There are no IPC restrictions on this topic that I am aware of… but I have some ‘old guy’ advice… for what it is worth.

 

Plated through hole thermal relief structures are designed to create some thermal resistance between the outer pad and the plane that it connects to internal or external to the board...

Why? Because the intended pad on the outer layers connected to the plated through hole needs to be able form a reliable solder joint with the lead of the thru hole component mounted on the surface of the board and soldered to that outer pad… the inner planes or outer planes act like heat sinks for the most part and lengthen the time required to heat up the pad to solder melting temps.

How does having a thermal relief help?  It allows the pad on the surface to heat up more quickly during the soldering process (reducing the heat sinking effect) so that the weld between the copper and the component lead all gets to the proper temperature at the proper time. A good solder joint happens when the lead and the pad and the solder all reach the proper melting point at the same time and then are allowed to cool together at the same rate. A full solid connection to the plane would sink the heat into the plane as fast or faster that you could heat the pad… and very likely you would end up with a cold solder joint which leads to premature failure of the assembly. This is true for surface mounted components too… surface mount pads that are soldered to pads in a copper plane need thermal relief connections to the plane to facilitate a good solder joint. It’s basically the same problem…

 

My thoughts on a thermal relief pad as a VIA…

WHY? You don’t solder to it… so what purpose could it possibly serve to reduce the thermal connection to the plane? Go ahead and connect them solidly to the plane they don’t need thermal relief.

Most applications for vias have no component leads in the holes(hence the term via instead of thru hole pad)… and often they are used to transfer heat from one side of the board to the other to cool a component that gets hot in normal operation like the belly thermal pads you find under some hotter chips.  A ‘thermal via’ with thermal relief connections would degrade the thermal conductivity of the via to the plane… making the situation worse and defeating the purpose of the thermal heat transfer purpose of the via. .

 

So I can’t see any benefit from having thermal relief on a via… and in some cases it could be worse to do so.

So I wouldn’t do it.

 

Best regards,