[popular science] main requirements for processing and production of printed circuit boards

1、 PCB size.

PCB size is limited to the capacity of electronic processing line equipment. Therefore, the appropriate size should be considered when designing the product system scheme.

(1) The maximum printed circuit board size that SMT equipment can install comes from pcb Standard sizes, mostly 20 inches × 24 inches, or 508 mm × 610 mm (rail width)

(2) It is recommended to compare the matching size of each equipment on the SMT production line, which can give full play to the production efficiency of each equipment and eliminate the equipment bottleneck.

(3) Designing a small PCB as a mosaic can improve the production efficiency of the whole production line.

[design requirements]:

(1) Generally, the maximum size of printed circuit board shall be limited to 460mm × Within 610 mm.

(2) The recommended size range is (200 ~ 250) mm × (250 ~ 350) mm, with length width ratio of 2.

(3) For dimensions 125 mm × 125 mm pcb , shall be spliced to a suitable size.

2、 PCB shape.

SMT production equipment is rail transmission pcb , irregular shaped printed circuit boards cannot be transmitted, especially at the corners Notched printed circuit board.

[design requirements]

(1)pcb The shape of the shall be square and the quadrangle shall be rounded.

(2) In order to ensure the stability in the transmission process, it should be considered to convert the irregular shaped printed circuit board into a standard square, especially the corner notch, so as to prevent the clamping plate in the transmission process of wave soldering claw.

(3) Notches are allowed for pure SMT plates, but the notch size shall be less than one third of the edge length. If this requirement is exceeded, the design process should be filled.

(4) In addition to chamfer design, chamfer (1 ~ 1.5) shall also be designed for the chamfer design of golden finger × 45 ° for easy insertion.

3、 Transfer side.

The key to the size of the transmission edge is the requirements of the equipment transmission guide rail. The transmission edge is generally required to be more than 3.5mm for printing machines, mounters and reflow welding furnaces.

[design requirements]

(1) Reduce welding time pcb The long side direction is usually used as the transmission direction of non composited printed circuit board; The direction of the long side should also be taken as the transmission direction.

(2) Generally, both sides of the transmission direction of PCB or composition are taken as the transmission direction. The minimum width of the transmission direction is 5.0mm. There shall be no parts or solder joints on the front and back of the transmission direction.

(3) On the non transmission side, SMT equipment is unlimited, and a 2.5mm element forbidden area shall be reserved.

4、 Locating hole.

The precise positioning of PCB is required in many processes such as assembly, assembly and testing. Therefore, the positioning hole should be designed as required.

[design requirements]

(1) Each printed circuit board shall be designed with at least two positioning holes, one is circular and the other is long groove. The former is positioning and the latter is guidance.

The positioning aperture has no special requirements and can be designed according to the specifications of your own factory. The recommended diameters are 2.4 mm and 3.0 mm.

Locating holes shall be non-metallic holes. if pcb It is to punch the printed circuit board, and the hole disc is designed for the positioning hole to enhance the stiffness.

The length of the guide hole is twice the diameter.

The center of the positioning hole shall be more than 5.0mm away from the transmission edge, and the two positioning holes shall be as far as possible. It is recommended to arrange it at the opposite corner of the printed circuit board.

(2) For hybrid PCB (PCBA with plug-ins installed), the positions of positioning holes shall be consistent as far as possible. In this way, the design of tooling can ensure that both sides are common. For example, the bottom bracket of mounting screw can also be used as the tray of plug-ins.

5、 Positioning symbol.

Modern mounter, printing press, optical inspection equipment (AOI) and solder paste inspection equipment (SPI) all use optical positioning system. Therefore, the optical positioning symbol must be designed on the printed circuit board.

[design requirements]

(1) Positioning symbols are divided into global positioning symbols and local positioning symbols. The former is the whole board positioning, and the latter is the patchwork sub board or fine spacing element positioning.

(2) The optical positioning symbols can be designed as square, diamond, cross, well, etc., with a height of 2.0mm. It is generally recommended to design a 1.0m circular copper definition pattern. Considering the comparison of material color and environment, a non resistance welding area 1mm larger than the optical positioning symbol is reserved, and no characters are allowed. Whether there is copper foil in the inner layer under the three symbols on the same plate surface shall be consistent.

(3) On the surface of the printed circuit board with patch elements, it is recommended to set three optical positioning symbols at the corners of the board to position the printed circuit board stereoscopically (three points determine a plane, and the thickness of solder paste can be detected).

(4) In addition to three optical positioning symbols for the whole plate, two or three optical positioning symbols for the plate shall be designed at the diagonal of each unit plate as far as possible.

(5) For QFP, BGA and other equipment with lead center distance ≤ 0.5mm, local optical positioning symbols shall be set at the opposite corners to facilitate accurate positioning.

(6) If there are mounting elements on both sides, each side shall have an optical positioning symbol.

(7) If pcb There is no positioning hole on the, and the center of the optical positioning symbol shall be more than 6.5mm away from the transmission edge of the printed circuit board. If there is a positioning hole on the printed circuit board, the center of the optical positioning symbol shall be designed on the side of the positioning hole close to the center of the printed circuit board.