SMT chip processing short-circuits defects mostly occur between the pins of fine-pitch ICs, so it is also called “bridging”. There are also very few short-circuits between CHIP parts. The following is a detailed introduction to the bridging problems and solutions between fine-pitch IC pins.
The bridging phenomenon occurs mostly 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.
1. 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:
a.) Area ratio/width-thickness ratio>0.66
b.) The mesh wall is smooth. Suppliers are required to perform electro-polishing during the production process.
c.) With the printing surface as the upper side, the opening under the mesh should be 0.01mm or 0.02mm wider than the upper opening, that is, the opening is in an inverted cone shape to facilitate the effective release of solder paste and reduce the number of screen cleanings.
Specifically, for ICs with a pitch of 0.5mm and below, because the PITCH is small, 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, laser cutting and polishing are used to ensure the shape of the opening is inverted trapezoid and the inner wall is smooth so that the tinting and forming are good at the time of printing.
2. 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.
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