General technical requirements:
1. Screen frame: The frame size is determined according to the requirements of the printing machine. Taking DEK265 and MPM UP 3000 as an example, the frame size is 29ˊ
29ˊ, using aluminum alloy, frame profile specification is 1.5ˊ 1.5ˊ.
Reference point: According to the size and shape provided by the PCB information, the opening is 1: 1, and the translucent is engraved on the reverse side of the printing. At the corresponding coordinates, the entire PCB opens at least two reference points.
Opening requirements: 1.41. The position and size ensure a high opening accuracy, and the opening is strictly in accordance with the prescribed opening method. 1.42. The size of the independent opening cannot be too large, the width cannot be greater than 2mm, and a 0.4mm bridge is required in the middle of the pad size greater than 2mm, so as not to affect the strength of the stencil.
1.43. The opening area must be centered.
Characters: In order to facilitate production, it is recommended to engrave the following characters in the lower left or lower right corner of the screen: Model; T; Date; the name of the company that made the screen.
Stencil thickness: In order to ensure the amount of solder paste printing and welding quality, the surface of the stencil is smooth and even, and the thickness is uniform. The stencil thickness refers to the above table. The stencil thickness should meet the premise of the finest pitch QFP BGA. If there are 0.5mmQFP and CHIP 0402 components on the PCB, the stencil thickness is 0.12mm; if there are 0.5mmQFP and CHIP 0603 or more components on the PCB, the stencil thickness is 0.15mm;
The opening shape and size requirements of printed tin screen:
2.1CHIP components: CHIP components above 0603, in order to effectively prevent the generation of tin beads.
2.2SOT89 components: Due to the large pad and small pad spacing between components, soldering quality problems such as tin beads are prone to occur.
2.3 SOT252 component: Because SOT252 has a large pad, it is easy to produce tin beads, and the reflow soldering tension causes displacement.
Look 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, IC with PITCH <= 005mm, because of its small PITCH, it is easy to produce bridging, the length direction of the steel mesh opening is unchanged, the opening width is 0.5PITCH, and the opening width is 0.25mm.
2.5 Other situations: One pad is too large, usually one side is greater than 4mm, and the other side is not less than 2.5mm, in order to prevent the generation of tin beads and displacement caused by the effect of tension, the mesh opening is recommended to be divided by grid lines In this way, the grid line width is 0.5mm and the grid size is 2mm, which can be evenly divided according to the pad size.
The shape and size requirements of the opening of the printed rubber screen: glue is used for simple PCB assembly. Dispensing is preferred. CHIP, MELF, SOT components are printed through the screen, and IC is used to avoid glue scraping. Here, only the recommended opening size and opening shape of CHIP, MELF and SOT offset printing plates are given.
1. Two diagonal positioning holes must be opened at the diagonal of the stencil. Select FIDUCIAL MARK to open the hole.