- PCB size
The size of the PCB is limited by the capacity of the electronic processing production line equipment. Therefore, the appropriate PCB size should be considered when designing the product system solution.
(1) The maximum PCB size that can be mounted by the SMT equipment is derived from the standard size of the PCB sheet, most of which are 20″×24″, that is, 508mm×610mm (guide rail width)
(2) The recommended size is the size that matches the various equipment on the SMT production line, which is conducive to the production efficiency of each equipment and eliminating equipment bottlenecks.
(3) For small-sized PCBs, they should be designed as a panel to improve the production efficiency of the entire production line.
Circuit board
[Design requirements]
(1) In general, 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 assembled.
- PCB shape
[Background]
SMT production equipment uses rails to transport PCBs. It cannot transport PCBs with irregular shapes, especially PCBs with notches at the corners.
[Design requirements]
(1) The PCB shape should be a regular square with rounded corners.
(2) To ensure stability during the transmission process, irregular PCBs should be converted into standard squares by assembly. In particular, it is best to fill the corner notches to avoid the board being stuck during the wave soldering clamp transmission process.
(3) For pure SMT boards, notches are allowed, but the notch size should be less than one-third of the length of the side. For those that exceed this requirement, the design process side should be filled.
(4) In addition to the chamfer design required for the insertion side, the chamfer design of the gold finger should also be (1~1.5)×45° on both sides of the plug-in board to facilitate insertion.
- Conveying edge
[Background]
The size of the conveying edge depends on the conveying rail requirements of the equipment. For printers, placement machines and reflow soldering furnaces, the conveying edge is generally required to be above 3.5mm.
[Design requirements]
(1) In order to reduce the deformation of the PCB during soldering, the long side direction of the non-paneled PCB is generally used as the conveying direction; for the panel, the long side direction should also be used as the conveying direction.
(2) Generally, the two sides of the PCB or panel conveying direction are used as conveying edges. The minimum width of the conveying edge is 5.0mm. There should be no components or solder joints on the front and back of the conveying edge.
(3) For non-conveying edges, there are no restrictions on SMT equipment. It is best to reserve a 2.5mm component prohibited area.
- Positioning hole
[Background]
Many processes such as panel processing, assembly, and testing require accurate positioning of the PCB. Therefore, it is generally required to design positioning holes.
【Design requirements】
(1) Each PCB should have at least two positioning holes, one circular and the other slotted. The former is used for positioning and the latter is used for guiding.
There are no special requirements for the positioning hole diameter. It can be designed according to the specifications of your own factory. The recommended diameter is 2.4mm and 3.0mm.
The positioning hole should be a non-metallized hole. 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 edge. The two positioning holes should be as far away as possible. It is recommended to be arranged at the diagonal corners of the PCB.
(2) For mixed PCBs (PCBAs with plug-ins installed), the positions of the positioning holes should be consistent on both sides. In this way, the design of the tooling can be shared by both sides, such as the screw base can also be used as a tray for plug-ins.
- Positioning symbols
[Background]
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 PCBs.
[Design requirements]
(1) Positioning symbols are divided into global positioning symbols and local positioning symbols. Fiducial). The former is used for whole-board positioning, and the latter is used for positioning of sub-boards or fine-pitch components.
(2) Optical positioning symbols can be designed into squares, diamond circles, crosses, tic-tac-toe shapes, etc., with a height of 2.0 mm. It is generally recommended to design a circular copper definition graphic of Ø1.0 mm. Considering the contrast between the material color and the environment, a solder mask-free area 1 mm larger than the optical positioning symbol is reserved. No characters are allowed in it. The inner layer of the three symbols on the same board should be consistent in terms of whether there is copper foil.
(3) On the PCB surface with SMD components, it is recommended to arrange three whole-board optical positioning symbols at the corners of the board to facilitate three-dimensional positioning of the PCB (three points determine a plane, which can detect solder paste). （4）For panelization, in addition to three full-board optical positioning symbols, it is best to design two or three panelization optical positioning symbols at the diagonal positions of each unit board.
（5）For QFP with lead center distance ≤0.5mm and BGA with center distance ≤0.8mm, local optical positioning symbols should be set at the diagonal positions to facilitate accurate positioning.
（6）If there are components 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.