When designing a PCB (printed circuit board), one of the most basic issues to consider is how many wiring layers, ground planes and power planes are needed to achieve the functions required by the circuit. The wiring layers, ground planes and power planes of the printed circuit board The determination of the number of plane layers is related to requirements such as circuit function, signal integrity, EMI, EMC, and manufacturing costs. For most designs, there are many conflicting requirements in factors such as PCB performance requirements, target cost, manufacturing technology, and system complexity. The stack-up design of the PCB is usually determined by a compromise after considering various factors. High-speed digital circuits and whisker circuits usually adopt multi-layer board designs. What factors affect PCB stackup design?

1. Layering

In a multi-layer PCB, it usually contains a signal layer (S), a power (P) plane and a ground (GND) plane. Power planes and ground planes are usually solid planes without separation, and they will provide a good low-impedance current return path for current from adjacent signal traces. The signal layer is mostly located between these power or ground reference plane layers, forming a symmetrical stripline or an asymmetrical stripline. The top and bottom layers of multi-layer PCBs are usually used to place components and a small number of traces. These signal traces must not be too long to reduce direct radiation generated by the traces.

2. Determine the single power supply reference plane (power plane)

The use of decoupling capacitors is an important measure to solve power supply integrity. Decoupling capacitors can only be placed on the top and bottom layers of the PCB. The traces, pads, and vias of the decoupling capacitor will seriously affect the effect of the decoupling capacitor. This requires that the traces connected to the decoupling capacitors should be as short and wide as possible during design, and the wires connected to the vias should also be considered. Keep it as short as possible. For example, in a high-speed digital circuit, you can place decoupling capacitors on the top layer of the PCB, assign layer 2 to the high-speed digital circuit (such as a processor) as the power layer, layer 3 as the signal layer, and layer 4 Set to high-speed digital circuit ground.

In addition, try to ensure that the signal traces driven by the same high-speed digital device use the same power layer as the reference plane, and this power layer is the power supply layer of the high-speed digital device.

3. Determine the multi-power reference plane

The multi-supply reference plane will be divided into several physical areas with different voltages. If there is a signal layer next to the multi-power layer, the signal current on the nearby signal layer will encounter an unideal return path, causing gaps in the return path. For high-speed digital signals, this unreasonable return path design may cause serious problems, so it is required that high-speed digital signal wiring should be kept away from multi-power reference planes.

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4. Determine multiple ground reference planes (ground planes)

Multiple ground reference planes (ground layers) can provide a good low-impedance current return path, which can reduce common-mode EMl. Ground planes and power planes should be tightly coupled, and signal layers should be tightly coupled to adjacent reference planes. This can be achieved by reducing the thickness of the media between layers.

5. Reasonable design of wiring combinations

The two layers spanned by a signal path are called a “routing combination.” The best routing combination design avoids return current flow from one reference plane to another, but flows from one point (plane) of a reference plane to another point (plane). In order to complete complex wiring, interlayer conversion of wiring is inevitable. When converting between signal layers, it is necessary to ensure that the return current can flow smoothly from one reference plane to another. In a design, it makes sense to treat adjacent layers as a routing group. If a signal path needs to span multiple layers, it is usually not a reasonable design to use it as a routing group, because a path through multiple layers is not clear for return current. Although the ground bounce can be reduced by placing decoupling capacitors near the vias or reducing the dielectric thickness between the reference planes, it is not a good design.

6. Set the wiring direction

On the same signal layer, the direction of most wiring should be consistent and orthogonal to the wiring direction of adjacent signal layers. For example, you can set the wiring direction of one signal layer to the “Y-axis” direction, and set the wiring direction of another adjacent signal layer to the “X-axis” direction.

7. Adopt an even-numbered layer structure

From the designed PCB stackup, it can be found that the classic stackup design is almost all even-numbered layers, not odd-numbered layers. This emergency is caused by a variety of factors, as shown below.

