Screaming Circuits: Via in Pad - Why and How

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Via in Pad - Why and How

There are many reasons you shouldn’t use via in pad. It’s not good practice, and those via holes act like BGA via in padlittle capillary straws and suck solder off of the pad or the BGA.

That said there are some applications that may require, or seem to require, via in pad. Here are a few examples of why you might need to use via in pad:

  • If there is not enough space on the board.
  • It can help with thermal management.
  • Trace routing may be easier with via in pad.
  • High frequency designs benefit from the shortest possible routing to bypass capacitors, which may indicate via in pad.

So if you don't have a choice, here are some methods you can try when using via in pad:

  • Filled viasHave the board fab house plug the via and then plate copper over it. This is our favorite option. It will give you all of the benefits of via in pad without causing problems in assembly. It's really our only recommended via-in-pad method.
  • Use a micro-via that only goes through one layer of the board. Although this may be an okay option, the solder can still wander down into the via, leaving voids.
  • Cap the underside of the board with solder mask. This is our least favorite option because sometimes the cap can pop open, and the void may be big enough to still pull too much of the solder off of the pad.

For more information on via in pad, check out this video. http://scrm.it/viainpad

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Comments

A very important reason for using via's in pads is mechanical stability. SMD USB connectors are relatively easy ripped of a PCB and putting some via's in the big mechanical solder pads on the sides makes them a lot stronger.

I've also seen this in a motor controller where thick copper cables (60A +) were guided through a hole in the PCB and directly soldered to pads (with via's) on the other side.

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