6 - Construction Techniques

Abstract

Parasitic reactances, material losses, and—for higher power devices—thermal limitations combine to make a package selection for RF transistors a technically challenging undertaking. Thermally, the package must allow the user to maintain die temperature below a prescribed maximum, generally 150°C for plastic packages and 200°C for metal-ceramic-type packages. Low power devices have their die mounted on the collector portion of the package lead frame encapsulated in plastic. For power dissipations less than 250 mW, no difficulties are typically encountered. Packages with thicker lead frames have been created to increase power dissipation limits to over 1 watt. The desirable characteristics of good RF power packages are good thermal properties, low interelectrode capacitance, low parasitic inductance, high electrical conductivity, reliability, low cost, and form factor suitable for customer application. Depending on frequency, power level, and voltage of operation, different requirements are dictated for RF circuit layouts. Practically all solid-state RF circuits today use some type of laminate of dielectric material and metal foil (usually copper). Circuit board layouts are made using computers. Low frequency and high power amplifier circuits are frequently realized in push-pull configurations with wideband transformers as matching elements, even if only single frequency operation is desired. Certain transistor packages are less critical in their mounting procedures than others. In order to improve the reliability and to minimize the lead inductance, the seating plane of the transistor lead frame should be close to that of the circuit board. A unique method, which is solderless, can be used for mounting flanged devices. Low power transistors are much less critical in mounting because of their low power dissipation. Two kinds of modules are common in today's RF world. One is a linear amplifier type used primarily for overcoming line losses in cable television systems. The second type is a power amplifier generally used in the transmitter portion of two-way radios.