It can be understood from the manufacturing process of printed circuit boards that all conductive layers in the circuit board are stored on the core layer. The material of the core layer is generally a double-sided cladding board. When the core layer is fully utilized, the conductive layer of the printed circuit board The number is even.

Even-layer printed circuit boards have cost advantages. Due to the lack of one layer of dielectric and copper coating, the cost of raw materials for odd-numbered printed circuit boards is slightly lower than the cost of even-numbered printed circuit boards. However, because odd-numbered layer printed circuit boards require a non-standard laminated core layer bonding process based on the core layer structure process, the processing cost of odd-numbered layer printed circuit boards is significantly higher than that of even-numbered layer printed circuit boards. Compared with the ordinary core structure, adding copper cladding outside the core structure will lead to a decrease in production efficiency and a prolongation of the production cycle. The outer core layer also requires additional processing before lamination and bonding, which increases the risk of scratches and incorrect etching of the outer layer. The added outer layer treatment will significantly increase manufacturing costs.

When the inner and outer layers of the printed circuit board are cooled after the multi-layer circuit bonding process, different lamination tensions will cause the printed circuit board to bend to varying degrees. And as the thickness of the board increases, the risk of bending of a composite printed circuit board with two different structures increases. Odd-numbered circuit boards are prone to bending, while even-numbered layers of printed circuit boards can prevent the circuit board from bending.

During design, if there are an odd number of layers, you can use the following method to increase the number of layers.

If you design a printed circuit board with an even number of power layers and an odd number of signal layers, you can use the method of adding signal layers. The added signal layer will not lead to an increase in cost, but can shorten the processing time and improve the quality of the printed circuit board.

If you design a printed circuit board with an odd number of power layers and an even number of signal layers, you can use the method of adding power layers. Another simple method is to add a ground layer in the middle of the stack without changing other settings, that is, first lay out the odd-numbered printed circuit board layers, and then copy a ground layer in the middle.

In microwave circuits and mixed media (media with different dielectric constants) circuits, a blank signal layer can be added near the center of the PCB stackup to minimize stackup imbalance.

8. Cost considerations

In terms of manufacturing cost, with the same PCB area, the cost of multi-layer circuit boards is definitely higher than single-layer and double-layer circuit boards, and the more layers, the higher the cost. However, when considering factors such as circuit function and circuit board miniaturization, and ensuring signal integrity, EMl, EMC and other performance indicators, multi-layer circuit boards should be used as much as possible. Comprehensive evaluation shows that the cost difference between multi-layer circuit boards and single- and double-layer circuit boards is not much higher than expected.

The thermal expansion coefficient of the PCB board refers to the change in the length of the material per unit length under unit temperature change when the temperature of the material changes. Simply put, it is the proportion of length change caused by temperature changes when a material is heated or cooled.

The thermal expansion coefficient is a material physical property parameter used to describe the behavior of materials during thermal expansion and contraction. It can affect the dimensional accuracy and performance stability of materials. Especially for electronic devices, accurate understanding and control of the thermal expansion coefficient is crucial to ensure product quality and reliability.

The coefficient of thermal expansion is usually expressed in ppm/°C (parts per million per degree Celsius). For example, assuming a material has a thermal expansion coefficient of 20 ppm/°C, the length of the material will increase by 0.002% of the original length for every 1°C increase in temperature.

PCB board is a common electronic component substrate and is widely used in the fields of electronics and communications. Electronic devices generate heat during operation, and thermal expansion and contraction are very common. Therefore, it is very important to select the appropriate PCB board material and accurately measure its thermal expansion coefficient.

There are many methods to measure the thermal expansion coefficient of PCB boards. Two common measurement methods will be introduced below.

1. Expanded cantilever beam method

The expansion cantilever beam method is an indirect method of measuring the thermal expansion coefficient and is often used for small sample testing of PCB board materials. The principle is to calculate the thermal expansion coefficient by measuring the deflection of the material under temperature changes.

