Sometimes during smt reflow soldering, it is found that the solder paste on the circuit board cannot be completely melted. This must be a serious defect in the poor SMT patch. Anhui Guangshengde will share with you the solder paste that does not melt after reflow soldering. Reasons and solutions.

1. After reflow soldering, when all or most of the solder joints on the PCB board have incomplete solder paste melting, it means that the peak temperature of reflow soldering is low or the reflow time is short, resulting in insufficient solder paste melting.

Solution: Adjust the reflow soldering temperature curve, the peak temperature is generally set at 30-40℃ than the solder paste melting, and the reflow time is 30-60s.

2. When reflow soldering a large-size smt circuit board, there is incomplete melting of the solder paste on both sides of the lateral direction, indicating that the lateral temperature of the reflow oven is uneven. This situation generally occurs when the furnace body is relatively narrow and the heat preservation is poor, because the temperature on both sides of the transverse direction is lower than the middle temperature.

Solution: You can appropriately increase the peak temperature or extend the reflow time, and try to place the smt processed circuit board in the middle of the furnace for welding.

3. After reflow soldering, when the solder paste is not completely melted 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 large heat capacity devices are attached , Is caused by excessive heat absorption or hindered heat conduction.

Solution: 1. When double-sided mounting the circuit board, try to place the large components on the same side of the SMT circuit board. If the arrangement does not open, they should be staggered; 2. Increase the peak temperature or extend the reflow time appropriately.

4. The problem of infrared reflow soldering causes the solder paste to be incompletely melted-due to the dark color absorbs more heat during infrared reflow soldering, the black device is about 30-40℃ higher than the white solder joint, so it is on the same SMT printed circuit board. The color and size of the device are different, the temperature is different.

Solution: In order to make the solder joints and large-volume components around the dark color reach the soldering temperature, the soldering temperature must be increased.

5. Solder paste quality problems cause the reflow solder paste to melt incompletely-the metal powder has high oxygen content, poor flux performance, or improper use of solder paste; if the solder paste is taken out of the low temperature cabinet and used directly, the temperature of the solder paste is higher than room temperature Low, produce water vapor condensation, that is, the solder paste absorbs moisture in the air, and the water vapor is mixed in the solder paste after stirring, or use recycled and expired solder paste.

Solution: Do not use inferior solder paste for smt solder paste printing, and develop a solder paste management system. For example, use within the validity period, remove the solder paste from the refrigerator one day before use, and open the container lid after reaching room temperature to prevent condensation; the recycled solder paste cannot be mixed with new solder paste, etc.