PCB design experience

From: 21ic

When it comes to PCB, many friends will think of it everywhere around us. From all household appliances, various computer accessories, to various digital products, PCB is almost used as long as it is an electronic product, so what is PCB? PCB is the printedcircuitblock, that is, the printed circuit board, for the electronic components to be installed, there is a line of the base version. The Base Plate of copper plating is printed with the anti-corrosion line in printing mode, and the outlet line is etching.

PCB can be divided into single, double, and multi-layers. All kinds of electronic components are integrated on the PCB. On the most basic Single-layer PCB, the parts are concentrated on one side, and the wires are concentrated on the other side. In this way, we need to punch holes in the board so that the PIN can pass through the board to the other side, so the pin of the parts is welded to the other side. In this case, the front and back sides of such a PCB are called the componentside and the solderside respectively ). The double laminate can be regarded as a combination of two single laminates, each of which has electronic components and cables. Sometimes you need to connect one side of the single line to the other side of the board, this is through the guide hole (). The guide hole is a small hole on the PCB that is filled with or coated with metal. It can be connected to both sides of the wire. Currently, many computer boards use 4-or even 6-layer PCB boards, while graphics cards generally use 6-layer PCB boards. Many high-end graphics cards, such as the nvidiageforce4ti series, use 8-layer PCB boards, this is the so-called multilayer PCB. On the multilayer PCB Board, the problem of connecting lines between layers may also be achieved through guide holes. Because it is a multilayer PCB, sometimes the guide hole does not need to penetrate the entire PCB. Such a guide hole is called a buried hole (buriedvias) and a blind hole (blinvias) because they only penetrate several layers. A blind hole is used to connect several layers of internal PCB to the surface PCB without penetrating the whole board. The buried hole is connected only to the internal PCB, so it cannot be seen from the surface alone. In multi-board PCB, the whole layer is directly connected to the top ground and power supply. Therefore, we classify each layer into signal layer, power layer, or ground layer ). If the components on the PCB need different power supplies, this type of PCB usually has more than two layers of power and wire layers. The more PCB layers, the higher the cost. Of course, the use of more layers of PCB is very helpful for providing signal stability.

The professional PCB production process is quite complex. Take the 4-layer PCB as an example. The PCB of the Main Board is mostly 4 layers. When manufacturing, the middle two layers are first crushed, cropping, etching, oxidation plating, the four layers are the component surface, power supply layer, formation layer and solder pressure layer. Then we put the four layers together and compress them into a PCB of the Main Board. Drilling and drilling are performed. After cleaning, print the two layers of lines on the outside, apply copper, etching, testing, solder mask, silk screen. Finally, press the entire PCB (including many main boards) into a PCB of the main board, and then perform the test for vacuum packaging. If the PCB is not properly coated with copper, it may be hard to paste, which may cause short-circuit or capacitive effect (interference ). The pass-through on the PCB must also be noted. If the hole playing is not in the middle, but in favor of one side, uneven matching will occur, or it is easy to be in contact with the power supply layer or formation in the middle, resulting in potential short-circuit or bad grounding factors.

Copper wiring Process

The first step is to establish cabling between parts. We use the negative transfer method to present the work film on the metal conductor. This technique is to pad the entire surface with a thin copper foil and remove the excess parts. Append transfer is another method that is rarely used. This is just a method to place copper wires where needed, but we will not talk about it here. A forward mask is made of a photosensitive agent, which is dissolved under illumination. There are a lot of ways to deal with copper surface light resistance agent, but the most common way is to heat it and scroll on the surface containing the light resistance agent. It can also be sprayed in a liquid way, but the Dry Film Type provides a relatively high resolution, you can also make a relatively fine wire. The hood is just a template for the PCB layer in manufacturing. The mask on the PCB is used to prevent some areas from being exposed before UV exposure. The places covered by the light resistance agent will change to wiring. Other bare copper parts to be etching after the light resistance agent is developed. The etching process may immerse the board in the etching solvent or spray the solvent on the board. It is generally used as an etching solvent, such as ferric chloride. After etching, remove the remaining photomasks.

