Very powerful PCB wiring technology

First:Preparations. This includes preparing the component library and schematic. "To do a good job, you must first sharpen the tool." To make a good board, you must draw a good picture in addition to designing the principles. Before designing a PCB, you must first prepare the component library of the schematic SCH and the component library of the PCB. You can use the library that comes with peotel, but it is difficult to find a suitable library. It is best to make the library based on the standard size of the selected device. In principle, the PCB Component Library and SCH component library are created first. The Component Library of PCB has high requirements, which directly affects the installation of the Board. The component library requirements of SCH are relatively loose. Just pay attention to defining the pin attributes and the relationship with the PCB components. PS: note the hidden pins in the standard library. After that, we will design the schematic diagram. After that, we will prepare for the PCB design.

Second:PCB structure design. This step draws the PCB Surface in the PCB design environment based on the determined circuit board size and various mechanical positioning, and place the required connectors, buttons/switches, screw holes, assembly holes and so on according to positioning requirements. And fully consider and determine the wiring area and non-wiring area, such as the scope around the screw hole is not wiring area ).

Third:PCB layout. In other words, the layout is to place the device on the board. In this case, if you have prepared all the preparations, you can generate a network Table Design> CreateNetlist on the schematic diagram, and then import the network Table Design> LoadNets on the PCB diagram ). We can see that all the devices are stacked up, and there is a flying line between each pin to prompt the connection. Then you can layout the device. The general layout follows the following principles:

①. Reasonably partitioned by electrical performance, generally divided into: digital circuit area (for fear of interference, and interference), analog circuit area (for fear of interference), power driving area interference source );

②. The circuit that completes 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 positions between functional blocks to make the connection between functional blocks the most concise;

③ For high-quality components, the installation position and installation strength should be taken into account; the heating elements should be separated from the temperature-sensitive components, and heat convection measures should also be taken into account when necessary;

④. I/O drive should be close to the side of the printed Board and to the egress plug-in;

⑤. The clock generator, such as crystal oscillator or clock vibration, should be as close as possible to the device that uses the clock;

6. when a decoupling capacitor is required between the power input foot and the ground of each integrated circuit, the high-frequency single-rock capacitor is usually used. When the space of the circuit board is relatively dense, you can also add a TA capacitor around several integrated circuits.

7. Discharge diode 1N4148 must be added to the relay coil );

Routing. The layout should be balanced, dense and orderly, and should not be heavy or heavy

-- Pay special attention to the fact that when placing components, the actual size and height of components must be taken into account, in order to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation, while ensuring that the above principles can be reflected, the placement of the device should be properly modified to make it neat and beautiful, for example, the same devices must be placed neatly and in the same direction, so they cannot be placed incorrectly ".

This step is related to the overall image of the board and the difficulty of wiring the next step, so it takes a lot of effort to consider it. During layout, you can make a preliminary wiring for some areas that are not certain, and make full consideration.

Fourth: Wiring. Wiring is the most important process in PCB design. This will directly affect the performance of PCB. In the PCB design process, there are generally three levels of wiring: first, wiring, then the most basic requirements for PCB design. If the line is not deployed and the result is a flying line, it will be an unqualified board. It can be said that there is no entry level. The second is the performance of electrical appliances. This is a measure of whether a printed circuit board is qualified. After the wiring is completed, adjust the wiring carefully so that it can achieve the best electrical performance. Then there is beautiful. If your wiring passes, there will be no impact on the electrical performance of the appliance, but with a glance at the mess in the past, and the colorful and colorful, green, then even if your electrical performance is good, in the eyes of others, it is still a piece of garbage. This will cause great inconvenience to testing and maintenance. The wiring should be uniform, and there is no way to do it. These must be achieved when the electrical appliance performance is guaranteed and other individual requirements are met. Cabling is performed based on the following principles:

①. In general, the power cord and ground wire should be first wired to ensure the electrical performance of the circuit board. In the range allowed by the conditions, the power supply and ground wire width should be widened as much as possible. It is better that the ground wire is wider than the power cord. Their relationship is: Ground Wire> power cord> signal line. Generally, the signal line width is 0.2 ~ 0.3mm. The shortest width can reach 0.05 ~ 0.07mm, power cord is generally 1.2 ~ 2.5mm. A circuit is formed for the PCB of a digital circuit, that is, the ground that forms a ground network to use a analog circuit cannot be used in this way)

②. Wiring of lines with strict requirements, such as high-frequency lines, should be performed in advance. The edge lines at the input end and the output end should be parallel to avoid reflection interference. Ground Line isolation should be applied when necessary. The wiring of two adjacent layers must be perpendicular to each other, and parallel cables are prone to parasitic coupling.

