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Jun 28 2018

PCB Design Tutorial and PCB Layout Software

Introduction

PCB (Printed Circuit Board) is an important electronic component, a support for electronic components, and a carrier for electrical connection of electronic components. Almost every kind of electronic equipment, such as electronic watches, calculators, computers, e-communication equipment, integrated circuits and so on. In order to make the electrical interconnection between the various components must be printed board. Therefore, it is very important to design a PCB board well.

Printed Circuit Board Design for Beginner Step by Step

The printed circuit board is composed of an insulating bottom plate, wires and pads for electronic components, which has the dual functions of a conductive circuit and an insulating bottom plate. It can replace complex wiring and realize the electrical connection between the components in the circuit. It not only simplifies the assembly and welding of electronic products, but also reduces the workload of wiring in the traditional way, and the labor intensity of workers. What’s more, it also reduces the whole machine size, the product cost, and improves the quality and reliability of electronic equipment. So that excellent PCB layout design can save production cost and achieve good circuit performance and heat dissipation performance.

Catalog

Introduction

Ⅰ PCB Layout Rules

Ⅱ PCB Layout Design Tips

Ⅲ Design Steps

3.1 Layout Design

3.2 PCB Components Placement

3.3 PCB Layout Check

Ⅳ PCB Board Design Details

4.1 Via

4.2 Silk Screen

4.3 External Plane and Fill

4.4 Pad

4.5 Place/Pad

Ⅴ PCB Design Basics

Ⅵ PCB Design Software

6.1 Altium Designer (AD)

6.2 Cadence spb

6.3 Cadence Allegro

6.4 EAGLE Layout

6.5 PADS


Ⅰ PCB Layout Rules

1) Under normal circumstances, all components should be arranged on the same side of the circuit board. Only when the top components are too dense, some devices with limited height and low heat generation, such as chip resistors, chip capacitors, and chip ICs can be put on the bottom.
2) To ensure electrical performance, the components should be placed on the grid and arranged parallel or perpendicular to each other to be neat. In general, they not allow to overlap. The arrangement of components should be compact and should be evenly distributed on the entire layout.
3) The minimum distance between adjacent land patterns of different components on the circuit board should be above 1mm.
4) The distance from the edge of the circuit board is generally not less than 2mm. The best shape of the circuit board is rectangular, and the aspect ratio is 3:2 or 4:3. When the size of the circuit board is larger than 200mm×150mm, the PCB mechanical strength should be considered.

 

Ⅱ PCB Layout Design Tips

In the design of the PCB layout, the units of the circuit board should be analyzed based on the circuit function. When laying out all the components of the circuit, the following tips may be useful:
1) Arrange the position of each functional circuit unit according to the circuit flow. So that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.
2) Take the core components of each functional unit as the center and place other components around it. The components should be uniformly, integrally and compactly arranged on the PCB to minimize and shorten the leads between the components.
3) For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible, so that it is easy to install and solder for mass production.
4) High-frequency components: The shorter the connection between high-frequency components, the better. Try to reduce the distribution parameters of the connection and the electromagnetic interference between each other. Components that are susceptible to interference should not be too close. In addition, the distance between the input and output components should be as large as possible.
5) Components with high potential difference: The distance between the components with high potential difference and the wiring should be increased to avoid short circuit. In order to avoid the occurrence of creepage, it is generally required that the distance between the copper film lines between the potential difference about 2000V should be greater than 2mm. For higher potential differences, the distance should be increased more. Devices with high voltage should be placed as hard as possible in a place that is not easily accessible during debugging.
6) Heavy weight components: This type of component should be fixed by a bracket. And components that generate a lot of heat easily should not be installed on the circuit board.
7) Heating and heat-sensitive components: Heating components should be kept away from heat-sensitive components.
8) The layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, micro switches, etc. should consider the structural requirements of the entire board. Some frequently used switches should be used if the structure permits. 

 

Ⅲ Design Steps

3.1 Layout Design

In the PCB, special components refer to the key components in the high-frequency part,  such as the core components in the circuit, the components that are susceptible to interference, the components with high voltage, the components with large heat generation, and some components of the opposite sex. The location of them needs to be carefully analyzed, and the belt layout meets the requirements of circuit functions and production requirements. Improper placement of them may cause circuit compatibility problems and signal integrity problems, which lead to the failure of PCB design.
When designing how to place special components, the first is the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-drying ability will decrease, and the cost will also increase. After determining the size of the PCB, next is the swing position of the special component. Finally, according to the functional unit, make overall layout of all the components on the PCB. 

