- What is a high-frequency, high-speed PCB board?
- The birth and development of high-speed PCBs
- The difference between high-frequency boards and ordinary PCB boards
- PCB high-speed definition
- Aspects of high-speed PCB design considerations
- General high-speed PCB material requirements
- What are the advantages of high-speed circuit boards?
- Need high-speed PCB design and production?
What is a high-frequency, high-speed PCB board?
High-speed PCB is a very common term, so what is a high-speed PCB? Explain our view of the term high-speed PCB.
There is a view that the speed of the digital circuit reaches or exceeds 45MHZ ~ 50MHZ; the signal at this speed accounts for more than one-third of the entire system, known as high-speed PCB or high-speed circuit.
At the same time, there is such a phenomenon: many people mention high-speed PCBs; it is instinctive that I have to pay great attention to this PCB. Otherwise, it is easy to have an accident. If my board is not a high-speed PCB board, then do it, no problem!
Now, if you want to make an electronic system, the highest rate of the signal is 44.99999999999999) MHz (infinity 9), but this signal drags a long string of loads (say 80), and each branch reaches more than 30 cm. From the above point of view, this is not a high-speed PCB (although the number 9 is a bit much, but less than 45MHz!) So full of confidence, oh, this is not a high-speed PCB, do it, it’s okay, not a high-speed PCB, how can you go wrong! Can this PCB be more perverted? Well, and overtime? Behind the project schedule, is there wood? Does delivery have a delay? Can’t get the bonus?
That’s why we have this – the question is:45MHZ~50MHZ How did this standard come about? What is the basis for the value of one-third?
Generally, come to the PCB industry says that the high-frequency, high-speed PCB board refers to the frequency of 1GHz or more printed circuit boards; this definition may not be the same in the industry. With the development of modern technology, device integration has increased significantly, and the operating frequency of various digital devices is also getting higher and higher; the signal alone can already reach the nanosecond level or even smaller. Hundreds of megahertz (MHz) or even gigahertz (GHz) of high-speed signals for the designer the need to consider the signal integrity (Signal Integrity) issues that do not need to be considered in the design of low-frequency circuits. These include delay, reflection, crosstalk, synchronous switching noise (SSN), and electromagnetic compatibility (EMC). However, a high-speed circuit is a digital logic circuit with a frequency of 50 MHz or more, and the circuit operating above this frequency accounts for more than 1/3 of the entire system; it can be called a high-speed circuit. If the line propagation delay is greater than 1/2 of the rise time of the driving end of the digital signal, such signals can be considered high-speed signals.
We believe that the concept of high-speed PCB only originated from a vague comparison in the minds of engineers. When digital circuits first emerged, the speed was really low, and the PCB was as long as it could be connected to the basic work. After the signal rate continues to increase, slowly before the old practice of drawing PCBs fails, the PCB will have problems. Speed up, the PCB is prone to problems, and this feeling continues to strengthen, so the term high-speed PCB was born. What is high speed? Maybe it is just a feeling. But later had to be explained in various ways. We want to say: There is no need to stretch. To add a new word to emphasize the sadness makes sense?
One of our points is that there is no need to play with concepts for engineering problems. All we care about is: Try not to let the PCB go wrong and make something on time. It’s not that painful for engineers to debug. For companies, the product can be marketed in time to capture the market, and for equipment suppliers, the product can be shipped in time to avoid losses.
So we advocate not focusing on nouns, just focusing on this phenomenon: If the signal distortion, PCB may be in trouble. For most PCBs, this possibility exists, and our goal is to eliminate this risk at design time.
The birth and development of high-speed PCBs
High-speed PCB has long been, and the earliest can be traced back to IBM, Cray, and other companies that design and manufacture large computers. But compared to other parts of the PCB industry, it is a fairly isolated niche market. For the rest of the world, high-speed PCBs became an issue in the early 1980s, when TTL was fast enough, and the path became longer. This is how we define high speed in terms of signal integrity; PCBs are high speed when the signal path is long relative to the rise time, and the path becomes longer when the signal reflects from the open end and causes problems.
In the exact math, if the rise time is one nanosecond, then 3 inches or longer for each path will fail due to reflections. Note:3 inches = 7.5 cm, 6 inches = 15 cm. By finding the speed of the path, the rise time is converted to length. In the PCB, this is equivalent to 6 inches per nanosecond. This is the starting point. In addition, the frequency of occurrence or clock frequency has no impact on the decision.
As Lee Ritchey, president and founder of Speeding Edge, says: “I’ve seen designs fail on the ‘power up’ reset line. This happens when the power is turned on. People think it doesn’t matter because it doesn’t happen often. The world has this habit of making snap judgments based on clock frequency, and that’s where they get into trouble.”
As an example, a few years ago, we had a problem with a malfunctioning pulse oximeter. The company that designed the product decided it was “slow” because it was clocked at 1 MHz. But no, because the memory portion of the design had a 350 picosecond rise time.
