For a long time, in various industry-related articles and reports, there will often be "high-frequency high-speed"/" high-speed high-frequency "related words, especially after 5G is proposed," high-frequency high-speed "is used a lot, it seems that this word is the standard of high-end PCB, no matter what the occasion to" rub ". I remember that in a public speech, there was once an industry veteran who said that people who use "high frequency and high speed" indiscriminately are people who do not understand high frequency and high speed. Indeed, in most cases, from the articles seen by the author and conversations with factory engineers, it is found that most people have a large misunderstanding and blind area of understanding of high frequency and high speed, a large part of them think that high frequency is equal to high speed, high frequency and high speed is a class of PCB; Or it is not clear what the high-frequency and high-speed are. What is the specific relationship between high frequency and high speed, and what are their specific differences and characteristics?

We know that no matter what scientific conclusion or law, it needs to be based on certain conditions or scenarios. Newton's famous three laws all require macroscopic, low-velocity conditions to hold. So to answer the above question, we first have to carry out a scene definition, careful friends must have noticed that the title of this article is the difference between high frequency and high speed in the context of PCB manufacturing. Context and scene should be distinguished here, precisely because in different contexts or scenes, the definition of high frequency and high speed is not exactly the same, the author believes that this is one of the reasons why the word "high frequency and high speed" is abused. The context or scene here is mainly divided into two parts, one is the design and application side, and the other is the manufacturing processing side. For design and application engineers, the role of high frequency or high speed PCB is to meet the needs of signal transmission, so the core of its concern lies in the properties of the signal transmitted by the PCB itself, which can only be defined from the signal properties themselves. For the engineers of the manufacturing board factory, the core of their concern is the processing requirements of the PCB; At the same time, the engineer of the board factory can not judge the signal properties required to be transmitted by the board through the processing instructions in most cases, which also forces the board factory to define the "high frequency" and "high speed" only from the perspective of its own ability to distinguish. Below we discuss the "high frequency" and "high speed" from the two scenarios of design application and manufacturing processing.

First of all, from the perspective of design and application, the signal transmitted by the board can be divided into two kinds, one is an analog signal (as shown below on the left), and the other is a digital signal (as shown below on the right), and they can be converted into each other.

For analog signals, generally speaking (not completely accurate in the absolute sense), we can understand that when the frequency of analog signals is high to a certain degree, that is, electromagnetic waves with a frequency above 300KHz (below 300GHz) can be transmitted wirelessly to the air for a longer distance, such signals are usually called Radio frequency signals (Radio frequency). Its core discriminant attribute is frequency, which can also be called a high-frequency signal, but this "high" is only a comparative concept, not an absolute scientific term. Higher frequency above 300MHz (below 300GHz) signals we commonly call Microwave signals (Microwave), can also be called high frequency signals, so this type of signal can be collectively referred to as high frequency signals.

However, for digital signals, although it has a more complex definition of higher frequencies (including rise time, frequency, and transmission line length), its core discriminant attribute is the signal rate, that is, the amount of signal capacity (potential information) transmitted per unit time, that is, how much Mbps is the download/upload speed that we usually understand. Like "high frequency", this "high" is only a comparative concept (different applications, different times), it is not a scientific term definition, such as our usual upload or download files, 100Mbps is already a very high speed, and for wired PCB network transmission or fiber optic transmission, it is not high speed. So even in the digital signal transmission scenario, we must also define the specific application. Even for the high-speed signal transmitted by PCB, there is no clear and unified rate standard at present, some places say that more than 1Gbps is high-speed, and some places say that more than 5Gbps can be defined as high-speed. However, for any electromagnetic wave signal, it has a frequency attribute, the same for digital high-speed signals, and as the high-speed signal rate increases, its frequency will also increase, so some high-speed engineers also call it a high-frequency signal. This is also the reason why high-speed signals in many cases are also called high-frequency signals, in this case, high frequency and high speed to achieve a relative unity.

