As a member of an application-support team, we regularly write and rewrite several application notes dealing with how to mount the RF transistors for maximum RF performance.
Through history, application notes such as AN1040, AN1041, AN1617, AN1673, AN1674, AN1907, AN1918, AN1923, and the latest versions of AN1949, AN3263 and AN3789, all deal with recommendations for device mounting.
The one item that is missing from all of these application notes is the total benefit of solder-mounting the devices when compared to any form of screw or clamp mounting. Some of these application notes compare various forms of thermal interface materials on RF performance, but they all stay within the confines of the particular mounting method.
What is missing is an application note that jumps across all three popular mounting methods and only compares the RF performance capability of the devices across mounting variations. If that were to occur, the clear winner would be the pure solder-mount method as mentioned in application note AN1907.
In the standard Freescale manufacturing test process, we typically clamp the device to a bare-copper heatsink under approximately 250 pound/in2 (PSI) average pressure, with either thermal grease or no thermal-interface material whatsoever. Under those test conditions, we get the standard RF-performance parameters that one sees in the data sheets and as reported on our manufacturing test data reporting system.
All well and good, but some of our more experienced customers know that those reported numbers are not the true capability of the devices. When one solder mounts the parts rather then clamping or bolting, the whole set of RF performance numbers goes up significantly. Performance increases on the order of 1 dB in gain, 1% in efficiency, and perhaps as much as 2 dBc in ACPR are quite common, along with a reduction in maximum die operating temperatures, by as high as 20°C.
This cooler die temperature is one of the key drivers that lead to the increased RF performance, along with the improved, lower-resistance common source impedance through the back-side solder-mounting of the device. In addition, for silicon-based devices, every 10 to 15°C reduction in junction temperature results in doubling of mean time to failure (MTTF), leading to a more reliable power amplifier.
So why would anyone bolt-down mount an RF device? Only because they did not know the significant benefits involved with solder mounting. And now you do.
Editor's Note: Leonard Pelletier is lead RF applications engineer at Freescale Semiconductor and will be a regular Guest contributor on RF topics for RFDesignLine. He can be reached for questions and suggestions for future topics by either commenting in the box below or by contacting him directly at leonard.pelletier@freescale.com. Alternatively, you can follow him on Twitter at www.twitter.com/RFLeonard. At IMS2009 his tag will be #IMS2009.
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