placing components above thermal vias

[veeresh]

Hi,
Please guide me regarding thermal vias.

Thermal vias are meant for dissipating heat to power/ground planes.

Suppose if a transceiver sort of IC has a thermal slug, and thermal
vias are provided in the pcb region below this, is it fine to place a
plastic body component on the other side of the PCB below this
transceiver?

thanks and regards,
Veeresh

 

[John Popelish]

Hi,
Please guide me regarding thermal vias.

Thermal vias are meant for dissipating heat to power/ground planes.

Suppose if a transceiver sort of IC has a thermal slug, and thermal
vias are provided in the pcb region below this, is it fine to place a
plastic body component on the other side of the PCB below this
transceiver?

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.

 

[David L. Jones]

Hi,
Please guide me regarding thermal vias.

Thermal vias are meant for dissipating heat to power/ground planes.

Suppose if a transceiver sort of IC has a thermal slug, and thermal
vias are provided in the pcb region below this, is it fine to place a
plastic body component on the other side of the PCB below this
transceiver?

thanks and regards,
Veeresh

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.

 

[Robert Baer]

Hi,
Please guide me regarding thermal vias.

Thermal vias are meant for dissipating heat to power/ground planes.

Suppose if a transceiver sort of IC has a thermal slug, and thermal
vias are provided in the pcb region below this, is it fine to place a
plastic body component on the other side of the PCB below this
transceiver?

thanks and regards,
Veeresh

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.

 

[Marc Guardiani]

Thermal vias are meant for dissipating heat to power/ground planes.

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.

 

[David L. Jones]

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.

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.

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.

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.

 

[veeresh]

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

 

[Brian]

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- Hide quoted text -

- Show quoted text -

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.

 

[David L. Jones]

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

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

Dave.

 

[Jasen Betts]

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.

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


Bye.
Jasen