How to add thermal via in PADS

Discussion in 'The Projects Forum' started by vivek20055, Feb 5, 2013.

  1. vivek20055

    Thread Starter Member

    Nov 4, 2012
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    Hi,

    I need to add thermal via in my 4 layer pcb using PADS software.

    What is thermal via????

    How to add thermal via in PADS???



    Regards,
    Vivek Alaparthi
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    A thermal via is just a regular via but its purpose is to spread heat between multiple layers vs just connecting traces between layers.
     
  3. MrChips

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    Oct 2, 2009
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    A thermal pad or via is a pad or via connected using radial spokes to a large copper plane. This is so that it is easier or solder and unsolder.
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    No, there is a difference.
    A pad with thermals is what you wrote, it uses spokes around it to decrease the thermal capactiy of the pad for soldering purposes.
    A thermal via is exactly the opposite, its purpose is to thermally and electrically connect the underside pad of an QFN package to multiple ground planes for better heatsinking.

    As for the question, if you want to do the thermal via, it is just a normal via, but size chosen with regards to the manufacturing process of the board and soldering process of the component to provide least thermal resistance.

    It seems to me your teacher is giving you hints of what you should / should not do, and you are too afraid to ask him what he means. If you don´t know what a thermal via is and are supposed to use it, why don´t you ask right away, instead of hunting for response here?
     
  5. JMac3108

    Active Member

    Aug 16, 2010
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    As MrChips said a "thermal" is a thru-hole pad or via with spokes to a surrounding plane or copper fill.

    For example, if you were designing a power supply and used a mosfet in a TO-220 package and connected the source of the mosfet to ground. If the thru-hole pad for the ground pin on the mosfet went straight into the ground plane you would have a hard time soldering it because the heat from the soldering iron (or wavesolder) would be wicked away into the ground plane. A "thermal" means that the thru-hole pin has short spokes (like a wagon wheel) that connect it to the ground plane. The spokes provide enough thermal isolation to be able to solder the component pin, but do not add too much electrical resistance to the circuit.

    You can also use "thermals" on your vias. But I don't generally do this because it really serves no purpose. I will not be soldering to the vias so I don't care if they go directly into planes or copper fills without a thermal.

    Sometimes people say "thermal vias" when they mean something else ... this is what MCGYVR was talking about. Lets say I'm using a surface mount mosfet in my power supply and it will get hot. I put a copper fill on the board to spread out the heat (acts like a heatsink). I also might want to drop a series of vias to copper on another layer to spread out the heat further. Another example is surface mount parts in a DFN package that have a thermal pad underneath them to dissipate heat. Usually you want to connect this pad to ground by dropping several vias to the ground plane to dissipate the heat from the part.

    So, the answer to your question is, as usual, it depends :D It depends on the context in which the term "thermal via" is used as to what it means.

    As far as how to implement them in PADs. I don't use PADs, but most layout packages have an option that you select that automatically puts thermals (spokes or wagon wheels) on any through connection to a plane.

    Thermal pads in the context of making a heat spreader are just regular vias you drop inside a copper pad to spread the heat to another layer. You do this the same way as any other via in PADs.

    Hope this helps.

    -------------------------------------------------------
    Hahahaha ... Kubeek and I were writing our posts simultaneously. As usual, he managed to say the same thing I did in about 200 less words ;-)
     
    Last edited: Feb 5, 2013
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Thermal Relief vs Thermal Via Image attached
     
    kubeek likes this.
  7. JMac3108

    Active Member

    Aug 16, 2010
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    A picture is worth a thousand words ;)
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Unless someone can point to a standards document or some kind of reasonably definitive and authoratative source, people may have to agree to disagree on what a "thermal via" is.

    For my $0.02 worth, I have generally seen the terminology that a typical pad has "thermal reliefs", while a "thermal pad" does not.
     
  9. MrChips

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    Oct 2, 2009
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    I have no problem with a "thermal relief".
    But why a special name for a "thermal via" if all it does is connect to another plane?
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    IPC-2221 covers thermal reliefs.
    IPC-7093 covers thermal vias
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Because its a via with the intended purpose to allow a better transfer of heat between the layers. A regular vias purpose is not that at all but the shape is identical. Thermal vias can make a HUGE difference.
     
  12. MrChips

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    Oct 2, 2009
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    How is a thermal via constructed that makes it different from a regular via?
    Just asking because I don't know.
     
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    no difference in the fabrication process.. But its "function" is different..

    A thermal via is just a via used solely for its decreased thermal resistance versus a layer of fr4. Thats it.

    And when heat is really an issue you can even have solder filled thermal vias which decreases the thermal resistance even more.
     
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