source: Microwaves&RF article
May 19, 2016 Paul Davidsson, Microwave Distributors Company | Microwaves and RF
Understanding the impact of selecting different materials and manufacturing processes can separate standard high-power resistive components from those designed and tested for true high-reliability applications.High reliability and long lifetimes, two product attributes that tend to go together, both can be enhanced through the thoughtful selection of materials for the design and fabrication of a high-frequency component. A 250-W termination will serve as an example to demonstrate how following proper design guidelines and how standardization of high-power testing methods aid in material selection for reliable, long-lasting high-power components.
Accelerated life testing, where a component is subjected to various operating conditions that may inevitably cause it to fail, can provide insight into how to make its next design iteration more robust and reliable. To better understand design and process issues that may ultimately limit product reliability, particularly for high-power, flange-mounted attenuators, resistors, and terminations, a 1000-h cyclic burn-in methodology was developed to address catastrophic failures.
Accelerated Life Test Specifics
The test procedure was developed to evaluate components for high-reliability (hi-rel) applications, but has merit when applied to new and existing component designs for both commercial and military use. The lifespan of these components depends solely on the end use of the product; e.g., it is known that steady-state operation will ultimately yield the highest reliability when a component is operated according to recommended limits (Fig. 1). In contrast, reliability will be compromised when components are subjected to operation that involves on/off power conditions and temperature cycling (Fig. 2).
Power testing involves two phases. The first phase is intended to verify the film topology of the termination and its power-handling capability for the intended maximum power levels of 250 W. During this test, a component is subjected to full (dc) input power while maintaining a flange temperature at +100°C (Table 1, again). The test runs for 30 min. The temperature of the resistive film is monitored (Fig. 6) to verify that the maximum operating-temperature boundary limits are not exceeded. If the resistive film stays within the boundary range, and no damage is evident to the film, power testing can proceed to the second phase.
Power-test cycling begins with a device under test (DUT) at room temperature (+25°C). Full input power is then applied to the DUT and the flange temperature is raised to +100°C and maintained at that temperature. Once the flange temperature stabilizes at +100°C, the DUT is pulsed with 10 cycles of power-on, power-off operation. This pulsing consists of full power on for 3 s and then zero power for 3 s. After 10 cycles of pulsing, the power to the DUT is turned off and the DUT is cooled down to +25°C. This process is repeated 300 times (Fig. 7, again).
In most cases, standard materials and manufacturing processes are suitable for producing standard high-power passive components. When higher-reliability performance is required, materials and manufacturing processes should be selected with the properties that can provide the target reliability for a given set of operating conditions. Cost of manufacturing the component should also be a design consideration. For high-power components that will have power and temperature cycling as part of the operating conditions, the use of closely matched CTE materials is paramount for long-term reliability, especially in high-power, hi-rel applications like military radar and communications systems.
Editor’s Note: The author has been involved in the development of new product lines of attenuators, resistors, and terminations based on the design methodologies presented above, with enhanced reliability for all commercial and hi-rel applications. The new product lines will be available in the June 2016 timeframe.
Paul Davidsson is President of Microwave Distributors Company, 9020 Kimberly Blvd., Boca Raton, FL 33434; (561) 948-2164, (800) MD-SHELF (800-637-4353).
Paul Davidsson, Application Note C-36, RF Power Components, 1996.
Anaren Microwave, Application Note ANN-8801, Rev. A, “Thermal Power Testing methodology for Chip, Flange, and PCB-Mounted Resistors.”
Barry Industries, “Finite Element Analysis of a High Power Resistor,” April 1996.
Barry Industries, “Pulsed Power Application Note.”
Tong Hsing Electronic Industries Ltd., “Key Process for Power Chip Attachment,” May 11, 2009.
Mini-Systems Inc., Thick-Film Div., “Anatomy of a High Reliability Chip Resistor.”
American Technical Ceramics, “Engineering Guidelines: Design, Test, and Measurement,” 2004.