Tutorial: Five Requirements Of PCB Processing And Production

PCB size

【 Background 】

The size of the PCB is limited by the capacity of the equipment in the electronic processing line. Therefore, the appropriate size of the PCB should be considered in the design of the product system scheme.

(1) The maximum PCB size that SMT equipment can mount is derived from the standard size of PCB sheet, most of which is 20 “×24”, namely 508mm×610mm (guide 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 size PCB should be designed into a patchwork, to improve the production efficiency of the whole production line.

【 Design Requirements 】

(1) Under normal circumstances, the maximum size of PCB should be limited to 460mm×610mm.

(2) The recommended size range is (200~250) mm× (250~350) mm, and the length-width ratio should be 2.

(3) For the size of 125mm×125mm PCB, it should be made to the appropriate size.

PCB outline

【 Background 】

SMT production equipment uses guide rail to transfer PCB, can not transfer irregular outline PCB, especially the corner notch PCB.

【 Design Requirements 】

(1) The shape of PCB should be regular square and rounded at four corners.

(2) In order to ensure the stability of the transmission process, the irregular shape of the PCB should be considered by the way of piecing it into the standard square, especially the corner gap is best to fill up, so as to avoid the peak welding jaw in the transmission process of the card plate.

(3) SMT board, allow to have a gap, but the gap size should be less than one third of the length of the side, for more than this requirement, should be the design process edge complement.

(4) For the design of the beveling of the golden finger, in addition to the chamfer design of the insert side, the chamfer of (1~1.5) ×45° should also be designed on both sides of the insert board to facilitate insertion.

Process board edge

【 Background 】

The size of the Process board edge depends on the requirements of the transmission guide rail of the equipment, printing presses, SMT machines and reflow welding furnaces. Generally, the transfer edge should be above 3.5mm.

【 Design Requirements 】

(1) In order to reduce the deformation of PCB during welding, the long side direction is generally used as the transmission direction for non-pieced PCB; The long side direction should also be used as the transmission direction for the composition.

(2) Generally, the two sides of the transmission direction of PCB or patchwork are used as the transmission side. The minimum width of the transmission side is 5.0mm. There can be no components or solder joints in the positive and negative sides of the transmission side.

(3) Non-transmission edge, SMT equipment has no restrictions, it is best to reserve a 2.5mm component forbidden area.

Positioning hole

【 Background 】

Plates processing, assembly, testing and many other processes require PCB accurate positioning, therefore, generally require the design of positioning holes.

【 Design Requirements 】

(1) Each PCB shall be designed with at least two positioning holes, one for round shape and the other for long groove shape, the former for positioning and the latter for guiding.

There is no special requirement for the positioning aperture, it can be designed according to the specifications of the factory. The recommended diameter is 2.4mm and 3.0mm.

Positioning holes shall be non-metallic holes. If the PCB is a blanking PCB, orifice plates shall be designed to enhance rigidity. The length of the guide hole is usually 2 times of the diameter.

The center of the positioning hole should be more than 5.0mm away from the transfer edge, and the two positioning holes should be as far away as possible. It is recommended to lay out on the diagonal of the PCB.

• For the mixed PCB (PCBA with plug-ins installed, the position of the positioning hole had better be consistent, so that the design of the tooling can be common to the front and back, such as the screw base can also be used for the tray of plug-ins.

Positioning symbols

【 Background 】

Modern mount machine, printing press, optical inspection equipment (AOI), solder paste inspection equipment (SPI) and so on all use optical positioning system. Therefore, the optical positioning symbol must be designed on PCB.

【 Design Requirements 】

(1) Positioning symbols are divided into global Fiducial and local Fiducial. The former is used for the positioning of the whole board, and the latter is used for the positioning of the pieced sub-board or finely spaced components.

(2) the optical positioning symbol can be designed into a square, diamond round, cross, well shape, etc., the height is 2.0mm. General recommendations designed to Ø 1.0 m round copper definition graphics, considering the material color contrast with the environment, set aside 1 mm greater than the optical position mark in welding area, which does not allow any characters, the same panel under the three symbols of the inner presence of copper foil should be consistent.

(3) On the PCB surface with patch components, it is recommended to lay three whole-board optical positioning symbols on the corner of the board to facilitate three-dimensional positioning of the PCB (three points determine a plane and can detect the thickness of solder paste).

(4) For the Mosaic, in addition to three optical positioning symbols for the whole board, it is better to design two or three optical positioning symbols for the Mosaic on the diagonal of each unit board.

(5) For QFP devices with lead center distance ≤0.5mm and BGA devices with lead center distance ≤0.8mm, local optical positioning symbols should be set diagonally so as to accurately position them.

(6) If there are mounting components on both sides, there should be optical positioning symbols on each side.

(7) If there is no positioning hole on 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 PCB, the center of the optical positioning symbol should be designed on the side of the positioning hole near the center of PCB.