Are they ever OK? Our columnist has an emphatic response.
The answer to the question “Is it ever okay to put open vias in BGA pads?” is simply “no.”
It’s no, no, no, no, not ever!!! That makes it easy. No technique to worry about. No tolerances. Nothing. Just don’t put an exposed via in a BGA pad. The only option is between the pads, with a complete solder mask dam between the pad and via, or have the vias filled and plated over at the board house. It doesn’t matter if the BGA is sub-millimeter pitch or larger-than-millimeter pitch. Nothing but metal is allowed on the BGA pad.
Now, other components offer more flexibility and thus require some choices and guidelines. Reader Andy B. asked about large components, such as voltage regulators, where the manufacturer has recommended vias to connect the thermal pad to the ground plane, or to additional thermal area on the back side of the PCB.
The easy answer is to just treat it like a QFN and read our various suggestions1 surrounding that form factor. Having the extra room does allow for additional flexibility, but if the vias are open, they still run the risk of sucking solder to the other side of the PCB. You can sometimes get away with really tiny vias.2 But it’s not best practice.
It’s really a matter of tradeoffs. Some say to never fill or cap the via because doing so might impede the thermal transfer. Well, power chip manufacturers, you shouldn’t rely on unbuildable design to meet product specs. Vias can be filled with thermally conductive material. Or the via can be capped with solder mask.2 Just make the via cap as small as possible – 100 to 125µm larger than the via.
Finally, segment the paste stencil layer. Solder paste on top of an open via or even on top of a masked via could be asking for trouble. In this power MOSFET footprint (Figure 1), there are four vias (which will be capped) between the openings of the stencil under the part. The original copper stops at the blue dashed lines. I’ve added the extra thermal area as a copper pour with vias to give additional cooling area, another technique sometimes used with power parts. The vias in the extra thermal area do not need to be filled, as they are in the solder mask covered area, not the paste covered area.
Ed.: Read Duane’s blog each week at circuitsassembly.com/blog.
1. Duane Benson, “Large Via In Pad,” June 11, 2010, circuitsassembly.com/blog/?p=992.
2. Duane Benson, “Open Small Vias,” Sept. 21, 2011, circuitsassembly.com/blog/?p=2195.