5
\$\begingroup\$

Several designs I've seen have a load of vias underneath high frequency or analog ICs. For instance, this is a part of a PCB design for a DDS unit:

DDS PCB layout

Design from http://rudius.net/oz2m/ngnb/dds.htm

Note the grid of vias underneath the AD9912. This is not a particularly good example, as the AD9912 has an exposed analog ground pad and the ton of vias is presumably just for a low impedance return path to the ground plane, but I'm sure I've seen this in lots of other places too, some of which don't have exposed pads underneath.

Is this perhaps to effectively move the ground plane closer to the sensitive IC, reducing impedance further?

Apologies if this has been asked before but I couldn't find it.

\$\endgroup\$
3
  • 6
    \$\begingroup\$ Electricity is not the only thing metal can conduct. \$\endgroup\$ Dec 1, 2015 at 12:55
  • 2
    \$\begingroup\$ Ooooh so it's a heat sinking thing? \$\endgroup\$
    – CharlieB
    Dec 1, 2015 at 12:56
  • 2
    \$\begingroup\$ @CharlieB Exactly. \$\endgroup\$
    – Marco
    Dec 1, 2015 at 13:04

1 Answer 1

8
\$\begingroup\$

With non-microwave devices, it's usually to conduct heat down into the copper planes within the board, or through to the heatsink attached to the back of it. This particular DDS device is fairly watty, so needs more cooling than the top surface and the leads can provide. One of the major manufacturers has registered the term 'power pad' for the exposed bit of metal under the IC.

Many microwave devices also have exposed pads on the back, but these are usually for grounding as well, where a good microwave match needs a very close ground.

Some surface mount power FETs have an exposed pad, and this can be intended to take tens of Amps (yes, scary), which needs a good array of vias down to multiple wide tracks.

Beware, almost all ICs with exposed pads are designed for them to be connected to ground. Just a very few are not. My team have been bitten twice by components, one amplifier, one analogue switch, where the exposed pad went to the -ve rail instead of ground. Much wailing and gnashing of teeth while we waited for the corrected board to get made. Check twice before you lay your board out.

\$\endgroup\$
3
  • \$\begingroup\$ "Usually" the pad goes to most negative voltage the chip is connected to- because it's connected to the substrate, but as 44635 says, check. I've seen datasheets where you it was not specified. There are also nightmare chips with multiple pads of different shapes on the bottom of one chip. \$\endgroup\$ Dec 1, 2015 at 14:16
  • \$\begingroup\$ Thanks for the clear answer, and for the warning too! I'll watch out \$\endgroup\$
    – CharlieB
    Dec 1, 2015 at 14:39
  • \$\begingroup\$ I had a Burr-Brown regulator where the pad was connected to the internal ground, but the datasheet specifically required no electrical connection to this pad to prevent any current at that node. \$\endgroup\$ Dec 1, 2015 at 14:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.