BGA, Blind via and Via in Pad

Blind vias in itself is a space saving and sometimes cost effective way of designing complex PCBs that have dense components. Blind vias are sometimes used as via in pads for BGAs of pitch 0.65mm, 0.5 mm or smaller. Via in pads adds complexities from the manufacturing point of view.

Via in pad is sometimes used in QFN type packages. The use of the via in pad for these SMT packages is often avoidable. The recommended practice is not to use via in pad for non BGA smt packaged from manufacturing perspective. The primary reason being, when left open, those via holes act like little capillary straws and suck off solder from the pad. This may result in some undesirable effects during the board fabrication process. If the vias are left open, solder will tend to wick down into the via hole. This may result in the board not having enough solder left to secure the component. If the via in pad is a through hole, it may create solder bump on the bottom side of the board which could interfere with other components or lead to shorts.

If the vias are partially filled, the trapped air inside the via will expand due to heat and pop off smaller components populated over it.

On the other hand vias in pad provide better thermal conduction. It is good from signal integrity point of view because of the reduced interconnect length and smaller parasitic inductance and parasitic capacitance.

Via in pad is often the only option left with BGAs of pitch sizes 0.65 mm, 0.5 mm and smaller. When we have to use via in pad, we must ensure to plug the via and then plate copper over it. The plugging can be done with metal or a thermally and electrically conductive epoxy before the final plating steps. This gives all of the benefits of via in pad without causing problems in assembly. The PCB designer must mention the vias that need to be plated in the drill drawing file.

A via in pad is usually done with a blind micro-via that only goes through one or two layers of the board. We should still fill the via so that no solder sucks down.


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