Blind Via Holes

by Brian Dornan

The exponential growth in the number of users in the communications markets in the past few years has forced system designers to look for ways to increase existing system capacity. Some of the avenues used are to employ frequency-hopping techniques, broader bandwidths and Picocell technology. All the above techniques place more and more demands on the overall system and ultimately on the power amplifier.

The function of the power amplifier is to provide distortion free amplification of the input signals. To accomplish this, amplifiers typically operate in a backed off condition, where the output power is a few dB below the maximum output power. This technique improves the linearity of the amplified but reduces the power-added efficiency of the amplifier. The reduction in efficiency of the amplifier, together with the demand for increased system capacity translates to an increase in the amount of heat that must be dissipated.

Power amplifier manufacturers have addressed the issue of increased heat dissipation with a number of design techniques which reduce the thermal path resistance between the active device and the mounting surface. This includes the use of heat spreaders and heat pipes within the power amplifier.

One such technique that has been utilized by the power amplifier designers is to fabricate the amplifier matching and combining circuits on substrate materials that are bonded directly to thick heat sinks. Some such substrate materials are ceramic filled PTFE (polytetrafluorothylene) or a thermal set polymer. The RF/microwave circuit topology utilized to realize the associated circuitry is a coplanar waveguide technology. Using this technique, both the RF/Microwave circuit traces and the ground path are located on the top side of the substrate. Reliable connection of the top side ground plane to the thick metal heat sink, where the active devices will be mounted, is essential for good RF/Microwave performance.

Filtran Microcircuits, Inc has developed techniques to fabricate blind via holes (without the need to drill through the heat sink), that provide the necessary connections of the top side ground plane to the heat sink. The first steps in the fabrication of blind via holes is the drilling of precision depth holes through the substrate material and just into the metal heat sink. The drilled holes are then prepared for a series of proprietary sputtering and plating operations that forms the via connection. A minimum wall thickness for these via's is ½ oz copper. The final finish of this via can be copper, tin, nickel or gold. The minimum via diameter of 0.020 inches (20 mils) has been fabricated with an aspect ratio (via length/via diameter) of up to 1 : 1. The process has been qualified by a number of customers for use in both commercial and military space programs.

For more information on Filtran, call Filtran Microcuits Inc. at (613) 737-0706, or send an e-mail to JKranitz@filtranmicro.com.

Brian Dornan is Vice President & Chief Engineer Microwave Products Group at Merrimac Industries, Inc.