PCB Assembly Services - Screaming Circuits: Via in pad refresher

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Via in pad refresher

It's a good time for a via-in-pad refresher. It's not quite as hot a topic as it has been in the past, but it's still a very important consideration. It's always a bit controversial. The assembly process folks pretty much always say: "don't ever do it." However, with certain parts, the part manufacturer strongly recommends it. Mutually exclusive requirements are always a source of "fun." Rather than just telling you "No, never", we have a few guidelines for those times when it is actually needed.

We don't like it, but with some designs, if done properly, there can be a number of compelling reasons for putting vias directly in the SMT pads for BGA and QFN packaged components.

  • Routing may be easier with big or fine pitch BGAs
  • Bypass capacitors can be placed closer to the chip that needs them
  • Thermal management
  • Grounding for high-frequency parts.

The primary reason we don't want to see vias in pads is that when left open, those via holes act like straws and capillary action draws solder off of the pad. A couple of undesirable events can happen depending on the method used during board fab.

  • If your vias are left open, solder will flow down into the via. The larger the via diameter, the worse the
    problem can be. You might end up without enough solder to give a solid mechanical or electrical connection. You may end up with a solder blob on the bottom of the board which could interfere with other components or cause to shorts.
  • If your vias are capped or partially filled, the caps might pop off due to thermal expansion or out-gassing. Internal air bubbles can migrate up, leading to voids in your solder joint. A capped via under a BGA ball will lead to a void or will not allow enough surface for a solid connection.

Open vias in pads are not an industry-accepted practice. In a perfect world, we'd like to never see one. However, the real-world is Plated over viasaying otherwise. Manufacturers of QFN parts often recommend vias in the heat-slug pad for improved thermal conductivity. High frequency designs benefit from the shortest possible routing, which may indicate via in pad. Super fine pitch BGAs may not leave any other options.

Our first recommendation would be to re-design the boards so the vias are in between the pads, plug or cap the vias at the board house or to use microvias that don't go all the way through the board. If you can't do any of that, we would suggest that you use as small a diameter as the design will allow and follow the component manufacturer's guidelines for placement and via capping or filling.

Regardless of via size, never put an open via in a BGA pad. Never. You must fill and plate over at the board house as shown in the illustration. With that taken care of, head over to Screaming Circuits and get a quote on assembly of your boards.

Duane Benson
Roads in pads are not an industry accepted practice either.

 

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