Short Circuit Phenomenon in SMT Patch Processing and Solution (Part 2)

3. Printing 　　

Printing is also a very important part. 　　

(1) Type of squeegee: There are two types of squeegee: plastic squeegee and steel squeegee. For IC printing with PTTCH less than or equal to 0.5, steel squeegee should be used to facilitate the formation of solder paste after printing.

(2) Adjustment of the squeegee: The operating angle of the squeegee is printed in the direction of 45 degrees, which can obviously improve the imbalance of the opening direction of the different stencils of the solder paste, and it can also reduce the damage to the fine-spaced stencil opening; the pressure of the squeegee is generally 30N /mm2.

(3) Printing speed: The solder paste will roll forward on the template under the push of the squeegee. Fast printing speed is beneficial to the rebound of the template, but at the same time it will prevent the solder paste from leaking. If the speed is too slow, the solder paste will be on the template. Will not roll, causing poor resolution of the solder paste printed on the pad, usually the printing speed range for fine pitch is 10~20mm/s 　　

(4) Printing methods: The current common printing methods are divided into “contact printing” and “non-contact printing”. The printing method where there is a gap between the template and the PCB is “non-contact printing”, and the general gap value is 0.5~1.0mm. Its advantage is that it is suitable for solder pastes of different viscosities. The solder paste is pushed into the template opening by the squeegee to contact the PCB pad. After the squeegee is slowly removed, the template will be automatically separated from the PCB, which can reduce the trouble of template contamination due to vacuum leakage.

The printing method with no gap between the template and the PCB is called “contact printing”. It requires the stability of the overall structure. It is suitable for printing the high-precision tin template to keep very flat contact with the PCB, and only separates from the PCB after printing. Therefore, the printing accuracy achieved by this method is relatively high, and it is especially suitable for fine pitch and ultra-fine pitch. Fine pitch solder paste printing.

4. Mounting height. For ICs whose PTTCH is less than or equal to 0.55mm, 0 distance or a mounting height of 0~0.1mm should be used when mounting to avoid the solder paste molding collapse due to too low mounting height. Causes a short circuit during reflow. 　　

5. Backflow 　　

(1) The heating rate is too fast 　　

(2) The heating temperature is too high 　　

(3) The solder paste heats up faster than the circuit board 　　

(4) The flux wetting speed is too fast.