Thermal Relief; Figure 2 - Example Of A Poor Rf Thermal Relief - Enfora Enabler LPP G Integration Manual

4.5.1.2. Thermal Relief

Because the ground plane acts as a large heat sink, it can affect the solderability of
components. A common method to reduce this effect is to use thermal relief around the pad
in question. However, great care must be taken when using thermal relief for high current or
high frequency applications
For example, a large thermal relief like the one shown in Figure 2 can serve the purpose for
general applications such as low current, low speed data lines, DC connections and audio
frequency applications. However, such thermal relief structures should be avoided for
applications where high current and/or high frequency is involved, such as those using the
Enabler Module. Depending on the frequency of operation, the long narrow thermal relief
traces between the pad and the ground plane act like an RF choke. These RF chokes
become higher impedance at harmonics of the fundamental frequency making it problematic
for high frequency suppression. This can make it difficult to pass type approval testing.
POOR RF thermal relief

Figure 2 - Example of a POOR RF Thermal Relief

If thermal relief is necessary, it is recommended that you use short, fat traces similar to those
shown in Figure 3. This will still provide a solderable connection, while providing a better RF
connection. Making them shorter also allows for a more continuous ground plane due to less
copper being removed from the area. It is also recommended to have ground vias around all
thermal relief of critical ground pins.
LPP0108IG001 Version - Preliminary – 06/20/08
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