However, from the perspective of manufacturing and processing, especially for some sample factories, the processing file can not accurately determine whether the processed board is a high-frequency signal or a high-speed signal, so most board factories can only discriminate according to the plate selected by the designer from the perspective of operational execution, that is, the board using high-speed plate is collectively referred to as high-speed plate. The boards using high-frequency plates are collectively referred to as high-frequency plates. However, with the continuous improvement of the frequency of high-speed signals, the use of high-speed plates and high-frequency plates is becoming more and more blurred, that is, high-frequency plates use high-speed plates (such as many antenna factories in South Korea designated to use D typical high-speed materials), and high-speed digital plates use typical high-frequency plates (such as many high-speed optical modules use R classic carbon and hydrogen materials). This often confuses high-frequency and high-speed products at the manufacturing end.

So next, we will talk about the distinguishing characteristics of high-frequency and high-speed boards from the perspective of PCB processing:

1. The first is product application, customers and industry. The high-frequency board is mainly used in the field of wireless transmission, such as operator base stations, automotive millimeter-wave radar and so on

The high-speed board is mainly used in wired transmission fields such as AI computing, data center server memory, and backbone network equipment.

It should be noted here that the industry attributes can only represent the main application of the product, with the development of the communication system, the complexity of the products continues to improve, the same module often has both high-frequency board and high-speed board, the same board in different layers or different areas have both high-frequency signals and high-speed signals. 2. Then the material, in most cases, the high-frequency plate is still used by the high-frequency plate, and the high-speed plate uses different grades of high-speed plate according to the specific rate. As for how to judge high-frequency plate or high-speed plate, this is still to be defined from the plate factory supplier, material manufacturers have its target market when launching a plate, and its plate factory characteristics are also special matching according to the corresponding application. In the material system, the high-frequency plate mainly uses hydrocarbon system and PTFE system materials, and the high-speed plate is mainly based on the PPO system. As for the characteristics of high-frequency plate and high-speed plate differences, involving more content, there is an opportunity to discuss separately later. For the high-frequency board, in addition to the dual panel, a large part of it is a digital analog hybrid design, so its materials are mostly used in the antenna layer of high-frequency materials, while the other layers are made of ordinary FR4 materials. In order to save costs, the design of mixed pressure of high-speed materials and ordinary materials has become more and more.

3, and then the graphic design, the most obvious difference is that the high-frequency plate transmission line is relatively large, the overall sparse graphic density, compared to our traditional line graphics, in order to achieve its special performance, the high-frequency plate will have some more "weird" (as shown below), such as there will be some open circuit design, horn, semi-independent pad antenna, in order to control the impedance of the cutting Angle line.

The high-speed plates are basically designed with dense fine lines and close-connected array holes.

4. The final processing requirement Angle, high-speed plate generally has a fine line (3/3mil), high thickness to diameter ratio (10:1 or more), fine hole (4-8mil), high precision back drilling (8~10mil below), high precision impedance (±5-10%), pressure hole design, with the server and AI high computing power application development, High order HDI, step gold finger, low coarse brown, insertion loss control and other high order requirements are becoming more and more popular.

For high-frequency plates, the process requirements have a large span of difficulty, the general requirements are high-precision line width, and the passive intermodulation PIM requirements in special scenarios, and the process difficulties are greater for high-order plates, such as buried resistance, buried copper blocks, multi-stage blind slots, metal holes in the bottom of the slot and graphic design, and a variety of special materials mixed pressure.

In summary, we can know that in the field of PCB manufacturing, high frequency and high speed are completely different two types of products (except for the mixed exception of digital analog), however, there is no scientific and clear rule for the definition of high frequency and high speed, even from the industry, materials, graphics, process multi-dimensional comprehensive analysis can not get a universal rule. This also requires the manufacturing industry engineers and customer engineers to carry out point-to-point in-depth exchanges, deepen the understanding of customer products, so as to slowly form the accumulation of experience, so as to create high-quality products.

Concluding remarks:

As various industries increasingly subdivide their PCB requirements, the previous "one-pot" way of making PCB is destined to lose its basic competitiveness. The ability to deeply understand customer products and clearly analyze industry and customer characteristics will become the core competitive elements that a company pursues. As a practitioner in the PCB manufacturing industry, understanding the industry and understanding the product will also create an important personal competitiveness.