The size of the PCB is limited by the capabilities of the electronic processing production line equipment. Therefore, suitable PCB sizes should be considered during product system design.
(1) The maximum PCB size that SMT equipment can mount originates from the standard size of PCB materials, most of which are 20″×24″, i.e. 508mm×610mm (rail width).
(2) Recommended sizes are those that match the various devices on the SMT production line, which helps to maximize production efficiency and eliminate equipment bottlenecks.
(3) For small-sized PCBs, they should be designed as panels to improve the overall production efficiency of the production line.
(1) Generally, the maximum size of the PCB should be limited to the range of 460mm×610mm.
(2) The recommended size range is (200~250)mm×(250~350)mm, with a length-to-width ratio of <2.
(3) For sizes <125mm×125mm, PCBs should be designed as panels to achieve suitable dimensions.
SMT production equipment uses rails to transport PCBs and cannot transport irregularly shaped PCBs, especially those with notches at the corners.
(1) The PCB shape should be a regular square with rounded corners.
(2) To ensure stability during the transport process, irregularly shaped PCBs should be converted into standard squares using paneling, especially notches at corners should be filled to avoid jamming during wave soldering.
(3) Pure SMT boards are allowed to have notches, but the notch size should be less than one-third of the length of the edge. For those exceeding this requirement, the design process edge should be filled in.
(4) The edge design for the gold finger should include a chamfer design for the insertion edge, and the edges on both sides of the insert should also be designed with a (1~1.5)×45° chamfer to facilitate insertion.
The size of the transport edge depends on the requirements of the transport rail of the equipment, such as printers, pick-and-place machines, and reflow soldering ovens, which generally require a transport edge of more than 3.5mm.
(1) To reduce PCB deformation during soldering, the long edge direction of non-panel PCBs should generally be used as the transport direction; for panels, the long edge direction should also be used as the transport direction.
(2) Generally, the two edges of the PCB or panel in the transport direction are used as the transport edges, with a minimum width of 5.0mm. There should be no components or solder points within the transport edge on both sides.
(3) For non-transport edges, there are no restrictions from SMT equipment, but it is best to leave a 2.5mm area free of components.
Many processes such as panel processing, assembly, and testing require accurate positioning of the PCB. Therefore, it is generally required to design positioning holes.
(1) Each PCB should design at least two positioning holes, one designed as circular and the other as a long slot. The former is used for positioning, and the latter for guiding.
The diameter of the positioning holes has no specific requirements and can be designed according to the factory specifications, with a recommended diameter of 2.4mm or 3.0mm.
Positioning holes should be non-plated holes. If the PCB is a punched PCB, the positioning holes should be designed with hole plates to enhance rigidity.
The length of the guiding holes should generally be twice the diameter.
The center of the positioning holes should be more than 5.0mm away from the transport edge, and the two positioning holes should be as far apart as possible, preferably laid out at the diagonals of the PCB.
(2) For mixed assembly PCBs (those with plug-in PCBA), the positioning holes should preferably be consistent on both sides, allowing for shared tooling designs, such as using the screw base for the plug-in tray.
Modern pick-and-place machines, printers, optical inspection equipment (AOI), and solder paste inspection equipment (SPI) all use optical positioning systems. Therefore, optical positioning symbols must be designed on the PCB.
(1) Positioning symbols are divided into global fiducials and local fiducials. The former is used for board-level positioning, while the latter is used for positioning sub-boards in paneling or for fine-pitch components.
(2) Optical positioning symbols can be designed as squares, diamonds, circles, crosses, or hash marks, with a height of 2.0mm. It is generally recommended to design them as Ø1.0mm circular copper-defined graphics, considering the contrast with the material color and environment, leaving a no-mask area larger than the optical positioning symbol by 1mm, where no characters are allowed. The copper foil under the three symbols on the same board surface should be consistent.
(3) On PCBs with mounted components, it is recommended to place three global optical positioning symbols at the corners of the board for three-dimensional positioning (three points determine a plane, which can detect the thickness of the solder paste).
(4) For panels, in addition to having three global optical positioning symbols, each unit board should also ideally have two or three local optical positioning symbols at the diagonals.
(5) For devices with a lead pitch ≤0.5mm QFP and a pitch ≤0.8mm BGA, local optical positioning symbols should be set at their diagonals for precise positioning.
(6) If there are mounted components on both sides, each side should have optical positioning symbols.
(7) If there are no positioning holes on the PCB, the center of the optical positioning symbol should be more than 6.5mm away from the PCB transport edge. If there are positioning holes, the center of the optical positioning symbol should be designed near the center side of the positioning hole.
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