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Thermal Pads on Some DC/DC Conveerters

dindea

E.g. Linear Technology's ADP2302/2303 has a thermal pad on the bottom side of the package. The data sheet says that this pad shall be soldered to a ground plane for thermal dissipation.

Two questions:

1)

Is the pad connected to the internal ground? Are this pad and pin 7 (SOIC-8) interconnected? Is it necessary, for proper operation, to connect the thermal pad to ground? To me it seems reasonable that it would not be necessary. No "strong" currents will flow through the GND connection(s). All "strong" currents will flow through the P-switch and the external catch diode.

2)

Wouldn't it be possible to make the thermal contact to a copper area with heat compound instead of soldering?

Parents

  • Usually manufacturers using odd packages have app notes detailing the steps required to get adequate thermal performance.

     

    Expect to require multiple small vias in the thermal pads to take heat away from the chip to copper on mulitple pcb layers.

     

    You will have to solder because there is no safe way of applying thermal grease so that it would not contaminate the soldering during reflow.

     

    For the ADP2302 there is a good diagram on page 22 of the data sheet showing how to lay out the pcb. Unless you are VERY confident that you know what you are doing you should use this as a starting point.

     

    MK

  • in reply to michaelkellett

    Thank you for your fast answer!

    I will hand-solder. Would it then work well to use thermal grease?

     

    /dindea

  • in reply to dindea

    No - use solder paste - for a hand assembly process use AD's suggested layout but with one hole 0.8mm or 1mm diameter at the center of the big pad.

     

    Apply solder paste to the pads on the pcb with a small blade - it doesn't need to be spread evenly. Apply paste to all the 9 pads for the chip. (If you can screen the paste on with a stencil then, of course, that is much better.)

     

    Reflow the paste by heating the board from the other side with a hot air device or a hotplate (hotplate is better but you need to be sure it's at the right temperature !!!).

     

    If you are allowed to use solder with lead in it you can work at lower temperatures and the solder flows better.

     

    The big hole in the pad allows excess solder to escape and allows you to inspect that the solder has flowed properly.

     

    You must take care to design your board so that there are no thermal breaks where the via though the big pad connects with copper planes.

     

    It isn't clear from the AD data sheet if an electrical connection is required to the pad, but since they show one I wouldn't take a chance on it.

     

    If you will only run the chip at reduced power you might not need to worry about thermal coupling to the pad.

     

    MK

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  • in reply to dindea

    No - use solder paste - for a hand assembly process use AD's suggested layout but with one hole 0.8mm or 1mm diameter at the center of the big pad.

     

    Apply solder paste to the pads on the pcb with a small blade - it doesn't need to be spread evenly. Apply paste to all the 9 pads for the chip. (If you can screen the paste on with a stencil then, of course, that is much better.)

     

    Reflow the paste by heating the board from the other side with a hot air device or a hotplate (hotplate is better but you need to be sure it's at the right temperature !!!).

     

    If you are allowed to use solder with lead in it you can work at lower temperatures and the solder flows better.

     

    The big hole in the pad allows excess solder to escape and allows you to inspect that the solder has flowed properly.

     

    You must take care to design your board so that there are no thermal breaks where the via though the big pad connects with copper planes.

     

    It isn't clear from the AD data sheet if an electrical connection is required to the pad, but since they show one I wouldn't take a chance on it.

     

    If you will only run the chip at reduced power you might not need to worry about thermal coupling to the pad.

     

    MK

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