Hi Ho, Hi Ho, It's off to Via I go
Here's just another via in pad story. In the first example (the green one), all those open vias are like 15 miles wide - okay, drop the "e", but they're really wide. You can't quite drive a truck through them, but solder will get sucked down into them. Lead-free solder will even go down something that wide. Without some modifications, this is not a very buildable board.
The second picture (the red one) also has "open" vias, but they are a whole lot smaller. We never like to see open vias, but in the second case, we'll probably just go ahead and use it as-is. It shouldn't be a problem with lead-free solder and may even be okay with leaded. With something like this, we might just run one, check it out and see how it goes and then decide if it's buildable as is based on that.
My guess is that the vias in the red board are about six mills. Maybe even a little smaller. When I hold it up to the light, I can't even see through it. That means that even if the vias weren't intentionally plugged, they effectively are. You will likely get some voiding under the part, but not much in this case.
If your application needs exact predictability in terms of the amount of voiding under the QFP or QFN, then your only choice is to have the vias filled and plated over at the board house. If you can get by with some variability (most component manufacturers say that around 50% is okay for most applications, but check with your part manufacturer to be sure), then a good rule of thumb might be that if you can't see light through it, it will likely build okay. If in doubt, get in touch with us and ask to speak with one of our manufacturing engineers about it.
There is not doubt about the green board. Those vias are too, too big to be left open. But, the vias in the red one might just be small and closed off enough to use.
When does 01631D = 8ABBA9?