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By Ginkgoem | 09 July 2021 | 0 Comments

Matters needing attention in the use of solder paste during SMT chip processing and production

Matters needing attention in the use of solder paste during SMT chip processing and production

The solder paste is mainly composed of flux and solder powder. The composition of the solder paste determines the quality of the solder paste, and directly determines the use and condition requirements of the solder paste. The metal composition of SAC305 solder paste includes: Sn96.5/Ag3/Cu0.5, of which tin is the most important , and the role of silver is to enhance the conductivity of solder and increase the gloss of solder. The function of the flux is to condense the solder paste particles into a paste shape, making it easier to print. In addition, it also has the function of removing surface contamination, preventing oxidation, and reducing surface tension expansion.

Before the solder paste is taken out of the refrigerator and used, many Guangzhou SMT chip processing plants will reheat under sealed conditions. The reheating time is not less than 4 hours to make the temperature close to the room temperature. The reason is that the temperature is close to the room temperature. During the temperature process, the solder paste is hygroscopic. If it is reheated in the air, the solder paste is likely to absorb moisture in the air. The residual solder paste will have a great impact on the reflow soldering process. During the reflow stage of the solder paste, excess moisture It will splash out and form a lot of tin beads, so special attention is needed.

1. The solder paste is easy to be oxidized when exposed to the air, and it is also easy to be hardened. Therefore, the amount of solder paste added on the stencil should not be too much, and the printing should be completed within 1 hour as the standard to supplement.

2. The cap of the solder paste bottle should be locked every two hours to prevent the remaining solder paste from being re-stirred by hand.

3. After each production is over, after the unused solder paste is recycled and put back into the bottle, the bottle should be shaken upside down a few times to avoid residual air in the solder paste.

4. When the solder paste is found to be hardened, stop using it immediately and discard it.

The influence of solder paste printing quality on the quality of products after the furnace far exceeds the influence of other processes. According to the quality of our products, 60~75% of quality problems are caused by poor printing. There are many factors that affect the quality of printing. The following table lists Several common factors that affect printing, the undesirable phenomena and the corresponding solutions are listed.

During the printing process, due to the thrust of the squeegee, the viscosity and rheology of the solder paste will change. When it reaches the opening position of the stencil, its viscosity will decrease, and it will settle to the PCB pad smoothly through the leakage of the mesh. At this time, the solder paste will slightly collapse and overflow, and then the viscosity will quickly rise under the action of the quenching agent, and form a shape corresponding to the mesh, so as to obtain a good printing effect.

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