Shenzhen Weisheng Electronic Technology Co., Ltd. has its own SMT chip processing plant, which can provide SMT chip processing services for 0201 components in the smallest package, and supports processing with supplied materials and OEM materials. The next is how to solve SMT processing. Missing printing, low tin defects?
Causes of SMT processing missed printing and low tin defects
1. The principle of solder paste printing
The solder paste is squeezed into the steel mesh through a squeegee, so that the solder paste touches the surface of the PCB and adheres to the PCB surface. The solder paste that adheres to the PCB surface during demolding overcomes the resistance of the steel mesh wall and is transferred to the PCB surface.
2. Observe, think, compare
a. Although part of the substrate around the pad is covered by the stencil opening during printing, the solder paste at the bottom of the stencil opening is difficult to contact the PCB pad and the surrounding substrate, and it is not enough to overcome the hole wall during demolding. Resistance (only a small amount of solder paste on the pad)?
b. There is a 35 um deep annular deep pit between the pad and the solder mask. Does the solder paste with the stencil opening on the pit not touch the bottom of the pit?
c. Why are other pads connected to the circuit not easy to miss?
3. Bare copper plate printing verification
The solder paste of 5 different brands of 4# powder can be stably tinned (laser + electro-polished steel mesh) on a circular hole with a thickness of 0.1 and an opening diameter of 0.28.
SMT processing leaks and poor tin solutions to solve the problem
1. Find all the pads that are not connected to the outer circuit, change the size of these pads from the original circle with a diameter of 0.27 to a circle with a diameter of 0.31, reduce the area of the deep pit around the pad, and make the opening area on the deep pit originally It becomes on the copper foil of the pad, so that the gap between the opening area on the deep pit and the bottom of the steel mesh is reduced. After the small batch verification is OK, the original stencil is used for mass production, and the soldering of the soldering pads that are difficult to solder is good (the area of the soldering pad is increased, and the batch verification does not find poor soldering).
2. Reduce the PCB solder mask thickness and reduce the influence of the higher height solder mask layer on the circuit near the pad. It is recommended that the PCB solder mask thickness be less than 25um.
3. The new PH steel mesh is used to eliminate printing gaps to the maximum extent. PH steel mesh is introduced.