General PCB basic design process is as follows:

Preliminary preparation →PCB structure design → guide list → rule setting →PCB layout → wiring → wiring optimization and silk screen → network and DRC check and structure check → output light drawing → light drawing review →PCB board production/proofing data →PCB board factory project EQ confirmation → patch data output → project completion.

1: Preparation

This includes preparing package libraries and schematics. Before PCB design, we should first prepare the logic package of schematic SCH and the package library of PCB. Package libraries can come with PADS, but it is difficult to find suitable libraries in general. It is best to make your own package libraries according to the standard size information of the selected devices. In principle, the PCB packaging library should be done first, and then the SCH logic packaging should be done. PCB packaging library has high requirements, which directly affects the board installation; SCH logical packaging requirements are relatively loose, as long as the definition of pin attributes and the corresponding relationship with PCB packaging on the line. PS: Note the hidden pins in the standard library. Then is the schematic design, ready to do PCB design.

2. PCB structure design

In this step, according to the circuit board size and mechanical positioning, PCB board surface is drawn in the PCB design environment, and connectors, buttons/switches, screw holes, assembly holes and so on are placed according to positioning requirements. And fully consider and determine the wiring area and non-wiring area (such as how much of the screw hole around the non-wiring area).

3: guide network table

It is recommended to route the net table into the board frame first. Import a board enclosure in DXF format or EMN format

4: Rule setting

Reasonable rules can be set according to the specific PCB design. These rules are PADS constraint managers, which can be used to restrict line width and safe spacing at any point in the design process. Non-conforming areas are marked by DRC Markers during SUBSEQUENT DRC testing.

The general rule setting is placed before the layout, because sometimes some fanout work needs to be completed during the layout, so the rules should be set well before the FANout. When the design project is larger, the design can be completed more efficiently. Note: rules are set for better and faster design, in other words, for the convenience of designers. Common Settings are: 1. Default line width/line spacing for common signals. Select and set the hole. 3. Set the line width and color of important signals and power supplies. 4. Board layer Settings.

5: PCB layout

Need to pay special attention to, in place of components, components should be considered when the actual size (in the area and height) and the relative position between the components, to ensure that the electrical properties and production of circuit board installation convenience and feasible sex at the same time, should be on the premise of guarantee the above principle to reflect, appropriate change device, make it tidy and beautiful, For example, the same device should be placed neatly and in the same direction, not “strewn at random”. This step concerns the difficulty of board integral figure and next wiring degree, want to spend big effort to consider so. When layout, can make preliminary wiring first to not quite affirmative place, sufficient consideration.

6: wiring

Wiring is the most important process in PCB design. This will directly affect the performance of PCB board. In the process of PCB design, wiring generally has such three levels of division: the first is the distribution, which is the most basic requirement of PCB design. If the line is not cloth, get everywhere is flying line, it will be a unqualified board, can say that there is no entry.

The second is the satisfaction of electrical performance. This is the standard to measure whether a printed circuit board is qualified. This is after the distribution, carefully adjust the wiring, so that it can achieve the best electrical performance. Then there is aesthetics. If your wiring cloth was connected, also do not have the place that what affects electric appliance performance, but look past desultorily, add colourful, brightly colored, that calculates how your electric appliance performance is good, still be rubbish in others eye. This brings great inconvenience to testing and maintenance. Wiring should be neat and uniform, not crisscross without rules. All these should be achieved in the context of ensuring electrical performance and meeting other individual requirements, otherwise it is to abandon the essence.

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. In the scope of conditions allow, as far as possible to widen the width of power supply, ground wire, the best ground wire is wider than the power line, their relationship is: ground wire > power line > signal line, usually signal line width is: 0.2 ~ 0.3mm (about 8-12mil), the narrowest width up to 0.05 ~ 0.07mm (2-3mil), the power cord is generally 1.2 ~ 2.5mm (50-100mil). The PCB of a digital circuit can be used as a circuit with wide ground conductors, that is, a ground network (analog circuit ground cannot be used in this way). (2) in advance to the more strict requirements of the line (such as high frequency line) wiring, input and output side line should avoid adjacent parallel, so as not to produce reflection interference. When necessary, ground wire should be added to isolate, and the wiring of two adjacent layers should be perpendicular to each other, which is easy to produce parasitic coupling in parallel. (3) the oscillator shell is grounded, and the clock line should be as short as possible, and it can’t be everywhere. Below the clock oscillation circuit, the special high-speed logic circuit should increase the area of the ground, and should not go to other signal lines, so that the surrounding electric field tends to zero;

