PCB design process

The general PCB basic design process is as follows:

Pre-preparation → PCB structure design → guide network table → rule setting → PCB layout → wiring → wiring optimization and screen printing → network and DRC check and structure check → output light painting → light painting review → PCB board production / sampling information → PCB board factory engineering EQ confirmation → SMD information output → project completion.

1: Pre-preparation

This includes preparation of package library and schematic. Before the PCB design, first prepare the schematic SCH logic package and PCB package library. Package library can PADS comes with the library, but in general it is difficult to find the right one, it is best to make your own package library based on the standard size information of the selected device. In principle, first do the PCB package library, and then do the SCH logic package. PCB package library is more demanding, it directly affects the installation of the board; SCH logic package requirements are relatively loose, as long as you pay attention to the definition of good pin properties and correspondence with the PCB package on the line. PS: pay attention to the standard library of hidden pins. After that is the design of the schematic, ready to start doing PCB design.

2: PCB structure design

This step according to the board size and the mechanical positioning has been determined, the PCB design environment to draw the PCB board surface, and positioning requirements for the placement of the required connectors, keys / switches, screw holes, assembly holes, etc.. And fully consider and determine the wiring area and non-wiring area (such as how much around the screw hole belongs to the non-wiring area).

3: Guide the netlist

It is recommended to import the board frame before importing the netlist. Import DXF format board frame or emn format board frame.

4: Rule setting

According to the specific PCB design can be set up a reasonable rule, we are talking about the rules is the PADS constraint manager, through the constraint manager in any part of the design process for line width and safety spacing constraints, does not meet the constraints of the subsequent DRC detection, will be marked out with DRC Markers.

The general rule setting is placed before the layout because sometimes some fanout work has to be completed during the layout, so the rules have to be set before the fanout, and when the design project is larger, the design can be completed more efficiently.

Note: The rules are set to complete the design better and faster, in other words, to facilitate the designer.

The regular settings are.

1. Default line width/line spacing for common signals.

2. Select and set the over-hole

3. Line width and color settings for important signals and power supplies.

4. board layer settings.

5: PCB layout

General layout according to the following principles.

(1) According to the electrical properties of a reasonable partition, generally divided into: digital circuit area (that is, the fear of interference, but also generate interference), analog circuit area (fear of interference), power drive area (interference sources).

(2) to complete the same function of the circuit, should be placed as close as possible, and adjust the components to ensure the most concise connection; at the same time, adjust the relative position between the functional blocks to make the most concise connection between the functional blocks.

(3) For the mass of components should consider the installation location and installation strength; heat-generating components should be placed separately from temperature-sensitive components, and thermal convection measures should be considered when necessary.

(4) I/O driver devices as close as possible to the side of the printed board, close to the lead-in connector.

(5) clock generator (such as: crystal or clock oscillator) to be as close as possible to the device used for the clock.

(6) in each integrated circuit between the power input pin and ground, you need to add a decoupling capacitor (generally using high-frequency performance of the monolithic capacitor); board space is dense, you can also add a tantalum capacitor around several integrated circuits.

(7) the relay coil to add a discharge diode (1N4148 can).

(8) layout requirements to be balanced, orderly, not head heavy or a sink.

Special attention should be paid to the placement of components, we must consider the actual size of the components (the area and height occupied), the relative position between the components to ensure the electrical performance of the board and the feasibility and convenience of production and installation at the same time, should ensure that the above principles can be reflected in the premise of appropriate modifications to the placement of the device, so that it is neat and beautiful, such as the same device to be placed neatly, the same direction. Can not be placed in a “staggered”.

This step is related to the overall image of the board and the difficulty of the next wiring, so a little effort should be put into consideration. When laying out the board, you can make preliminary wiring for places that are not so sure, and give it full consideration.

6: Wiring

Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board is good or bad. In the design process of the PCB, wiring generally has so three realms of division.

First is the cloth through, which is the most basic requirements for PCB design. If the lines are not laid through, so that everywhere is a flying line, it will be a substandard board, so to speak, has not been introduced.

The next is the electrical performance to meet. This is a measure of whether a printed circuit board qualified standards. This is after the cloth through, carefully adjust the wiring, so that it can achieve the best electrical performance.

Then comes the aesthetics. If your wiring cloth through, there is nothing to affect the electrical performance of the place, but a glance at the past disorderly, plus colorful, flowery, that even if your electrical performance how good, in the eyes of others or a piece of garbage. This brings great inconvenience to testing and maintenance. The wiring should be neat and tidy, not crisscrossed without rules. These are to ensure the electrical performance and meet other individual requirements to achieve the case, otherwise it is to put the cart before the horse.

Wiring according to the following principles.

(1) In general, the first should be wired for power and ground lines to ensure the electrical performance of the board. Within the limits of the conditions, try to widen the power supply, ground line width, preferably wider than the power line, their relationship is: ground line > power line > signal line, usually the signal line width: 0.2 ~ 0.3mm (about 8-12mil), the thinnest width up to 0.05 ~ 0.07mm (2-3mil), the power line is generally 1.2 ~ 2.5mm (50-100mil). 100mil). The PCB of digital circuits can be used to form a circuit of wide ground wires, that is, to form a ground network to use (analog circuit ground can not be used in this way).

(2) pre-wiring of the more stringent requirements of the line (such as high-frequency lines), the input and output side lines should be avoided adjacent to parallel, so as not to produce reflected interference. If necessary, ground isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, parallel to easily produce parasitic coupling.

