/Solder paste incomplete melting causes and countermeasures
1.When all the solder joints or most of the solder joints of the pcb assembly board are incompletely melted, indicating the peak temperature of the reflow soldering is low or the reflow time is short, which causes the solder paste to melt insufficiently.
Preventive measures: adjust the temperature curve. The peak temperature is generally set at 30-40 °C for the solder paste, and the reflow time is 30~60s.
2.When the pcb manufacturer is welding large-size PCB circuit boards, there is incomplete melting phenomenon of the solder paste on both sides of the lateral direction, indicating that the lateral temperature of the reflow soldering furnace is not uniform. This situation generally occurs when the furnace body is relatively narrow and the insulation is poor, because the lateral sides are lower than the intermediate temperature.
Preventive measures: Increase the peak temperature or extend the reflow time as appropriate. Trying to place the PCB in the middle of the furnace for soldering
3.When the solder paste is not completely melted in the fixed position of the pcb assembly board, such as large solder joints, large components and large components, or occurs on the back of the printed board with a large heat capacity device, it is because of excessive heat absorption or heat conduction blockage.
A. When designing the pcb circuit board on both sides, place the large components on the same side of the PCB as much as possible. If too many to be arranged , it should be staggered.
B . Properly increase the peak temperature or extend the reflow time.
4.Infrared furnace problem—–Infrared furnace welding due to deep color absorption of heat, black device is about 30-40 °C higher than white solder joint, so on the same PCB, due to the different color and size of the device, Its temperature is various
Preventive measures: In order to achieve soldering temperatures around the solder joints and large volume components around the dark color, the soldering temperature must be increased.
5.Solder paste quality problem —- high metal powder oxide content, poor flux performance, or improper using of solder paste; if the solder paste is used directly from the low temperature cabinet, the temperature of the solder paste is lower than room temperature. The condensation of water vapor, that is, the solder paste absorbs the moisture in the air, and the water vapor is mixed in the solder paste after stirring, or the solder paste which is recycled and expired is used.
6.Preventive measures: Do not use inferior solder paste to establish a management system for solder paste usage. For example, if it is used within the validity period, remove the solder paste from the refrigerator the day before using, and then open the container lid to prevent condensation; the recycled solder paste cannot be mixed with the new solder paste.
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