Here in the Freescale Application Support Department, we get lots of calls from RF design engineers, who are in a desperate, crises mode. They have created a design using the best simulation tools possible and after having just built up their first prototype units, they are having some sort RF performance issues of varying degrees. Sometimes it is poor RF performance numbers not in line with the data sheet, sometime it is spurious oscillations, or shifting RF performance numbers with temperature or maybe even, in extreme cases, total device failures.
By a wide margin, the number one cause for most of the above mention design problems can be traced back to a serious shortcoming in the back side RF grounding connection of the device and its’ contact with the associated PCB ground plane.
There are two key assembly requirements that must be accomplished by the devices’ back side interface.
The interface must provide a good thermal contact between the RF device and heatsink. This interface needs to be flat, smooth, and maybe have a microscopic gap filing interface material like solder, thermal grease or a very thin elastomeric pad of some sort. All of these attributes are needed in order to facilitate the heat flow out of the device and into the heatsink-to-air assembly.
Some common mistakes made here include excessive solder voiding, excessive thermal grease thickness and excessively rough machined surfaces with no gap filling material.
The second item that must be created in the back side interface is a solid, consistent RF plainer ground. Not a DC point ground but a broad, ultra low electrical resistance connection between the back side of the RF device and the back side of the PCB. Notice I did not say heatsink. The heatsink may be used as part of the grounding structure or it may not, but the key required function is to mesh the PCB ground plain and the RF device’s ground plain with a low resistance, high currant carrying assembly method.
Most common mistakes that occur in this regard are associated with the PCB grounding. At low frequencies and low RF powers, one can screw the PCB to the heatsink and create point contact DC grounds. That just does not work at the higher frequency above 500 MHz and higher powers above 100W. One needs to solder the back side ground of the PCB to the back side ground of the RF device thru a copper carrier or heat spreader assembly in order to be able to handle the very high RF circulating currants that are created in this area.
At RF power levels above 100W and with low RF impedances at the device’s leads( near 1 ohm), the RF currents tend to be very large.
P= I**2/R or I=10 Amps in the given example. Designers take great pains to make sure the top side PCB impedance matching structure is consistent, robust, repeatable and has a ultra low RF resistance connection. But if there is 10 amps of RF current flowing on the top side of the PCB, then there is also 10 amps flowing on the back side ground plain between the PCB and the RF device and yet that interface structure is often largely uncontrolled. The back side ground consistency is just as important as the top side matching consistency and yet the back side ground is never documented, rarely measured and very often overlooked.
Another common mistake is to apply a chemical chromate finish to the aluminum heatsink to prevent oxidation. Class 3 is the insulating version and Class A1 is consider electrically conducting but the reality is that both prevent the ultra low contact resistance that is required in a back side RF grounding application. That is why the heatsink is a poor choice in providing the electrical connection between the PCB and the RF part. A much better design methodology is to have a consistent, robust, fully soldered assembly that controls both the top side matching and the back side return path thru a carrier, coin, heat spreader or PCB back side metal layer.
Picture 1, Three methods of mounting a RF power Device. From the left, True Surface Mount Soldered with Gull-Wing Leads, Back Side Solder Down, and Screws with a Center-Pushing Clamp.
Conclusions
Clearly a fully soldered assembly in the critical area between the back side source of the device and the PCB ground plane thru a heat spreader is the design optimum solution. Anything less and one is asking for trouble.
For reference, here is a list of mounting application notes that are available from the freescale.com/rfpower web site: AN1908, AN1940, AN2467, AN3778 and AN3789