A System of Producing High-Power RF Circuit Boards

Al Wasserzug
Cirexx International

The paper will propose to present a technology for the fabrication of Printed Circuit Boards (PCBs), used primarily in high-power RF/millimeter wave applications, which involves the use of a thermally engineering metalized layer with superior thermal characteristics and a ceramic-matched co-efficient of thermal expansion (CTE).

The resulting PCBs allow the user to direct die-attach high-power RF die, such as GaA and GaN devices through a cavity in the outer core layer(s), directly to the thermal layer below; and then wire bond to the surface conductive layer. The thermal characteristics of the engineered material quickly and efficiently evacuate the significant heat generated by the die while CTE "anchors" the resulting PCB substrate assuring the reliability of the die-attach wire bonds.

These PCBs are commonly constructed with other high technology materials such as polyimides, PTFEs and ceramic filled dielectrics and adhesive systems. Advanced process techniques are also employed in the fabrication of these PCBs: depth laser ablation and NC routing, blind and buried vias, via hole fill, edge plating, mixed finishes and multiple sequential lamination cycles. The presentation will explain the construction and provide general design guidelines.

This technology replaces the need for bulky, heavy heat sinking schemes around the high-power devices and ceramic hybrid packages. And since the item is essentially a Printed Circuit Board, made from otherwise typical Printed Circuit Board materials, the other electronic components in the design and on the board can be standard "plastic parts" attached in a standard process such as a vapor phase soldering. This results in an electronic board system that is smaller, lighter weight, and certainly much less costly than the ceramic hybrid, all-die alternative.

Thermal management of Printed Circuit Board (PCB) structures is a critical and ever-present issue, but perhaps never more than in the high power amplifier world of long-range communications devices. These designs operate on a constant delivery of localized high power feeding into precision RF (radio frequency) circuitry, usually on a PCB. The heat dissipated by these high power devices is significant and detrimental to the reliability of all of the components of the system.

There have been many methods developed and employed to measure, manage and mitigate this dissipated heat. Most involving thermally conductive materials such as aluminum, copper, graphite and composite combinations thereof. And while these approaches have been successful in addressing the heat dissipation factor in most of the applications, they all, however, lack a vital characteristic for the most sever of the environments: very high mechanical stability to protect against interconnect stress and component fatigue due to prolonged high thermal exposure.

In the most advanced high power designs the amplifiers are attached to the circuit substrate as bare die and wire bonded to the surface. The die-attach method is used both for device performance and miniaturization of the entire system. This has traditionally meant that the circuit substrate containing the RF circuitry is something on the order of a ceramic hybrid. Something made of materials with a dimensional stability - measured in CTE (coefficient of thermal expansion) - in the range of common ceramics, which is generally 5 to 20 ppm/degrees C.

This is necessary for not only accurate die placement, but also subsequent reliability of the unit in operation. The "ribbons" commonly used for this wire bonding process are typically 0.001" (25.4 i) in diameter and can be as small as 0.00025" (6.35 i). Bonding these wires and then having them stay in place without breaking over the life of the system - many of which are designed to operate in an environment with dramatic temperature extremes and high shock and vibration - is of the utmost importance.

In summary, a PCB constructed with TEML can solve the difficult challenges of efficient heat sinking with superior dimensional stability. The resulting system can be lighter, easier to manufacture, more reliable over time and less costly. The process to fabricate such PCBs is similar to ordinary techniques - with a few special steps - and does not require any special equipment.