placing components above thermal vias

Unless you are providing a blast of air to that side of the board to cool the hot spot, it may actually be helpful. Most component materials, especially those with metal lead frames passing through them, are more thermally conductive than the air they replace. So their extra surface area just makes contact with the air through a larger boundary area.

The exceptions would be components that are sensitive to the hot board, or components that add heat to that spot.

It depends entirely on your circumstances. There can be a dozen factors involved, some of which are which are:

- How is your boards mounted (vertical or horizontal)?

- What layers connect to the thermal vias?

- Are they blind vias?

- Do you have airflow?, and where?

- How much heat is your part dissipating?

- What is the steady state temp on that side of the board?

- What is the plastic body component?, and does it have a tempco that you care about? etc

If your board got so hot that the plastic body on the component is your concern, then that is the least of your worries.

But as a general guide, no, you wouldn't put a component over thermal via's.

Dave.

Use your head! *Anything* placed on the other side will severely reduce convection, make conduction to a heatsink impossible, and will reduce radiant transfer. Might as well as forgo the thermal vias.

Sorry, but that is not the purpose of a "thermal" via. In a thermal, part of the copper is removed to prevent the heat of soldering from dissipating into the plane(s) or other traces. This makes it easier to solder a leaded component to the circuit board.

In addition, I have also been told that thermals allow the via to expand while soldering a lead into the hole thereby preventing damage to the circuit board.

So if you want to pull heat away from a surface mount component, I would recommend you put solid vias to the appropriate plane(s) in the copper under the device's thermal slug. Some time ago I read an app note that suggested using small vias that would fill with solder and form a better thermal connection. Others advocate the use of large vias because they have more surface area to connect to the plane(s). YMMV.

Also, as others have noted, you should not put components directly on the other side of the board as this will reduce the heat dissipation.

Ah, no, you have your terminology a bit mixed up. You are talking about a "thermal relief" on a pad or via. That is completely different to a "thermal via" which is indeed designed to transfer the heat into a larger copper plane.

That is precisely what a "thermal via" is. The exact terminology can vary of course, but it's the one I use and have most often heard used.

It is clear the OP is talking about "thermal vias", not "thermal reliefs".

Dave.

that's correct sir, thermal relief is like opening solder mask to expose copper in that area of IC's slug, and soldering it there. Since I don't have any air flow mechanism in the closed box, i thought heat would be dissipating to planes, so keeping a plastic body component should not affect much. But now I think it's better to leave that area

~veeresh

Poo poo on the answers you got so far. Many designers could use a lesson in thermal dynamics ;)

The question is, what heat am I producing and how much of the copper plane do I need to cool it? The answer could be "none". Then blocking it, or even having it, is not an issue. On the other hand, I have used the heatsink from one regulator drawing very small current to help cool another running hot. One post here was correct, the part over the thermal may in fact REMOVE more heat rather than block it. So go back and ask these questions of your design.

Could have sworn I asked the OP to ask himself all these sorts of questions in my first post...

Dave.

the best solution seems to me to be one large plated hole backfilled with solderor possibly a soldered slug of copper.

Bye. Jasen