What is the cause of the short circuit in the PCBA / SMT chip assembly? How to deal with it?

2019-07-25 16:02:36

PCBA/SMT chip assembly short circuit This kind of defect mainly occurs between the pins of the fine pitch IC, so it is also called "bridge". Of course, there is also a short circuit between CHIP components, which is very rare. Let's talk about the causes and solutions for the bridging problem between fine pitch IC pins.

Bridging occurs primarily between IC pins with a pitch of 0.5 mm or less. Due to the small pitch, the template design is unreasonable or the print is slightly leaky.

A. Template

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

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 to facilitate the effective release of the solder paste, and the number of cleaning of the stencil can be reduced.

Specifically, for ICs with a pitch of 0.5 mm or less, since the PITCH is small, bridging is easy, and 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. print

    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) Adjustment of the scraper: The operation angle of the scraper is printed in the direction of 45°, which 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 is generally 30N. /mm?.

    (3) Printing speed: The solder paste will roll forward on the template under the push of the scraper. 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 is 10~20mm/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. Mounting height

For ICs with PITCH ≤ 0.5mm, the mounting height should be 0 distance or 0~-0.1mm to avoid slumping of solder paste due to low placement height, resulting in short circuit during reflow.

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 circuit board. 

4. The flux is too fast.

Shenzhen Juding Circuit Technology Co., Ltd. Professional: PCBA / SMT patch assembly, SMT patch PCBA prototype assembly, small and medium batch SMT patch assembly manufacturers

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