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Advantage of choosing full thermal instead of thermal relief

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psr

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Hi all,

Greetings.
Is there any advantage in choosing full thermal instead of using thermal relief.
Only for fine pitch components, we prefer full thermal. Why this is so?
Thanks,
Psr
 

House_Cat

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Re: Thermal relief

The word "thermal" is used in two different contexts when talking about PCB layout. It isn't clear from your question which one you are asking about.

There are thermal reliefs which are thru-hole pads or vias with reduced copper contacts to the plane layers to reduce the amount of thermal energy (heat) necessary to solder and desolder components. You choose the number, angle, and size of the "spokes" depending on the current being carried, and the density of the components on the board. The more components you have, the less room there is for a thermal without isolating the connection from the rest of the plane.

There are also thermal pads which are used to conduct the heat of a component such as QFN IC packages to the copper of the PCB board for enhanced chip cooling. A thermal pad would not have thermal relief - it would be directly connected to a solid plane. This is because you want maximum heat conduction away from the device.
 

    psr

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FvM

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Thermal relief

To my opinion, the only plausible purpose for thermal relief is with through-plated pins for the reasons said by House_Cat. Although thermal relief vias can be found sometimes, they bring mostly unwanted effects as parasitic inductance and waste of board space.

As a disadvantage, thermal relief with through plated pins is considerably reducing the current capacity and may be unsuitable for high current pins.
 

cyberrat

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Thermal relief

You also have to take thermal relief into account with SMT.
I.E. if you have a resistor to 0v & the bottom pin has thermal relief spokes on, this will increase the copper amount & you get an imbalance & potential tombstoning etc.
 

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