The bad phenomenon of SMT processing short circuit often occurs between the pins of fine pitch IC, so it is also called "bridging". Of course, there are also short circuits between chip parts, which are very few. Here is a brief discussion on the cause and solution of the bridging problem between the pins of fine pitch IC.
Bridging often occurs between IC pins with a spacing of 0.5mm or less. Due to its small spacing, improper template design or slight omission in printing are very easy to occur.
According to the requirements of ipc-7525 steel mesh design guide, in order to ensure that the solder paste can be smoothly released from the mesh plate opening to the PCB pad, the mesh plate opening mainly depends on three factors:
1. ) area ratio / width thickness ratio > 0.66
2. ) the mesh wall is smooth. The supplier is required to perform electropolishing during the manufacturing process.
3. ) take the printing surface as the top, and the lower opening of the mesh shall be 0.01mm or 0.02mm wider than the upper opening, that is, the opening shall be inverted conical to facilitate the effective release of the solder paste and reduce the cleaning times of the mesh plate.
Specifically, for ICs with a spacing of 0.5mm or less, due to their small pitch, bridging is easy to occur. The length direction of the steel mesh opening mode remains unchanged, and the opening width is 0.5 ~ 0.75 pad width. The thickness is 0.12 ~ 0.15mm. It is best to use laser cutting and polishing to ensure that the opening shape is inverted trapezoid and the inner wall is smooth, so as to facilitate tin deposition and good forming during printing.
B. Solder paste
The correct selection of solder paste also has a great relationship to solve the bridging problem. When using solder paste for IC with a spacing of 0.5mm or less, the particle size should be 20 ~ 45um and the viscosity should be about 800 ~ 1200pa. S. the activity of solder paste can be determined according to the cleanliness of PCB surface, and RMA grade is generally used.
Printing is also a very important part.
(1) Type of scraper: there are two types of scraper: plastic scraper and steel scraper. For IC with pitch ≤ 0.5mm, steel scraper shall be selected during printing to facilitate the formation of solder paste after printing.
(2) Adjustment of scraper: the running angle of the scraper is printed in the direction of 45 °, which can significantly improve the imbalance in the direction of different template openings of solder paste, and reduce the damage to the template openings with fine spacing; the scraper pressure is generally 30n / mm2.
(3) Printing speed: the solder paste will roll forward on the template under the push of the scraper. Fast printing speed is conducive to the rebound of the template, but it will prevent the solder paste from missing printing; if the speed is too slow, the solder paste will not roll on the template, resulting in poor resolution of the solder paste printed on the pad. Generally, the printing speed range for fine spacing is 10 ~ 20mm / s.
(4) Printing method: at present, the most common printing method is divided into "contact printing" and "non-contact printing". The printing method with gap between template and PCB is "non-contact printing" The general clearance value is 0.5 ~ 1.0mm, which has the advantage that it is suitable for solder pastes with different viscosities. The solder paste is pushed into the opening of the template by the scraper and contacts with the PCB pad. After the scraper is slowly removed, the template will be automatically separated from the PCB, which can reduce the disturbance of template pollution caused by vacuum air leakage.
The printing method without gap between template and PCB is called "contact printing". It requires the stability of the overall structure and is suitable for printing high-precision solder paste. The template maintains very flat contact with PCB and is separated from PCB after printing. Therefore, this method achieves high printing accuracy, especially for solder paste printing with fine spacing and ultra-fine spacing.
D. For the mounting height of IC with pitch ≤ 0.5mm, the mounting height of 0 distance or 0 ~ -0.1mm shall be adopted during mounting, so as to avoid the formation collapse of solder paste due to low mounting height, resulting in short circuit during reflux.
1. The heating speed is too fast. 2. The heating temperature is too high. 3. The heating speed of solder paste is faster than that of circuit board. 4. The wetting speed of solder is too fast.