The specific experimental steps are as follows:

(1) Prepare a thin slice sample of appropriate size and fix it on the support beam to form a cantilever.
(2) Place the cantilever in a constant temperature environment and record the baseline deflection.
(3) Gradually heat or cool the cantilever and record the deflection at different temperatures.
(4) Use Hooks’ law to calculate the thermal expansion coefficient based on the deflection changes at different temperatures.

The advantage of this method is that the testing process is relatively simple and the equipment requirements are low. However, measurement accuracy may be limited because of the small sample size.

2. Thermal expansion coefficient meter method

The thermal expansion coefficient meter method is a method of directly measuring the thermal expansion coefficient and is suitable for testing large-size PCB board materials. This method calculates the thermal expansion coefficient by accurately measuring the linear expansion of materials at different temperatures.

The specific experimental steps are as follows:

(1) Prepare PCB board samples of suitable size.
(2) Fix the sample in the thermal expansion coefficient meter and establish the zero position of the measuring instrument.
(3) Adjust the temperature and record the sample length at different temperatures.
(4) Calculate the thermal expansion coefficient based on the recorded length changes.

The advantage of this method is that the test accuracy is relatively high and the thermal expansion coefficient of large-size samples can be accurately measured. However, the instruments and equipment are relatively expensive, and the testing process requires operators to have a certain technical level.

In addition to the above common measurement methods, there are other methods such as cantilever beam method, thermal difference method and expansion volume method. Each method has its applicable scope and advantages, and the choice should be based on actual needs.

To sum up, the thermal expansion coefficient of PCB board refers to the proportion of material length change per unit length under unit temperature change. Accurate measurement of thermal expansion coefficient is very important to ensure the quality and reliability of electronic devices. Common measurement methods include the expansion cantilever beam method and the thermal expansion coefficient meter method. Each method has its advantages and disadvantages and needs to be selected according to the actual situation.

Automobile pcb board, that is, printed circuit board for automobiles. As the carrier of on-board electronic equipment, it plays a vital role in the operation of the car. However, what is so special about it that makes it stand out from ordinary circuit boards?

1. Concept and composition of automobile pcb board
Automotive PCB boards are important components used to install electronic components and provide connection paths for various electronic devices and systems in vehicles. It usually consists of a substrate, conductive paths, circuit components, etc. The substrate materials of automotive PCB boards mainly use high-temperature, heat-resistant, moisture-resistant, and corrosion-resistant materials to cope with various challenges in the automotive environment. Conductive paths are usually printed on the substrate. These copper conductive paths are called printed circuits, which connect circuit components and form paths for current flow. Circuit components, such as resistors, capacitors, inductors, diodes, transistors, etc., are key components of automotive circuit boards. They are inseparable from controlling automotive power systems, environmental sensing, navigation and other functions.

2. Application fields of automotive circuit boards
The complexity of modern automobiles far exceeds the mechanical equipment visible to the naked eye of drivers in the past, and automobile PCB boards are used in many fields. For example, the power control system is used to control the entire power system of the car, including the engine, transmission, brakes, etc.; the on-board information system is used to realize navigation, communication, entertainment and other functions; the safety control system involves here including the body stability system, Airbag control, etc.; environment perception systems, such as reversing radar, raindrop detection, etc. Each function requires targeted design and production of automotive PCB boards, which have strict requirements on the performance of automotive PCB boards.

3. Functions and characteristics of automobile pcb boards
The biggest function of the automobile PCB board is undoubtedly to provide connections for electronic components and form a current path for various electronic devices and systems in the automobile. Its performance parameters mainly include high temperature resistance, vibration resistance, humidity resistance, corrosion resistance and other properties. Because of the complexity of the automotive working environment, automotive PCB boards need to cope with a variety of harsh environments such as high temperature, humidity, vibration and impact. Therefore, in terms of material selection and design, there are very high requirements for weather resistance and strength, which are much higher than those of ordinary PCB boards. The circuit board is more powerful.

