Views:41 Author:Site Editor Publish Time: 2021-01-22 Origin:Site
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1. PCB size
The size of the PCB is limited by the capabilities of the electronic processing production line equipment. Therefore, the appropriate PCB size should be considered when designing the product system scheme.
(1) The maximum PCB size that can be mounted on SMT equipment comes from the standard size of PCB materials, most of which are 20"×24", that is, 508mm×610mm (rail width)
(2) The recommended size is the size that matches the equipment of the SMT production line, which is conducive to the production efficiency of each equipment and eliminates equipment bottlenecks.
(3) For small-size PCBs, it should be designed as an imposition to improve the production efficiency of the entire production line.
(1) Generally, the maximum size of the PCB should be limited to 460mm×610mm.
(2) The recommended size range is (200~250)mm×(250~350)mm, and the aspect ratio should be <2.
(3) For PCBs with a size of <125mm×125mm, the appropriate size should be set up.
2. PCB shape
SMT production equipment uses guide rails to transport PCBs, and cannot transport PCBs with irregular shapes, especially PCBs with gaps in the corners.
(1) The shape of the PCB should be a regular square with rounded corners.
(2) In order to ensure the stability of the transmission process, the irregular shape of the PCB should be considered to be converted into a standardized square by means of imposition, especially the corner gaps that should be filled in order to avoid the transmission process of wave soldering jaws Cardboard.
(3) For pure SMT boards, gaps are allowed, but the gap size should be less than one-third of the length of the side where it is located. For those exceeding this requirement, the design process side should be filled.
(4) In addition to the chamfering design for the inserting side, the chamfering design of the golden finger should also be designed with (1~1.5)×45° chamfering on both sides of the board to facilitate insertion.
3. Transmission side
The size of the conveying edge depends on the requirements of the conveying rail of the equipment, printing presses, placement machines and reflow soldering furnaces generally require the conveying edge to be above 3.5mm.
(1) In order to reduce the deformation of the PCB during soldering, the long side direction of the non-imposed PCB is generally used as the transmission direction; for the imposition PCB, the long side direction should also be used as the transmission direction.
(2) Generally, the two sides of the PCB or imposition transmission direction are used as the transmission side. The minimum width of the transmission side is 5.0mm. There should be no components or solder joints on the front and back of the transmission side.
(3) There are no restrictions on the non-transmission side and SMT equipment. It is best to reserve a 2.5mm component forbidden area.
4. Positioning hole
Many processes such as imposition processing, assembly, and testing require accurate positioning of the PCB. Therefore, positioning holes are generally required to be designed.
(1) For each PCB, at least two positioning holes should be designed, one is circular and the other is long groove shape, the former is used for positioning and the latter is used for guiding.
There is no special requirement for the positioning aperture, it can be designed according to the specifications of your own factory, and the recommended diameter is 2.4mm and 3.0mm.
The positioning holes should be non-metalized holes. If the PCB is a punched PCB, the positioning hole should be designed with a hole plate to strengthen the rigidity.
The length of the guide hole is generally 2 times the diameter.
The center of the positioning hole should be more than 5.0mm away from the transmission edge, and the two positioning holes should be as far away as possible. It is recommended to layout at the opposite corner of the PCB.
(2) For mixed PCB (PCBA with the plug-in installed, the position of the positioning hole should be the same so that the design of the tooling can be shared between the front and the back. For example, the screw base can also be used for the tray of the plug-in.
5. Positioning symbol
Modern placement machines, printing machines, optical inspection equipment (AOI), solder paste inspection equipment (SPI), etc. all use optical positioning systems. Therefore, optical positioning symbols must be designed on the PCB.
(1) The positioning symbols are divided into global positioning symbols (Global Fiducial) and local positioning symbols (Local Fiducial). The former is used for the positioning of the whole board, and the latter is used for the positioning of imposition sub-boards or fine-pitch components.
(2) The optical positioning symbol can be designed into a square, diamond-shaped circle, cross-shaped, tic-tac-toe, etc., with a height of 2.0mm. Generally, it is recommended to design a Ø1.0m round copper definition pattern. Taking into account the contrast between the material color and the environment, leave a non-soldering area 1mm larger than the optical positioning symbol. No characters are allowed inside. Three on the same board The presence or absence of copper foil in the inner layer under each symbol should be consistent.
(3) On the PCB surface with SMD components, it is recommended to place three optical positioning symbols on the corners of the board for the three-dimensional positioning of the PCB (three points determine a plane, which can detect the thickness of the solder paste).
(4) For imposition, in addition to three optical positioning symbols for the entire board, it is better to design two or three imposition optical positioning symbols at the diagonal corners of each unit board.
(5) For devices such as QFP with lead center distance ≤0.5mm and BGA with center distance ≤0.8mm, local optical positioning symbols should be set at the diagonal corners for precise positioning.
(6) If the components are mounted on both sides, there should be optical positioning symbols on each side.
(7) If there is no positioning hole on the PCB, the center of the optical positioning symbol should be more than 6.5mm away from the PCB transmission edge. If there is a positioning hole on the PCB, the center of the optical positioning symbol should be designed on the side of the positioning hole close to the center of the PCB.