Hi folks,
What’s the best way to add thermal reliefs to an existing board?
Our assembly house has recommended that we put thermal reliefs on all PTH & SMD connections to planes, fills, large traces, etc. Setting aside for a moment any debates over their necessity or electrical performance, what’s the best way to do this for a board that has approximately 200 components of various package sizes?
Proper sizing of thermal reliefs seems to be based on IPC-2222 (which my company does not own, so I haven’t read it). But from what’s available online, the approach determines thermal relief settings from the component pad size. So the spoke widths will be different for an 0402 package than, say, an 0805. (There are other considerations as well, such as ensuring the spokes can handle the necessary current.)
To implement this in Kicad, it’s not as simple as setting the spoke & gap sizes at the zone level because then all pads connecting to that zone would have the same thermal relief size regardless of whether they were 0402, 0603, high-current, etc. Also, they can’t be set at the footprint level because footprints don’t have spoke/gap dimension settings. So it looks like the only way to do this is to go component by component through the entire board and set the thermal relief dimensions for each pad.
I’m not opposed to a brute force approach like this - I’m totally willing to do the work. But it seems time-consuming and error-prone (i.e. approximately 200 pad settings to get right for my current PCB). It also makes things very difficult to change in the future as it would again require component-by-component changes.
FWIW, one approach I did contemplate was to set the thermal relief dimensions at the zone level, and based on my most-used package size. I would then need to update only the remaining packages one by one. This would save me about half the work, as approximately half my components are 0402. The problem is that an 0402 thermal relief would have the smallest spoke size on my board, so if I made any mistakes with the remaining components, they could end up under-sized with respect to current-carrying capacity. This seemed riskier than just leaving the zone connections defined as “solid” and going one-by-one through every pad.
Anyway, am I missing something? Is there a better way to handle this? I’ve gotta be making this harder than it is… 
Thanks!
Steve
