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.
- Stretching the net: Adopt the red glue + aluminum tape method, at the junction of the aluminum frame and the glue, a layer of protective paint must be evenly scraped. At the same time, in order to ensure that the screen has sufficient tension and good flatness, it is recommended that the stainless steel plate be kept 25mm-50mm away from the inside of the screen frame.
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:
- General principle: According to the requirements of the IPC-7525 steel mesh design guide, in order to ensure that the solder paste can be smoothly released from the opening of the stencil to the PCB pad, in the opening of the stencil to view polyester 1. The area ratio / width-to-thickness ratio area ratio is greater than 0.66 2. The mesh hole wall is smooth. Especially for QFP and CSP with a pitch of less than 0.5mm, suppliers are required to perform electro-polishing during the manufacturing process. 3.) With the printing surface as the upper side, the lower opening of the mesh opening should be 0.01mm or 0.02mm wider than the upper opening, that is, the opening is inverted tapered, which facilitates the ineffective release of solder paste and reduces the number of times of cleaning the stencil. Normally, the size and shape of the opening of the SMT element is the same as the pad, and the opening is 1: 1. Under special circumstances, for some special SMT components, the size and shape of the screen openings are specified.
No. 2 Special SMT component screen opening:
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.
- The openings are all elongated. Inspection method 1) Visually check that the opening is centered and the net is flat. 2) Check the correctness of the stencil opening through the PCB entity. 3) Examine the length and width of the screen opening and the smoothness of the hole wall and the surface of the steel sheet with a scaled high power microscope. 4) The thickness of the steel sheet is verified by detecting the thickness of the solder paste after printing tin, that is, the result verification. Conclusion The technical requirements for screen design have been tested for a period of time, and the print quality has been well controlled. The PPM of SMT welding quality has dropped from about 1300ppm to about 130ppm. Due to the development of the packaging direction of modern electronic components, higher requirements have also been placed on the design of steel mesh. Is a subject that we need to focus on in the future