High Frequency Circuit Materials With Increased Thermal Conductivity

Thermal management has always been a major concern in the design of high frequency, high power electronic devices. Historically, in most power amplifier designs, the vast majority of the power dissipated is from the high power field effect transistors (FET) themselves. In these cases, the high power dissipation requires direct attachment of the FETs to a finned heat sink, or conduction through copper “coins” placed in areas where the polymer composite dielectric circuit substrate has been machined away. Direct connection is required because the dielectric material is two orders of magnitude lower in thermal conductivity than copper. There has recently been increasing interest in higher z-axis thermal conductivity high frequency circuit laminate materials for applications where the heat removal path needs to be through the thickness direction of the dielectric material itself. A number of recent studies have modeled different applications where the thermal conductivity of the dielectric material is of interest [1-3]. One example of such an application is a high power circuit where RF trace heating can be significant [4]. In this case, in addition to high thermal conductivity, the lowest possible conductor and dielectric losses are desirable to minimize power dissipated. Other examples include remote radio head [5] and similar applications where weight and size are critical. Lower power FET devices that may not require a thermal coin may benefit from higher thermal conductivity dielectric substrates, as well as some high speed digital devices. In some cases, heat dissipation through a dielectric laminate is improved by the use of multiple thermal vias, also known as a “via farm.” While this obviously cannot be used to control RF trace heating, many devices can be soldered to a pad connected to the thermal vias to greatly increase heat dissipation through the laminate. In the present article, we demonstrate the large reduction in the temperature rise of a surface-mounted resistor when high thermal conductivity dielectric laminate materials are used. Surprisingly, the higher thermal conductivity dielectric materials significantly reduce the temperature rise even in the case of mounting the resistor on a via farm. The effects are demonstrated for both a Dk = 3.5 material and a Dk = 6.15 material, useful for size reduction in circuits with wavelength dependent geometries.