- Buyers Guide
Military Microwaves Supplement
Thermal Overdrive Firmware to Improve Thermal Test and Conditioning Throughput
Sigma Systems Corp.
San Diego, CA
Thermal testing and conditioning involve moving heat. Moving heat into or out of a device under test (DUT) takes time. That time affects production throughput and, ultimately, production cost. However, there are methods available to help minimize the time required and improve the traditional approach to thermal processing.
The special features of the C4 controller are designed to reduce the time required for thermal testing or conditioning when using a temperature chamber or thermal platform. The newest of these features is T-Drive thermal overdrive and thermal stress management software, which provides dramatic improvements in thermal test and conditioning times for high mass and thermally latent DUTs. The new feature is supplied as firmware for Sigma C4 and CC-3.5 controllers. While the concept of thermal overdriving is not new, until now controlling thermal overdrive to provide useful benefits has proven largely elusive. The new T-Drive software makes managing thermal overdrive both easy and effective.
Traditional Thermal Control
Common single-probe thermal control strives to maintain the setpoint temperature in either the thermal chamber air stream or at the thermal platform surface. The DUT is then left in the chamber or the platform long enough to allow heat transfer in or out so it will assume the same temperature. However, if the DUT is massive or a poor thermal conductor, its internal temperature can lag the chamber or platform temperature considerably for a long time. If the thermal conditioning is terminated too quickly, the DUT may not actually have achieved the desired setpoint test temperature. Therefore, most users of this traditional approach elect to soak the DUT at the setpoint for more time than it actually takes to achieve the required DUT temperature. As a result, test times are long and productivity is low.
Another approach is the use of a second temperature sensor probe buried inside the DUT (or in a thermal imposter) to control the temperature. This technique may achieve better interior DUT temperature control, but it does so at the risk of extreme temperatures in the chamber or on the platform and, thus, on the DUT surface as well. This method is fraught with risks and control problems and, hence, is seldom used once these difficulties are revealed.
Thermal Overdrive Control
T-Drive is a new firmware feature for Sigma C4 (or CC-3.5) controllers designed to achieve the setpoint temperature inside the DUT either as quickly as possible or at a controlled ramp rate while always respecting the limits of the DUT, controller, chamber or platform. The user may specify not only the absolute limits of the DUT, but also limit thermal shock by specifying a proportionally applied maximum temperature differential for the DUT skin-to-core temperature. T-Drive will maximize speed in achieving internal setpoint temperatures while simultaneously controlling the thermal stress on the DUT.
T-Drive requires the internal temperature of the DUT (or thermal imposter) to be used in the temperature control algorithm. Both the primary temperature probe (located in the chamber air stream or in the platform) and the secondary probe (typically located inside the DUT/imposter) are used to provide a chamber or platform response that can accelerate testing or conditioning while respecting the absolute and relative limits of all affected components.
In simple terms, the software takes advantage of the fact that the temperature differential between two objects determines the rate of heat transfer between them. For example, if a thick and heavy object is to be heated from 0"‑ to 100"‑C and the object is placed in a temperature chamber with a 100"‑C internal air stream, the temperature of the object will rise quickly at first because of the large temperature differential between the chamber air stream and the object. However, as the object continues to absorb heat, its temperature rises and the temperature differential in the constant 100"‑C chamber air decreases. Thus, the rate of heat transfer decreases. The closer the object's temperature comes to the chamber air temperature, the more slowly the object absorbs heat.
Figure 1 shows a typical example of a cold/hot thermal cycle using standard single-probe control and the method for setting a setpoint and letting the DUT soak until it reaches the setpoint temperature. Note that the rate of DUT heating slows substantially as the DUT temperature approaches the chamber temperature. The heat transfer rate to the DUT could be maintained by ensuring a constant temperature differential between the chamber air and DUT. However, because the object is thick and heavy, there is likely a large temperature differential between the object's surface temperature and the core temperature that is being measured. In fact, if the object is a poor thermal conductor, the surface temperature may approach the air temperature and a substantial part of the object's mass may become significantly overheated even though the core temperature has yet to reach the setpoint temperature.
By contrast, T-Drive controlling reduces the amount of overheating of the chamber air or platform surface and, thus, the object's surface, as shown in Figure 2 . The closer the object's core temperature gets to the setpoint, the less overheating is applied by the software. Eventually, the overheating reaches zero just as the object's core temperature reaches the setpoint. If the object has substantial thermal latency, the software can be told to undershoot the setpoint by a fixed amount so that the chamber or platform will stop overdriving the temperature and return to the setpoint temperature early enough to allow the object to homogeneously stabilize at or near the setpoint. Thus, the ramp rate of the object's core is maximized without significant overshoot, as shown in Figure 3 .
Another problem with thermal overdriving occurs when trying to achieve the core setpoint temperature in as rapid a time as possible. The induced surface temperature due to thermal overdrive may be more than the object can tolerate. If this limit is known, T-Drive can be set to restrict the overheating to the limits specified for the object. In this case, a slightly longer time is required to achieve the core setpoint temperature, however, damage to the DUT is avoided.
In a somewhat related problem, it may be necessary to limit the thermal stress to the DUT during the testing process while striving to achieve the DUT's core setpoint temperature as rapidly as possible, or the DUT may have more tolerance for thermal differentials when hot rather than cold or vice versa. T-Drive enables the user to specify these differential limits and still utilize thermal overdriving to improve productivity.
In addition to T-Drive, two other features are available to shorten the thermal testing or conditioning process. The Adjustable Settling Band function may be employed if more than one temperature is to be used in the test or conditioning profile. This software feature does not change the way the C4 controller controls the setpoint temperature. However, when there are multiple steps in the profile, the controller must finish one step before starting the next. The Adjustable Settling Band function allows the user to specify how close the chamber or platform must get to the setpoint and how stable the temperature must be before the next command is processed. If extreme accuracy and stability are required, the controller will meet the setpoint exactly. However, if less rigid standards may be tolerated, the controller will assume the setpoint is achieved once the temperature is within the tolerance and proceed to the next setpoint. Figure 4 shows a chamber's typical settling time characteristics. Significant time may be saved by setting the settling band parameters to the largest value appropriate for the process.
If a fast cycling program between two (or more) temperatures is required but precise settling is not needed at each temperature, the Touch 'n Go feature of the C4 contoller may be employed. This feature allows extremely rapid turnaround for multicycle or multitemperature profiles where it is important to instantaneously reach a temperature before moving on to the next setpoint. Whereas most contollers require the full settling process for each temperature, this feature will simply touch a temperature and then move on to the next step, as shown in Figure 5 . Thus, many more temperature cycles may be achieved in a specified time period, speeding the process substantially.
New firmware is now available for the Sigma C4 controllers that can significantly speed up the temperature testing or conditioning process for high mass and thermally latent devices and provide dramatic improvements in production throughput and costs. Existing owners of C4 controllers may obtain the firmware upgrade free of charge by supplying an appropriate electronically erasable programmable read-only memory (EEPROM) device to hold the firmware and paying the return postage or, for a $50 charge and a C4 serial number, a new EEPROM with the firmware installed may be obtained. Contact the factory for price and delivery information for purchase of a new C4 controller with the firmware included.
Sigma Systems Corp.,
San Diego, CA
Get access to premium content and e-newsletters by registering on the web site. You can also subscribe to Microwave Journal magazine.