1. Wiring width and current
Generally, the width should not be less than 0.2mm (8mil)
On high-density and high-precision PCB, the gap and line width are generally 0.3mm (12mil ).
When the thickness of copper foil is about 50 um, the wire width is 1 ~ 1.5mm (60mil) = 2a
Generally, 80 mil is used in the public, and more attention should be paid to applications with microprocessor.

2. How high is the frequency?
When the signal goes up/down <3 ~ When the signal transmission time is six times, it is considered as a high-speed signal.
For digital circuits, the key is to look at the steep edge of the signal, that is, the time when the signal rises or drops,
According to the theory of a very classic book "high speed Digtal design>, the signal is increased from 10% to 90%, and the time delay of the wire is less than 6 times. It is a high-speed signal! ------ That is! Even if the square wave signal of 8 kHz is steep enough, it is also a high-speed signal, and the transmission line theory needs to be used during cabling.

3. Stack and hierarchy of PCB
The four-layer laminate has the following stacking sequence. The following describes the advantages and disadvantages of different layers:
First case
Gnd
S1 + power
S2 + power
Gnd

Case 2
Sig1
Gnd
Power
Sig2

Case 3
Gnd
S1
S2
Power

Note: Layer 1 of S1 signal cabling, Layer 2 of S2 signal cabling, and power supply layer of Gnd Formation
The first case should be the best case of a four-layer board. Because the outer layer is the formation, it shields EMI, and the power supply layer is also very reliable with the formation, which makes the internal resistance of the power supply small and gets the best effect. However, the first case cannot be used when the density of the current Board is large. In this way, the integrity of the first layer cannot be guaranteed, so that the second layer signal will become worse. In addition, this structure cannot be used when the power consumption of the entire board is relatively large.
The second case is the most commonly used method. From the structure of the Board, it is not applicable to high-speed digital circuit design. In this structure, it is difficult to maintain low power impedance. Take a Single Board 2mm as an example: z0 = 50ohm. The width of the board is 8mil. The copper foil thickness is 35 μm. In this way, the signal layer is 0.14 in the middle of the formation. The formation and power supply layer are 1.58mm. This greatly increases the internal resistance of the power supply. In this structure, as the radiation is directed to the space, a shield plate must be added to reduce EMI.
In the third case, the signal line on the S1 layer has the best quality. S2 followed. Shielding EMI. However, the power impedance is large. This board can be used when the power consumption of the entire board is large, and the Board is a source of interference or is closely connected to the source of interference.

4. Impedance Matching
The amplitude of the reflected voltage signal is determined by the source side's reflection coefficient p S and the load reflection coefficient p L.
P l = (RL-z0)/(RL + z0) and p s = (RS-z0)/(RS + z0)
In the above formula, if RL = z0, the load reflection coefficient p L = 0. If rs = z0 source side reflection coefficient p S = 0.
Because the normal transmission line impedance z0 should generally meet the requirements of 50 Ω about 50 Ω, and the load impedance is usually several thousand ohm to several tens thousand ohm. Therefore, it is difficult to achieve impedance matching at the load end. However, the signal source (output) impedance is usually relatively small, roughly 10 ohm. Therefore, it is much easier to achieve impedance matching at the source end. If the load side is connected to the resistor, the resistor will absorb some signal which is unfavorable for transmission (my understanding). When the TTL/CMOS standard 24mA drive current is selected, its output impedance is approximately 13 Ω. If the transmission line impedance z0 = 50Ω, a 33 Ω source end matching resistance should be added. 13 Ω + 33 Ω = 46 Ω (similar to 50 Ω, weak underdamping will help the setup time of the signal)
The matching impedance varies with other transmission standards and Driving Currents. In high-speed logic and circuit design, we recommend that you add source-end matching resistance to some key signals, such as clock and control signals.
In this way, the signal will be reflected back from the load. Because the source impedance matches, the reflected signal will not be reflected back.

