In PCB design, Design Rule is the key to the success or failure of a PCB design. All designers' intentions and functional manifestations of design are driven and realized by the soul of design rules. Exquisite and detailed rule definition can help designers to be handy in the work of PCB layout and wiring, save a lot of energy and time of engineers, help designers achieve excellent design intentions, and greatly facilitate the design work.
After the design data is transferred from the schematic stage to the PCB design stage, when the PCB design layout is performed, the Design Rule needs to be defined in advance. The entire subsequent PCB design needs to comply with the rule definitions. Including the most basic electrical rules (spacing, short circuit and open circuit), wiring rules (line width, trace style, via style, fan-out, etc.), plane rules (power ground plane layer connection method, copper laying connection method); and other Commonly used auxiliary rules such as layout rules, manufacturing rules, high-speed design rules, signal integrity rules and so on.
After the rule-driven design is completed, you can also perform a rule check Design Rule Check to re-examine your design to see if there are any rule violations and improve and perfect them. The final design is an excellent work that fully complies with the definition of the rules and meets the design intent.
This is especially relevant with respect to design rules related to PCB manufacturing. If the design rules do not meet the requirements of PCB process manufacturing, it will not only affect the function of the product, but even fail to process and achieve the engineer's design intent. Therefore, when defining design rules, it is crucial to understand the process manufacturing requirements of downstream manufacturers for the design.
1. What are the precision requirements for the manufacturing process of PCB processing?
Process requirements of PCB board manufacturers. Including the basic parameter requirements such as the number of circuit board layers, thickness, aperture, minimum line width and line spacing, copper thickness, etc.; also includes special requirements such as sheet type, surface treatment, and special processing. Generally, when PCB is processed, it is divided into proofing processing for testing and batch product processing for final molding. For designers, what has practical significance and needs to be strictly followed is the process requirements of batch product processing.
For the process requirements related to manufacturing accuracy, the most basic and most important are the line width and line spacing and the minimum aperture. That is, the processing plant can handle the smallest line width and the smallest hole. If the line width does not meet the requirements in the design, it cannot be processed correctly if it is too thin. The accuracy of line width and line spacing also affects whether the text pattern on the silk screen layer is clear. And if the aperture is too small, there is no corresponding drill support. The drill size corresponding to the minimum hole diameter also affects the tolerance accuracy of various types of plate shears such as mechanical holes and mounting holes.
Precautions for setting line width, line spacing and aperture rules
Learn how to set line width, line spacing and aperture rules that meet the requirements and design intent in terms of PCB manufacturing accuracy according to PCB manufacturing process requirements.
Minimum line width/spacing 4mil
In PCB design, the highest precision that batch processing can support is 4mil line width and line spacing. That is, the width of the wiring must be greater than 4 mil, and the spacing between the two lines also needs to be greater than 4 mil. Of course, it is only the minimum limit value of line width and line spacing. In actual work, the line width needs to be defined as different values according to the design requirements. For example, the definition of the power network is wider, and the definition of the signal line is thinner.
For these different requirements, you can define different line width values for different networks in the rules Design - Rules - Routing - Width, and then set the priority of the rule application according to the importance. Similarly, for wire spacing, define the electrical safety clearance between different networks in the rules page Design - Rules - Electrical - Clearance, including wire spacing of course.
There is also a special case. For components with high-density pins, the spacing between the pads in the device is generally very small, such as 6 mils. Although it meets the manufacturing requirements of minimum line width or spacing greater than 4 mils, it may not meet the requirements for designing PCBs. Rule design requirements. If the minimum safe spacing setting of the entire PCB is 8mil, then the spacing of the component pads clearly violates the rule setting.
Violations are highlighted in green during rule checking or online editing. This violation obviously doesn't need to be dealt with and we should fix the rule settings to remove the green highlighting. In the original processing method, the query language is used to define different safe spacing rules for this device and set it as a high priority. In the new version, this problem can be solved by simply ticking the option, Ignore Pad to Pad clearance within a footprint.
It is very easy to check with this option. There is no need to use the Query statement InComponent('U1') as before, and then set its minimum safe spacing to 6mil and set it as the highest spacing priority.
2. The minimum mechanical aperture is 0.2mm (8mil) and the minimum laser aperture is 4mil
Drilling holes are inevitably used in PCB design. In terms of setting design rules and even specific drilling operations, you know exactly what kind of holes to drill (through holes, blind holes, buried holes, or back-drilling holes?) and what size holes to drill. Yet? Do you see how big the holes are drilled by others, or just fill in the size to meet the convenience of board layout and wiring?
Type of hole. Typically less complex designs, and designs with few board stack-ups often use vias. In complex designs, especially multi-layer boards, high-speed and high-density designs, and high requirements for PCB wiring space, blind holes or buried holes can be set according to actual needs. Of course, because the manufacturing process of blind buried vias is more complicated than that of through vias, the manufacturing cost will increase accordingly.
The difference between mechanical drilling and laser drilling
First understand the process of drilling down. Drilling is done with different drill sizes as shown in the picture below. If the size of the via hole diameter in your design is not the same as the size of the drill bit available in the shop, then the drill bit size closest to your design values will be selected to drill the hole. The Entry panel is used to protect the drill bit and the table, reduce burrs and reduce the temperature of the drill bit. Backup bottom plate is used to protect the board surface from indentation, prevent slippage and guide and reduce burrs.
Drills for mechanical drilling usually have ST type and UC type. Generally speaking, the UC type has higher drilling accuracy than the ST type.
Laser drilling is generally used for micro-vias. With the development of PCBs in the direction of miniaturization and high-density interconnection, more and more board processing adopts the connection method of via holes to realize high-density interconnection. The small hole capability of traditional mechanical drilling is almost at its limit. With the development of blind hole design, the reliability of high density needs to be improved by new technology, and laser drilling came into being. The following figure shows the method of laser drilling.
The precision of laser drilling is much higher than that of mechanical drilling. Therefore, the hole diameter of the minimum mechanical drilling shall not be less than 0.2mm (8mil) in the rule setting. The minimum laser aperture is set in the rules and cannot be less than 4mil.
3. The relationship between the aperture setting and the thickness of the board, the number of layers, etc.
The setting size of the aperture is first determined according to the required accuracy of the board, the thickness of the board, and the number of stacks, etc., under the condition that the minimum process requirements are met.
Therefore, when setting the aperture size, you can refer to the above table to set the corresponding size that meets the requirements according to the thickness of the plate, the number of layers, etc. It is also possible to define the appropriate hole diameter size roughly according to the entire plate thickness dimension by the following simple plate thickness/aperture ratio.
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