The quality of a circuit board assembly depends on several factors, such as the interface between the board and the component. This makes the pad an essential part of PCB design and development, as it serves as a designated surface area for electrical contact between the component and the board.
What is a pad in PCB design?
A pad is an exposed area of metal traces on a circuit board to which component leads are soldered. Multiple pads are used in conjunction to generate a component footprint, or land pattern, on a PCB. The two types of pads available are through-hole pads and surface mount pads.
Pad Design for Surface Mount Pads
Pad Design for Through-hole Pads
Surface Mount Pads
The pads used to mount surface mount components are called surface mount pads. These pads have the following features:
1. The pad shows the copper trace area. It can be rectangular, circular, square or oblong.
2. Solder mask
3. Solder paste
4. Pad number (the number of pads where the component is present)
Special Features of BGA Pads
SMD Pads vs NSMD Pads
Proper pad design is essential to ensure the manufacturability of BGA components. There are basically two types of BGA pads - solder mask defined pads (SMD) and non-solder mask defined pads (NSMD).
Solder Mask Defined (SMD) BGA Pads
SMD pads are defined by solder mask holes applied to the BGA pads. These pads have solder mask holes so that the mask openings are smaller than the diameter of the pads they cover. This is done to reduce the size of the copper pads that the part will be soldered to.
The image shows how the solder mask is specified to cover a portion of the copper pad underneath. This can provide two benefits - first, the overlapping mask helps prevent the pad from lifting off the board due to mechanical or thermal stress. The second benefit is that as the part moves throughout the soldering process, the openings in the mask will align with a channel for each solder ball on the BGA.
Traditionally, the copper diameter of SMD BGA pads is equal to the pad diameter on the BGA. To generate an SMD coverlay, this is traditionally reduced by 20%.
SMD and NSMD Pads
Non-Solder Mask Defined BGA Pads (NSMD)
NSMD pads differ from SMD pads in that the solder mask is defined not to touch the copper pad. Instead, the mask is formed so that a gap is created between the edge of the pad and the solder mask.
Cross section of NSMD pad
Here, the copper pad size is defined by the copper pad diameter rather than the mask layer.
NSMD pads can be smaller than the diameter of the solder ball, and this reduction in pad size is 20% of the solder ball diameter. This approach leaves more space between adjacent pads, makes routing easier, and is used for high-density and small-pitch BGA chips. One disadvantage of NSMD pads is that they are extremely susceptible to delamination due to thermal and mechanical stresses. However, delamination of NSMD pads can be prevented if standard manufacturing and handling practices are followed.
Through-hole pads
The pads used to mount through-hole components are called through-hole pads, and there are two types:
Plated Through Hole (PTH)
A PTH is a pad with a through hole. The hole walls will be plated with copper and sometimes solder or other protective plating. The hole plating is done through an electrolytic process. This plating provides an electrical connection between different layers of the board.
Non-plated through hole
NPTH refers to a pad that has no plating in the hole. This pad is mainly used in single-sided boards, or these holes are used to mount the PCB in the housing, and screws are installed through these holes. Usually, the non-plated hole will have an area without any copper around the hole (similar to the edge gap of the board). This is done to prevent short circuits between the copper layer and the part to be placed.
The different parts of the through-hole pad are usually called the pad stack, which consists of the following parts:
1. Top pad
2. Bottom pad
3. Inner pad
4. Drill bit
5. Annular ring
6. Pin number
Can I place a via in a pad? – Yes, as a through-hole
In HDI designs, it is necessary to place vias in pads when space is limited. Traditional vias have a signal-carrying trace from the pad to the trace. Via-in-pad can minimize the size of the PCB by reducing the space occupied by trace routing. Via-in-pad is used for BGA components with a pitch of 0.5 mm or less.
Through hole in pad
Traditional via vs. via-in-pad