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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

Copper pours

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  • \$\begingroup\$ How will the board be assembled? Will you be soldering the parts by hand, with a soldering iron, or using reflow or wave soldering? AFAIK, thermal relief is to slow the transfer f heat while soldering with a soldering iron. \$\endgroup\$ – gbulmer Oct 23 '15 at 12:26
  • \$\begingroup\$ The first prototype will be soldered by hand entirely. Aftewards all SMD components would be soldered using reflow and the leaded components would be added by hand. Thtas why the two capacitors C1 and C2 which is connected to a large ground plane is having thermal relief. \$\endgroup\$ – Carsten Farving Oct 26 '15 at 9:50
  • \$\begingroup\$ Those thermal reliefs certainly look a bit badly rendered in the image. I wonder just how much copper connects the pads to the trace/plane in the pad library as it looks like it might be a minimal tangential contact that works by luck and copper swell rather than actual design. \$\endgroup\$ – KalleMP Aug 30 '16 at 5:51
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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.

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  • \$\begingroup\$ tombstoning, isn't it mainly an issue with capacitors and resistors rather than more heavier components as inductors or diodes? And my issue here is that there is no recommendations at all. I'd just prefer to go with the best option. But as this is a switching application with a low switching frequency, low current, what you basically say is that it's more a matter of personal preference? \$\endgroup\$ – Carsten Farving Oct 26 '15 at 9:52
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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.

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