1. General principles:

According to the requirements of the Jialichuang laser stencil tapping design guide, in order to ensure that the solder paste can be smoothly released from the laser stencil tapping to the PCB pad, the laser stencil tapping mainly depends on three factors:

1. ) Area ratio/width thickness ratio>0.66

2. ) The mesh wall is smooth. Especially for QFP and CSP with spacing less than 0.5mm, suppliers are required to conduct electropolishing treatment during the manufacturing process.

3. ) With the printing surface as the upper part, the lower opening of the mesh shall be 0.01mm or 0.02mm wider than the upper opening, that is, the opening shall be inverted tapered, which is convenient for ineffective release of solder paste, and can reduce the cleaning times of SMT steel mesh.

In general, the size and shape of the laser steel mesh opening of the smt element are consistent with that of the pad, and the opening is 1:1.

In special cases, the opening size and shape of the laser steel mesh of some special SMT elements are specially specified.

2 Special SMT element laser steel mesh opening

2.1 CHIP element:

For CHIP components above 0603, in order to effectively prevent the generation of tin beads.

2.2 SOT89 element: due to large pad and element

The space between pads is small, which is easy to cause solder bead and other welding quality problems.

2.3 SOT252 element: because SOT252 has a large pad, it is easy to produce solder beads, and the large reflow tension causes displacement.

2.4IC：

A. For the standard pad design, PITCH=0.65mm IC, the opening width is 90% of the pad width, and the length is unchanged.

B. For the standard pad design, the IC with PITCH=005mm is easy to be bridged due to its small PITCH. The length direction of the steel mesh opening mode remains unchanged, the opening width is 0.5PITCH, and the opening width is 0.25mm.

2.5 Other situations:

When one pad is too large, usually one side is more than 4mm, and the other side is not less than 2.5mm, in order to prevent the generation of solder beads and displacement caused by tension, it is recommended to use grid line division method for laser mesh opening. The grid line width is 0.5mm, and the grid size is 2mm, which can be equally divided according to the size of the pad.

Requirements for the shape and size of the opening of stencil stencil:

Glue process is adopted for simple PCB assembly, and glue dispensing is preferred. CHIP, MELF and SOT components pass through laser stencil printing glue, and IC uses glue dispensing as far as possible to avoid laser stencil scraping. Here, only the recommended opening size and shape of CHIP, MELF and SOT printing laser stencil are given.

1. Two diagonal positioning holes shall be opened at the opposite corners of the laser steel mesh, and the FIDUCIAL MARK point shall be selected for opening.

2. The openings are all long.