4). Layer settings.
(1) According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (that is, fear of interference and interference), analog circuit area (fear of interference), and power drive area (interference source);
(2) The circuit that completes the same function should be placed as close as possible, and each component should be adjusted to ensure the simplest connection; at the same time, the relative position between each functional block should be adjusted to make the connection between the functional blocks the simplest;
(3) For components with large mass, the installation position and installation strength should be considered; heating components should be placed separately from temperature sensitive components, and heat convection measures should also be considered if necessary;
(4) The /0 drive device should be as close to the edge of the printed board as possible, and close to the lead-out connector;
(5) The clock generator (such as: crystal oscillator or clock oscillator) should be as close as possible to the device that uses the clock;
(6) Between the power input pin of each integrated circuit and the ground, a decoupling capacitor (generally a monolithic capacitor with good high-frequency performance) is required; Add a button capacitor around it.
(8) The layout requirements should be balanced, dense and orderly, not top-heavy or heavy
Special attention should be paid to the actual size (area and height) of the components when placing them. The relative position between components is to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, on the premise of ensuring that the above principles can be reflected, the placement of the components should be properly modified to make it neat and beautiful, such as The same devices should be placed neatly and in the same direction, and cannot be placed in a "patchwork". This step is related to the overall image of the board and the difficulty of the next step of wiring, so it takes a lot of effort to consider. When laying out, you can make preliminary wiring for places that are not sure, and give full consideration.
Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board. In the PCB design process, the wiring generally has such three levels of division: the first is the routing, which is the most basic requirement for PCB design. If the lines are not connected and there are flying wires everywhere, it will be a substandard board, and it can be said that it has not been started yet.
The second is the satisfaction of electrical performance. This is the standard to measure whether a printed circuit board is qualified. This is after Bouton, carefully adjust the wiring so that it can achieve the best electrical performance. Then there is beauty. If your wiring is clear, there is nothing that affects the performance of the electrical appliances, but at a glance, it is messy, colorful and colorful, then no matter how good your electrical appliances are, it is still a piece of garbage in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, not criss-cross and disorderly. All of these must be realized under the condition of ensuring the performance of electrical appliances and meeting other individual requirements, otherwise it will be a waste of money.
The wiring is mainly carried out according to the following principles:
(1) In general, the power line and ground wire should be wired first to ensure the electrical performance of the circuit board. As far as conditions permit, try to widen the width of the power supply and ground wires. It is best that the ground wires are wider than the power wires. Their relationship is: ground wire>power wire>signal wire, and the signal wire width is usually: 0.2~0.3mm (about 8-12mil), the thinnest width can reach 0.05~0.07mm (2-3mil), and the power cord is generally 1.2~2.5mm (50-100mil). For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground network (the ground of the analog circuit cannot be used in this way).
(2) Pre-wire the lines with strict requirements (such as high-frequency lines), and the side lines of the input end and the output end should avoid being adjacent to each other in parallel to avoid reflection interference. If necessary, ground isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.
(3) The shell of the oscillator is grounded, and the clock line should be as short as possible, and cannot be drawn everywhere. Under the clock oscillation circuit and the special high-speed logic circuit, the area of the ground should be increased, and other signal lines should not be used to make the surrounding electric field approach zero;
(4) Use 45° broken line wiring as much as possible, and do not use 90° broken line to reduce the radiation of high-frequency signals; (the line with high requirements should also use double arc lines)
(5) Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible; the vias of the signal line should be as few as possible;
(6) The key line should be as short and thick as possible, and protective ground should be added on both sides.
(7) When transmitting sensitive signals and noise field signals through flat cables, use the "ground wire-signal-ground wire" method to lead out.
(8) Test points should be reserved for key signals to facilitate production and maintenance testing
(9) After the wiring of the schematic diagram is completed, the wiring should be optimized; at the same time, after the preliminary network inspection and DRC inspection are correct, the ground wire is filled in the
unwired area, and a large area of copper layer is used as the ground wire. Connect the unused places to the ground as the ground wire. Or make a multi-layer board, the power supply and the ground wire each occupy one layer. PCB layout process requirements (can be set in the rules)
(1) Under normal circumstances, the signal line width is 0.3mm (12mil), and the power line width is 0.77mm (30mil) or
1.27mm (50mil); the distance between the line and the line and between the line and the pad is greater than or equal to 0.33mm (13mil). In practical applications, when conditions permit, the distance should be considered. When the wiring density is high, it can be considered ( But it is not recommended) Use two wires between the IC pins, the width of the wires is 0.254mm (10mil), and the distance between the wires is not less than 0.254mm (10mil). In special cases, when the device pins are denser and the width is narrower, the line width and line spacing can be appropriately reduced.
