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Thermal relief dilemma

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Pedro:
Hello all,

I have a doubt about thermal relief in the specific situation of power components.

Thermal relief is recommended to allow for better soldering of both thru-hole and SMD parts. Without thermal relief, the pad has a very high thermal conductivity to the plane and it is very difficult to heat, it cools down much quicker than the other pad's, creating all sorts of problems. So as I understand it, by adding thermal relief we are effectively increasing the pad's thermal resistance, solving many of these assembly problems. Up to this point everything is fine.

The problem comes when we have a heat-producing part, such as a D2-PAK or SO-8 power transistor. In these type of components, the tab or bottom pad is used to dissipate heat. We might have connected such thermal pad to a big plane to lower thermal resistance. But if we placed a thermal relief to help manufacturing, isn't the added thermal resistance of the relief making dissipation worse? Wouldn't it be better to have a direct connection of the power part to the plane?

I guess there must be some accepted compromise in this regard.

Thanks guys and gals,
Pedro

Mechatrommer:
i think you've questioned and answered it in one post.

cyberfish:
Usually the pad on D2PAK is connected to plane without relief. It just takes a long time (~30 seconds with a 50W iron) to solder.

There won't be quality problems if you make sure the solder flows completely, just like soldering with thermal relief. Also, the temperature cannot be too high or it may damage the part.

ivan747:
What about these SMD mpdified "power" SSOP or SOIC motor drivers with huge pins at the sides connected to ground to disipate heat. It must be quite tricky to solder in some situations, because there are two huge tabs and some tiny pins next to them. The situation gets even worse when one of these "Powerpads" (TI) are included in the package.

Mechatrommer:
once the whole thing goes into reflow machine, everything will go above the solder melting point, no heat will be able to be "relieved", no issue.
the same with wave soldering for through hole, the heat is just too massive at mass production scale. my imagination tells me from looking at documentaries.

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