PCB design glossary
Pad
AKA: pads
An exposed copper area on a PCB that a component lead solders to.
A pad is the piece of exposed copper a component lead lands on. Surface-mount pads sit on top of the board; through-hole pads have a plated hole through them. The size, shape, and finish of pads is set by the footprint and constrained by IPC-7351 recommendations.
Pad geometry matters for assembly yield: too-small pads cause tombstoning on small SMD parts, too-large pads waste space and can cause solder bridging.
Kinds of pad
- SMD pads sit on the surface; the component's terminals solder onto them.
- Through-hole pads surround a plated hole that a lead passes through.
- Thermal pads (exposed pads) under QFN and power parts carry heat into the board and usually to ground.
- Test pads and fiducials: copper with no component, used for probing and machine alignment.
Soldermask-defined vs non-soldermask-defined pads
On a non-soldermask-defined (NSMD) pad the mask opening is slightly larger than the copper, so the copper edge defines the pad. This is the usual choice, because the solder wraps the pad edge. On a soldermask-defined (SMD) pad the mask overlaps the copper and defines the opening; it is sometimes used on BGA pads to anchor them against peeling.
Pads and assembly yield
- Pads that are too small or uneven on tiny parts cause tombstoning: the part stands up on one end.
- Pads that are too large or too close cause solder bridges between pins.
- A large pad joined straight to a plane is hard to hand-solder; thermal relief spokes help.
- Thermal pads under QFNs often need a windowed paste pattern, not one big opening, to avoid floating the part.
Work it out with a free tool
Pin pitch decides the pads
Header and connector pads have to match the part's pitch exactly. The pitch reference lists the common ones on one page: pin headers, JST, Molex and USB.
See connector pitches