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Expert layout design of components for SMT patch processing

2022/11/22

Component layout shall be designed according to SMT SMT processing and production equipment and process characteristics and requirements. Different processes, such as reflow and wave soldering, have different layouts for components. There are different requirements for the layout of side A and side B in double-sided reflow welding. Selective wave soldering and traditional wave soldering, also have different requirements.

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The basic requirements of SMT process for component layout design are as follows:


The distribution of components on the printed circuit board should be as uniform as possible. The heat capacity of heavy components during reflow welding is large, and too much concentration is easy to cause low local temperature and lead to virtual welding. At the same time, the uniform layout is also conducive to the balance of the center of gravity. In the vibration and impact experiment, it is not easy to appear the destruction of components, metallized holes and pads.


The components shall be arranged in the same direction as far as possible on the printed circuit board, and the characteristic direction shall be the same to facilitate the installation, welding and testing of the components. For example, the positive electrode of electrolytic capacitor, the positive electrode of diode, the single pin end of triode and the first pin arrangement of integrated circuit should be consistent as far as possible. All component numbers are printed in the same direction.


Large components should be set aside around the SMD repair equipment heating head can operate size.


The heating components should be kept away from other components as far as possible. Generally, the heating components should be placed in the corners and ventilated positions in the chassis. The heating component should be supported by other leads or other supports (such as heat sink) to keep a certain distance between the heating component and the printed circuit board surface, the most small distance is 2mm. The heating component body is connected with the printed circuit board in the multilayer board. The metal welding plate is made during the design, and the soldering tin is connected during the processing, so that the heat is distributed through the printed circuit board.


Keep temperature sensitive components away from heating components. For example, transistors, integrated circuits, electrolytic capacitors and some plastic-case components, etc. should be as far away from the bridge pile, high-power components, radiators and high-power resistors as possible.


The layout of components and parts that need to be adjusted or frequently replaced, such as potentiometers, adjustable inductance coils, variable capacitor micro switches, fuse, keys, plugers and other components, should consider the structural requirements of the whole machine, and be placed in a position that is convenient for adjustment and replacement. If the machine adjustment, should be placed on the printed circuit board to facilitate the adjustment of the place; In case of external adjustment, its position should be adapted to the position of the adjustment knob on the chassis panel to prevent the conflict between three-dimensional space and two-dimensional space. For example, the panel opening of a toggle switch should match the empty position of the switch on the printed circuit board.


Fixing holes should be set near wiring terminals, plug and pull parts, the center of long series terminals and the parts under constant force, and corresponding space should be left around the fixing holes to prevent deformation due to heat expansion. If the expansion of long string terminals is more serious than that of printed circuit boards, it is easy to warp during wave soldering.


Some components and components (such as transformers, electrolytic capacitors, varistors, bridge reactors, radiators, etc.) that need to be processed twice have large volume (surface) product tolerances and low precision, and the spacing between them and other components should be increased with a certain margin on the basis of the original setting.


It is recommended that the increase margin of electrolytic capacitors, varistors, bridge piles and polyester capacitors should be no less than 1mm, and that of transformers, radiators and resistors exceeding 5W (including 5W) should be no less than 3mm


Electrolytic capacitors cannot touch heating components, such as high-power resistors, thermistors, transformers, radiators, etc. The most small interval between the electrolytic capacitor and the radiator is 10mm, and the most small interval between other components and the radiator is 20mm.


Stress-sensitive components should not be placed at the corners, edges or near the connectors, mounting holes, slots, incisions, gaps and corners of the printed circuit board. These positions are high stress areas of the printed circuit board, which are easy to cause cracking or cracks of solder joints and components.


The layout of components shall meet the technical requirements and spacing requirements of reflow welding and wave soldering. Reduce the shadow effect during wave soldering.


The positioning holes and fixing supports of the printed circuit board should be reserved.


In the design of a large area of printed circuit board with an area of more than 500cm2, in order to prevent the bending of the printed circuit board when the tin furnace, a gap of 5~10mm wide should be left in the middle of the printed circuit board, not to put the components (can be wired), to be used in the tin furnace plus to prevent the bending of the printed circuit board.


Component arrangement direction of reflow welding process.


① The layout direction of components should consider the direction of the printed circuit board entering the reflow furnace.


(2) In order to heat the welding ends on both sides of the two end chip components and the pins on both sides of the SMD components simultaneously, and reduce the welding defects such as standing tablet, displacement and welding end from the welding plate caused by the synchronous heating of the welding ends of the components, the long axis of the two end chip components on the printed circuit board should be perpendicular to the direction of the conveyor belt of the reflow furnace.


(3) The long axis of the SMD component should be parallel to the transmission direction of the reflow furnace, and the long axis of the Chip component and the SMD component at the two ends should be perpendicular to each other.


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