The last memory component data from Micron Technology showed a slow edge of 100 picoseconds and a standard edge of 50 picoseconds. No fast edge is specified. If you start with nanoseconds, the slow edge is 1/10th of an inch, which means that for a slow edge, there could be reflection failure for a path 3/10th of an inch in length. In this case, no product will be fast at the clock frequency.
When they think the product designer is not “fast” enough because of their final product implementation, they are still in trouble, indicating that the production speed is not high. In addition, there are areas where people tend to make mistakes. Including:
Not following signal integrity rules. This includes not controlling impedance, not using the right terminals, and using application notes as a design guide. Many excuses for design failure start with “I followed the application notes, and the product didn’t work. (Many application notes do not contain valid signal integrity recommendations.)
Many technical product ideas come from people who don’t understand the technical rules. Over the past 30 years, many product ideas have come from computer science engineers without training in signal integrity. Only mastered a bunch of rules, but many people in high-speed design, without knowing how things work, applied to the design process.
The difference between high-frequency boards and ordinary PCB boards
With the continuous progress of high-tech, electronic products are becoming more and more sophisticated, ordinary circuit boards are far from being able to meet the needs of the market, and with the emergence of high-frequency circuit boards to meet these needs, the high-frequency circuit board is the line width / inter moment than the customary board to narrow, fine hole / narrow ring width and buried blind hole and other skills to reach the high-density board, high-frequency PCB board has a “fine, small, narrow, thin “and several other features, to 0.2m line width, for example, the normal production between O.16 ~ 0.24mm is considered qualified, the error of (O.20 earth 0.04) mm, and 0.1mm line width, the same error of (0.10 ± O.02) mm, the data can be seen in the latter high precision circuit board line width is twice as high as the customary board. The difference between high-frequency circuit boards and ordinary circuit boards is that high-frequency circuit board lines are more precise, the number of layers is higher, and the process is more difficult.
PCB high-speed definition
The Circuit board size is increasingly smaller, and the circuit is more powerful, but the clock speed and device rise time is increasingly fast. The high-speed design has become an important part of the design process. So how to define whether the PCB is designed with high speed?
Suppose the clock frequency of a digital system reaches or exceeds 50 MHz, and the circuitry operating above this frequency has accounted for a certain number (e.g., 1/3) of the entire electronic system. In that case, it is called a high-speed circuit.
The harmonic frequency of the signal is higher than the repetition frequency of the signal itself, which is an unintended consequence of the rapid changes in the rising and falling edges of the signal leading to its transmission. Therefore, it is generally accepted that if the routing propagation delay is greater than 20% of the rising time of the signal at the driver’s end, such signals are considered to be high-speed signals and may produce transmission line effects.
Having defined the prerequisites for transmission line effects, how can we determine whether the propagation delay is greater than 20% of the signal rise time at the driver? Typical values of signal rise time can generally be found in the device manual. In contrast, the PCB design’s actual wiring length and propagation speed determine the signal propagation time. For example, the signal propagation speed on the “FR4” board is about 6in/ns (1in = 2.54 cm), but if there are more holes and more device pins, the speed will drop, the signal rise time of high-speed logic devices is about 0.2ns, so the safe route length will not exceed 0.24in.
Assuming that “Tr” for the signal rise and “TD” for the signal line propagation delay, the following rule of thumb: if the signal ≥ 5TD, the signal falls in the safe area; if 2TD ≥ Tr ≥ 5TD, the signal falls in the uncertainty region; if Tr ≤ 2TD, then the signal falls in the problem region. A high-speed circuit design method should be used for signals that fall in uncertain and problematic regions.
In contrast to low-speed digital design, the high-speed digital design emphasizes the path and interconnections between digital circuits for signal transmission and the complete current path from the signal-sending chip to the signal-receiving chip, including various structures such as packages, wiring, connectors, and sockets. High-speed digital circuit design focuses on the impact of interconnections on signal propagation and the interaction of signals with the outside world.
Aspects of high-speed PCB design considerations
For modern electronic components, high-speed PCB design is very complex and requires in-depth study of the field, whether it is a consumer product or industrial-grade, military-grade products are the same, CPU or FPGA and other key devices are the object of our concentrated research. However, we are now not correct, standardized design of the board, which may have many problems at a later stage, so we should pay more attention to modern high-speed PCB design, mainly through the following aspects to design.
PCB lamination
The cornerstone of the entire PCB defines the number of layers within the high-speed PCB (the more layers, the higher the cost) while allowing the creation of characteristic impedance on the desired layer. As with many things in engineering, this is a trade-off between the manufacturing process and several layers to achieve reliability, yield and cost goals.
Types of vias
There are different types of interconnections through layers and devices, mainly through holes, buried holes, and blind holes (these types are mainly used for single-layer, multilayer, etc.). The best design minimizes the different types of vias, and it is also important to communicate with your PCB supplier to ensure that your vias are within their functional limits. You also need to ensure the current carrying capacity of the different visa types to ensure a high current path through which your circuit can pass.
