Quality control knowledge of SMT stencil design

I. General technical requirements

   1, mesh frame: frame size according to the requirements of the printing machine, DEK265 and MPMUP 3000 models, for example, the frame size of 29ˊ 29ˊ, the use of aluminum alloy, frame profile specifications for 1.5ˊ 1.5ˊ.

　2, taut mesh: the use of red glue + aluminum tape way, in the aluminum frame and glue adhesive joints, must be evenly scraped with a layer of protective paint. At the same time, in order to ensure that the net plate has enough tension and good flatness, it is recommended that the stainless steel plate from the inside of the frame to retain 25mm-50mm.

　3, the reference point: according to the size and shape provided by the PCB information according to the 1:1 way openings, and in the printing of the reverse side of the engraved semi-transparent. In the corresponding coordinates, the whole PCB open at least two datum points.

　4, opening requirements:
　　Position and size to ensure a high degree of opening accuracy, strictly in accordance with the prescribed opening mode opening.
　　Independent opening size can not be too large, the width can not be greater than 2mm, pad size greater than 2mm in the middle of the need to set up a 0.4mm bridge, so as not to affect the strength of the stencil.
　　The opening area must be centered.

　5, characters: for the convenience of production, it is recommended that the lower left corner or lower right corner of the stencil engraved with the following characters: Model; T; Date; stencil production company name.

　6, stencil thickness: in order to ensure that the amount of solder paste printing and welding quality, stencil surface smooth and uniform, uniform thickness, stencil thickness refer to the above table, stencil thickness should be used to meet the * fine pitch QFP BGA as a prerequisite.
　　If there are 0.5mm QFP and CHIP 0402 components on the PCB, the thickness of the stencil is 0.12mm;
　　If there are 0.5mmQFP and CHIP 0603 components on the PCB, the thickness of the stencil is 0.15mm;

II, the shape and size requirements of the opening of the printed tin mesh plate

　1, the general principle:
　　According to IPC-7525 stencil design guide requirements, in order to ensure that the solder paste can be smoothly released from the stencil openings to the PCB pads, in terms of stencil openings, mainly depends on three factors:
　　1,) area ratio / width to thickness ratio area ratio > 0.66
　　2,) mesh hole hole wall smooth. Especially for pitch less than 0.5mm QFP and CSP, the production process requires suppliers to make electropolishing treatment.
　　(3,) the printing surface as above, the mesh hole under the opening should be wider than the upper opening 0.01mm or 0.02mm, that is, the opening into an inverted cone, to facilitate the release of solder paste ineffective, while reducing the number of times the stencil can be cleaned.
　　Usually, the SMT components of its mesh plate opening size and shape and pads consistent with the 1:1 way to open.
　　Under special circumstances, some special SMT components, the size and shape of the stencil opening has special provisions.

　2 Special SMT components stencil opening
　　CHIP components:
　　CHIP components above 0603, in order to effectively prevent the generation of tin beads.
　　SOT89 components: due to larger pads and components
　　Pad spacing is small, easy to produce tin beads and other welding quality problems.
　　SOT252 components: due to SOT252 has a pad is very large, easy to produce tin beads, and reflow soldering tension caused by displacement.
　　IC:
　　A. For standard pad design, PITCH》=0.65mm的IC，the opening width is 90% of the pad width and the length is unchanged.
　　B. For standard pad design, PITCH "= 005mm IC, due to its PITCH is small, easy to produce bridging, stencil opening way length direction remains unchanged, opening width of 0.5PITCH, opening width of 0.25mm.

Other cases:
　　A pad is too large, usually more than 4mm on one side, the other side is not less than 2.5mm, in order to prevent the generation of tin beads and displacement caused by tension, stencil openings are recommended to use a grid line division of the way, the grid line width of 0.5mm, grid size of 2mm, according to the size of the pads can be divided equally.
　　Printing glue stencil opening shape and size requirements:
　　For simple PCB assembly using the glue process, give priority to the use of dispensing, CHIP, MELF, SOT components through the stencil printing glue, IC will try to use dispensing to avoid stencil scraping. Here, only give CHIP, MELF, SOT printing and gluing stencil recommended opening size, opening shape.
　 1, stencil diagonal must open two diagonal positioning holes, select the FIDUCIAL MARK point openings.
     2, the opening are long strip.

　Inspection methods
　　(1) Visually check the opening centered on the taut mesh leveling.
　　(2) Through the PCB entity to check the correctness of the opening of the stencil.
　　(3) with a graduated high-powered microscope to check the length and width of the opening of the stencil as well as the hole wall and the smoothness of the surface of the steel.
　　4) Thickness of steel sheet is verified by testing the thickness of solder paste after tin printing, i.e. result verification.
　　Stencil design technical requirements after a period of trial, the printing quality has been well controlled, as shown in the SMT solder quality defects PPM from about 1300ppm down to about 130ppm. Due to the development of modern electronic components in the direction of encapsulation, the stencil design also puts forward higher requirements.