③. The oscillator housing is grounded, and the clock line should be as short as possible and cannot be cited everywhere. The area of the special high-speed logical circuit under the clock oscillating circuit must be added to the earth, rather than other signal lines, so that the surrounding electric field approaches zero;

④. Use line cabling of 45o as much as possible. Do not use line cabling of 90o to reduce the radiation of high-frequency signals. Dual Arc must be used for lines with high requirements)

⑤ Do not form a loop for any signal line. If it is inevitable, the loop should be as small as possible; the signal line should be as few as possible through holes;

6. The key lines should be as short and rough as possible, and the protected area should be added on both sides.

7. When a flat cable is used to transmit sensitive and noisy field band signals, it must be obtained using the "ground-signal-ground" method.

Testing points should be reserved for key signals to facilitate production and maintenance testing

Invalid. after the schematic wiring is completed, the wiring should be optimized. At the same time, after the preliminary network check and DRC check are correct, the uncabled area should be ground filled and a large area copper layer should be used as the ground wire, on the Printed Board, the unused parts are connected to the ground as the ground. Or make a multi-layer board, power supply, ground each occupies a layer.

-- PCB wiring process requirements

①. Line

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 the wire and the pad is greater than or equal to 0.33mm (13mil). In practice, increase the distance when conditions permit;

When the wiring density is high, consider but not recommended) use the IC foot to walk two lines, the line width is 0.254mm (10mil), the line spacing is not less than 0.254mm (10mil ). In special cases, when the device pin is relatively dense and the width is narrow, the line width and line spacing can be appropriately reduced.

2. PAD)

PAD) and transition hole VIA) The basic requirements are: the disk diameter is greater than 0.6mm than the hole diameter; for example, general pin type resistance, capacitance and integrated circuit, etc, use Disk/hole size 1.6/0.8mm63mil/32mil), socket, plug-in, diode 1N4007, etc., with 1.8/1.0mm71mil/39mil ). In actual application, the size of the pad should be determined based on the size of the actual component. when conditions are met, the size of the pad can be appropriately increased;

The size of the component installation aperture designed on the PCB must be 0.2 ~ larger than the size of the component pins ~ About 4mm.

③.)

Generally 1.27/0.7 (50mil/28mil );

When the wiring density is high, the passing hole size can be appropriately reduced, but not too small. 1.0/0.6 (40mil/24mil) can be considered ).

④. Welding pad, wire, and pass-through spacing requirements

PADandVIA: ≥0.3mm12mil)

PADandPAD: ≥0.3mm12mil)

PADandTRACK: ≥0.3mm12mil)

TRACKandTRACK: ≥0.3mm12mil)

When the density is high:

PADandVIA: ≥0.254mm10mil)

PADandPAD: ≥0.254mm10mil)

PADandTRACK: ≥0.254mm10mil)

TRACKandTRACK: ≥0.254mm10mil)

Fifth:Wiring optimization and silk screen printing. "No best, only better "! No matter how hard you design it, after you finish the painting, you will still feel that many places can be modified. The general design experience is that the cabling optimization time is twice the initial cabling time. I feel that there is no need to modify it, so I can lay the copper Place-> polygonPlane ). Lay copper usually lay the ground line to pay attention to the analog ground and digital ground separation), the multi-layer board may also need to lay the power. For Silk Screen, be sure not to be blocked by the device or be removed by the through hole and pad. At the same time, the design should face up to the component surface, and the underlying words should be mirrored to avoid confusion.

Sixth:Network and DRC check and structure check. First, check the physical connection relationship between the generated PCB network file and the schematic network file on the premise that the circuit schematic design is correct. NETCHECK ), correct the design in time based on the output file results to ensure the correctness of the wiring connection relationship;

After the network check is passed correctly, perform a DRC check on the PCB design and correct the design according to the output file results in time to ensure the electrical performance of the PCB wiring. Finally, the mechanical installation structure of PCB should be further checked and confirmed.

Seventh: Plate. Before that, it is best to have a review process.

PCB design is a kind of hard work. It is easy to design a board because of its strong thinking and experience. Therefore, we should be extremely careful when designing, taking into full consideration the factors of various aspects such as convenience of maintenance and inspection, which many people will not consider), and strive for perfection, we will be able to design a good board.

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