Epoxy sealed, non-hermetic RF power packages

AN3232

6/26

Doc ID 17594 Rev 3

Power RF packages typically require a reliable, low thermal resistance attachment to a

heatsink. In this respect, nothing out-performs a direct attachment of the device flange, or

thermal base, to the heatsink by means of soldering. This can be accomplished using any of

the PbSn or Pb-free soldering methods used throughout the electronic industry. The final

soldering step may be preceded by a hot-solder dip of the package base and/or leads, as

required for situations that require attention to Au content of the solder joint. Alternatively,

the packages may be inserted into an amplifier using a pick-and-place methodology, so that

complete soldering is accomplished in a single reflow.

Some amplifier assemblies require a manual approach for device positioning on a PCB

while soldering bases to copper core heatsinks. In such manual operations STAC boltdown

style packages have the unique possibility of serving as a built-in clamp to accomplish both

tasks expeditiously. These manual efforts are rewarded by an extremely low thermal

clamping fixtures is eliminated.

The approaches to soldering RF transistors in amplifier housings are as numerous as the

quantity of RF package outlines. While the traditional demand for high levels of ruggedness

in RF devices assures an intrinsic high tolerance to a wide variety of assembly methods, the

high-stress nature of any soldering operation requires careful consideration. Users

ultimately assume responsibility for developing and qualifying their soldering processes, and

therefore it is strongly recommended to fully evaluate temperature profiles at all steps in the

soldering procedure to avoid excessive peak temperatures and ramp rates.

When amplifier assemblies require cleaning after soldering operations, the use of a non-

reactive cleaning agent is recommended. All residue from reactive flux should be removed

according to the flux supplier's recommendations.

If a non-aqueous cleaning method is employed, it must be followed by cleaning with DI water

to remove all ionic contamination. It is recommended to periodically check the conductivity

of the DI rinse to insure that levels of ionic contamination are as low as possible. In cases

where heavy ionic contamination is present, multiple DI rinses in isolated baths should be

used. Proceed from the most to the least contaminated rinsing system to maximize cleaning

effectiveness.

In general it is recommended to minimize the use of flux, to use no-clean flux, or to adopt

fluxless soldering techniques. When there is doubt as to the behavior of residual flux or flux

levels, consider a two-step solder approach. Since the PCB and other components may

involve flux, an initial soldering step of the majority of non-critical components can be done,

followed by a thorough cleaning. The second solder step, with greatly reduced flux content,

can be planned for more critical components such as epoxy sealed RF transistors. An

added benefit of a two-step operation is that non-solder thermal interface materials (TIMs),

such as thermal compounds, are not influenced by aggressive flux cleaning.

Following DI rinsing operations, the entire assembly should be baked, as determined by

user experimentation, before power is applied. It is suggested to perform this drying at 125

°C for 24 hrs, if all components are rated for this condition. A longer bake time is

recommended at lower temperatures. Refer to Figure 2 for some suggestions.

OEM equipment operating conditions should be specified for operation only in a “non-

condensating” environment. If there is a question of humidity in ambient conditions in power

cycling, a heating/drying cycle is recommended before power is applied.