What is a PCB Pad?

A pad is the exposed copper area on a printed circuit board (PCB) where component leads are soldered. Electrical connections for all components are made through pads. The design and placement of pads directly influence the solderability, stability, and heat dissipation of components. Depending on the type of components and their packaging methods, PCB pads are generally categorized into through-hole pads and surface-mount pads.

Figure 1: PCB Pad

Through-hole Pad

Pads used to mount through-hole components are known as through-hole pads. During soldering, component leads must be inserted into the via holes of the pad.

Soldering components via through-hole pads results in strong, reliable solder joints, creating a long-lasting mechanical and electrical connection between the components and the PCB. However, due to the leads and the number of holes required, routing space on multilayer boards can become limited.

Figure 2: Through-hole Pad

Surface-mount Pad

Pads that allow for the direct mounting of electronic components on the surface of the board are called surface-mount pads. Unlike through-hole pads, surface-mount pads are used for smaller components and allow for more compact placement of components in limited space. This enables better functionality and performance in smaller designs. Surface-mount pads are particularly advantageous in the design of complex, multi-layer PCBs. However, they may not be suitable for components that generate significant heat.

Figure 3: Surface Mount Pad

BGA Pad

For BGA (Ball Grid Array) components, proper pad design is essential to ensure manufacturability and reliability of the PCB. Two common types of BGA pads are Solder Mask Defined Pads (SMD) and Non-Solder Mask Defined Pads (NSMD).

Figure 4: BGA Pad

Solder Mask Defined Pads (SMD):

SMD pads are defined by the solder mask aperture applied to the BGA pads. In this design, the solder mask aperture is smaller than the copper pad itself, partially covering the pad. SMD pads offer several benefits, such as preventing pad displacement and improving BGA component positioning. However, this design reduces the contact area between the copper pad and the solder joint, limits the space between adjacent pads, and can affect the routing of signal traces and the use of through-holes.

Figure 5: SMD & NSMD

Non-Solder Mask Defined Pads (NSMD):

NSMD, or Non-Solder Mask Defined Pads, are defined by the diameter of the copper pad itself, rather than by the solder mask aperture. This creates a small gap between the edge of the pad and the solder mask, with the copper surface fully exposed. NSMD pads offer a larger surface area for solder joints and provide more space between adjacent pads, making them ideal for high-density and fine-pitch BGA components. However, NSMD pads are more prone to delamination due to thermal and mechanical stresses.

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