5. Considerations for power cord and ground layout
The power cord should be as short as possible and take a straight line, and it is best to take a tree shape instead of a ring
Ground loop problem: for digital circuits, the ground Circulation Caused by the ground loop is dozens of millivolt levels, and the anti-interference threshold of TTL is 1.2 V, the CMOS circuit can reach 1/2 power supply voltage, that is to say, the ground circulation will not affect the operation of the circuit. On the contrary, if the ground wire is not closed, the problem will be greater, because the pulse power supply current generated by the digital circuit during operation will cause the ground potential imbalance of each point, for example, I tested the ground current 1.2a when 74ls161 was reversed (measured by 2gsps oscilloscope, the pulse width of the ground current is 7ns ). Under the impact of large pulse current, if a branch-Shaped Ground Line (25 mil in width) is used, the potential difference between various points of the ground line will reach a hundred millivolt level. After the ground wire loop is adopted, the pulse current will be distributed to various points of the ground wire, greatly reducing the possibility of interference circuit. Using Closed ground wires, the maximum instantaneous potential difference of the Ground Wires of each device is measured from 1/2 to 1/5 of the non-closed ground wires. Of course, the actual measurement data of circuit boards with different density and speed varies greatly. As I mentioned above, it refers to the level of Z80 Demo Board attached to Protel 99se. For low frequency analog circuits, I think that the power frequency interference after the ground is closed is induced by space, which cannot be simulated or calculated in any case. If the ground wire is not closed, it will not generate ground eddy current. beckhamtao says, "but the power frequency induction voltage of the ground ring opening will be larger ." ? Taking two examples, I took over a project of another person seven years ago. The precise pressure gauge uses a 14-bit A/D converter, but the actual measurement only has 11-bit effective precision. After investigation, the geographic line has 15mvp-p power frequency interference. The solution is to open the analog ground loop of the PCB, and the front-end sensor uses the flying line as the branch distribution to the ground line of A/D, later, the mass-produced PCB models were re-produced according to the flying line. No problems have occurred so far. In the second example, a friend loves fever and DIY a power amplifier, but the output always has a sound of communication. I suggest him cut the ground loop and solve the problem. Afterwards, this guy checked dozens of "hi-fi host" PCB diagrams and confirmed that none of them used a ground loop in the simulation.

6. design principles and anti-interference measures of printed circuit boards
A Printed Circuit Board (PCB) is a supporting component of circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electricity and technology, the density of pgb is getting higher and higher. The quality of PCB design has a great impact on the anti-interference capability. Therefore, the general principles of PCB design must be observed during PCB design and must comply with the anti-interference design requirements.
General principles of PCB design

To achieve optimal performance of electronic circuits, it is very important to deploy components and lay wires. The following general principles should be followed in order to design high-quality and low-cost PCB:

1. Layout
First, consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-noise capability decreases, and the cost increases. If the PCB size is too small, the heat dissipation is poor and the adjacent lines are vulnerable to interference. After determining the PCB size, determine the position of the Special component. Finally, all components of the circuit are arranged according to the functional units of the circuit.

Observe the following principles when determining the location of a special element:

(1) try to shorten the connections between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are vulnerable to interference cannot be too close to each other, and the input and output components should be kept away as far as possible.
(2) Some components or wires may have a high potential difference, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be arranged as far as possible in areas not easily accessible during debugging.
(3) components weighing more than 15 GB should be fixed with a bracket, and then welded. Those components that are large, heavy, and generate much heat should not be mounted on the printed board, but should be mounted on the chassis of the whole machine, and heat dissipation should be considered. Thermal components should be kept away from heating components.
(4) the structural requirements of the entire machine should be considered for the layout of adjustable components such as potentiometer, adjustable Inductance Coil, variable capacitor, and micro switch. If it is internal adjustment, it should be placed above the Printed Board to facilitate adjustment; if it is external adjustment, its position should be adapted to the position of the adjusting knob on the chassis panel.
(5) the position occupied by the printed positioning hole and the fixed bracket should be set aside.

