Vias not covered
It means that the vias are exposed and the surface finish is applied to via barrel.
The solder paste might get into the vias and cause poor or non-existent solder joints.
It means that the vias are simply covering by soldermask ink.No additional process steps is required during fabrication.
Covering the annular ring and vias with soldermask ink in order to prevent exposure to the elements and reduce accidental shorting or contact with the circuit.
Plugged Vias with non conductive media (eg. Epoxy/resin, soldermask ink)
The conductive vias in BGA requires plugged vias. Because solder paste might wick away from the intended pad and flow down into the via, then creating poor or non-existent solder joints during assembly.
The diameter of the plugged vias requires to be smaller than 0.5mm.
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