Summary of Three Precautions for SMT Processing

1、 Correct components

It is required that the type, model, nominal value, polarity and other characteristic markings of each assembly position number component comply with the requirements of the product's assembly drawing and detailed list, and cannot be pasted in the wrong position.

2、 Accurate location

The ends or pins of the components should be aligned and centered with the solder pad pattern as much as possible, and it is also necessary to ensure that the solder ends of the components are in contact with the solder paste pattern. The installation position of components should meet the process requirements. The requirements for the mounting positions of the Chip components, wing shaped and J-shaped pin devices, and spherical pin devices with two ends are as follows:

1. Chip component with two ends: The self positioning effect of the Chip component with two ends is relatively large. When mounting, more than 1/2 of the component width direction is lapped on the bonding pad. As long as the two ends in the length direction are lapped on the corresponding bonding pad and contacted with the solder paste pattern, reflow soldering can self position. However, if one of the ends is not lapped on the bonding pad or contacted with the solder paste pattern, reflow soldering will produce displacement or suspension bridge.

2. Wing pin and J-pin devices: The self positioning effect of SOP, SOJ, QFP, PLCC and other devices is relatively small, and the mounting offset cannot be corrected by reflow soldering. If the mounting position exceeds the allowable deviation range, it must be adjusted manually before entering the reflow soldering furnace for welding. Otherwise, it must be repaired after reflow soldering welding, which will result in waste of man hours and materials, and even affect product reliability. If the installation position is found to exceed the allowable deviation range during the production process, the installation coordinates should be corrected in a timely manner.

When manually installing or aligning, it is required to have an accurate installation position, align the pins with the solder pad, and center them. Do not place them incorrectly. Drag and align them on the solder paste to avoid sticking solder paste patterns and causing bridging.

3. Spherical pin devices: Due to the relatively large solder pad area of BGA, CSP, and other spherical pin devices compared to the area of the component body, the self positioning effect is very good. Therefore, as long as these two points are met, one is that the solder balls of BGA correspond to the corresponding solder pads one by one; The second is that the maximum deviation between the center of the solder ball and the center of the solder pad is less than 1/2 of the diameter of the solder ball.

3、 The pressure (patch height) is appropriate.

The mounting pressure (Z-axis height) should be appropriate and appropriate. When the mounting pressure is too low, the soldering end or pin of the component floats on the surface of the solder paste, and the solder paste cannot stick to the component. It is easy to cause position movement during transmission and reflow soldering. In addition, due to the high height of the Z-axis, the component is dropped from a high position during mounting, which can cause the mounting position to shift;

Excessive pressure on the patch and excessive extrusion of solder paste can easily cause solder paste adhesion. During reflow soldering, bridging can also occur, and the placement of the patch may shift due to sliding. In severe cases, it can also damage components.