In our common SMT chip processing process, there will be more or less poor soldering after reflow. Therefore, it is necessary for us to manually use tools for rework to obtain qualified PCBA solder joints.
1. During manual rework and soldering, the SMT components should be soldered according to the principle of first small, then large, and low and then high. First, solder chip resistors and chip capacitor transistors, and then solder small IC devices and large IC devices.
2. When soldering chip components, the width of the selected soldering iron tip should be consistent with the width of the component. If it is too small, it will not be easy to locate during soldering.
3. When welding SOP, QFP, PLCC and other devices with pins on both sides or four sides, you should first weld several positioning points on both sides or four sides. After careful inspection to confirm that each pin is consistent with the corresponding pad, proceed Drag soldering completes the soldering of the remaining pins.
4. The speed of dragging should not be too fast. The IC can drag over 1-2 solder joints in about 1second.
5. After welding, use a magnifying glass of 4 to 6 times to check whether there is a bridge between the solder joints, and the welding of the same part should not be more than two consecutive times. If it is not soldered at one time, it should be cooled before soldering to prevent the pad from falling off.
6. When soldering IC devices, evenly apply a layer of flux paste on the pads, which can not only play the role of wetting and fluxing on the solder joints, but also improve the maintenance efficiency.
Rework process requirements
1. The operator should wear an anti-static wristband. Generally, it is required to use an anti-static constant temperature electric soldering iron. When using an ordinary electric soldering iron, it must be well grounded.
2. A low-power soldering iron of 15 to 20W should be used when repairing chip components. The temperature of the soldering iron tip is controlled below 250°C.
3. The welding time should not be too long, otherwise the components will be easily scalded. The welding time of chip components should not exceed 5 seconds, and the number of welding of the same solder joint should not exceed two times.
4. The solder joint should be in the shape of a sine wave, the surface should be bright and smooth, without tin thorns, and the amount of tin should be moderate.
5. After the soldering is completed, use alcohol to clean the residual flux on the circuit board to prevent the carbonized flux from affecting the normal operation of the circuit.
6. When removing components for SMT patch repair, you should wait until the solder is completely melted before removing the device to prevent damage to the coplanarity of the device pins.