1. PCB size
【Background Description】The size of PCB is limited by the capability of electronic processing production line equipment. Therefore, appropriate PCB size should be considered in the design of product system scheme.
(1) The maximum PCB size that SMT equipment can mount is derived from the standard size of the PCB sheet, most of which are 20″×24″, that is, 508mm×610mm (rail width)
(2) The recommended size is the matching size of each equipment in the SMT production line, which is conducive to the production efficiency of each equipment and the elimination of equipment bottlenecks.
(3) For small-sized PCBs, it should be designed as an imposition to improve the production efficiency of the entire production line.
【Design requirements】
(1) In general, the maximum size of the PCB should be limited within the range of 460mm×610mm.
(2) The recommended size range is (200~250)mm×(250~350)mm, and the aspect ratio should be <2.
(3) For a PCB with a size of “125mm×125mm”, it should be made into a suitable size.
2. PCB shape
[Background Description] SMT production equipment uses guide rails to transport PCBs, and PCBs with irregular shapes cannot be transported, especially PCBs with notches at the corners.
【Design requirements】
(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 imposition method should be considered to convert the irregularly shaped PCB into a standardized square shape, especially the corner gaps should be filled to avoid the wave soldering jaws during the transmission process. Medium card board.
(3) For pure SMT boards, gaps are allowed, but the size of the gap should be less than one-third of the length of the side. For those exceeding this requirement, the design process side should be filled.
(4) The chamfering design of the golden finger is not only required to design chamfering on the insertion side, but also to design (1~1.5)×45° chamfering on both sides of the plug-in board to facilitate insertion.
3. Transmission side
[Background Description] The size of the conveying edge depends on the requirements of the conveying guide rail of the equipment. For printing machines, placement machines and reflow soldering furnaces, the conveying edge is generally required to be more than 3.5mm.
【Design requirements】
(1) In order to reduce the deformation of the PCB during soldering, the long side direction of the non-imposition PCB is generally used as the transmission direction; for the imposition, 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 must be no components or solder joints in the front and back of the transmission side.
(3) On the non-transmission side, there is no restriction on SMT equipment, and it is best to reserve a 2.5mm component prohibition area.
4. Positioning hole
[Background Description] Many processes such as imposition processing, assembly, and testing require accurate positioning of the PCB. Therefore, positioning holes are generally required to be designed.
【Design requirements】
(1) For each PCB, at least two positioning holes should be designed, one is designed as a circle, and the other is designed as a long groove. The former is used for positioning and the latter is used for guidance.
There is no special requirement for the positioning aperture, it can be designed according to the specifications of your own factory, and the recommended diameters are 2.4mm and 3.0mm.
Positioning holes shall be non-metallized holes. If the PCB is a punched PCB, the positioning hole should be designed with a hole plate to enhance rigidity.
The length of the guide hole is generally twice the diameter.
The center of the positioning hole should be more than 5.0mm away from the transmission side, and the two positioning holes should be as far away as possible. It is recommended to arrange them at the opposite corners of the PCB.
(2) For mixed PCBs (PCBAs with plug-ins installed, the position of the positioning holes should be the same as the front and back, so that the design of the tooling can be shared between the front and back, such as the screw bottom bracket can also be used for the plug-in tray.
5. Positioning symbols
[Background Description] 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.
【Design requirements】
(1) The positioning symbols are divided into global positioning symbols (Global Fiducial) and local positioning symbols (Local Fiducial)
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) Optical positioning symbols can be designed as squares, diamonds, circles, crosses, wells, etc., with a height of 2.0mm. Generally, it is recommended to design a circular copper definition pattern of Ø1.0m. Considering the contrast between the material color and the environment, a non-soldering area 1mm larger than the optical positioning symbol is reserved, and no characters are allowed in it. Three on the same board The presence or absence of copper foil in the inner layer should be the same under the symbol.
(3) On the PCB surface with SMD components, it is recommended to arrange three optical positioning symbols on the corner of the board for stereo positioning of the PCB (three points determine a plane, and the thickness of the solder paste can be detected).
(4) For the imposition, in addition to having three optical positioning symbols on the entire board, it is better to design two or three imposition optical positioning symbols at the opposite corners of each unit board.
(5) For devices such as QFP with a lead center distance of ≤0.5mm and BGA with a center distance of ≤0.8mm, local optical positioning symbols should be set on the diagonal for precise positioning.
(6) If there are mounted components on both sides, each side should have optical positioning symbols.
(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 transmission side of the PCB. 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 near the center of the PCB.
Post time: Apr-08-2023