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I'm working on a charger circuit which basically works as a buck-converter. I'm pouring my components with copper planes.
However as some of the components are connect to a large ground plane on bottom layer I suspect the best choice is to use thermal relief to ease soldering. However, should I do the same on the top SMD components?
I know there is pros and cons using thermal relief, but when should it be used and when not?
See below for the layout described above
So a thermal relief is useful for hand soldering but in automated assembly like those top parts it can help prevent tombstoning. That's where one side of the smd liquified first and the surface tension causes it to stick straight up in the air :)
My advice would be to use it in circuits or applications that do not need a solid connection to the plane for either the lowest possible impedance or a large current carrying capability.
For regulators I'd follow the mfg guidelines in the datasheet, and you may find rf applications that want direct connection as well. But for everyday parts like say pull-ups etc if go thermal relief.
Tying the pad to a large copper plane makes hand soldering difficult; the iron has to heat up the entire plane rather than just the device pad. This can be overcome with larger irons, and in some cases that's the only way to proceed. But if the pad doesn't have to be solidly tied to the plane, it's best to avoid that.
Make your thermal reliefs large enough to carry the current that connection actually expects to see. (With reasonable overhead, of course.) Any more copper than that is just making things more difficult to assemble for no good reason.
