Design and Quality Control of SMT Fabricated Wire Mesh

Design and Quality Control of SMT Fabricated Wire Mesh
General technical requirements: strip the net: use red glue+aluminum tape method, a layer of protective paint must be evenly scraped and applied when the aluminum frame is bonded with glue At the same time, in order to ensure that the screen has sufficient tension and good flatness, it is recommended to keep the stainless steel plate 25mm-50mm away from the inside of the screen frame
1. Zhang Wang
For the method of red glue+aluminum tape, a layer of protective paint must be evenly applied on the joint between aluminum frame and glue. At the same time, in order to ensure that the filter screen has sufficient tension and good flatness, it is recommended to keep the stainless steel plate 25mm-50mm away from the inside of the screen frame.
2. Screen frame
The frame size shall be determined according to the requirements of the printing machine. Take DEK265 and MPM UP 3000 models for example, the frame size is 29 Ë 29 Ë 138;, Aluminum alloy is used, and the frame profile specification is 1.5 Ë 1.5 Ë 138;.
3. Reference point
According to the size and shape PCB data provided by the manufacturer, the opening is 1:1, and the back of the printed matter is engraved with a translucent pattern At the corresponding coordinates, the entire PCB should have at least two reference points

4. Opening requirements
(1) The position and size ensure a high opening accuracy, and the opening is carried out in strict accordance with the specified opening method.
(2) The size of the independent opening should not be too large, and the width should not be greater than 2mm A 0. A 4mm bridge shall be built between pads larger than 2mm to avoid affecting the strength of the mold
(3) The opening area must be centered.
5, characters
In order to facilitate production, it is recommended to engrave the following characters in the lower left or right corner of the screen: model; T Date The name of the screen production company.
6. Screen thickness
In order to ensure the printing quantity and welding quality of solder paste, the screen surface is smooth and even, and the thickness is even. Refer to the table above for the thickness of the screen. The thickness of the screen shall conform to the best spacing of QFP BGA.
If there are 0.5mm QFP and chip 0402 components on the PCB, the thickness of the mold is 0.12mm; If there are 0.5mm QFP and chip 0603 components on the PCB, the thickness of the mold is 0.15mm;
Shape and size requirements for the second wire mesh opening
1. General principles
According to the requirements of IPC-7525 Mold Design Guide, in order to ensure that the solder paste can be released smoothly from the mold opening to the PCB pad, the mold opening mainly depends on three factors:
1) Area ratio/aspect ratio Area ratio>0.66
2) The grid wall is smooth. Especially for QFP and CSP with spacing less than 0.5mm, suppliers need to conduct electrolytic polishing during production.
3) With the printing surface as the upper side, the lower opening of the screen 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 ineffective release of solder paste and reduce the number of times to clean the screen.
Under normal conditions, the size and shape of screen opening of SMT components are the same as that of pads, and the opening is 1:1.
Under special circumstances, some special SMT elements have special regulations on the size and shape of screen openings.
2. Special SMT element screen opening
Wafer assembly:
Chip modules above 0603 can effectively prevent the production of tin beads.
SOT89 assembly: due to large pads and assemblies
The space between pads is small, and welding quality problems such as solder balls are easy to occur.
SOT252 assembly: Since SOT252 has a large pad, solder beads are easy to be produced, and reflow welding tension will lead to displacement.
A. For the standard pad design, the spacing is "=0.65mm IC, the opening width is 90% of the pad width, and the length remains unchanged.
B. For standard pad design, IC with spacing less than or equal to 005mm is easy to bridge because of its small spacing. The length of the sample opening method remains unchanged, the opening width is 0.5PITCH, and the opening width is 0.25mm.
Other information:
When the pad is too large, one side is usually larger than 4mm, and the other side is not smaller than 2.5mm. In order to prevent displacement caused by solder beads and tension, it is recommended to use grid lines to divide grid openings. The width of the grid is 0.5mm, and the grid size is 2mm, which can be divided equally according to the size of the pad.
Requirements for opening shape and size of printing rubber screen:
For simple PCB assembly, glue technology is adopted, and glue is preferred. The chip, MELF and SOT components are printed on the screen, and the IC is used as far as possible to avoid scratching the screen. Only the recommended opening size and shape of the wafer, MELF and SOT printed rubber screen are given here.
1. The screen must have two diagonal positioning holes. Select the datum mark point to open the hole.
2. The openings are all long.
3 Verification method
Inspection method of steel wire mesh
1) Visually check whether the opening is centered and whether the stretched net is flat.
2) Check the correctness of the screen opening through the PCB entity
3) The length and width of the sieve plate opening and the smoothness of the hole wall and the steel plate surface shall be checked with a graduated high-power microscope.
4) Verify the thickness of steel plate by detecting the thickness of solder paste after soldering, that is, the verification result.