PCBA Tech - What to do if a short circuit occurs in SMT patch processing

In SMT patch processing, there will be short circuits, mainly between the pins of fine-pitch ICs, so it is also called "bridging." The occurrence of short-circuit phenomenon will directly affect the performance of the product and cause defective products. The short-circuit phenomenon of SMT patch processing needs to be paid attention to. Here is an introduction to the causes and solutions of short circuits in SMT chip processing.

One, template

The bridging phenomenon in SMT patch processing is mostly due to the small IC pin spacing, which usually occurs when the pin spacing is 0.5mm or less. Therefore, if the template design is improper or the printing is slightly omitted, it is very easy to cause a short circuit. Phenomenon.

Solution: For ICs with a pitch of 0.5mm and below, due to their small PITCH, bridging is easy to occur. Keep the length of the stencil opening mode 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 the effective release of solder paste and good molding during printing, and also reduce the number of screen cleanings.

2. Printing

In SMT patch processing, printing is also a very important link. In order to avoid short circuits due to improper printing, the following issues need to be paid attention to:

1. Type of squeegee: There are two types of squeegee: plastic squeegee and steel squeegee. For IC with PITCH≤0.5mm, steel squeegee should be used for printing 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°, which can significantly improve the imbalance of the opening direction of the different stencils of the solder paste, and it can also reduce the damage to the opening of the fine-spaced stencil; 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 conducive to the rebound of the template, but at the same time it will prevent the solder paste from being printed. If the speed is too slow, the solder paste will not roll on the template, causing poor resolution of the solder paste printed on the pad. The printing speed range of the pitch is 10~20mm/s.

Three, solder paste

The correct choice of solder paste is also very important for solving bridging problems. When using solder paste for ICs with a pitch of 0.5mm and below, the particle size should be 20~45um, and the viscosity should be around 800~1200pa.s. The activity of the solder paste can be determined according to the cleanliness of the PCB surface, generally RMA grade is used.

Four. Mounting height

For ICs with PITCH≤0.5mm, 0 distance or 0~-0.1mm mounting height should be used when mounting, to avoid the solder paste molding collapse due to too low mounting height, causing short circuit during reflow.

5. Reflow

During the reflow process of SMT chip processing, if the following conditions occur, short circuits may also occur, such as:

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

Regarding the causes and solutions of short circuits in SMT patch processing, there are many process procedures in SMT patch processing, and each link needs to be treated carefully by the operator to reduce the occurrence of undesirable phenomena.