PCB design is very important for power circuit design, and it is also one of the necessary technologies for beginners. In this article, Xiaobian will share some highlights about PCB design.
PCB structure design
In this step, according to the determined circuit board size and various mechanical positioning, the PCB board surface is drawn in the PCB design environment, and the required connectors, keys/switches, screw holes, assembly holes and so on are placed according to the positioning requirements. And fully consider and determine the wiring area and the non-wiring area (such as how far around the screw hole belongs to the non-wiring area).
PCB layout
① According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (i.e. fear of interference and interference), analog circuit area (fear of interference) and power drive area (interference source);
② The circuits with the same function should be placed as close as possible, and the components should be adjusted to ensure the most concise connection; At the same time, adjust the relative position of each functional block to make the connection between functional blocks the most concise;
③ For components with high quality, the installation position and intensity should be considered; Heating elements should be placed separately from temperature sensitive elements, and heat convection measures should be considered when necessary;
④I/O drive device should be as close to the edge of the printed board and the lead-out connector as possible;
⑤ The clock generator (such as crystal oscillator or clock oscillator) should be as close as possible to the device using the clock;
⑥ A decoupling capacitor should be added between the power input pin of each integrated circuit and the ground (generally, a monolithic capacitor with good high frequency performance is used); When the circuit board space is dense, a tantalum capacitor can be added around several integrated circuits.
⑦ Add a discharge diode (1N4148) at the relay coil;
End layout requirements should be balanced, dense and orderly, not top-heavy or heavy.
Special attention should be paid to the fact that when placing components, the actual size (occupied area and height) of components and the relative position between components must be considered to ensure the electrical performance of circuit boards and the feasibility and convenience of production and installation. At the same time, on the premise that the above principles can be embodied, the placement of devices should be properly modified to make them neat and beautiful. For example, the same devices should be placed neatly and in the same direction, and should not be “patchy”.
This step is related to the overall image of the board and the difficulty of wiring in the next step, so we should make great efforts to consider it. When layout, you can make preliminary wiring for places that are not sure, and give full consideration to it.
Wiring is the most important process in the whole PCB design. This will directly affect the performance of PCB. In the process of PCB design, wiring is generally divided into three levels: first, routing, which is the most basic requirement of PCB design. If the lines are not routed and flying wires are everywhere, it will be an unqualified board, so to speak, it has not been started yet.
Secondly, the satisfaction of electrical performance. This is the standard to measure whether a printed circuit board is qualified or not. This is to carefully adjust the wiring after wiring, so that it can achieve the best electrical performance.
Then there is beauty. If your wiring is well-laid, there is nothing that will affect the performance of electrical appliances. But if you look at the past in disorder, plus colorful and colorful, then even if your electrical appliances have good performance, they will still be rubbish in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring should be uniform, not crisscross and disorganized. All these should 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.
Wiring is mainly carried out according to the following principles:
① In general, the power cord and ground wire should be wired first to ensure the electrical performance of the circuit board. Within the permitted range, try to widen the width of power supply and ground wire, preferably the ground wire is wider than the power wire. Their relationship is: ground wire > power wire > signal wire. Usually, the signal line width is 0.2 ~ 0.3 mm, the thinnest width can reach 0.05～0.07mm, and the power wire is generally 1.2 ~ 2.5 mm. For the PCB of digital circuit, a wide ground wire can be used to form a loop, that is, a ground net can be used (the ground of analog circuit cannot be used in this way).
②PCB wiring process requirements:
Generally, the signal line width is 0.3mm(12mil) and the power line width is 0.77mm(30mil) or 1.27mm(50mil). The distance between wire and wire and between wire and pad is greater than or equal to 0.33mm(13mil). In practical application, the distance should be increased when conditions permit;
When the wiring density is high, it may be considered (but not recommended) to use two wires between IC pins, with the width of the wires being 0.254mm(10mil) and the spacing between the wires not less than 0.254mm(10mil). Under special circumstances, when the device pins are dense and the width is narrow, the line width and line spacing can be appropriately reduced.
