PCB Tech - ​Short-circuit phenomenon in SMT patch processing?

Causes and solutions for short circuits in SMT chip processing? The short-circuit in SMT chip processing is often found between the pins of fine-pitch ICs, so it is also called "bridging". Of course, there are also short circuits between CHIP parts, which is very rare. Now let's talk about the reason and solution of the bridging problem between fine-pitch IC pins.

SMT chip processing

The bridging phenomenon mostly occurs between IC pins with a pitch of 0.5mm and below. Because of the small pitch, improper template design or slight omissions in printing can easily occur.

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 openings to the PCB pads, the opening of the stencil mainly depends on three factors:

(1) Area ratio/width-thickness ratio>0.66 

(2) The mesh hole wall is smooth. Suppliers are required to perform electro-polishing during the production process. 

(3) With the printing surface as the upper side, the lower opening of the mesh should be 0.01mm or 0.02mm wider than the upper opening, that is, the opening is in an inverted cone shape, which facilitates the effective release of solder paste and reduces the frequency of cleaning the screen.

Specifically, for ICs with a pitch of 0.5mm and below, due to their small PITCH, bridging is easy to occur, the length of the stencil opening method is unchanged, and the opening width is 0.5 to 0.75 pad width. The thickness is 0.12~0.15mm. 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 as to facilitate the tinting and forming well at the time of printing.

B. Solder Paste 

The correct choice of solder paste also has a lot to do with 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.

C. 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, a 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. The fast printing speed is beneficial to the rebound of the template, but it will also 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 method: At present, the most common printing method is divided into "contact printing" and "non-contact printing". The printing method with 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 different viscosity solder pastes. 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 problem 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, and is suitable for printing high-precision tin templates to maintain very flat contact with the PCB board, and then separate 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 solder paste printing.

D. Mounting height. For ICs with PTTCH less than or equal to 0.55mm, 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. Causes a short circuit during reflow.

E. Reflow

(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.