3.2 PCB Components Placement

The success of a product must first focus on its internal quality; second, it must take into account the overall design.
First, place components that closely match the structure, such as power sockets, indicator lights, switches, connectors, etc.
Next, place special components, such as large components, heavy components, heating components, transformers, ICs, etc.
Finally, place small components.

3.3 PCB Layout Check

1) Whether the size of the circuit board meets the processing size required by the product drawing.
2) Whether the layout of the components is balanced, neatly arranged, and whether they have all been laid out.
3) Whether there are conflicts at all levels. For example, the components, frame, and silk screen should be reasonable.
3) Whether the commonly used components are convenient to use. Such as switches, plug-in board insertion equipment, components that must be replaced frequently, etc.
4) Whether the distance between thermal components and heating components is reasonable.
5) Whether the heat dissipation is good.
6) Whether the line interference problem needs to be considered.

 

Ⅳ PCB Board Design Details

4.1 Via

A via is mainly composed of two parts, one is the drill hole in the middle, and the other is the pad area around the drill hole, as shown in the figure below. The size of these two parts determines the size of the via. Obviously, in high-speed and high-density PCB design, designers always hope that the smaller the via hole is, the better, so that more wiring space can be left on the board. In addition, the smaller the via hole, the smaller its own parasitic capacitance. What’s more, the smaller it is, the more suitable it is for high-speed circuits. However, the reduction of hole size also brings about an increase in cost, and the size of vias cannot be reduced indefinitely. It is limited by process technologies such as drilling and plating: the smaller the hole, the longer the drilling time takes, also the easier it is to deviate from the center position. When the depth of the hole exceeds 6 times the diameter of the drilled hole, it cannot be guaranteed that the hole wall can be uniformly plated with copper. For example, the thickness (through hole depth) of a normal 6-layer PCB board is about 50mil, so the minimum drilling diameter that PCB manufacturers can provide can only reach 8mil.

pcb board

Figure 1. PCB Board

Similarly, there are parasitic capacitances and parasitic inductances in vias. In the design of high-speed digital circuits, the harm caused by the parasitic inductances of vias is often greater than the impact of parasitic capacitance. Because it will weaken the contribution of the bypass capacitor and weaken the filtering effect of the entire power system.

In high-speed PCB design, generally, simple vias often bring great negative effects to circuit design. In order to reduce the bad effects caused by the parasitic effects of vias, the following tips can be done in the design:
1) Considering the cost and signal quality, choose a reasonable via size. For example, for the 6-10 layer memory module PCB design, it is better to use 10/20mil (drilled/pad) vias. For some high-density small-size boards, you can try to use 8/18mil. According to current technical conditions, it is difficult to use smaller vias. For power or grounding vias, you can consider using a larger size to reduce impedance.
2) Based on PCB technique, a thinner PCB is beneficial to reduce the two parasitic parameters of the via.
3) Try not to change the layers of the signal traces on the PCB board, that is, reduce unnecessary vias.
4) The power and ground pins should be drilled nearby. And the lead between the via and the pin should be as short as possible, because they will increase the inductance. At the same time, the power and ground leads should be as thick as possible to reduce impedance.
5) Place some grounded vias near the vias of the signal change layer to provide the nearest loop for the signal. It is even possible to place a large number of redundant ground vias on the PCB board. Of course, the design needs to be flexible. The via model discussed earlier is the case where there are pads on each layer. Sometimes, we can reduce or even remove the pads of some layers. Especially when the density of vias is very high, it may lead to the formation of a broken groove in the copper layer to isolate the loop. To solve this problem, in addition to moving the position of the via, we can also consider reducing the via size on the copper layer.

4.2 Silk Screen

Silk screen refers to the required logo patterns and text codes printed on the upper and lower surfaces of the printed circuit board, such as component label and nominal value, component outline shape and manufacturer logo, production date, etc. In order to facilitate the installation and maintenance of the circuit. So silk screen must pay great attention to details.

Big Part: Placement
Generally speaking, when placing the silk screen of resistors, capacitors, tubes and other components, do not use four directions. This will lead to wrong welding directions during debugging, maintenance, and welding. Therefore, it is recommended to place them in two directions, as shown in the figure below. In this way, the silk screen will be viewed very clearly.

PCB Board

As shown in the figure below, if the components are too dense to fit the silk screen, you can write a silk screen in a blank place nearby. Marking the arrow, it is best to draw a box so that it is easier to identify.

PCB Board


Try not to make the vias on the silk screen
As shown in the figure below, the via hole is punched in the number 8. After punching the board, you will not be able to tell whether it is R48 or R49.

PCB Board

Do not press the silk screen on high-speed signal lines (such as clock lines, etc.)
It is for high-speed signal lines on the top or bottom layer, because such signal lines can be regarded as microstrip lines. The speed (phase speed) of the signal on the microstrip line is related to the medium. If the silk screen is pressed to the line, as shown in the figure below, the medium will become uneven, resulting in a change in the phase speed, which will eventually appear as discontinuous impedance, affecting the signal quality. Of course, there will be no such problem in the signal line of the inner electric layer.

PCB Board


The reading direction of the silk screen should be consistent with the direction of use.
As shown in the figure below, the reading direction of the silk screen is the same as the using direction of the chip, which is mainly to reduce the probability of reverse soldering when soldering. However, such as electrolytic capacitors, can not follow this recommendation, because you can indicate the positive and negative polarity.

PCB Board


The pin number should be clearly marked on the silk screen.
As shown in the figure below, 4 pin numbers are marked on the P3 connector to facilitate debugging/installation. In addition, it is best to mark the places with dense pins, such as chips, FPC sockets, etc. At the same time, the reading direction of P3 is consistent with the direction of use of the connector.

PCB Board


Silk screen for special package
For special packages such as BGA and QFN, the size of the silk screen must be exactly the same as the size of the chip (as shown in the figure below), otherwise it will be difficult to align, which will affect the soldering.

PCB Board


Silk screen of mounting hole
Here, the silk screen of the screw is added near the mounting hole, and the length and the total number of screws are also indicated for easy installation.

PCB Board


Mark Function
Some components, such as buttons, lights, knobs, etc., need to indicate functions and uses.

Add LOGO
If there is space, you can add the company's LOGO, anti-static logo, one-dimensional code, and QR Code on the board. For certification, the certification LOGO should be added. There are also some warning signs.

eco logo

 

4.3 External Plane and Fill

External plane is to process a large area of copper foil into a network, and the fill area only keeps the copper foil intact. Beginners often can't see the difference between the two on the computer in the design process. Because it is not easy to see the difference between the two in normal use, so when using it, it is even more careless to distinguish between the two. It should be emphasized that the former has a strong function of suppressing high-frequency interference in circuit, and is suitable for applications that require large areas, especially when certain areas are used as shielding areas, partitioning areas, or high-current power lines. The latter is mostly used in places where a small area is required such as general line ends or turning areas.

4.4 Pad

The selection of the pad types should comprehensively consider the shape, size, layout, vibration and heating conditions, and force direction of the component. There are a series of pads of different sizes and shapes in the package library, such as round, square, octagonal, round and positioning pads, but sometimes this is not enough and needs to be edited by yourself. Generally speaking, in addition to the above, the following principles should be considered when editing the pad by yourself:
(1) When the length of the shape is inconsistent, the difference between the width of the wire and the specific side length of the pad should not be too large.
(2) It is often necessary to use asymmetric pads with asymmetrical length when routing between component lead angles.
(3) The size of each component pad hole should be determined separately according to the thickness of the component pins. The principle is that the size of the hole is 0.2 to 0.4 mm larger than the pin diameter.

4.5 Place/Pad

1) Placing Method
You can execute the Place/Pad command in the main menu, or use the component to place the Place/Pad button in the toolbar. After entering the pad placement state, the mouse will turn into a cross shape. Move the mouse to a suitable position and click to complete the placement of the pad.
2) Property Settings
When using the mouse to place the pad, the mouse will turn into a cross shape. Press the Tab and the Pad (pad property) setting dialog box will pop up. Double-click the pad that has been placed on the PCB, and the pad property setting dialog box can also pop up. There are several settings in the pad property setting dialog box as following:
Hole Size: used to set the inner diameter of the pad.
Rotation: Use to set the rotation angle of the pad placement.
Location: Used to set the x and y coordinates of the center of the pad.
Layer drop-down list: From this drop-down list, you can select the wiring layer where the pad is placed.
Net drop-down list: This drop-down list is used to set the net of the pad.
Electrical Type drop-down list: Used to select the electrical characteristics of the pad. It has 3 selection methods: Load, Source and Terminator.
Testpoint multiple option: Used to set the pad is used as a test point. Only the top and bottom pads can be used as test points.
Locked check option: Select this check option, it means that the position of the pad will be fixed after placement.
Size and Shape option area: Used to set the size and shape of the pad
X-Size and Y-Size: Set the x- and y-size of the pad respectively.
Shape drop-down list: Used to set the shape of the pad, including Round, Octagonal and Rectangle.
Paste Mask Expansions: Used to set the attributes of the paste mask.
Solder Mask Expansions: Used to set the properties of the solder mask.

 

Ⅴ PCB Design Basics

(1) Avoid arranging important signal lines, such as clock and reset signals, on the edge of the PCB.
(2) The distance between the chassis ground wire and the signal wire is at least 4 mm. Keep the length-to-width ratio of the chassis ground wire less than 5:1 to reduce the inductance effect.
(3) The devices and lines whose positions have been determined are locked with the LOCK function to prevent them from being moved by mistake.
(4) The minimum wire width should not be less than 0.2mm (8mil). In high-density and high-precision printed circuits, the wire width and spacing can generally be 12mil.
(5) The 10-10 and 12-12 principles can be applied to the wiring between the IC pins of the DIP package. That is, when two wires pass between the two pins, the pad diameter can be set to 50mil, and the line width and line spacing are both 10mil; when only one wire passes between the two pins, the pad diameter can be set to 64mil, and the line width and line spacing are both 12mil.
(6) When the diameter of the pad is 1.5mm, in order to increase the stripping strength of the pad, an pad with a length of not less than 1.5mm and a width of 1.5mm can be used.
(7) When having the thinner trace of the pad connection, the connection between the pad and the trace should be designed into a drop shape, so that the pad is not easy to peel, and the trace and the pad are not easy to disconnect.
(8) When designing large-area copper coating, there should be opening windows on the copper coating, adding heat dissipation holes, and design the opening windows into a mesh.
(9) Shorten the connections between high-frequency components as much as possible, reduce their distribution parameters and avoid mutual electromagnetic interference. The components that are susceptible to interference should not be too close to each other. And the input and output components should be as far away as possible.

Free PCB Design Software

Ⅵ PCB Design Software

6.1 Altium Designer (AD)

The design software that most PCB engineers come into with basically is Altium Designer. AD is a simple and easy-to-learn PCB design software. It is suitable for drawing simple single and double panels and four to six-layer boards through schematic design and circuit simulation. The perfect integration of technologies such as, PCB drawing, signal integrity analysis, etc., enables PCB engineers to design easily. If they can use this software proficiently, the quality and efficiency of circuit design will be greatly improved.

6.2 Cadence spb

Cadence spb software, the current version is Cadence SPB 16.5. Its ORCAD schematic design is an international standard. PCB design and simulation are very complete, and it is more complicated than Protel, because of the requirements and settings. What’s more, there are ready-made rules for the design, so the design is more effective.

6.3 Cadence Allegro

The biggest advantage of this software is that it is very convenient to repair the wire. And it is intelligent, and it can display DRC in real time. In addition, it is more convenient to attach, route and wind wires. What’s more, Allegro has advantages over other PCB software in multi-person cooperation.

6.4 EAGLE Layout

EAGLE Layout is the most widely used PCB design software in Europe. If you are a beginner in PCB design, Cadence spb is a not bad choice. It can develop a good design habit for designers and ensure good design quality.

6.5 PADS

There are many people using this software. Relatively speaking, it is more convenient and easy to operate than Protel. This software is more suitable for low-end PCB design. Its characteristic is that it is easy to interact with the schematic, unlike Allegro, sometimes the schematic cannot be in accord with the PCB. In addition, it also supports real-time modification of the network, which is suitable for small changes or simple board routing.

Also, except the mentioned design software above, there are also other better choices, such as Cadstar, cr5000, PCAD, mentor EE, mentor WG, mentor en, pads professional and so on. If you have any interest, you can use them based on actual requirements.

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