General basic PCB design flow as follows: preparation -> PCB design -> PCB Layout - > Cabling - > routing optimization and silk - > Network and DRC checks and structural inspection - > plate.
First: preparation. This includes preparing the library and schematics. " We must first sharpen his tools " to make a good board, in addition to good design principles , should also paint it . Before PCB design , the first to be ready SCH schematic and PCB library component library . Library can be used peotel own library, but under normal circumstances it is difficult to find a suitable , preferably their own standard size based on the information of the selected device's own component libraries. In principle, the PCB component libraries do first , do SCH component library . Higher PCB component library requirements, which directly affects the board installed ; SCH component library requires relatively loose , just pay attention to the definition of a good correspondence between the pin and the PCB component properties on the line . PS: Note that the standard library hidden pins. After that schematic design , ready to start doing well after the PCB design .
Second : PCB design . This step has been determined according to the size of the circuit board and the mechanical positioning , draw PCB board in the PCB design environment , and then positioning requirements connectors, buttons / switches, screw holes , place the required mounting holes so . And to fully consider and determine the routing area and the non- routing area ( eg screw holes around the extent to belong to the non- routing area ) .
Third : PCB layout. Layout that white is on the board to put the device . Then if mentioned earlier preparations are good , then you can generate a netlist (Design-> Create Netlist) on the schematic , and then import the netlist (Design-> Load Nets) on the PCB. I saw the device crashed whole heap up, as well as fly line connection between the pin tips . Then you can pair the device layout . The general layout of the following principles:
①. By electrical performance and reasonable zoning, generally divided into : digital circuit area ( ie fear of interference, and interference ) , analog circuit region ( fear of interference ) , power driver area ( sources of interference ) ;
②. Perform the same function in the circuit should be placed as close as possible , and adjust the various components to ensure the connection of the most concise ; while adjusting the relative position of each functional block to make the connection between the functional blocks among the most concise ;
③. For large mass components should consider the installation location and mounting strength ; heating element should be placed separately from the temperature-sensitive components, thermal convection measures should also be considered , if necessary ;
④. I / O -driven device as close to the edge of the printing plate , close to the lead connector ;
⑤. Clock generator ( eg : Crystal or Zhong ) to try to close the device clock used ;
⑥. Between each IC power input pin and ground , need to add a decoupling capacitor ( typically a good high-frequency performance monolithic capacitors ) ; dense when board space , but also add a few integrated circuits around tantalum capacitors.
⑦. To add a relay coil at discharge diode (1N4148 can ) ;
⑧. Layout requirements should be balanced , density and orderly , not top-heavy or a sink
- Require special attention when placing components , be sure to consider the actual size of the ( occupied area and height ) components , the relative position between components , in order to ensure the feasibility of the electrical properties of the circuit board and production installation and convenience at the same time , it should be able to reflect the principle of ensuring the above premise , the proper place to modify the device to make it neat appearance, such as the same device to be placed neatly in the same direction , you can not put too " patchwork ."
This step is related to the degree of difficulty and the overall image of the next wiring board , so it takes a great effort to consider. When the layout , not sure where to preliminary wiring can be fully considered.
Fourth: cabling. PCB design layout is the most important step . This will directly affect the quality of the performance of the PCB . In the PCB design process , so the wiring are generally divided into three phases : the first is through the cloth , the basic requirement of PCB design time . If the line did not pass the cloth , and made full fly line , it would be a failure of the board, it can be said yet started. Followed by the electrical performance of the meet . It is a measure of a printed circuit board eligibility criteria. This was after Bouton , carefully adjust the wiring so that it can achieve the best electrical performance. Next is beautiful. If your wiring through the cloth , there is no impact on electrical performance places, but a glance disorganized , plus colorful, colorful , and that even if you how good electrical properties , in the eyes of others or refuse a . To test and repair this great inconvenience . Wiring to uniform, not criss-cross clueless . These should be in ensuring the electrical performance and meet other requirements of the individual case to achieve , otherwise the forest for the trees . The main wiring according to the following principles:
①. Under normal circumstances, the first response to power and ground wiring to ensure that the electrical performance of the circuit board. In the conditions permit, try to widen the power , ground width , preferably ground than the power line width, their relationship is : Ground > Power cable> signal line , usually signal line width : 0.2 ~ 0.3mm , the smallest width of up to 0.05 ~ 0.07mm, the power cord is generally 1.2 ~ 2.5mm. Available on a wide PCB ground wire digital circuits to form a loop , constitutes a ground network to use ( analog circuit ground so you can not use )
②. More stringent requirements in advance of the line ( such as high-frequency cable ) wiring , input and output terminals of adjacent parallel edges should be avoided to avoid reflection interference. Necessary, add ground isolation , wiring two adjacent layers perpendicular to each other and parallel prone to parasitic coupling.
