The RF performance of a surface-mount-technology (SMT) device depends on how well the thermal coefficients of expansion (TCEs) between device and board are matched as well as the design principles and consistency of materials in the device's construction. Other factors impacting performance include the thermal-management techniques that are used to direct the device's dissipated energy and the circuit-level methods that minimize reflective losses. In the 20-page white paper titled, "Understanding the Basic Thermal Properties of SMT Devices," International Manufacturing Services delves into three different scenarios in which the power capacity of an SMT device can be optimized through the appropriate use of thermal-conduction principles.
Specifically, the paper focuses on the following situations: basic thermal management on the circuit board to which an alumina SMT device is mounted; the implementation of improved processing methods on the part of the SMT device itself; and the use of alumina nitride rather than alumina as the substrate for the devices. For SMT ceramic devices, thermal conduction is the primary mode of heat transport. Making good use of conductive design principles and applying them to the device's operation in the circuit will thus optimize its overall RF performance.
This paper reviews the basic principles of thermal physics (i.e., radiation, convection, and conduction). It then investigates and compares examples of heat flow for a typical SMT ceramic substrate of 25-mil thickness using radiation, convection, and conduction for cooling. Conduction is found to be the most efficient cooling mechanism, as most SMT ceramics have relatively high thermal conductivity. In addition, most surface-mount devices are quite thin. As a result, they cannot take advantage of convection like an SIP or DIP can.
The note provides some additional observations as well. For example, a chip with a very small area and a certain thickness will not be able to take advantage of conduction cooling to the same extent as a chip with a larger area and same thickness. Essentially, the benefit of conduction is expected to decrease for a given thickness when the area of the chip in question decreases. The paper concludes that it is in the designer's best interest to facilitate heat conduction to the highest degree practicable. It is then possible to enhance both the thermal and overall RF performance of the SMT devices employed in the circuit boardno matter what device substrate material is used.
International Manufacturing Services, Inc., 50 Schoolhouse Ln., Portsmouth, RI 02871; (401) 683-9700, FAX: (401) 683-5571, Internet: www.ims-resistors.com.