The basic requirements of PAD and VIA are: the diameter of pad is larger than that of via by 0.6mm；; For example, general pin resistors, capacitors, integrated circuits, etc., adopt 1.6mm/0.8mm(63mil/32mil) disk/hole size, socket, pin and diode 1N4007, etc., and adopt 1.8mm/1.0mm(71mil/39mil). In actual use, it should be determined according to the size of the actual components. When conditions permit, the pad size can be appropriately increased; The mounting aperture of components designed on PCB should be about 0.2 ~ 0.4 mm larger than the actual size of component pins.
③ VIA: generally 1.27mm/0.7mm (50mil/28mil); When the wiring density is high, the via hole size can be appropriately reduced, but it should not be too small, and 1.0mm/0.6mm(40mil/24mil) can be considered.
④ Spacing requirements of pads, wires and vias
PAD and VIA ：≥ 0.3mm(12mil)
PAD and PAD ：≥ 0.3mm(12mil)
PAD and TRACK ：≥ 0.3mm(12mil)
TRACK and TRACK ：≥ 0.3mm(12mil)
When the density is high:
PAD and VIA ：≥ 0.254mm(10mil)
PAD and PAD ：≥ 0.254mm(10mil)
PAD and TRACK ：≥ 0.254mm(10mil)
TRACK and TRACK ：≥ 0.254mm(10mil)
Routing optimization and silk screen printing
“There is no best, only better”! No matter how hard you try to design, after you finish painting, you will still feel that there are many places that can be modified. The general design experience is that the time to optimize the wiring is twice that of the initial wiring.
After feeling that there is nothing to be modified, you can lay copper (Place->polygon Plane). Generally, copper is used to lay the ground wire (pay attention to the separation of analog ground and digital ground), and power supply may also be needed for multilayer boards. For screen printing, it should be noted that it cannot 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 at the bottom should be mirrored to avoid confusing the layers.
And network DRC check and structure check.
Firstly, on the premise that the circuit schematic design is correct, the generated PCB network file and the schematic network file are NETCHECK, and the design is corrected in time according to the output file results to ensure the correctness of the wiring connection relationship; After the network check is passed correctly, the PCB design will be checked by DRC, and the design will be corrected in time according to the output file results to ensure the electrical performance of PCB wiring. Finally, the mechanical installation structure of PCB should be further checked and confirmed.
plating
① Before that, it is better to have an audit process.
PCB design is a thoughtful job. Whoever has a dense mind and a high experience will design the board. Therefore, when designing, we should be extremely careful, give full consideration to various factors (for example, many people don’t think about the convenience of maintenance and inspection), and strive for perfection, and we will certainly be able to design a good board.
② Wire the lines with strict requirements (such as high frequency lines) in advance, and the edges of the input end and the output end should be avoided to be adjacent and parallel, so as to avoid reflection interference. When necessary, ground wire should be added for isolation. The wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling will easily occur when parallel.
③ The oscillator shell is grounded, and the clock line should be as short as possible, and can’t be attracted everywhere. Under the clock circuit, the special high-speed logic circuit should increase the ground area, instead of other signal lines, so that the surrounding electric field approaches zero.
④ As far as possible, a 45-degree broken line should be used instead of a 90-degree broken line to reduce the radiation of high-frequency signals; (Double arcs should be used for demanding lines);
⑤ Do not form a loop in any signal line. If it is inevitable, the loop should be as small as possible; The number of signal line vias should be as small as possible;
⑥ The key line should be as short and thick as possible, and protected areas should be added on both sides;
⑦ When transmitting sensitive signals and noise field signals through flat cables, they should be led out in the form of “ground wire-signal-ground wire”;
⑧ Test points should be reserved for key signals to facilitate production and maintenance testing;
Pet-name ruby schematic wiring is completed, the wiring should be optimized; At the same time, after the preliminary network inspection and DRC inspection are correct, the ungrounded areas will be filled with ground wires, and a large area of copper layer will be used as ground wires, and all unused places on the printed board will be connected to the ground as ground wires. Or it can be made into a multi-layer board, with one layer for power supply and one layer for ground wire.
The steps of PCB design, layout, routing analysis and optimization all require long-term experience accumulation to achieve a design overall view.