PCB Tech - Five requirements for PCB processing and production

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 when designing the product system scheme.

(1) The maximum PCB size that can be mounted on SMT equipment comes from the standard size of PCB materials, 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 eliminates equipment bottlenecks.

(3) For small-size PCBs, they should be designed as an imposition to improve the production efficiency of the entire production line.

【Design requirements】

(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, and the aspect ratio should be "2.

(3) For a PCB with a size of "125mm*125mm, it should be set up to a suitable size.

2. PCB shape

[Background description]

SMT production equipment uses guide rails to transport PCBs, and cannot transport PCBs with irregular shapes, especially PCBs 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 shape of the PCB should be considered to be converted into a standardized square by means of imposition, especially the corner gaps should be filled in order to avoid the transmission process of wave soldering jaws Card board.

(3) For pure SMT boards, gaps are allowed, but the gap size should be less than one-third of the length of the side where it is located. For those exceeding this requirement, the design process side should be filled.

(4) In addition to the chamfering design for the inserting side, the chamfering design of the golden finger should also be designed with (1~1.5)*45° chamfering on both sides of the board 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 3.5mm or more.

【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 PCB, the long side direction should also be used as the transmission direction.

(2) Generally, the two sides of the PCB or imposition transmission direction are used as the transmission side. The minimum width of the transmission side is 5.0mm. There should be no components or solder joints on the front and back of the transmission side.

(3) There is no restriction on the non-transmission side and SMT equipment. It is best to reserve a 2.5mm component forbidden area.

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 circular and the other is long groove shape, the former is used for positioning and the latter is used for guiding.

There are no special requirements 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 positioning 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 transmitting edge, and the two positioning holes should be as far away as possible. It is recommended to lay out at the opposite corner of the PCB.

(2) For mixed PCBs (PCBA with plug-ins installed, the position of the positioning holes should be the same, so that the design of the tooling can be shared between the front and the back. For example, the screw bottom bracket can also be used for the tray of the plug-in.

5. Positioning symbol

[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 symbols are divided into global positioning symbols (GlobalFiducial) and local positioning symbols (LocalFiducial). 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 a square, a diamond-shaped circle, a cross, a tic-tac-toe, etc., and the height is 2.0mm. Generally, it is recommended to design a Ø1.0m round copper definition pattern. Taking into account 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. Three on the same board The presence or absence of copper foil in the inner layer under each symbol should be consistent.

(3) On the PCB surface with SMD components, it is recommended to place three optical positioning symbols on the corners of the board for the three-dimensional positioning of the PCB (three points determine a plane, which can detect the thickness of the solder paste).

(4) For imposition, in addition to three optical positioning symbols for the whole board, it is better to design two or three optical positioning symbols for imposition 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 diagonal corners for accurate positioning.

(6) If there are mounted components 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.