SMT Short Circuit Causes and Solutions

Pick and place machine and other SMT equipment in the production and processing will appear a lot of bad phenomena, such as monument, bridge, virtual welding, fake welding, grape ball, tin bead and so on. SMT SMT processing short circuit is more common in fine spacing between IC pins, more common in 0.5mm and below the spacing between IC pins, because of its small spacing, improper template design or printing is easy to produce a slight omission.

Causes and solutions:

Cause 1: Stencil template

Solution:

The hole wall of the steel mesh is smooth, and the electropolishing treatment is required in the production process. The opening of the mesh should be 0.01mm or 0.02mm wider than the opening of the mesh. The opening is inverted conical, which is conducive to the effective release of tin paste under the tin, and can reduce the cleaning times of the mesh plate.

Cause 2: solder paste

Solution:

0.5mm and below the pitch of IC solder paste should be selected in the size of 20~45um, viscosity in 800~1200pa. S

Cause 3: Solder paste printer printing

Solution:

1. Type of scraper: the scraper has two kinds of plastic scraper and steel scraper. The 0.5 IC printing should choose the steel scraper, which is conducive to the solder paste forming after printing.

2. Printing speed: the solder paste will roll forward on the template under the push of the scraper. The fast printing speed is conducive to the springback of the template, but it will hinder the solder paste leakage; But the speed is too slow, the solder paste will not roll on the template, resulting in poor resolution of the solder paste printed on the solder pad. Usually, the printing speed range of fine spacing is 10~20mm/s

3 printing mode: at present the most common printing mode is divided into “contact printing” and “non-contact printing”.
There is a gap between the template and PCB printing mode is “non-contact printing”, the general gap value is 0.5~1.0mm, its advantage is suitable for different viscosity solder paste.

There is no gap between the template and PCB printing is called “contact printing”. It requires the stability of the overall structure, suitable for printing high precision tin template and PCB to keep a very flat contact, after the printing and PCB separation, so this way to achieve high printing accuracy, especially suitable for fine spacing, ultra-fine spacing of solder paste printing.

Cause 4: SMT machine mount height

Solution:

For 0.5mm IC in the mounting should be used 0 distance or 0~0.1mm mounting height, in order to avoid due to the mounting height is too low so that solder paste forming collapse, resulting in reflux short circuit.