The layout of all components of the circuit should comply with the following principles according to the functional units of the circuit:
(1) arrange the positions 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) The core components of each function circuit are centered around it. Components should be arranged evenly, neatly, and compact on the PCB. Minimize and shorten the lead and connection between components.
(3) distribution parameters between components should be considered for circuits working at high frequencies. The general circuit should arrange components in parallel as much as possible. In this way, it is not only beautiful, but also easy to assemble and weld. It is easy to produce in bulk.
(4) components located at the edge of the circuit board are generally no less than 2mm away from the edge of the circuit board. The Optimal Shape of the circuit board is a rectangle. The aspect ratio is. When the PCB Surface is larger than 150mm X, the mechanical strength of the PCB should be considered.

2. Wiring
The cabling principles are as follows:
(1) cables used at the input and output ends should be avoided as close as possible. It is best to add a line between lines to avoid feedback coupling.
(2) The minimum width of the printed wire is determined by the adhesion strength between the wire and the insulation tray and the current value flowing through them. When the copper foil thickness is 0.05mm and the width is 1 ~ At 5mm mm, the temperature is not higher than 3 ℃ through 2a, so the wire width is. For integrated circuits, especially digital circuits, 0.02 ~ 3mm wire width. Of course, as long as you allow, use the wide wire as much as possible, especially the power cord and ground wire. The minimum wire spacing is determined by the insulation resistance and breakdown voltage in the worst case. For integrated circuits, especially digital circuits, as long as the process permits, the spacing can be as small as 5 ~ 8mm.
(3) The turns of printed wires are generally arc-shaped, and the right angle or angle in the high-frequency circuit will affect the electrical performance. In addition, avoid using a large area of copper foil if possible. Otherwise, copper foil may expand or fall off easily When it is heated for a long time. It is best to use a grid shape when a large area of copper foil must be used. This will help eliminate the volatile gas produced by heat of the adhesive between the copper foil and the substrate.

3. Pad
The center hole of the pad is slightly larger than the diameter of the device lead. The pad is too large to form virtual soldering. The outer diameter D of the pad is generally not less than (D + 1.2) mm, where D is the lead aperture. For high-density digital circuits, the minimum diameter of the pad can be (D + 1.0) mm.

PCB and Circuit Anti-interference measures

The anti-interference Design of the printed circuit board is closely related to the specific circuit. Here, we will only describe some common measures of the PCB anti-interference design.

1. Power cord Design
Based on the current size of the printed circuit board, try to rent the power line width to reduce the loop resistance. At the same time, the direction of the power cord and ground wire is consistent with that of data transmission, which helps to enhance noise resistance.
2. Ground Design
The principle of Ground Design is:
(1) numbers are separated from analog data. If both logical and Linear Circuits exist on the circuit board, separate them as much as possible. The ground of the low-frequency circuit should adopt single point parallel grounding as much as possible. If the actual wiring is difficult, it can be partially connected before parallel grounding. The high-frequency circuit should be connected to ground by multiple points. The ground wire should be short and rented, and the grid-like large area of ground foil should be used around the high-frequency components as much as possible.
(2) the ground line should be bold as much as possible. If the ground wire uses a very dense line, the Grounding Potential changes with the change of current, reducing the noise resistance. Therefore, the grounding wire should be roughened so that it can pass three times the allowable current on the printed board. If possible, the grounding line should be 2 ~ More than 3mm.
(3) the ground line forms a closed loop. The grounding circuit is a printed board composed of only digital circuits. It can build a loop to improve noise resistance.
3. backcoupling capacitor Configuration
One of the general practices of PCB design is to configure the appropriate backcoupling capacitor in each key part of the printed board.
The general configuration principle of the backend capacitor is:
(1) 10 ~ 100uf electrolytic capacitor. If possible, it is better to connect 100uf or above.
(2) In principle, each IC chip should be equipped with a porcelain chip capacitor of 0.01pf. In case of gaps in the printed board, each 4 ~ One for eight chips ~ 10pf but capacitor.
(3) For devices with weak noise resistance and great changes in power supply upon shutdown, such as Ram and Rom memory, the chip's power cord and ground wire should be directly connected to the backcoupling capacitor.
(4) capacitor leads cannot be too long, especially high-frequency bypass capacitors cannot have leads.

