A finished PCB is covered with holes of different sizes. Some are for soldering components, some hold the board to a chassis, and some look minor but carry the electrical connection between layers. Those holes fall into two groups: plated through-holes (PTH) and non-plated through-holes (NPTH).
The difference is not cosmetic. Plating on the barrel decides whether the hole is a conductor or only a mechanical opening. If that class is wrong in the design data, the board can lose a layer-to-layer net, or a screw hole can become an unintended short to an inner plane.
How to Tell PTH from NPTH
The main difference is whether the hole wall has a plated copper layer.
Figure 1 | Left: PTH with plated copper. Right: NPTH with no copper on the wall
A PTH barrel is copper-plated and conductive. Copper on the wall ties annular rings or pads on different layers into one electrical node. That is why a PTH can solder a lead and why a via can carry a signal or a return from one layer to another.
An NPTH barrel is bare and insulating. There is no copper on the wall, so the hole is not part of a net. It is used only as a mechanical feature: a clearance for a screw, a locating pin, or a fixture.
On a finished board the two are usually easy to distinguish. PTH holes sit in copper pads and show a plated barrel. NPTH holes are often larger, may have no pad at all, and show laminate or exposed dielectric on the wall. In the CAD and Gerber data the distinction must be explicit. Diameter alone does not define plating.
What NPTH Is For
An NPTH hole is often large and is not part of the netlist. Its jobs are still specific.
- Screw holes that fasten the board to a chassis, heat sink, or other structure
- Mechanical fixing of an external connector or interface
- Tooling or fixture locating holes
Figure 2 | Typical NPTH screw holes and corner locating holes
NPTH diameters are generally larger than signal vias because they exist for strength or for assembly, not for a minimum electrical drill. They do not need to connect copper inside the board. In many designs the region around an NPTH screw hole is kept clear of copper so the hardware cannot short to a plane. In other designs a grounded metal standoff is intended to contact a pad, but that pad is a separate copper feature. The hole wall itself remains unplated.
Locating and tooling holes follow the same rule. They register the panel to a fixture or align a connector body. Plating those holes would add cost without adding function, and it could create an unwanted conductor around a mechanical pin.
What PTH Is For
PTH is how the board connects electrically from layer to layer. Two common uses cover most of the plated holes on a product board.
Through-hole component soldering
Typical parts include through-hole resistors, capacitors, and inductors, along with headers and other DIP-style devices. The hole must be larger than the lead so the part can be inserted and soldered. Solder wets the lead and the plated barrel, forming a joint that is both mechanical and electrical. Without plating on the wall, the lead would have no reliable conductive path to inner layers and a weaker fillet on the barrel.
Signal vias
The small plated holes used as vias are PTH structures. They connect copper on different layers and are required in high-speed and high-density designs. A via is simply a smaller, denser form of PTH: the same plated barrel, used for routing rather than for a component lead.
Power vias, ground stitch vias, and high-speed signal vias all belong to this class. Their drill is chosen for current, density, or aspect ratio rather than for a lead diameter, but the electrical requirement is the same. The barrel must be plated, and the pads or capture lands on the connected layers must belong to the same net.
PTH versus NPTH
| Item | PTH (plated through-hole) | NPTH (non-plated through-hole) |
|---|---|---|
| Plating | Copper plated | No copper plating |
| Conductive | Yes | No |
| Use | Soldering and layer-to-layer connection, including vias | Screws, locating, mechanical support |
| Typical size | Generally smaller; vias smaller still | Generally larger, for structure |
| When required | Required on every multilayer board | Used when the mechanics need it |
Every multilayer board needs PTH because inner layers have no other path to the outer surfaces. NPTH appears only where the mechanics need an unplated opening. Mixing the two classes is the usual documentation error: a screw hole exported as plated can short inner copper, and a via exported as non-plated becomes an open between layers.
Why the CAD and CAM Definition Has to Match
PTH and NPTH are only holes in the board, but they set both the electrical and the mechanical behavior. The two types have to be specified correctly so fabrication and function do not fail for a hole that was defined as the wrong class.
On a released file, CAM does not infer plating from barrel appearance or from diameter. The process stops when the plated flag, pad, net, and mask do not agree. A hole drawn at via size but marked NPTH will not receive a barrel. A mounting hole marked PTH will be plated even if the designer intended only a screw clearance. Those conflicts are resolved in CAM review, or they ship as a board that does not match the schematic.
Clear fabrication notes remove the ambiguity. PTH holes should carry pads and net connectivity consistent with plating. NPTH holes should be called out as non-plated, with pad and plane clearances that match the mechanical hardware. When those attributes agree, the finished board has conductive barrels where nets need them and insulating holes where screws and fixtures pass through.
