pad

A pad is a small, flat area of exposed copper on a printed circuit board (PCB) designed to accept a solder joint. It serves as the physical and electrical connection point where component leads, wires, or traces meet the board. Pads are typically circular, rectangular, or oblong, with diameters or dimensions ranging from 0.3 mm to several millimetres depending on the current requirement and assembly method.

Pads fall into two main categories based on their function: through-hole pads, which have a plated hole running through them to allow component leads to pass and solder on both sides of the board; and surface-mount pads (also called lands), which sit flat on one side of the board to receive surface-mount device (SMD) leads or balls. The size and shape of a pad is determined by design rules that account for the component lead diameter, the solder fillet needed for mechanical strength, and the PCB manufacturing tolerances.

Pad degradation and defects

Common failure modes include pad lifting, where thermal cycling causes the copper to separate from the substrate due to stress; and pad cratering, a depression that forms around the pad when excessive mechanical stress is applied during component removal or rework. Poor solder wetting, where the solder fails to properly adhere to the pad surface, often results from oxidation, contamination, or inadequate pad surface finish (such as HASL, ENIG, or OSP coating).

The copper content and plating thickness of a pad directly affect its solderability and current-carrying capacity. High-current applications may use thicker copper layers (2 to 3 ounces per square foot) or dedicated power planes connected to pads via thermal reliefs, which are small copper bridges that allow heat to dissipate during soldering without making the joint so thermally massive that solder flows unevenly.

Pad layout and spacing are controlled by design rules specific to the manufacturing process. In fine-pitch designs, pad-to-pad spacing may be as small as 0.15 mm for ball grid arrays (BGAs), requiring precision laser or mechanical drilling and tight process controls. Inadequate spacing risks solder bridges, where molten solder flows between adjacent pads and creates an unintended short circuit.