PCB Technical - Understand the common problems of SMT solder paste

1. Components fall off when PCBA double-sided SMD soldering

PCBA double-sided soldering is more and more common in the SMT surface mount process. In general, users will print, mount components and solder on the first side first, and then process the other side. In this process, the problem of component falling off is not very common; and some customers, in order to save process and cost, save the first welding of the *** surface, but simultaneously perform the welding on both sides, as a result The component falling off during soldering becomes a new problem. This phenomenon is due to insufficient vertical fixing force of the solder to the component after the solder paste is melted. The main reasons are:

1. PCB components are too heavy;

2. The solderability of the solder feet of PCB components is poor;

3. PCB solder paste has poor wettability and solderability;

We always put the solution to the ****** reasons later, but first start to improve the second and third reasons. If the second and third reasons are improved, this phenomenon still exists. We would suggest that when soldering these components that fall off, they should be fixed with red glue first, and then reflow and wave soldering. The problem can basically be solved.

Second, there are tin beads on the PCB surface after welding

This is a relatively common problem in the SMT welding process, especially in the initial stage of the user using a new supplier’s product, or when the production process is unstable, it is more likely to cause such a problem. After using the customer’s cooperation, it will be communicated to us. After a large number of experiments, we finally analyzed the reasons for the production of tin beads, which may have the following aspects:

1. The PCB board is not fully preheated during reflow soldering;

2. The setting of the reflow soldering temperature curve is unreasonable, and there is a big gap between the board surface temperature before entering the soldering area and the soldering area temperature;

3. The solder paste failed to return to room temperature when it was taken out of the cold storage;

4. The solder paste is exposed to the air for a long time after opening;

5. There is tin powder splashing on the surface of the PCB board during the patch;

6. In the process of PCB printing or transportation, oil stains or water stick to the PCB board;

7. The flux itself in the solder paste is unreasonably formulated and contains non-volatile solvents or liquid additives or activators;

The above points of attention and the second reason can explain why the newly replaced solder paste is prone to such problems. The main reason is that the currently set temperature curve does not match the solder paste used, which requires the customer to change the PCB supplier At the time, be sure to ask the solder paste supplier for the temperature curve that the solder paste can adapt to; the third, fourth and sixth reason may be caused by improper operation of the user; the fifth reason may be due to the solder paste Improper storage or expiration of the shelf life causes the solder paste to fail or cause the solder paste to have no stickiness or low viscosity, which causes the splash of tin powder during SMT placement; the seventh reason is the production technology of the solder paste PCB supplier itself .