A pad is the exposed area of copper on a circuit board, where the component lead is soldered. The electrical connection of all components is made via pads. The design and placement of pads directly affect the solderability, stability, and heat transfer of components. According to different components and packaging methods, PCB pads can be typically divided into through-hole pads and surface mount pads.
Figure1: PCB Pad
Pads used to mount through-hole components are called through-hole pads. When soldering, the component pins need to be inserted into the via holes of the pad.
Soldering components via through-hole pads can create reliable solder joints between the components and the PCBs, providing a long-term mechanical and electrical connection. However, due to the component leads and the number of holes, the available routing areas on a multilayer board will be limited.
Figure2: Through-hole Pad
Pads that allow electronic components to be mounted directly on the board surface are called surface mount pads. Unlike through-hole pads, surface mount pads are suitable for mounting smaller components. They allow more components to be placed in a smaller space but at the same time provide larger functionality and performance. This type of pad is a great advantage in the design of complex multi-layer boards. However, surface mount pads commonly cannot be used for components that will generate high levels of heat.
Figure3: Surface Mount Pad
When connecting BGA-style components, the proper pad design is also crucial to ensuring the manufacturability of the boards. There are two common types of BGA pads: Solder Mask Defined Pads (SMD) and Non-Solder Mask Defined Pads (NSMD).
Figure4: BGA Pad
SMD pads are defined by the solder mask apertures applied to the BGA pads. In this pad design, the mask aperture is smaller than the actual copper pad, and the pad is partially covered by the surrounding solder mask. The use of SMD pads has some advantages. For example, the overlapping mask helps prevent the pad from shifting or coming off the board. Also, the pad is useful for BGA positioning. However, this type of pad reduces the contact area between the pad and the copper surface, as well as the space between adjacent pads. This limits the thickness of the tracks between the pads and can affect the use of through-holes.
Figure5: SMD &NSMD
Non-Solder Mask Defined Pads (NSMD):
NSMD stands for Non-Solder Mask Defined Pad or Copper Defined Land Pattern. As the name suggests, the size of the pad is defined by the diameter of the pad itself rather than the solder mask aperture and there is a small gap between the edge of the pad and the solder mask. The pad is independent and the copper on the surface is fully exposed. This pad design provides a larger surface area for solder joint connections and offers greater clearance between pads and is widely used in high-density and fine-pitch BGA chips. A disadvantage of NSMD pads is that they are prone to delamination due to thermal and mechanical stresses.
