with instant-on operation, a long wave detector, and a microbolometer detector. The one-piece, solid-state system features automatic image correction and calibration functions. It stores up to 1,000 voice-annotated images and provides 14-bit digital image storage with 320 × 240 pixel resolution. The unit weighs less than 5 lb. FLIR Systems, (503) 684-3731.
Thermography System Performs
Device-Level Characterization
The Model TG01000A ThermoView™ Thermography System has 1-micron spatial and 0.1°C thermal resolution for chips, components, modules, hybrid devices, PCBs, and subassemblies. The system comes with Windows-based software. The modular hardware uses visible-light optics that can be integrated into existing test stations or used as a stand-alone measurement system. ThermoView’s imaging technique processes 30 color images/s. Temptronic, (617) 969-2501.
Copyright 1998 Nelson Publishing Inc.
August 1998