Factors Affecting the Quality of BGA Patch Assembly （2/3）

Through holes are allocated to BGA pads from the outside.

This type of pad is most suitable for BGA components with a small number of I/Os. This type of pad design provides convenience for soldering and provides more free space for the pad size. Of course, basic requirements must be met in terms of tracking. Therefore, it is almost impossible to utilize such pads on BGAs with higher I/O counts.

The vias in the pads develop with the development of microvia technology in PCB manufacturing.

In addition to the pad type, the position of the solder mask and the BGA pad is directly related to the BGA soldering. According to different solder mask positions, BGA pads are divided into two types: SMD (defined as solder mask) pads and NSMD (undefined solder mask) pads, which respectively play a role in BGA soldering. When using SMD pads, the bonding area between two bonding pads is relatively large, resulting in a relatively large bonding area between the solder joint and the PCB board. However, as the size of the pad increases, the distance between adjacent pads becomes smaller, which affects the distribution and tracking ability of the pads.

In the PCB manufacturing process, if the solder mask deviates in the same direction, it will not affect the BGA pads, which is beneficial for BGA soldering.However, this type of pad tends to crack when the solder mask is reworked at the edge, which is detrimental to the rework effect. Once NSMD pads are used, the pads will be relatively small, which is conducive to the distribution and tracking of via pads. However, this type of pad structure leads to a reduction in the bonding area between the bonding point and the bonding pad, and further reduces the bonding strength of the bonding point. In short, both pads have their own advantages and disadvantages, and the corresponding pad can be determined based on technical considerations.

Solder Paste Printing

Solder paste printing plays a key role in determining the quality of soldering. Solder paste printing is the accurate conversion of solder paste from template to pad, which includes solder paste and printers. The precision of the solder paste printer should first meet the requirements of BGA assembly. The template determines the amount of solder paste by its thickness and opening size. The amount of solder paste required for BGA packaging is usually determined by the following three aspects:

• Sufficient solder should be used to ensure good BGA soldering.
• The amount of solder paste should compensate for the coplanar error of the solder balls of BGA components (usually 0.1mm).
• When there are other fine-pitch components on the circuit board, the amount of solder paste should be considered comprehensively to prevent more soldering defects from occurring.