Design Rules
These design rules are mainly the following constraints, i.e. device layout, crosstalk budget, layer assignment, length matching/run time analysis, etc. It also includes design for manufacturing rules to ensure that the design comes out with a file compliant with manufacturing requirements, such as the correct spacing between vias.
Layout Wiring Planning
Before you can start verifying signal and power integrity, you must first ensure that you can lay out and route all signal lines on a high-density device. This has a lot to do with stacking the PCB board; for example, if you use buried blind vias (when possible), these are how many layers you need to plan for layout wiring. Once you have defined the number of layers for a high-speed PCB, you can route the wires according to the pre-planned alignment layers, ultimately ensuring that our planning is correct.
Signal Integrity
The most common aspects of good high-speed PCB design consider the signal rise and fall times, wiring length and characteristic impedance, drive capability, and conversion rates of drivers and terminals. SI simulations will be performed after the PCB layout and layout to ensure optimal performance. In addition to the SI aspects of the simulation, you will also need to consider the crosstalk budget.
Power Integrity
High-performance devices, especially modern FPGAs and ASICs, may require high currents at low voltages, among other issues. These all boil down to the realm of power integrity through the simulation of signal integrity to ensure the DC and AC performance of the power distribution network, etc.
Of course, in addition to the knowledge points listed above, but also mentioned a good starting point. Many other design specifications need attention after continuous refinement of high-speed PCB design in the relevant issues, or you can design a more level of work.
General high-speed PCB material requirements
As a qualified and excellent PCB design engineer, I not only need to master high-speed PCB design skills but also need to have an understanding of other related knowledge, such as the selection of high-speed PCB materials. This is because the wrong choice of PCB materials can also hurt the signal transmission performance of high-speed digital circuits.
General high-speed PCB material requirements are as follows.
1, low loss, CAF/heat resistance and mechanical toughness (stickiness) (good reliability)
2, stable Dk / Df parameters (small coefficient of variation with frequency and environment)
3, small material thickness and glue content tolerance (good impedance control)
4, low copper foil surface roughness (reduce loss)
5, try to choose a flat open window with small glass fibre cloth (reduce skew and loss)
The integrity of high-speed signals is mainly related to impedance, transmission line loss and time delay consistency. If the appropriate waveform and eye diagram can be received at the receiving end, it can be considered that the signal integrity is guaranteed. Therefore, the main parameter indicators for high-speed digital circuit PCB selection are Dk, Df, loss, etc.
Whether analogue or digital circuits, the PCB material dielectric constant Dk is an important parameter for material selection because the Dk value is closely related to the actual circuit impedance value applied to this material. When the Dk value of the PCB material changes, either with frequency or with temperature, the transmission line impedance of the circuit can change unexpectedly, adversely affecting the signal transmission performance of high-speed digital circuits if the Dk of the PCB material exhibits different values for harmonic components at different frequencies, then the impedance will also exhibit different resistance values at different frequencies. Unexpected Dk values and impedance changes can result in a degree of loss and frequency shift in the harmonic components, distorting the analogue harmonic components of high-speed digital signals and thus reducing signal integrity.
Dispersion is closely related to the Dk value and is a material characteristic. The smaller the change in Dk value with frequency, the smaller the dispersion and the better the application for high-speed digital circuits. Various factors, such as dielectric polarization, material loss, and surface roughness of high-frequency copper conductors, can cause circuit dispersion. Therefore, the Dk value of high-speed materials should be stable, and the smaller the fluctuations, the better in different frequency bands and temperatures.
What are the advantages of high-speed circuit boards?
1. High efficiency
Generally speaking, because the material constants are small, the loss in high-frequency, high-speed circuit boards is naturally lower than in other boards. In such excellent innate conditions, induction heating technology at the forefront of scientific and technological development can also meet the needs of the target heating so that the high-frequency circuit board is very high efficiency.
2.Fast speed
Fast speed is well known, and the transmission speed positively correlates with the dielectric constant. In the principle of electricity, the transmission speed and the square root of the dielectric constant are inversely proportional; that is, the more dielectric constant, the transmission speed becomes slower; the smaller the dielectric constant, the faster the transmission speed. This is one of the reasons for the popularity of high-frequency, high-speed circuit boards. The use of special materials ensures the characteristics of the small dielectric constant and the transmission speed so that the board’s operation is relatively stable.
3. Large degree of dispatchability
High-frequency circuit board is widely used in various industries for precision metal heating treatment; in the process industry, they can not only achieve the heating of different deep parts but also pay attention to the characteristics of partial heating. It can easily complete the surface or deep, concentrated or dispersed heating.
4. Strong tolerance
The environment affects the composition of the medium. Therefore, the high-frequency, high-speed circuit board still has certain environmental provisions, especially in the south, where there is more humid weather. High-frequency circuit boards can be well adapted to such an environment. High-frequency circuit boards made of very low water absorption materials can challenge this environment. At the same time, it is desirable to make high-frequency, high-speed circuit boards with the advantages of chemical resistance to moisture and high-temperature resistance in a humid environment and a large peel strength.
Need high-speed PCB design and production?
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