PCB Tech - Reasons for insufficient melting of SMT solder paste

Some SMT chip processing plants may encounter insufficient melting of the flux paste during soldering. Why is this? How to avoid this kind of situation? Here are a few cases to analyze.

1. When all solder joints or most solder joints on the PCB have insufficient flux melting, it means that the peak temperature of reflow soldering is low or the reflow time is short, which leads to insufficient flux melting.

Preventive measures: adjust the temperature curve, the peak temperature is generally set at 30-40 degree Celsius than the flux melting, and the reflow time is 30-60s.

2. When the SMT chip processing manufacturer is welding large-size smt circuit boards by electric welding, there is insufficient melting of the fluxing paste on both sides, indicating that the temperature of the reflow soldering furnace is uneven. This kind of situation generally occurs when the furnace body is relatively narrow and the heat preservation is poor, because the temperature on both sides is lower than the middle temperature.

Preventive measures: appropriately increase the peak temperature or extend the reflow time, try to place the smt processed circuit board in the middle of the furnace for electric welding.

3. Insufficient melting of the solder paste occurs at the fixed position of the smt patch assembly board, such as large solder joints, large components and large components, or when it occurs on the back of the printed board where a large heat capacity device is attached, it is It is caused by excessive heat absorption or hindered heat conduction.

Preventive measures: 1. When SMT chip processing factory double-sided mounting circuit boards, try to place large components on the same side of the SMT circuit board. 2. Properly increase the peak temperature or extend the reflow time.

4. Infrared furnace situation-----The dark color absorbs more heat when the infrared furnace is welded. The black device is about 30-40 degree Celsius higher than the white solder joint. Therefore, on the same SMT printed circuit board, due to the color of the device And the size is different, the temperature is different.

Preventive measures: In order to make the solder joints and large-volume components around the dark color reach the electric welding temperature, the electric welding temperature must be increased.

5. The quality of SMT flux paste-the metal powder has high oxygen content, poor flux performance, or improper use of flux paste; if the flux paste is taken out of the low temperature cabinet and used directly, due to the temperature of the flux paste It is lower than the room temperature, and condensation occurs, that is, the solder paste absorbs moisture in the air, and the water vapor is mixed in the solder paste after stirring, or the recycled and expired solder paste is used.

Preventive measures: do not use inferior solder paste, formulate a management system for the use of SMT solder paste. For example, use within the validity period, take out the soldering paste from the refrigerator one day before use, and open the container lid after reaching room temperature to prevent condensation; the recycled soldering paste cannot be mixed with the new soldering paste, etc.