How to avoid soldering defects such as bridging, tin bead, and less tin? Pay attention to the following factors:

 

1. Alloy particle size

The shape, diameter and uniformity of alloy particles affect its printing performance.

If the diameter is too large, it is easy to block the opening of the steel mesh during printing, resulting in incomplete printing, burrs, and difficult to demold; too small, the surface area of ​​the alloy particles is relatively increased, the possibility of oxidation increases, and the possibility of tin beads is increased.

 

 

Generally, the particle size range is selected according to the opening size of the steel mesh corresponding to the PITCH of the IC used. The particle diameter is about 1/5 or less of the opening size.

The currently used solder paste particle size is -325/+500 mesh, which is in the range of 25um to 45um.

The requirement for uniformity is generally that the number of particles with the largest size and the smallest size should not exceed 10%.

 

2. The demoulding performance is not good

This is related to the condition of the inner wall of the stencil opening, the shape and size of the solder particles, and whether vibration is selected when the Zhengshi solder paste printer is demolded, and it is mainly related to the adhesion of the solder paste.

 

Good adhesion performance makes the adhesion to the pad greater than the adhesion to the sidewall of the stencil opening during printing, so that the solder paste adheres firmly to the pad and improves the mold release. Adhesion depends on the composition of the flux and the proportion of other additives. Adhesion should be appropriate, too small to stick to the component, too large, not easy to demould, easy to adhere to the scraper and not fall off.

 

 

3. Improper viscosity

If it is too large, the solder paste will not easily pass through the stencil, and the printed lines will be incomplete; if it is too small, it will flow easily, resulting in poor shape retention. As with adhesion, in addition to the ratio of itself, it is also related to the use process:

1) Only use certified solder paste. At present, SMT01, SMT02, and SMT07 only use Xiaoming solder paste, and the other lines use Alpha solder paste. When producing MBC products, only Chenri solder paste can be used.

2) Welding Xiaoming solder paste must be stored in the refrigerator. The temperature is 2℃～8℃. Before use, ensure that the thawing time is 4-8 hours, and in principle, it should not exceed 48 hours. It is strictly forbidden to open the solder paste that has not been completely thawed. The solder paste should be leveled, the inner cover should be pressed to the bottom as far as possible, and the outer cover should be tightened. In addition to reducing the amount of oxidation, it can also reduce the amount of solvent volatilization.

3) The amount of solder paste on the stencil should not be too much, about 10mm in diameter. When adding a small amount multiple times, it can reduce the exposure time of the solder paste added at the same time; when the machine is not working, the scraper should be immediately removed When the solder paste is recovered, it should be noted that the old and new solder pastes cannot be mixed, and it is strictly forbidden to mix the solder pastes of different brands.

4) Solder paste beyond the expiration date can no longer be used

 

With the extension of the delivery time, the solder paste will precipitate and chemical reactions between the components. If the sealing is not good, the solvent will volatilize and the solder powder will oxidize, which will reduce the performance of the solder paste and even become unusable.

The storage period of solder paste is 6 months to one year; generally 6 months. The shorter the delivery time of the solder paste, the better, so you should buy a small amount of it several times when you buy it, and pay attention to the principle of "buy first, use first" when using it.

The solder paste cleaned from the stencil and scraper will be scrapped every weekend;

The sooner the solder paste after opening is used up, the better, and the solder paste after opening cannot be used for more than 24 hours.

4. External factors

1) Temperature and humidity (workshop, printing press).

Avoid moisture absorption and excessive flux volatilization.

2) The cleanliness of the environment.

Avoid using solder paste in a dusty environment.

3) Steel mesh, residues on PCB.

There should be no fibers left on the stencil during cleaning; there should be no residual solder paste (including squeegee) left by the last use in the stencil; there should be no greasy debris on the PCB to ensure the printing effect.