The Nuts and Bolts of a Via Padstack

The humble via comes in many flavors. By connecting one layer of a conductor pattern to another, vias have connected the world. My whole career has depended on them as part of the hardware I used to design for everyone else to use. A foundational innovation in electronics brought plated through holes to the masses. As a leap forward from wire-wrap technology, multi-layer printed circuit boards put a “mainframe” in each of our pockets almost overnight.

From the first plated through-hole to the latest, the trend is to support higher density interconnect. The key driver in plating holes is the aspect ratio, the hole’s width over depth. For a through-hole, the depth is the thickness of the PCB. Most reputable fab houses can handle a 10 to 1 ratio such that a popular .062” board thickness will require a minimum finished hole size 0.006”

Pressing on the factory capabilities inevitably leads to more X-outs. As yield drops, unit prices rise.

Finished hole size is just what it says on the tin. The initial drill will be one or two-thousandths of an inch larger than the finished size to account for the thickness in the copper “barrel”. The barrel is created by drilling the through holes and electroplating the top and bottom of the board prior to etching away the circuit pattern. So there are these two complete sheets of copper of a starting thickness. As those are plated up, the prepared holes get their plating. That is all of the copper that is added to form the barrel. Other protective layers come later.

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