PCB manufacturing SMT printing fixture – stencil
The metal template (Stencil) used in PCB manufacturing SMT processing, also known as drain plate, steel plate and steel mesh, is used to quantitatively distribute solder paste. It is the key fixture for ensuring the printing quality of SMT patch solder paste for PCB manufacturing. The template is formed on a piece of metal, chemically etched out of the hollow or laser engraving machine to engrave the leak hole, and the aluminum alloy frame is stretched to make a suitable size metal plate.
According to the PCB manufacturing SMT template material and fixing method, the template can be divided into three categories: mesh/latex template, full metal template, flexible metal template. The mesh/latex template is made in the same way as the screen plate, except that the opening portion is completely etched through, that is, the mesh at the opening is also etched away, which will deteriorate the stability of the screen, and this SMT processing template is also relatively expensive. The full-metal SMT processing template is to fix the metal steel plate directly on the frame. It can not withstand the tension and can only be used for contact printing, also called rigid metal template. This template has a long service life but is expensive. The flexible metal template is made of polyester and stretched on the frame with a tension of (30~223) N/cm2. The width of the mesh is 30~40mm to ensure the certain elasticity of the stencil during using. PCB manufacturing stencils are currently the most widely used.
Metal stencils in SMT processing are generally made of thinner sheets of nickel, brass or stainless steel with better elasticity. Stainless steel stencils are superior to brass stencils in terms of hardness, stress resistance, etching quality, printing effect and service life, while nickel electroforming molds are the most expensive. Therefore, stainless steel stencils are widely used in solder paste printing.
There are three kinds of opening methods for metal template in SMT process: chemical etching method, laser cutting method and electroforming method. At present, most of the pcb manufacturer use the laser cutting method to make PCB SMT processing stencils, except for a small amount of advanced fine pitch CSP, and the through hole reflow device which requires a large amount of solder paste is almost impossible to use,almost can cover all smt components. The size of the template opening is directly related to the quality of the solder paste printing, which affects the quality of the solder. The shape of the opening affects the release effect of the solder paste, and the opening design elements of the stainless steel smt processing stencils are as follows.
(1) The shape of the opening. SMT stencil openings are usually available in rectangular, square and circular shapes. Rectangular openings have better demolding efficiency than square and circular openings. When the opening is vertical or the bell mouth is down, the solder paste is released smoothly :
(2) Opening size design. In order to control soldering defects such as solder balls or bridges connection during soldering, the size of the template opening is usually slightly smaller than the size of the land pattern. When the SMT PCB manufacturing factory prints tin-lead solder paste, the general template opening size is 0.92* pad size.
(3) Stencil aspect ratio and area ratio. In the SMT processing factory solder paste printing process, when the adhesion tension between the solder paste and the PCB pad is greater than the friction between the solder paste and the opening wall, there is a good printing effect.
(4) The thickness of the SMT metal template. The thickness of the stencil is directly related to the amount of solder paste after printing. It has a great influence on the soldering quality of electronic products. Under normal circumstances, if there are no BGA, CSP, FC and other devices, the thickness of the template is generally 0.15mm.
The above is the industry information provided by FASTPCBA, I hope to help you!
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