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PCB | September 03, 2009

Methods for dissipating heat from RF Circuit Boards

Further expansion of the cellular telephone networks meant that increasingly more compact high-frequency transceivers were needed. The power amplifiers for these transceivers are equipped with RF power transistors which generate high heat losses.
Large quantities of heat are generated when signals are processed in high-frequency applications, and particularly in the amplification of RF signals. This heat needs to be removed quickly and efficiently from the components. RF circuits are very often built into special shielding housings (Figure 1). This makes it difficult to mount a simple cooling element on the component needing to be cooled, or heat removal by the element is inadequate.


Figure 1: RF power amplifier

In RF technology, the heat from power transistors is normally dissipated via the base of the component to the heat sink, e. g. the housing, for which materials with a high thermal capacity are used. This principle allows the heat dissipation system to be integrated into the mechanical construction of RF circuit boards.

For heat dissipation, the whole circuit board is very often mounted on a thick metal plate. Various methods are available. Bare boards, for example, can be "sweat soldered“ onto the heat sink. Or they are simply screwed to a heat sink after circuit board assembly.

We know from experience that such methods involve a lot of time and effort on the part of our customers, especially when large piece numbers are processed and high quality standards need to be met. For that reason, we have developed methods and models that make it possible to integrate the heat sink or other local elements for heat dissipation, into the structure of the RF circuit board during the production stage.

Pre-bonded or post-bonded
Several material manufacturers offer RF materials that have already been laminated to a flat metal plate for heat dissipation. The RF substrate is located directly on a thick layer of metal, with no layer in between. Materials used for the metal plate are copper, aluminium or brass. Because this composite structure is obtained as such from the material manufacturer, the method has become known as the "pre-bonded“ technique.


Figure 2: Metal-backed RF substrate

Figure 2 shows a pre-bonded RF substrate with a thick metal backing of copper. For circuit boards, this base material is processed in the same way as a double-sided circuit board. Plated through holes in the form of blind holes or through holes can be made. The circuit board features a conductive pattern on one side only. However, the processing of such metal-backed accessories, typically 1 mm to 3 mm thick, is time-consuming in circuit board production.

The starting material for the pre-bonded version is a composite material consisting of a relatively soft RF substrate and a thick metal plate. The optimum parameters for mechanically processing these materials differ widely. For that reason, compromises have to be made in production or special machine equipment or configurations have to be used.

The metals to choose from have very different plating properties. Usually, copper and brass can be through-plated without difficulty. However, aluminium requires special pre-treatment and coatings before copper plating. The same applies to the surface finish. The base material in the pre-bonded construction is normally more expensive than when the materials are procured singly.

In the post-bonded method, the finished RF circuit board is bonded by means of an adhesive film to a separately made heat sink. The adhesive film can be selected according to requirements; electrically conductive, thermally conductive and non-conductive films are available. Electrically and thermally conductive adhesive films are normally used for RF power amplifiers. Since the adhesive film is electrically conductive, the heat sink is conductively connected to the grounding of the RF circuit board.


Figure 3: RF circuit board with heat sink

Figure 3 shows a segment of an RF circuit board with electrically and thermally conductive adhesive film and heat sink. The heat sink contains a cavity in which an RF power transistor with flange is mounted. The bond between circuit board and heat sink must be of very high quality. There should be no voids to avoid hot spots. After lamination, the thickness of the adhesive film must be very uniform. Appropriate process management will ensure that these requirements are met.

The post-bonded method has considerable advantages over the pre-bonded method in terms of both the design and production of circuit boards. The RF circuit board can be of any design. There is no limit to the number of conductive pattern layers nor to the choice of RF substrates. The heat sink can also be of any shape and design. In production, the three components are manufactured separately in optimally geared processes. After completion, the single parts are joined by laminating. Any defective parts can be rejected before assembly.

Local heat dissipation by copper coins
In many cases, mounting of a heat sink over the full board area is not necessary or practical, for example, when the RF circuit board has yet to be mounted on a metal holder or in a metal housing. A local heat sink integrated in the RF circuit board will then suffice to transfer heat away from the component to the underside of the circuit board and then to an external heat sink.

Thus, for example, areas with thermal vias can dissipate the heat through the circuit board. If their thermal conductivity is not high enough, pieces of solid copper (Cu coins) are inserted in the circuit board. Copper has high electrical and thermal conductivity properties and can be readily integrated into circuit board designs.

One method is to embed the copper pieces in an RF multilayer circuit board. High-frequency circuits are often stacked up to form a so-called hybrid design. An RF substrate is bonded to a laminate of a different material. Before the complete composite construction is laminated, the Cu coins are placed in an opening in the second substrate. The Cu coins can be connected by plated-through holes and the metallic coating of the circuit board to the circuit ground avoiding ground loops (Figure 4).


Figure 4: RF circuit board with embedded Cu coin

As shown on the right in Figure 4, the Cu coin lies flush with the plane on the rear side of the circuit board. However, the method also allows the Cu coin to finish flush with both surfaces of the circuit board. This design then allows the use of SMT versions of RF power transistors.

Another method is to bond the Cu coins after the RF circuit board has been completed. For this purpose the RF circuit board contains cavities to hold prefabricated Cu coins in place with a conductive adhesive. The adhesive can be thermally and electrically conductive (Figure 5).


Figure 5: RF circuit board with bonded Cu coin

The bond strength of the bonded Cu coins depends on the adhesive used, the type of surfaces, and also on the size and geometry of the bonded area. In the configuration shown, a vertical peel force of 600 N is attained.

Cu coins in press-fit technique
The insertion of Cu coins in circuit boards by means of the press-fit method is a fairly recent method that is practised on circuit boards for motor controls in the automotive industry. This method can also be used for RF circuit boards. Round or rectangular Cu coins are pressed into appropriate openings in the RF circuit boards. The Cu coins can be press-fitted into plated or non-plated openings. Copper plating of the circuit board after the coins have been press-fitted is just as possible as is the application of the usual surface finishes.

The following Figure 6 shows a segment of an RF circuit board with press-fitted Cu coins. Additionally, the coins are surrounded by thermal vias to boost heat dissipation. However, they are not absolutely necessary and in this instance were only fitted because space was available. The thermal conductivity of Cu coins is far higher than that of thermal vias.


Figure 6: RF circuit board with press-fitted Cu coin

The better efficiency of heat dissipation of Cu coins compared to thermal via arrays can be measured with thermographic cameras (Figure 7).


Figure 7: (left) Thermal vias; (right) Cu Coin

Figure 7 shows two thermographic pictures of the power transistor stage that is also shown in Figure 6. In the left hand picture the power transistor is mounted onto an array of thermal vias. The maximum temperature of the component was measured to 105 °C. The power transistor in the right hand picture is placed over a Cu coin that was press-fitted into the RF circuit board. The maximum temperature in this case is only 90 °C.

As the Cu coin has a higher thermal conductivity than an array of thermal vias of the same size the temperature of the power transistor can be reduced by 15 °C in this example. This is a significant reduction of the temperature of the component that could increase the life time and reliability of the component and the whole system.

Summary
In addition to using conductive adhesive films to bond RF circuit boards to heat sinks, new methods have been developed to integrate local heat removal systems in the form of Cu coins. These techniques give the circuit designer flexibility in terms of board design and choice of materials. The models presented are already being used for various RF components in base stations for cellular telephone networks and WiMax services

Author: Markus Wille, product manager, Schoeller Electronics, Wettern, Germany
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