Five requirements for PCB Production Processing

1. PCB size

【Background Description】

The size of the PCB is limited by the capabilities of the electronic processing production line equipment. Therefore, the appropriate PCB size 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 the PCB sheet, most of which are 20″×24″, that is, 508mm×610mm (rail width)

(2) The recommended size is the size that matches the equipment of 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 to be imposition to improve the production efficiency of the entire production line.

【Design requirements】

(1) Under normal circumstances, 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 a PCB with a size of <125mm × 125mm, the layout should be a suitable size.

2. PCB shape

【Background Description】

SMT production equipment uses guide rails to transfer PCBs, and cannot transfer PCBs with irregular shapes, especially those with gaps in 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 irregular-shaped PCB should be converted into a standard square by imposition, especially the corner gap should be filled to avoid the wave soldering jaw 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) In addition to the chamfering required for the insertion edge, the chamfering design of the gold finger should also be designed with (1~1.5) × 45° chamfering on both sides of the insert plate 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-imposed 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 transfer direction are used as the transfer side, the minimum width of the transfer side is 5.0mm, and there can be no components or solder joints on the front and back of the transfer side.

(3) For the non-transmission side, there are no restrictions on SMT equipment. It is best to reserve a 2.5mm forbidden area for components.

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 slot. The former is used for positioning and the latter is used for guidance. There is no special requirement for the positioning aperture, and it can be designed according to the specifications of your own factory. The recommended diameter is 2.4mm and 3.0mm. The pilot holes should be non-metallized holes. If the PCB is a punched PCB, the positioning hole should be designed with a hole plate to strengthen the rigidity. The length of the guide hole is generally 2 times the diameter. The center of the positioning hole should be more than 5.0mm away from the transmission edge, and the two positioning holes should be as far apart 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 is preferably the same as the front and back, so that the design of the tooling can be shared on the front and back, such as the screw bottom bracket can also be used for the tray of the plug-in.

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 symbol is divided into the global positioning symbol (Global Fiducial) and the local positioning symbol (Local 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) The optical positioning symbol can be designed into square, diamond-shaped circle, cross-shaped, well-shaped, etc., with a height of 2.0mm. It is generally recommended to design a circular copper definition graphic with a diameter of 1.0m. Considering the contrast between the material color and the environment, leave a non-soldering area 1mm larger than the optical positioning symbol. No characters are allowed in it. The presence or absence of copper foil on the inner layer under the symbol should be consistent.

(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 for stereo positioning of the PCB (three points determine a plane, which can detect the thickness of the solder paste).

(4) For imposition, in addition to having three whole-board optical positioning symbols, it is better to design two or three imposition optical positioning symbols at the diagonal corners of each unit board.

(5) For devices such as QFP with lead center distance ≤ 0.5mm and BGA with center distance ≤ 0.8mm, local optical positioning symbols should be set at the opposite corners to facilitate accurate positioning.

(6) If there are mounted components on both sides, there should be an optical positioning symbol 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 at the positioning hole on the center side of the PCB.