(4) Use 45° broken line wiring as far as possible, not 90° broken line, in order to reduce the radiation of high-frequency signal; (5) Any signal line should not form a loop, if unavoidable, the loop should be as small as possible; Signal line through the hole should be as little 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, it is necessary to use the way of “ground line – signal – ground line”. (8) Test points should be reserved for key signals to facilitate production and maintenance testing. (9) After schematic wiring is completed, wiring should be optimized; At the same time, after the preliminary network check and DRC check is correct, the ground wire is filled in the area without wiring, and a large area of copper layer is used as ground wire, and the unused places are connected with the ground as ground wire on the printed board. Or make it multi-layer board, power supply, grounding line each occupy a layer.

(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 wire and wire and between wire and pad should be greater than or equal to 0.33mm(13mil). In practical application, it should be considered to increase the distance when conditions permit; When the cabling density is high, it is advisable (but not recommended) to use two cables between IC pins. The width of the cables is 0.254mm(10mil), and the distance between the cables is not less than 0.254mm(10mil). Under special circumstances, when the pin of the device is dense and the width is narrow, the line width and line spacing can be appropriately reduced. (2) PAD (PAD) PAD (PAD) and transition hole (VIA) the basic requirements are: the diameter of the disk than the diameter of the hole is greater than 0.6mm; For example, universal pin type resistors, capacitors and integrated circuits, using disk/hole size 1.6mm /0.8mm (63mil/32mil), socket, pin and diode 1N4007, using 1.8mm/1.0mm (71mil/39mil). In practical application, it should be determined according to the size of the actual components. If conditions are available, the size of the pad can be appropriately increased. The installation aperture of the components designed on the PCB should be about 0.2 ~ 0.4mm (8-16mil) larger than the actual size of the pins of the components. (3) The perforation (VIA) is generally 1.27mm/0.7mm(50mil/28mil); When the wiring density is high, the hole size can be appropriately reduced, but not too small, can consider 1.0mm/0.6mm(40mil/24mil). PAD and VIA: ≥ 0.3mm (12mil) PAD and PAD: ≥ 0.3mm (12mil) PAD and TRACK: ≥ 0.3mm (12mil) TRACK and TRACK: ≥ 0.3mm (12mil) ≥ 0.3mm (12mil) PAD and VIA: ≥ 0.254mm (10mil) PAD and TRACK: ≥ 0.254mm (10mil) PAD and TRACK: ≥ 0.254mm (10mil) TRACK and TRACK: ≥ 0.254mm (10mil)

7: wiring optimization and screen printing

“There is no best, only better”! No matter how much effort you put into the design, when you’re done, look at it again, and you’ll still feel you can change a lot. A general design rule of thumb is that optimal wiring takes twice as long as initial wiring. After feeling that nothing needs to be changed, you can lay copper. Laying copper generally laying ground wire (pay attention to the separation of analog and digital ground), multilayer board may also need to lay power. For screen printing, we should pay attention to not be blocked by the device or removed by the hole and pad. At the same time, design to face the component surface, the bottom of the word should be mirror processing, so as not to confuse the level.

8: Network, DRC and structure inspection

Before light painting, it is generally necessary to Check. Every company has its own Check List, including the requirements of principle, design, production and other links. The following is an introduction to the two main inspection functions provided by the software. DRC check:

9: output light painting

Before the output of light painting, ensure that the veneer is the latest version that has been completed and meets the design requirements. The output file of light painting is used for the production of board in the plate factory, the production of steel net in the steel net factory, and the production process file in the welding factory.

The output files are as follows (take the four-layer board as an example) : 1). Wiring layer: refers to the conventional signal layer, mainly wiring. They are named L1,L2,L3, AND L4, where L represents the layer of the wiring layer.

2). Screen printing layer: refers to the layer in the design document that provides information for the processing of screen printing. Usually, there will be top screen printing and bottom screen printing if there are devices or marks on the top and bottom layer. Naming: the top layer is named SILK_TOP; The underlying name is SILK_BOTTOM.