PCB components pad design is a key point, and the quality of the final product depends on the quality of the solder joints. Therefore, whether the pad design is scientific and reasonable is of vital importance. So, today we will classify and talk about the pad design requirements of different types of components.
Chip pcb components pad design should master the following key elements
1.Symmetry: The pads at both ends must be symmetrical to ensure the balance of the surface tension of the molten solder; for small-sized components 0603, 0402, 0201, etc., the imbalance of the surface tension of the molten solder at both ends can easily cause the component to form a “tombstone” defect.
2. Pad spacing: ensure the proper overlap size of the component end or pin and the pad.
3.The remaining size of the pad: the remaining size after the lap must ensure that the solder joint can form a meniscus;
4.Pad width: It should be basically the same as the width of the component tip or pin.
A: pad width
B: Pad length
G: Pad pitch
S: Remaining size of pad
SOP and QFP design principles
- The center distance between pads is equal to the center distance between pins;
- General principles of single pin pad design
Y=T+b1+b2=1.5~2mm (b1=0.3~1.0mm b2=0.3~0.7mm)
- The distance between the inner sides of the two rows of pads is calculated by the following formula (unit: mm)
Where: G—the distance between two rows of pads,
F—Package size of component shell,
K-factor, generally 0.25mm
BGA pad design principles
- The center of each pad on the PCB coincides with the center of the corresponding solder ball at the bottom of the BGA;
- The PCB pad pattern is a solid circle, and the via hole cannot be processed on the pad;
- The width of the wire connected to the pad should be the same, generally 0.15mm~0.2mm;
- The solder mask size is 0.1mm~0.15mm larger than the pad size;
- The via hole near the pad must be blocked with solder resist after metallization, and the height must not exceed the height of the pad;
- Process the screen pattern on the four corners of the BGA device outline, and the line width of the screen pattern is 0.2mm~0.25mm.
Setting of via hole for reflow soldering process
- Generally, the diameter of the via hole is not less than 0.75mm.
- Except for SOIC, QFP, PLCC and other devices, no via holes can be made under other components.
3.The via hole cannot be designed in the middle of the two pads of the chip component on the soldering surface.
- It is not allowed to directly use the vias as the pads of BGA devices.It is absolutely necessary to avoid setting through holes within the surface mounting pads, and setting through holes within 0.635mm from the surface mounting pads, which should be connected by a small piece of wire, and the solder loss channel should be blocked with a solder resist, otherwise it is easy to cause “stand-up” Defects such as “chips” or “insufficient solder”.
Pad design of plug-in PCB components
- If the hole pitch is 5.08mm or more, the diameter of the pad shall not be less than 3mm;
- For holes with a pitch of 2.54mm, the minimum pad diameter should not be less than 1.7mm;
- The distance between the pads connected to the 220V voltage on the circuit board should not be less than 3mm;
- The diameter of the pad whose current exceeds 0.5A (including 0.5A) should be greater than or equal to 4mm;
- The pad should be as large as possible. For general solder joints, the minimum diameter of the pad should not be less than 2mm.
When the wave soldering process is used, the pad design of the chip component
When using wave soldering to solder chip components, pay attention to the occurrence of “shadow effect (missing solder) and’bridging’ (short circuit)”. For CHIP components, the axial direction of the component should be perpendicular to the transfer direction of the PCB. Small components It should be in front of large components, and the spacing should be greater than 2.5mm.
The above is about the basic content of component pad design, I hope I can bring you some help.