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The cause and solution of short circuit in SMT chip processing

SMT processing short circuit This kind of defect occurs mostly between the pins of the fine pitch IC, so it is also called "bridge". Of course, there is also a short circuit between CHIP parts, which is very rare. Let's talk about the cause and solution of the bridging problem between fine pitch IC pins.

The bridging phenomenon occurs mostly between IC pins with a pitch of 0.5 mm or less. Because of the small pitch, the template is not designed properly or the printing is slightly leaky.

A. Template

According to the requirements of the IPC-7525 stencil design guide, in order to ensure that the solder paste can be smoothly released from the stencil opening to the PCB pad, the opening of the stencil mainly depends on three factors: p>

1) Area ratio / width to thickness ratio >0.66

2) The mesh hole wall is smooth. Suppliers are required to perform electropolishing during the manufacturing process.

3)) With the printed surface as the top, the opening under the mesh should be 0.01mm or 0.02mm wider than the upper opening, that is, the opening is inverted and tapered to facilitate the effective release of the solder paste and reduce the number of times the stencil is cleaned.

Specifically, for ICs with a pitch of 0.5 mm or less, since the PITCH is small, bridging is easy, the length direction of the stencil opening method is constant, and the opening width is 0.5 to 0.75 pad width. The thickness is 0.12~0.15mm, and it is best to use laser cutting and polishing to ensure that the shape of the opening is inverted trapezoid and the inner wall is smooth, so that the tin is well formed during printing.

B. Solder paste

The correct choice of solder paste also has a lot to do with solving the bridging problem. ICs with a pitch of 0.5mm or less should have a particle size of 20~45um and a viscosity of 800~1200pa.s. The activity of the solder paste can be determined according to the cleanliness of the PCB surface, generally using RMA grade.

C. Printing

Printing is also a very important part.

(1) Type of scraper: The scraper has two kinds of plastic scraper and steel scraper. For ICs with PITCH≤0.5mm, steel scraper should be used for printing to facilitate the molding of solder paste after printing.

(2) Adjusting the scraper: The operating angle of the scraper in the direction of 45° can significantly improve the imbalance of the different template opening directions of the solder paste, and also reduce the damage to the fine-pitch template opening; the blade pressure Usually 30N/mm2.

(3) Printing speed: The solder paste will roll forward on the stencil under the push of the squeegee. Fast printing speed is good for the rebound of the template, but it will also hinder the solder paste leakage; and the speed is too slow, the solder paste will not roll on the template, causing poor solder paste resolution on the pad, usually for fine The printing speed of the pitch ranges from 10 to 20 mm/s.

(4) Printing method: The most common printing methods at present are classified into "contact printing" and "contactless printing". The printing method in which there is a gap between the template and the PCB is "contactless printing". The general gap value is 0.5~1.0mm, which has the advantage of being suitable for different viscosity solder pastes. The solder paste is pushed into the template opening by the doctor blade to contact the PCB pad. After the blade is slowly removed, the template is automatically separated from the PCB, which can reduce the problem of template contamination caused by 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, is suitable for printing high-precision solder paste, the template and the PCB maintain a very flat contact, and is separated from the PCB after printing, so the printing precision achieved by this method is high, especially suitable for fine pitch. Ultra-fine pitch solder paste printing.

D. For the height of the mounting, for ICs with a PITCH ≤ 0.5mm, the mounting height should be 0 distance or 0~-0.1mm to avoid the solder paste forming collapse due to the low placement height, resulting in reflow. A short circuit is generated.

E. Reflux

1. The heating rate is too fast 2. The heating temperature is too high 3. The solder paste is heated faster than the board 4. The solder wetting speed is too fast.

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