Thermal relief for a chip without metal plate on the bottom

Question

I have a chip in which I have selected this footprint: Package_SO: SOIC-8_3.9X4.9mm_P1.27mm. This is what it looks like:

Because this chip gets pretty hot, I thought I should keep the footprint but with vias for thermal relief. Something like this:

However, the chip is all plastic. There is no metal on the bottom to be soldered. Does it make sense to make a change like this? Will it help reduce the temperature?

Or because it cannot be soldered on the bottom, it will have no effect at all?

EDIT: This is a picture of the surrounding layout:

EDIT2: This is the layout for the whole portion that gets heated up:

EDIT 3: Following are images of the schematic topology, as well as a proposed layout for this boost converter.

EDIT 4: This is my attempt in copying user's user1850479 layout.

Answer 1

The effect will be minimal without anything more thermal conductive than air between the chip and that thermal pad. What you can do is to increase the width of the traces, that will work way better.

Answer 2

As mentioned you’d have to have grease or another thermal interface material to make this work. There should be some glues or epoxies that are compatible with a production process. You could also glue a small Heatsink to the top.

Also maximize heat flow through the Vcc and GND pins. Tie them well thermally to the planes/pours and you’ll be able to decrease temps a little.

But they do add those power pads for a reason. You’ll want to change packages if you have significant dissipation.

Answer 3

If you cannot change to a different package, you'll get best heat conduction by having a large copper area for each pin:

If the package had exposed metal pad, that would have a direct connection to the chip. Then it would make sense to just connect that to a big ground plane and have normal traces for other pins.

But the side pins connect to the chip through thin bond wires. Heat will have to transfer through the plastic enclosure to the pins, and then to the PCB. When each individual pin is a poor heat conductor, you want to use them all in parallel to make the best out of a poor situation.

Thermal conductive glue between the chip and PCB would be a good idea, but usually comes at an extra manufacturing cost. If you add the glue, adding ground plane vias under the chip is a good idea.

Answer 4

I think the problem is actually the enormous high current switching loop you have routed through your board. It is so large it does not fit in your picture, but here is the part of it I can see:

Depending on how far it goes to the right of the board before meeting back up and closing the loop it is actually a little surprising the converter works at all.

Here is a better layout for that converter:

By putting the diode and MLCC capacitor right up against the switch, current only flows a few millimeters and all on one layer, so it encounters only a small amount of resistance and inductance which leads to lower losses and better performance. Furthermore that distance is mostly connected with wide planes that have lower resistance/inductance. Note that depending on the switching frequency you may also want to put that big electrolytic capacitor in too, but you can parallel the MLCC to keep the loop area small and put the electrolytic slightly further out.

When I built that converter like this the chip did not get particularly hot.

Edit: If you wanted to try and salvage the existing board, you could try adding an MLCC and a (short!) jumper wire here:

That would reduce the loop to the green area. You could test that and see if the temperature is reduced.

Answer 5

Does it make sense to make a change like this? Will it help reduce the temperature?

Or because it cannot be soldered on the bottom, it will have no effect at all?

It won't if the package doesn't have a pad directly on the bottom, there will still be air and plastic between the die and the PCB and neither of those will be great a transferring heat.

If you want to get rid of the air, use thermal heatsink compound or a thermal glue on the bottom of the chip. But the PCB isn't the best place to dump heat, because it simply heats up the PCB. The best place is to dump it in a heatsink and the heat ends up in the air (If you dump heat in a pcb it just heats up the PCB and then the PCB heats up the air, but you end up with a hot PCB). Even a small piece of metal can make a significant difference in transfering heat to the air. Even better would be a small heatsink.

The best thing to do for a DC DC switcher would be to reduce the current or make sure the design is correct.