(2) The basic requirements of the pad (PAD) and the transition hole (VIA) are: the diameter of the pad is greater than the diameter of the hole by 0.6mm; for example, general-purpose pin-type resistors, capacitors and integrated circuits, etc., use Disk/hole size 1.6mm/0.8mm (63mil/32mil), socket, pin and diode 1N4007, etc., use 1.8mm/1.0mm (71mil/39mil). In practical applications, it should be determined according to the size of the actual component, there are If necessary, the size of the pad can be appropriately increased; the component mounting aperture designed on the PCB should be about 0.2~0.4mm (8-16mil) larger than the actual size of the component pin.
(3) The via hole (VIA) is generally 1.27mm/0.7mm (50mil/28mil); when the wiring density is high, the size of the via hole can be appropriately reduced, but it should not be too small, and 1.0mm/0O.6mm can be considered (40mil/24mil).
(4) Spacing requirements for pads, lines, and vias PAD and VIA: z 0.3mm (12mil) PAD and PAD: ≥ 0.3mm (12mil PAD and TRACK: ≥0.3mm (12mil) TRACKand TRACK: 2 0.3mm (12mil ）When the density is high: PAD and VIA: 20.254mm (10mil) PAD and PAD: 20.254mm (10mil)PAD and TRACK: ≥ 0.254mm (10mil)TRACKand TRACK: ≥0.254mm (10mil)
7: Routing optimization and silk screen
"There is no best, only better"! No matter how hard you try to design, after you finish the drawing, you will still feel that there are many places that can be modified. The experience of general design is: the time of optimizing wiring is twice of the time of initial wiring. After feeling that there is nothing to modify, you can lay copper. Copper laying generally lays ground wires (note the separation of analog ground and digital ground), and power supply may also be laid when multi-layer boards are used. For silk screen printing, be careful not to be blocked by devices or removed by vias and pads. At the same time, when designing, face up to the component surface, and the words on the bottom layer should be mirrored to avoid confusing the layer.
8: Network · DEC inspection and structure inspection
Before making light painting, it generally needs to be checked, and each company will have its own Check List, which includes the requirements of principles, design, production and other links. The following is an introduction from the two main inspection functions provided by the software. DRC check:
9: Output light painting
It is necessary to ensure that the veneer is the latest version that has been completed and meets the design requirements before the light drawing output. The light drawing output files are used for board making in the board factory, steel mesh in the stencil factory, and process documents in the welding factory.
The output files are (take the four-layer board as an example):
1) Wiring layer: refers to the conventional signal layer, mainly wiring. Named L1, L2, L3, L4, where L represents the layer of the wiring layer.
2) Silk screen layer: refers to the layer in the design document that provides information for processing silk screens. Usually, when there are devices or logos on the top and bottom layers, there will be top and bottom silk screens. Naming: The top layer is named SILK_TOP; the bottom layer is named SILK_BOTTOM.
3）Solder mask layer: refers to the layer in the design document that provides processing information for green oil coating. Naming: The top layer is named SOLD_TOP; the bottom layer is named SOLD_BOTTOM.
4) Stencil layer: refers to the layer in the design document that provides processing information for tin coating. Usually, when there are SMD devices on the top and bottom layers, there will be a top layer of stencil and a bottom layer of stencil. Naming: The top layer is named PASTE_TOP; the bottom layer is named PASTE_BOTTOM.
5) Drilling layer (including 2 files, NC DRILL numerical control drilling file and DRILL DRAWING drilling map) named NCDRILL and DRILL DRAWING respectively.
10: Light Painting Review
After the light painting is output, inspection of the light painting, Cam350 open and short circuit and other inspections must be carried out before it can be sent to the board factory for board manufacturing. In the later stage, it is necessary to pay attention to the board manufacturing project and answer questions.
11 : PCB board making data (Gerber light drawing data + FCB board making requirements + jigsaw puzzle)
12: FCE board factory project Q confirmation (board making project and question reply)
13: PCEA patch data output (stencil data, patch bitmap, component coordinate file)
At this point, all the workflow of the FCB design of a project is completed
PCB design is a very meticulous work, so the design must be extremely careful and patient, and fully consider various factors, including production, assembly and processing, and ease of maintenance in the later stage. In addition, developing some good working habits during design will make your design more reasonable, more efficient, easier to produce, and better in performance. When good design is applied to daily products, consumers will feel more assured and trustworthy.