In addition, pay attention to the following two points:
(1) When the Printed Board contains components such as contactor, relay, and button, a large spark will be generated during operation. The RC Circuit shown in the figure must be used to absorb the discharge current. Generally, R is 1 ~ 2 K, C get 2.2 ~ 47 UF.
(2) The input impedance of CMOS is very high and easy to sense. Therefore, when using cmos, the power supply must be grounded or positive.

7. Design tips and key points for achieving efficient automatic PCB cabling
Although the current EDA tool is very powerful, the PCB design is not difficult as the PCB size requirements become smaller and the device density increases. How can we achieve high PCB distribution rate and shorten the design time? This article introduces the design techniques and key points of PCB planning, layout and wiring. Now the PCB design is getting shorter and shorter, the Board space is getting smaller and smaller, and the device density is getting higher and higher. The extremely demanding layout rules and large size components make the work of the designer more difficult. In order to solve the design difficulties and speed up the product market, many manufacturers now prefer to use special EDA tools to achieve PCB design. However, specialized EDA tools cannot produce ideal results or reach a 100% distribution rate. They are messy and usually take a lot of time to complete the rest of the work.
Nowadays, there are many popular EDA tools and software on the market. However, apart from the terms used and the positions of function keys, they are all similar. How can we use these tools to better implement PCB design? Carefully analyzing the design and setting the tool software before wiring will make the design more compliant with requirements. The following describes the general design process and steps.

1. Determine the PCB Layers

The circuit board size and wiring layers must be determined at the initial stage of design. If the design requires the use of a high-density grid array (BGA) component, the minimum number of cabling layers required for these devices must be considered. The number of cabling layers and the stack-up method directly affect the wiring and impedance of printed cables. The size of the Board helps determine the stacked mode and the width of the printed line to achieve the expected design effect.

Over the years, people have always believed that the less the number of PCB layers, the lower the cost, but there are many other factors affecting the manufacturing cost of the PCB. In recent years, the cost difference between multiple layers has been greatly reduced. At the beginning of the design, it is best to use a large number of circuit layers and make the copper fl evenly distributed to avoid the occurrence of a small amount of signals that do not meet the defined rules and spatial requirements when the design is nearing the end, thus, a new layer is forced to be added. Careful planning before design will reduce a lot of trouble in wiring.

2. design rules and restrictions

The Automatic wiring tool itself does not know what to do. To complete the cabling task, the cabling tool must work under the correct rules and restrictions. Different signal lines have different wiring requirements. To classify all signal lines with special requirements, different design classifications are different. Each signal class should have a priority. The higher the priority, the stricter the rule. The rules involve the printing line width, the maximum number of passing holes, parallelism, mutual influence between signal lines, and layer limitations. These rules have a great impact on the performance of the cabling tool. Careful consideration of design requirements is an important step in successful cabling.

3. Component Layout

To optimize the assembly process, the manufacturing design (DFM) rules impose limitations on the component layout. If the assembly department allows components to move, you can optimize the circuit to facilitate automatic wiring. The defined rules and constraints affect the layout design.

During the layout, you must consider the routing path and the passing area. These paths and areas are obvious to the designer, but the automated cabling tool considers only one signal at a time, by setting wiring constraints and setting the layer of the printable signal line, cabling tools can be used to complete cabling as the designer imagined.

4. fan-out Design

In the fan-out design phase, to enable the Automatic wiring tool to connect component pins, each pin of the surface mount device should have at least one pass hole so that when more connections are needed, the circuit board can be connected to the inner layer, online testing (ICT), and circuit reprocessing.

