PCB edge design

When PCB is to be processed by SMT, PCB is transported by guide rail on SMT production line. Therefore, a pair of edges of forbidden cloth elements must be reserved as transport edges. Usually, the two long sides of the PCB or the rear panel of the jigsaw puzzle are used as its conveying edges.
The width of the fixing plate of SMT track is 3.0mm, and theoretically the limit value of the transmission edge is 3.0mm. However, it is suggested that you should not take this limit to increase the difficulty of mounting. To reserve more margins, it is suggested that 5.0mm should be the “forbidden area” of the conveying edge.
If this allowance of forbidden cloth is insufficient, the interference part will affect the solder paste or the already placed components after the PCB is mounted on the conveying guide rail, and the interfered part of the board edge can only be post-soldered or needs to be additionally clamped, which increases the production cost.
If the distance between the patch parts in the board and the board edge is far enough to be out of the track range, there is no need to add additional craft edges. If the parts are in the track range, craft edges must be added, and the craft edges are used as clamping areas. According to the characteristics of the machine, the process width usually needs 3-10mm, with 5mm being the most common one.
Generally, the process edge is added to the longer side, and the board enters the SMT machine vertically, so that the board has a higher hardness, and it won’t bounce due to the light pressure of the machine probe when pasting. However, the enlarged process edge has a larger area, which increases the average price of a single board in disguise. When the hardness of the board is enough, it can be added in the short direction, the area of the process edge is small, the average cost of a single board is reduced, and the board enters the SMT machine horizontally.
Usually, MARK points are used for alignment in SMT. If there are no MARK points in the board, 2-4 MARK points must be added diagonally at the process edge. Generally, the size of MARK points is 1.0mm, and the exposed copper is coated with tin. In some cases in the board, the process edge can be added or not.
When the circuit board is produced, it needs to be positioned for molding and testing. Adding a special positioning hole at the edge of the process makes the shape relatively standard, and it is convenient to position. Therefore, 3-4 positioning holes with a diameter of 2.0-4.0mm will be added to the process edge, with the diameter of 3mm being the most common and the best.
Precautions for PCB edge design:
Patches or machine-inserted components cannot be arranged in the process edge, and the entities of patches or machine-inserted components cannot enter the process edge and its space.
The entity of hand-inserted components can’t fall in the space within 3mm height above the upper and lower process edges, and can’t fall in the space within 2mm height above the left and right process edges.
The conductive copper foil inside the process should be as wide as possible. The lines smaller than 0.4mm need to be insulated and wear-resistant, and the line at the outermost edge should be no less than 0.8mm.
The edge and PCB can be connected by stamp hole or V-shaped groove, which is generally used.
There should be no pads or through holes on the edge of the process.
A single board with an area greater than 80mm requires that the PCB itself has a pair of parallel process edges, and no component entities enter the upper and lower spaces of the process edges.
The width of the process edge can be appropriately increased according to the actual situation.