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I'd like to use a 0.35mm BGA in a new design. If possible, I'd like to do this without resorting to an expensive manufacturing process with microvias. This is possible for my particular application, if I can pull a little trick.

All of the pads I need for my application are around the perimeter of the chip, except one. The other internal pads I don't need. Perhaps I can just miss out the pad that's in the way of my track, and hope that the solder resist prevents a short.

BGA with naughty trick

Question: Is this a sensible thing to do?

Rocketmagnet
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    A ball that doesn't have any exposed pad to wet onto will probably want to remain taller than those that do. One in isolation will be squashed down by the pull of it's neighbors, and you may get away with that, but even beside the relying on soldermask issue, this seems an idea that should be used very sparingly and cautiously. – Chris Stratton Jul 20 '17 at 16:05
  • Heh, just few hours ago I saw a similar solution on cheap USB power bank - traces are routed under unused D+ and D- pins of microUSB socket. To make isolation between unused pins and traces a little bit "stronger" manufacturer adds white filled square on overlay layer. – Jakub Rakus Jul 20 '17 at 18:25

1 Answers1

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Yes and no. You can omit pads if you wish however, the following issues can occur.

  1. If you run traces under the ball and trust the solder resist to act as an insulator you are likely to get shorts or insulation breakdown/capacitive coupling through the resist. It simply is not intended for that purpose.

  2. If you remove too many pads you are reducing the mechanical stability of the device attachment.

  3. If you remove too many pads you will be reducing the thermal conductivity of the device to the PCB and may incur greater heating problems with the device.

As such, it is recommended that missing pads be kept to a minimum and no traces should be routed under the pins.

EDIT: If the pin on the device is an unused input and there is no issue if there is a short to the underlying trace, fan-out etc, you could get away with it... but really, if that is the case, then just wire it that way and keep the pad in.

Trevor_G
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