Small Open Vias

摘要

PAD PARTS CHANGE and so do vias. Our standard policy is that open vias in pads are bad. We from time to time recommend ways to plug them. Generally, you have several options, such as capping the top or bottom of the via with solder mask. However, with vias in the pads of really small parts, those solder mask options will probably not work. Solder mask generally isn't put down with enough precision to cover holes on tiny pads, and further, the solder mask would probably mess with the clearance, especially with very small QFNs. Immersion silver finishes may develop corrosion in sealed vias. FIGURE 1 is an example of a small QFP with open vias in the pads. Those are some small vias. If solder mask isn't going to work, what will? Filling and plating over them, that's what. You really have only two choices: fill and plate, or live with a bunch of voids under the part and solder slopped on the bottom side of the printed circuit board.