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Basic technical requirements for PCB design
The ideal dimensions for production are: width (200mm-250mm), length (250mm-350mm). For PCBs with a length less than 125mm or a width less than 100mm, the panel method can be used to convert the PCB size to the ideal value according to production needs, to facilitate component insertion and soldering.
When designing PCBs, do not exceed the manufacturer's mass production technical level; otherwise, the PCB may not be able to be processed, or it may result in high costs.
1.1 Size Range
The ideal size for production is: width (200mm-250mm), length (250mm-350mm). For PCBs with a length less than 125mm or a width less than 100mm, according to production needs, the panel method can be used to convert the PCB size to the ideal value to facilitate component insertion and welding.
1.2 Shape
a) The PCB shape is rectangular. If the PCB does not need paneling, the four corners of the board must be rounded; if paneling is required, the four corners of the PCB after paneling must be rounded, with a fillet radius of 1mm-2mm.
In order to ensure the stability of the transmission process, the irregular PCB is modified using a penalty method. Specifically, the gaps at the corners must be filled; otherwise, special tooling design is required.
b) To ensure the stable transmission of the PCB in the chain, the length of any gap on the pure SMT must be less than 1/3 of the corresponding edge.
c) Design requirements for gold fingers: chamfer the edge of the plug-in board as needed; (1-1.5) x 45° chamfer or (R1-R1.5) ball should be designed on both sides of the plug-in board to facilitate plug-in.
1.3 Technical Advantages
For PCBs without process edges, there should be no components or solder joints within 5mm or more of the positive or negative edge of the board; the routing position should be at least 3mm from the edge of the board. If short-insertion wave soldering is used, the board must meet the general transfer edge width requirements. Considering the characteristics of short-insertion wave soldering, the height of components within 10mm of the edge of the board is limited to within 40mm (including board thickness).
If the size of the PCB board's transfer edge restricted area does not meet the above requirements, a processing edge with a width of 5mm or more must be added to the corresponding board edge, and the fillet radius of the processing edge is 2mm.
In order to meet the special requirements of structural design, if the components protrude from the transfer edge of the PCB, the width of the auxiliary edge must be satisfied.
1.4 Reference Marks
Equipment using optical positioning needs to place reference marks for fully automated positioning of the pick-and-place machine. These marks need to have high contrast when illuminated by the pick-and-place machine.
1.4.1 Design of Reference Marks
Appearance design requirements for reference marks:
1. Solid circle;
2. Inner diameter = 1mm;
3. Solder mask ring shape radius is 0.5mm.
1.4.2 Application of Reference Marks
Reference marks are mainly used on panels, license plates, and local positions.
1.4.2.1 Global Reference Marks
Three reference points must be selected at the four corners of the board. If there are surface mount components on both sides of the board, each side must have reference marks.
1.4.2.2 Panel Reference Points
Three boards require overall reference points. Each board's diagonal points need at least two reference points. In special cases, it needs to be negotiated with the process personnel whether the two reference points of two boards can be omitted, but the overall reference points of the board must be retained.
1.4.2.3 Local Reference Points
For QFP packaged chips with a pin pitch less than 0.4mm and more than 144 pins, two marks need to be added to the diagonal corners of the chip. If the spacing of the above components is small (distance less than 100mm), it can be regarded as a whole, and two local reference points need to be added to the diagonal positions.
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