4. The difference between automobile pcb board and ordinary circuit board
Compared with ordinary circuit boards, automotive PCB boards have significant differences in many properties. The main differences are greater weather resistance and strength. There are higher requirements for the design and production of automobile PCB boards, which need to meet the harsh working environment of automobiles and their complex and changing functional requirements. Automotive PCB boards also have higher vibration resistance and waterproof performance, and pay more attention to environmental protection, because for outdoor equipment such as cars, vibration resistance, waterproofness and environmental protection are particularly important. Automotive PCB boards may be more complex and sophisticated than ordinary circuit boards, because various precise control and adjustment functions in the automotive system need to be implemented by electronic circuits, which requires the circuits to be determined to be more sophisticated and complex.
High temperature resistance, vibration resistance, humidity resistance, corrosion resistance and other properties enable automotive PCB boards to work stably in complex and harsh automotive environments. Compared with ordinary circuit boards, automotive PCB boards are more sophisticated and complex, and meet various functional requirements at the same time, showing their superiority and complexity. Understanding these is of great significance for us to better understand automotive PCB boards and their important role in automotive electronic systems.

In high-speed PCB design, critical high-speed signal lines such as clocks need to be shielded. If there is no shielding or only part of it is shielded, it will cause EMI leakage. It is recommended that shielded wires be drilled and grounded every 1000mil.

Rule 2

High-speed signal routing closed-loop rules

As the density of PCB boards becomes higher and higher, many PCB LAYOUT engineers are more likely to make a mistake during the routing process, that is, high-speed signal networks such as clock signals produce closed-loop results when routing multi-layer PCBs. , such a closed-loop result will produce a loop antenna, increasing the radiation intensity of EMI.

Rule three

Open-loop routing rules for high-speed signals

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Rule 2 mentions that the closed loop of high-speed signals will cause EMI radiation, but the open loop will also cause EMI radiation. In high-speed signal networks such as clock signals, once an open loop occurs during multi-layer PCB routing, a linear antenna will be generated, increasing the radiation intensity of EMI.

Rule 4

Characteristic impedance continuity rules of high-speed signals

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For high-speed signals, the characteristic impedance must be continuous when switching between layers, otherwise the EMI radiation will be increased. In other words, the width of wiring on the same layer must be continuous, and the impedance of wiring on different layers must be continuous.

Rule five

Wiring direction rules for high-speed PCB design

The wiring between two adjacent layers must follow the principle of vertical wiring, otherwise it will cause crosstalk between lines and increase EMI radiation. In short, adjacent wiring layers follow the horizontal and vertical wiring directions, and vertical wiring can suppress crosstalk between lines.

Rule 6

Topology rules in high-speed PCB design

In high-speed PCB design, the control of the circuit board’s characteristic impedance and the design of the topology under multiple loads directly determine the success or failure of the product. The diagram shows a daisy chain topology, which is generally beneficial when operating at a few Mhz. Star symmetry of the backend is recommended in high-speed PCB designs.

Rule 7

Resonance rules for trace length

Check whether the length of the signal line and the frequency of the signal constitute resonance. That is, when the wiring length is an integer multiple of 1/4 of the signal wavelength, the wiring will resonate, and the resonance will radiate electromagnetic waves and cause interference.

Rule 8

Return path rules

All high-speed signals must have good return paths. Ensure that the return path of high-speed signals such as clocks is as small as possible. Otherwise, radiation will be increased, and the size of the radiation is proportional to the area enclosed by the signal path and the return path.

Rule 9

Device decoupling capacitor placement rules

The placement of the decoupling capacitor is very important. Unreasonable placement will not achieve the decoupling effect at all. The principle is: be close to the pins of the power supply, and the area surrounded by the power supply traces and ground wires of the capacitor is small.