(3) oscillator shell grounding, the clock line should be as short as possible, and can not be led everywhere. Clock oscillation circuit below, special high-speed logic circuit part to increase the area of the ground, and should not go other signal lines to make the surrounding electric field tends to zero;.

(4) as far as possible using 45 ° fold wiring, do not use 90 ° fold, in order to reduce the radiation of high-frequency signals; (high requirements of the line also use double arc line)

(5) any signal lines do not form loops, such as unavoidable, loops should be as small as possible; signal lines should have as few holes as possible.

(6) the key line as short and thick as possible, and on both sides with a protective ground.

(7) through the flat cable transmission of sensitive signals and noise field band signal, to use the “ground – signal – ground” way to lead out.

(8) Key signals should be reserved for test points to facilitate production and maintenance testing

(9) After the schematic wiring is completed, the wiring should be optimized; at the same time, after the initial network check and DRC check is correct, the unwired area for ground filling, with a large area of copper layer for ground, in the printed circuit board is not used on the place are connected to the ground as ground. Or make a multilayer board, power and ground each occupy a layer.

PCB wiring process requirements (can be set in the rules)

(1) Line

In general, the signal line width of 0.3mm (12mil), the power line width of 0.77mm (30mil) or 1.27mm (50mil); between the line and the line and the distance between the line and the pad is greater than or equal to 0.33mm (13mil), the actual application, the conditions should be considered when the distance is increased.

Wiring density is high, can be considered (but not recommended) to use IC pins between the two lines, the line width of 0.254mm (10mil), the line spacing is not less than 0.254mm (10mil). In special cases, when the device pins are denser and narrower width, the line width and line spacing can be reduced as appropriate.

(2) Solder pads (PAD)

Solder pad (PAD) and transition hole (VIA) the basic requirements are: the diameter of the disk than the diameter of the hole to be greater than 0.6mm; for example, general-purpose pin resistors, capacitors and integrated circuits, etc., using the disk / hole size 1.6mm / 0.8mm (63mil / 32mil), sockets, pins and diodes 1N4007, etc., using 1.8mm / 1.0mm (71mil / 39mil). Practical applications, should be based on the actual size of the components to determine, when available, can be appropriate to increase the size of the pad.

PCB board design component mounting aperture should be larger than the actual size of the component pins 0.2 ~ 0.4mm (8-16mil) or so.

(3) over-hole (VIA)

Generally 1.27mm/0.7mm (50mil/28mil).

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

(4) The spacing requirements of the pad, line 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)

At higher densities.

PAD and VIA : ≥ 0.254mm (10mil)

PAD and PAD : ≥ 0.254mm (10mil)

PAD and TRACK : ≥ 0.254mm (10mil)

TRACK and TRACK : ≥ 0.254mm (10mil)

7: Wiring optimization and silkscreen

“There is no best, only better”! No matter how much you dig into the design, when you finish drawing, then go to take a look, you will still feel that many places can be modified. The general design experience is that it takes twice as long to optimize the wiring as it does to do the initial wiring. After feeling that there is no place to modify, you can lay copper. Copper laying generally laying ground (pay attention to the separation of analog and digital ground), multi-layer board may also need to lay power. When for silkscreen, be careful not to be blocked by the device or removed by the over-hole and pad. At the same time, the design is looking squarely at the component side, the word on the bottom layer should be made mirror image processing, so as not to confuse the level.

8: Network, DRC check and structure check

Out of the light drawing before, generally need to check, each company will have their own Check List, including the principle, design, production and other aspects of the requirements. The following is an introduction from the two main checking functions provided by the software.

9: Output light painting

Before light drawing output, you need to ensure that the veneer is the latest version that has been completed and meets the design requirements. The light drawing output files are used for the board factory to make the board, the stencil factory to make the stencil, the welding factory to make the process files, etc.

The output files are (taking 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 alignment layer.

2). Silk-screen layer: refers to the design file for the processing of silk-screening information in the level, usually the top and bottom layers have devices or logo case, there will be a top layer silk-screening and bottom layer silk-screening.

Naming: The top layer is named SILK_TOP ; the bottom layer is named SILK_BOTTOM .

3). Solder resist layer: refers to the layer in the design file that provides processing information for the green oil coating.

Naming: The top layer is named SOLD_TOP; the bottom layer is named SOLD_BOTTOM.

4). Stencil layer: refers to the level in the design file that provides processing information for solder paste coating. Usually, in the case that there are SMD devices on both the top and bottom layers, there will be a stencil top layer and a stencil bottom layer.

Naming: The top layer is named PASTE_TOP ; the bottom layer is named PASTE_BOTTOM.

5). Drill layer (contains 2 files, NC DRILL CNC drilling file and DRILL DRAWING drilling drawing)

named NC DRILL and DRILL DRAWING respectively.

10: Light drawing review

After the output of light drawing to light drawing review, Cam350 open and short circuit and other aspects of the check before sending to the board factory board, the later also need to pay attention to the board engineering and problem response.

11: PCB board information (Gerber light painting information + PCB board requirements + assembly board diagram)

12: PCB board factory engineering EQ confirmation (board engineering and problem reply)

13: PCBA placement data output (stencil information, placement bit number map, component coordinates file)

Here all the workflow of a project PCB design is complete

PCB design is a very detailed work, so the design should be extremely careful and patient, fully consider all aspects of the factors, including the design to take into account the production of assembly and processing, and later to facilitate maintenance and other issues. In addition, the design of some good work habits will make your design more reasonable, more efficient design, easier production and better performance. Good design used in everyday products, consumers will also be more assured and trust.