②. More stringent requirements in advance of the line ( such as high-frequency cable ) wiring , input and output terminals of adjacent parallel edges should be avoided to avoid reflection interference. Necessary, add ground isolation , wiring two adjacent layers perpendicular to each other and parallel prone to parasitic coupling.
③. Oscillator shell ground , the clock line as short as possible , and can not be attracted everywhere. Clock oscillation circuit below , a special high-speed logic circuit part to increase the land area , and should not take the other signal lines to the surrounding farms close to zero ;
④. As far as possible 45o polyline wiring , do not use a line 90o to reduce high-frequency signal radiation ; ( lines also require high double arc )
⑤. Do not any signal line to form a loop , as is inevitable , the loop should be as small as possible ; vias to minimize signal line ;
⑥. The key lines as short and thick, and on both sides of a protective ground.
⑦. When sending sensitive signal and noise signal through the field with a flat cable , use the "ground - signal - ground " approach leads.
⑧. The key signal test points should be set aside to facilitate the production and maintenance of detection
⑨. After completion of wiring schematics , wiring respond optimized ; Meanwhile , after a preliminary examination and DRC network after checking correct wiring area on the ground is not filled with a large area for the ground layer of copper on the PCB is not spend the places connected with the land use as a ground . Or made of plywood , power , occupied the ground floor of each .
- PCB layout process requirements
①. Line
In general, the signal line width is 0.3mm (12mil), the power supply line width 0.77mm (30mil) or 1.27mm (50mil); distance between lines and between the lines and the pad is not less than 0.33mm (13mil ) , practical application , conditions permitting, should be considered to increase the distance ;
At higher wiring density can be considered ( but not recommended ) using IC foot walk between two lines , the line width of 0.254mm (10mil), line spacing is not less than 0.254mm (10mil). Under special circumstances, when the device pins dense , narrow width , the line may be appropriate to reduce the line width and spacing .
②. Pad (PAD)
Pad (PAD) and the transition pore (VIA) The basic requirements are: disc diameter than the diameter of the hole is larger than 0.6mm; For example , GM pins resistors , capacitors and integrated circuits , the use of disk / Hole Size 1.6mm/0.8 mm (63mil/32mil), sockets, pins and diodes 1N4007 , etc., using 1.8mm/1.0mm (71mil/39mil). Practical application, should be based on the size of the actual components to be, when conditions may be appropriate to increase the pad size ;
PCB board design element mounting aperture should be larger than the actual size of the component pin is about 0.2 ~ 0.4mm.
③. Vias (VIA)
Generally 1.27mm/0.7mm (50mil/28mil);
When high wiring density , vias may be appropriate to reduce the size , but not too small , consider using 1.0mm/0.6mm (40mil/24mil).
④. Spacing pad lines , vias request
PAD and VIA: ≥ 0.3mm (12mil)
PAD and PAD: ≥ 0.3mm (12mil)
PAD and TRACK: ≥ 0.3mm (12mil)
TRACK and TRACK: ≥ 0.3mm (12mil)
Higher density :
PAD and VIA: ≥ 0.254mm (10mil)
PAD and PAD: ≥ 0.254mm (10mil)
PAD and TRACK: ≥ 0.254mm (10mil)
TRACK and TRACK: ≥ 0.254mm (10mil)
Fifth : wiring optimization and printing . "There is no best , only better !" No matter how you rack their brains to design, etc. After you finish the painting , go take a look , still feel a lot of places that can be modified . Experience in general design are: optimizing routing time is twice the initial wiring time . I feel no need to modify the following places , you can shop the copper (Place-> polygon Plane). General Shop Shop copper ground wire ( Note that the analog and digital ground separation ) , plywood may also involve laying off. When the screen , be careful not to be blocked by the device or vias and pads removed. Meanwhile, the design element face to face , mirroring the underlying word processing should be done to avoid confusion levels.
Sixth : Network and DRC checks and structural inspection . First, in determining circuit schematic design premise is correct , the generated PCB schematics network file and check the physical network connection for network file relationships (NETCHECK), and timely correction of design based on the results of the output file to ensure that wiring connection correctness relations ;
After checking the correct network through , for PCB design DRC, and timely correction of design based on the results of the output file to ensure that the performance of PCB electrical wiring . Finally, the mechanical mounting structure for further inspection and confirmation of the PCB .
Seventh : plate. Prior to this, there is also preferably an audit process.
PCB design is a work of reference mind , who thought dense, high experience, designed like a board . So be extremely careful design , fully consider all aspects of the factors ( for example, ease of maintenance and inspection which a lot of people do not go to be considered ) , excellence , we will be able to design a good board .