In order to make the Automatic wiring tool more efficient, make sure to use the largest pass-through size and printed line as much as possible, the interval is set to 50mil is more ideal. Use the type of pass hole that maximizes the number of cabling paths. During fan-out design, the online circuit test should be taken into account. The test fixture may be expensive and is generally ordered only when it is about to be put into full production. It would be too late to consider adding nodes to achieve 100% testability at this time.

After careful consideration and prediction, the design of on-line circuit testing can be carried out at the initial stage of design. After implementation in the later stage of the production process, the type of through-hole fan-out is determined based on the wiring path and On-line circuit testing, power Supply and grounding will also affect wiring and fan-out design. In order to reduce the inductance produced by the filter capacitor connection line, the pass hole should be as close as possible to the pin of the surface mount device. Manual wiring can be used if necessary, which may affect the original wiring path, it may even cause you to reconsider which type of pass-through, so you must consider the relationship between the pass-through and pin inductance and set the priority of the pass-through specification.

5. Manual cabling and Key Signal Processing

Although this article mainly discusses the issue of Automatic wiring, manual wiring is an important process for the design of printed circuit boards in the present and future. Manual cabling helps the automatic cabling tool to complete cabling. As shown in Figure 2a and Figure 2B, You can manually route and fix the selected network (net) to form a path that can be used for automatic cabling.

Regardless of the number of critical signals, you can first wiring these signals, either manually wiring or using an automatic wiring tool. Key signals must be carefully designed to achieve the expected performance. After wiring is completed, the relevant engineers will check the signal wiring, which is much easier. After the check is passed, fix these wires and then start to automatically route other signals.

6. Automatic cabling

For key signal cabling, you need to control some electrical parameters during cabling, such as reducing the distribution inductance and EMC, and wiring other signals is similar. All EDA vendors provide a way to control these parameters. After learning about the input parameters of the Automatic wiring tool and the impact of the input parameters on the wiring, the quality of the Automatic wiring can be guaranteed to a certain extent.

General rules should be used to automatically route signals. By setting constraints and disallow wiring areas to limit the layers used by a given signal and the number of holes used, the wiring tool can automatically route according to the engineer's design philosophy. If there is no limit on the layers used by the Automatic wiring tool and the number of passing holes, each layer will be used for Automatic wiring, and many passing holes will be generated.

After the constraints and rules created by the application are configured, the Automatic wiring will achieve the same result as expected. Of course, you may need to sort out the rules, at the same time, other signals and network cabling space must be ensured. After a part of the design is completed, fix it to avoid being affected by the wiring process.

Use the same steps to route other signals. The number of cabling times depends on the complexity of the circuit and the number of general rules you define. After a type of signal is completed, the constraints on other network cabling are reduced. However, many signal cabling requires manual intervention. The current Automatic wiring tool is very powerful, and can usually complete 100% of the wiring. However, when the Automatic wiring tool does not complete all signal cabling, it needs to manually route the remaining signals.

7. Key Points of automatic Cabling Design include:

7.1 slightly changed the settings and tried multiple path wiring;
7.2 keep the basic rules unchanged. Try different wiring layers, different printing lines, different spacing widths, different linewidth, and different types of passing holes, such as blind holes and buried holes, observe how these factors affect the design results;
7.3 let the wiring tool handle the default network as needed;
The less important the signal is, the more freedom the Automatic wiring tool gives to the device.

8. Wiring

If the EDA tool software you use can list the cabling length of signals and check the data, you may find that the signal cabling length is very long with few constraints. This problem is easy to handle. Manual Editing can shorten the signal wiring length and reduce the number of passing holes. During the sorting process, you need to determine which cables are reasonable and which ones are unreasonable. Like the manual wiring design, the Automatic wiring design can also be organized and edited during the inspection process.

9. Appearance of the circuit board

In the past, the visual effects of the circuit board were often paid attention. The automatically designed circuit board is no better than the manually designed circuit board, but it can meet the specified electronic characteristics, and the complete design performance is guaranteed.