Surface mount pads
Pads used to mount surface mount components are called surface mount pads. These pads have the following features:
- Pad which shows the copper area. This can be rectangle, round, square, or oblong.
- Solder mask layer
- Solder paste
- Pad number (number of pads present for the component)
SMD pad vs NSMD pad
Proper pad design is critical to ensure the manufacturability of BGA components. There are basically two types of BGA pads – the solder mask-defined pad (SMD) and the non-solder mask-defined pad (NSMD).
Solder mask defined (SMD) BGA pads
SMD pads are defined by the solder mask apertures applied to the BGA pads. These pads have the solder mask aperture such that the mask opening is smaller than the diameter of the pad they cover. This is done to shrink the copper pad size that the part will be soldered to.
The image shows how the solder mask has been specified to cover a part of the copper pad underneath. This can lead to two advantages – firstly, the overlapping mask helps prevent the pads from lifting off the board because of mechanical or thermal stress. The second advantage is that the opening in the mask will create a channel for every ball on the BGA to align with while the part moves through the soldering process.
The copper layer of an SMD BGA pad conventionally has a diameter equal to the pad on the BGA. To generate the SMD overlay, a reduction of 20% is traditionally used.
Non-solder mask defined BGA pads (NSMD)
NSMD pads vary from SMD pads in that the solder mask is defined to not contact the copper pad. The mask is instead created such that a gap is generated between the pad edge and the solder mask. Here the copper pad size is defined by the copper pad diameter instead of 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 ball diameter. This approach leaves more room between adjacent pads enabling easier trace routing and is used for high density and fine pitch BGA chips. One disadvantage of NSMD pads is their high susceptibility to delamination due to thermal and mechanical stresses. However, NSMD pad delamination can be prevented when standard manufacturing and handling practices are followed.
Pads used to mount through-hole components are called through-hole pads and are of two types: