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About PCB design and development

2019-07-22 13:45:26 419

About PCB design and development:

The design of the printed circuit board is based on the circuit schematic diagram to achieve the functions required by the circuit designer. The design of the printed circuit board mainly refers to the layout design, and the layout of the external connection needs to be considered. Optimized layout of internal electronic components. Optimized layout of metal wiring and vias. Electromagnetic protection. Various factors such as heat dissipation. Excellent layout design can save production costs and achieve good circuit performance and heat dissipation. Simple layout designs can be implemented manually, and complex layout designs need to be implemented with computer-aided design (CAD).

In high-speed designs, the characteristic impedance of the controllable impedance plates and lines is one of the most important and common problems. First, let's look at the definition of a transmission line: a transmission line consists of two conductors of a certain length, one for transmitting signals and the other for receiving signals (remember that "loop" replaces the concept of "ground"). In a multi-layer board, each line is part of the transmission line and the adjacent reference plane can be used as a second line or loop. The key to a line becoming a "good performance" transmission line is to keep its characteristic impedance constant throughout the line.

The key to making a board a "controlled impedance board" is to make the characteristic impedance of all lines meet a specified value, typically between 25 ohms and 70 ohms. In a multilayer circuit board, the key to good transmission line performance is to keep its characteristic impedance constant throughout the line.

But what exactly is the characteristic impedance? The easiest way to understand the characteristic impedance is to see what happens to the signal during transmission. This is similar to the microwave transmission shown in Figure 1 when moving along a transmission line having the same cross section. Suppose that a 1 volt voltage step wave is applied to the transmission line, such as connecting a 1 volt battery to the front end of the transmission line (which is located between the transmission line and the loop). Once connected, the voltage wave signal follows the line at the speed of light. Propagation, its speed is usually about 6 inches / nanosecond. Of course, this signal is indeed the voltage difference between the transmission line and the loop, which can be measured from any point on the transmission line and the point at which the loop comes. Figure 2 is a schematic diagram of the transmission of the voltage signal.

Zen's approach is to "generate a signal" and then propagate along the transmission line at 6 inches per nanosecond. The first 0.01 nanosecond advances by 0.06 inches, when the transmission line has excess positive charge and the loop has excess negative charge. It is these two charge differences that maintain a voltage difference of 1 volt between the two conductors. The two conductors in turn constitute a capacitor.

In the next 0.01 nanoseconds, the voltage of a 0.06 inch transmission line is adjusted from 0 to 1 volt. This must add some positive charge to the transmission line and add some negative charge to the receiving line. For every 0.06 inch of movement, more positive charge must be applied to the transmission line and more negative charge added to the loop. Every 0.01 nanoseconds, another segment of the transmission line must be charged, and then the signal begins to propagate along this segment. The charge is from the battery at the front end of the transmission line, and as it moves along this line, the continuous portion of the transmission line is charged, thus creating a voltage difference of 1 volt between the transmission line and the loop. Every 0.01 nanoseconds advance, some charge (±Q) is obtained from the battery, and a constant current (±Q) flowing out of the battery at a constant time interval (±t) is a constant current. The negative current flowing into the loop is actually equal to the positive current flowing out, and just at the front end of the signal wave, the alternating current passes through the capacitor composed of the upper and lower lines, ending the entire cycle.


Layout design

In the PCB, special components refer to the key components of the high-frequency part, the core components in the circuit, the components that are susceptible to interference, the components with high voltage, the components with large heat, and some heterogeneous components. The location of these special components needs to be carefully analyzed to make the layout conform to the requirements of the circuit function and the production requirements. Improper placement of them can create circuit compatibility problems, signal integrity problems, and lead to PCB design failure.

Consider how the PCB size is considered when placing special components in the design. Quick and easy to buy indicates that when the pcb size is too large, the printed lines are long, the impedance is increased, the anti-drying ability is lowered, and the cost is also increased; when it is too small, the heat dissipation is not good, and the adjacent lines are easily disturbed. After determining the size of the PCB, the position of the particular component is determined. Finally, according to the functional unit, all the components of the circuit are laid out. The location of special components is generally subject to the following principles when laying out:

1. Minimize the connection between high-frequency components and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and the input and output should be as far away as possible.

2 Some components or wires may have a higher potential difference, and their distance should be increased to avoid accidental short circuit caused by discharge. High-voltage components should be placed where they are not accessible by hand.

3. Components weighing more than 15G can be fixed by brackets and then soldered. Those components that are heavy and hot should not be placed on the circuit board and should be placed on the bottom plate of the main chassis, and the heat dissipation problem should be considered. Thermal components should be kept away from hot components.

4. For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, microswitches, etc., the structural requirements of the entire board should be considered. Some switches that are often used should be allowed when the structure permits. Place it in a place where you can easily reach it. The layout of the components is balanced, the density is dense, and it is not top-heavy.

The success of a product is to focus on the inherent quality. It is to take into account the overall beauty, both of which are perfect boards to become a successful product.


Placement order

1. Place components that closely match the structure, such as power sockets, indicators, switches, connectors, etc.

2. Place special components, such as large components, heavy components, heating components, transformers, ICs, etc.

3. Place small components.


Layout check

1. The board size and drawings require the processing dimensions to match.

2. Whether the layout of the components is balanced, neatly arranged, and whether they have all been finished.

3. Whether there are conflicts at all levels. Such as components, frames, and the level of private printing is reasonable.

4. Whether the commonly used components are convenient to use. Such as switches, boards inserted into the device, components that must be replaced frequently.

5. Is the distance between the thermal components and the heating components reasonable?

6. Is the heat dissipation good?

7. Does the interference problem of the line need to be considered?


Use less holes

Once the via hole is selected, it is necessary to handle the gap between it and the surrounding entities, especially the gap between the line and the via which is easily ignored in the middle layer and the via hole. If it is automatic wiring, the number of vias can be In the "Mini Minimiz8tion" submenu, select the "on" item to solve it automatically. (2) The larger the required current carrying capacity, the larger the required via size, such as the vias used to connect the power and ground layers to other layers.

Overlay

In order to facilitate the installation and maintenance of the circuit, the required logo pattern and character code, etc., such as the component number and the nominal value, the component outline shape and the manufacturer logo, the date of manufacture, and the like are printed on the upper and lower surfaces of the printed board. When many beginners design the relevant content of the silk screen layer, they only pay attention to the neat and beautiful text symbols, ignoring the actual PCB effect. On the plates they design, the characters are not blocked by the components or invaded the soldering area, and the components are marked on the adjacent components. Such designs will bring great assembly and maintenance. inconvenient. The correct silk screen layer character layout principle is: "No ambiguity, see the stitches, beautiful appearance."


SMD package

Particularity

The Protel package has a large number of SMD packages, surface soldered devices. The biggest feature of this type of device in addition to its small size is the single-sided distribution element pin hole. Therefore, to select such devices, you must define the surface of the device to avoid "Missing Plns". In addition, the relevant text labels for such components can only be placed with the surface of the component.


Filled area

Grid Filled Area (External Plane) and Filled Area (Fill)

As the name suggests, the network-like filling zone treats a large area of copper foil into a mesh, and the filling zone is only a complete copper foil. In the design process of beginners, the difference between the two is often not seen on the computer. In essence, as long as you enlarge the drawing, you can see at a glance. It is precisely because it is not easy to see the difference between the two, so the use of it does not pay attention to the distinction between the two, it should be emphasized that the former has a strong role in suppressing high-frequency interference in the circuit characteristics, suitable for the need to do Large areas of filling, especially when using certain areas as shields, partitions, or high-current power lines. The latter is often used in places where a small area is required, such as a general line end or a turning area.


Pad

Pads are the most common and important concept in PCB design, but beginners tend to ignore its choices and corrections, using circular pads in the design. The type of the pad of the selected component should take into account factors such as the shape, size, arrangement, vibration and heat, and direction of the force. Protel presents a range of pads of different sizes and shapes in the package library, such as circles, squares, octagons, circles and positioning pads, but sometimes this is not enough and you need to edit it yourself. For example, a pad that generates heat and has a large force and a large current can be designed as a "teardrop shape". In the design of the output transformer pin pad of the familiar color TV PCB, many manufacturers are This form is adopted. In general, in addition to the above, when editing the pad yourself, consider the following principles:

(1) When the length of the shape is inconsistent, the difference between the width of the wire and the length of the specific side of the pad should not be considered too large;

(2) It is necessary to use a long and short asymmetric pad when routing between the component lead angles, which is often more effective;

(3) The size of each component pad hole should be edited and determined according to the component pin thickness. The principle is that the hole size is 0.2-0.4 mm larger than the pin diameter.


PCB placement pad:


1. Method of placing a pad

You can execute the command Place/Pad in the main menu, or you can use the Place Pad button in the component placement toolbar.

After entering the pad state, the mouse will become a cross shape, move the mouse to the appropriate position and click to complete the pad placement.


2. Pad property settings

There are two ways to set the properties of the pad:

● When placing the pad with the mouse, the mouse will change to a cross shape. Pressing the Tab key will bring up the Pad (Pad Attributes) Settings dialog box.

7-24 Pad Property Settings Dialog Box

● Double-click the pad that has been placed on the PCB to pop up the pad property setting dialog box. In the Pad Properties Settings dialog box, there are the following settings:

● Hole Size: Used to set the inner diameter of the pad.

● Rotation: Use a set rotation angle to set the pad.

● Location : Used to set the position of the x and y coordinates of the pad center.

● Designator text box: used to set the serial number of the pad.

● Layer drop-down list: From this drop-down list, you can select the wiring layer on which the pad is placed.

● Net drop-down list: This drop-down list is used to set up the network of pads.

● Electrical Type drop-down list: Used to select the electrical characteristics of the pad. There are 3 selection methods for this drop-down list: Load, Source, and Terminator.

● Testpoint option: used to set whether the pad is used as a test point. Only the top and bottom pads can be used as test points.

● Locked option: Check this option to indicate that the position will be fixed after the pad is placed.

● Size and Shape option area: used to set the size and shape of the pad

● X-Size and Y-Size: Set the size of the x and y of the pad, respectively.

● Shape drop-down list: used to set the shape of the pad, round (round), Octagonal (octagonal) and Rectangle

(rectangle).

● Paste Mask Expansions option area: Used to set the solder layer properties.

● Solder Mask Expansions option area: Used to set the solder mask properties. Abc


Mask

These films are not only essential for the PcB manufacturing process, but also necessary for component soldering. According to the position and function of the "film", the "film" can be divided into the component surface (or soldering surface) soldering film (TOp or Bottom and the solder mask (TOp or BottomPaste Mask). As the name suggests, the solder mask is a film that is applied to the pad to improve solderability, that is, a light colored spot on the green board that is slightly larger than the pad. The solder mask is the opposite, in order to The prepared board is adapted to the soldering form such as wave soldering, and the copper foil on the non-pad on the board is required to be non-sticky. Therefore, a layer of paint is applied to each part of the pad to prevent tin on these parts. It can be seen that the two membranes are a complementary relationship. From this discussion, it is not difficult to determine the menu.


PCB prototype board

The Chinese name of the PCB is a printed circuit board, also known as a printed circuit board. The printed circuit board is an important electronic component that is a support for electronic components. It is a provider of electrical connections for electronic components. Because it is made by electronic printing, it is called a "printing" circuit board.

PCB prototyping refers to the pre-production of printed circuit boards before mass production. The main application is for PCB engineers to design a good circuit and complete the PCB Layout and then perform small-scale trial production to the factory. The number of PCB proofing productions generally has no specific boundaries. Generally, engineers call PCB prototypes before the product design is confirmed and completed.