PCB Tech - PCB process General steps for drawing PCB

PCB process General steps for drawing PCB

Each time you design a PCB, you should proceed in the following order, so you can save time and get the best results.

1. Choose the name of SCH, PCB and other files (in English, numbers), and add the extension.

2. Schematic
First design the size of the deleted grid, the size of the drawing, select the metric system, and add the library components. Draw the diagrams, components, and lines according to the circuit function modules so that people can easily see the principles. Try to be even and beautiful. Do not run wires inside the components. Be careful not to run wires between the pins, because there is no electrical connection. It is best not to connect the two component pins directly. After drawing, you can automatically number (except for special requirements), and then add the corresponding nominal value. It is best to change the nominal value to red and bold, so that it can be distinguished from the label. open. It is best to put the label and the nominal value in a suitable place. Generally, the left side is the label and the right side is the nominal value, or the upper side is the label, and the lower side has no nominal value. Habitual preservation during the process! First, ensure that the schematic diagram is completely correct, perform ERC check without error, and then print it for verification. Secondly, it is best to figure out the circuit principle, for high and low voltage; large and small currents; analog and digital; large and small signals; large and small power blocks, so as to facilitate the layout later.

3. Make PCB component library
For the production of component packages that are not available in the standard library and your own common library, pay attention to drawing a top view, paying attention to the size, pad size, location, number, inner hole size, direction, (printing method and size). The name is in English, which is easy to read. It is better to have the corresponding size so that you can find it when you use it next time (you can save it in the form of a table corresponding to the name and the corresponding size). For commonly used diodes, transistors should pay attention to the labeling method. It is best to have commonly used series of diodes in your own library, such as 9011-9018, 1815, D880, etc. For light-emitting diodes, LED, RAD0.1, RB.1/.2, and other commonly used component packages that are not available in the standard library, they should all be in their own library. You should be familiar with the sealed forms of commonly used components (resistors, capacitors, diodes, and triodes).

4. Generate the network table
Add package, save, ERC check in the schematic diagram, generate component list check. Generate a netlist.

5. Build the PCB
Choose the metric system, capture and visible grid size, design the outer frame as required (guide or draw by yourself), and then place the position and size of the fixed hole (3.0mm screws can use 3.5mm inner hole pads, 2.5mm You can use the inner hole of 3), first change the pad on the edge, the size of the hole, and the position are fixed. Add the libraries you need to use.

6. Layout
Call the netlist, load the components, modify the size of some pads, set the wiring rules, you can change the size, thickness, and hide the nominal value of the label. Then first put the components that need special positions and set them up. Then according to the layout of the functional module, (you can use the SCH to select the way to select the PCB), generally do not use X, Y to flip the components, but use the space to rotate, or the L key, (because some components cannot be flipped, Such as integrated blocks, relays, etc.). For a functional module, put the central component or the large component first, and then put the small component next to it. (For example, put the integrated block first, and then put the components directly connected to the two pins of the integrated block, and then put the components connected to one pin of the integrated block, and put similar components together as much as possible, and it is more beautiful to consider the subsequent wiring Convenience). Of course, some special relational components are put first, such as some filter capacitors and crystal oscillators, which need to be placed close to some components first. There are also components that will interfere with the overall consideration to be far away. The high and low voltage modules should be spaced at least 6.4mm apart. Pay attention to the position of the heat sink, connectors, and fixing frame. FILL can be used in some places that cannot be wired. Also consider heat dissipation, heat-sensitive components. Resistor and diode placement methods: divided into two types: horizontal placement and vertical placement.

(1) Horizontal placement: When the number of circuit components is small and the size of the circuit board is large, it is generally better to use horizontal placement; for resistors below 1/4W, the distance between the two pads is general Take 4/10 inches, when the 1/2W resistor is placed flat, the distance between the two pads is generally 5/10 inches; when the diode is placed flat, 1N400X series rectifier tubes, generally take 3/10 inches; 1N540X series rectifier tubes, generally Take 4 to 5/10 inches.

(2) Vertical installation: When there are a large number of circuit components and the size of the circuit board is not large, the vertical installation is generally adopted, and the distance between the two pads is generally 1 to 2/10 inches in the vertical installation.

7. Wiring
First set the content in the rules, VCC, GND and other high current lines can be set wide points (0.5mm-1.5mm), generally 1mm can pass 1A current. For large voltage line spacing, you can set a larger point, generally 1mm is 1000V. Once the settings are complete, first route some important lines such as VCC and GND. Pay attention to the distinction between modules. It is best to add some lines to a single panel. Add vias, not necessarily horizontal and vertical. Generally, there is no wiring between the pads of the integrated block. Wide lines with high current can be drawn on the solder layer for tinning on the back; the wiring uses a 45-degree angle

8. Manually modify the line
Modify the width, corners of some lines, fill the tear ground or wrap the land (single-sided board must be done), lay copper, and deal with the ground wire.

9. Check
DRC, EMC and other inspections can then be printed for inspection and network table comparison. Check the component list.

10. Plus model (usually made of silk screen)

11. The adjustment of the potentiometer is generally clockwise to increase (voltage, current, etc.)

12. High frequency (>20MHz) is generally grounded at multiple points. <10MHz or <1MHz single point grounding. In the meantime, it is mixed grounding.

13. According to needs, not all devices need to be packaged according to the standard, which can be jumper or vertical welding.

14. When wiring the printed circuit board, the circuit board manufacturer should first determine the position of the components on the board, and then lay out the ground wire and the power wire. When arranging high-speed signal lines, it is best to consider low-speed signal lines. The position of vitality parts is grouped according to power supply voltage, digital simulation, speed, current size, etc. Under safe conditions, the power cord should be as close to the ground as possible. Reducing the loop area of differential radiation also helps to reduce the crosstalk of the circuit. When it is necessary to arrange fast, medium-speed, and low-speed logic circuits on the circuit board, the high-speed logic circuit should be placed close to the edge connector, and the low-speed logic and memory should be placed far away from the connector. This is beneficial to the common impedance coupling, the reduction of radiation and crosstalk. Grounding is the most important thing. It is necessary to have a backup at almost the same time, or some steps are prone to crashes, and backups are required when files are damaged.