PCB Tech - Common problems and causes analysis of solder paste for SMT in use

Common problems and causes analysis of solder paste for SMT in use
There are tin beads on the PCB surface after soldering:
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 fully 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 PCB surface during patching;

6. During printing or transportation, oil stains or moisture 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 first and second reasons above can also explain why the newly replaced solder paste is prone to such problems. The main reason is that the currently set temperature profile does not match the solder paste used, which requires customers to change suppliers At the time, be sure to ask the solder paste supplier for the temperature profile that the solder paste can adapt to;

The third, fourth, and sixth reasons may be caused by improper operation of the user; the fifth reason may be caused by improper storage of the solder paste or failure of the solder paste due to the expiration date. The solder paste has no viscosity or low viscosity. The tin powder splashes during the patch; the seventh reason is the production technology of the solder paste supplier itself.

There are many residues on the board surface after welding:

After soldering, there are more residues on the PCB board surface, which is also a problem that customers often report. The existence of more residues on the board surface not only affects the smoothness of the board surface, but also has a certain impact on the electrical properties of the PCB itself; The main reasons for multiple residues are as follows:

1. When promoting solder paste, do not understand the condition of the customer's board and the customer's requirements, or the selection error caused by other reasons; for example: the customer's request is to use no-clean and residue-free solder paste, and the solder paste manufacturer provides rosin Resin type solder paste, so that customers report that there are more residues after soldering. In this regard, solder paste manufacturers should pay attention to when they promote their products.

2. The content of rosin resin in the solder paste is too much or its quality is not good; this should be a technical problem of the solder paste manufacturer, and SMT is recommended.

Problems such as tailing, sticking, blurry image, etc. occur during printing:

This reason is often encountered in the printing process. After summarizing, we found that the main reasons are as follows:

1. The viscosity of the solder paste itself is low, which is not suitable for the printing process; this problem may be the wrong selection of the solder paste, or it may be that the solder paste has expired, etc., which can be solved by coordinating with the supplier.

2. It is caused by poor machine setting or improper operation method of the operator during printing. Improper settings such as the speed and pressure of the squeegee are likely to affect the printing effect. In addition, the proficiency of the operator (including the speed, pressure during printing, repeated printing, etc.) also has a great influence on the printing effect.

3. The gap between the screen and the substrate is too large;

4. Poor overflow of solder paste;

5. The solder paste is not fully stirred before use, resulting in uneven mixing of the solder paste;

6. When using screen printing, the latex mask on the screen is not evenly coated;

7. The metal composition in the solder paste is too low, that is, it is caused by the high proportion of the flux composition;

Insufficient tin on solder joints:

The main reasons for the insufficient tin on the solder joints are as follows:

1. The activity of the flux in the solder paste is not enough to completely remove the oxides on the PCB pads or SMD soldering positions;

2. The wetting performance of the flux in the solder paste is not good;

3. The PCB pads or SMD soldering positions are seriously oxidized;

4. The preheating time is too long or the preheating temperature is too high during reflow soldering, which causes the flux activity in the solder paste to fail;

5. If there is insufficient tin on some solder joints, it may be that the solder paste has not been fully stirred and the solder powder has not been fully fused before use;

6. The temperature of the reflow soldering zone is too low;

7. The amount of solder paste at the solder joint is insufficient;

The solder joints are not bright:

In the SMT welding process, general customers have requirements for the brightness of the solder joints. Although this is also a problem in normal work, it is often just a subjective consciousness of the customer, or only through comparison. Draw the conclusion that the solder joints are bright or not bright, because there is no standard to follow the brightness of the solder joints; roughly speaking, the reasons for the solder joints not bright are as follows:

1. If the products after soldering without silver solder paste are compared with the products after soldering with silver solder paste, there will be some gaps. This requires customers to explain their solder joint requirements to the supplier when choosing solder paste;

2. The tin powder in the solder paste is oxidized;

3. The flux in the solder paste itself has additives that cause the matting effect;

4. There are rosin or resin residues on the surface of the solder joints after soldering. This is a phenomenon we often see in actual work, especially when rosin-type solder paste is selected, although rosin-type flux is better than no-clean flux Make the solder joints a little brighter, but the presence of its residues often affects this effect, especially in larger solder joints or IC foot parts; if it can be cleaned after soldering, I believe that the gloss of the solder joints should be improved;

5. The preheating temperature is low during reflow soldering, and there are non-volatile residues on the surface of the solder joints;

Component shift:

"Component displacement" is a foreshadowing of other problems in the soldering process. If this problem is not detected before entering the reflow soldering process, it will cause more problems. The main reasons for component displacement are as follows:

1. The viscosity of the solder paste is not enough, and no parts are displaced after handling and vibration;

2. The solder paste has exceeded the expiration date, and the flux has deteriorated;

3. The air pressure of the suction nozzle is not adjusted properly during placement, and the pressure is not enough, or the placement machine has a mechanical problem, causing the component to be placed in the wrong position;

4. Vibration or incorrect handling occurs during the handling process after printing and patching;

5. The flux content in the solder paste is too high, and the flux flow during the reflow soldering process will cause the components to shift;

Tombstones of components after welding:

Compared with other welding methods, "post-welded component tombstone" is a unique phenomenon in the SMT welding process, and this problem is often encountered. After analysis, we believe that the main reasons for this problem are as follows:

1. The reflow soldering temperature zone line setting is unreasonable, and the drying and penetration work before entering the soldering area is not done well, so that there is still a "temperature gradient" on the PCB, and the solder paste melting time on each soldering point is inconsistent in the soldering area, thus As a result, the stress on both ends of the component is different, which causes the phenomenon of "tombstone";

2. The preheating temperature is too low during reflow soldering;

3. The solder paste is not fully stirred before use, and the flux distribution in the solder paste is uneven;

4. Before entering the reflow soldering area, there is a component misalignment;

5. Poor solderability of SMD components may also cause this phenomenon.