PCB Technical - High-speed and high-density PCB design faces new challenges

With the increasing complexity and performance of electronic products, the density of printed circuit boards and the frequency of their related devices are constantly increasing, and the various challenges that engineers face in the design of high-speed and high-density PCB s are also increasing. In addition to the well-known signal integrity (SI) issues, the next hot spot in high-speed PCB technology should be power integrity (PI), EMC/EMI, and thermal analysis.

With the ever-increasing competition, manufacturers are facing increasing pressure on product launch time. How to use advanced EDA tools and optimized methods and processes to complete the design with high quality and efficiency has become a system manufacturer and Problems that design engineers have to face.

Hot spot: shift from signal integrity to power integrity

When it comes to high-speed design, the first thing people think of is the signal integrity problem. Signal integrity mainly refers to the quality of signal transmission on the signal line. When the signal in the circuit can reach the receiving chip pin with the required timing, duration and voltage amplitude, the circuit has good signal integrity. When the signal can't respond normally or the signal quality can't make the system work stably for a long time, there will be signal integrity problems. The signal integrity is mainly manifested in several aspects such as delay, reflection, crosstalk, timing, and oscillation. It is generally believed that when the system works at 50MHz, signal integrity problems will occur, and as the system and device frequencies continue to rise, the signal integrity problems will become more prominent. The parameters of the components and the PCB board, the layout of the components on the PCB board, and the wiring of high-speed signals can cause signal integrity problems, resulting in unstable operation of the system, or even failure of normal operation at all.

After decades of development of signal integrity technology, its theory and analysis methods have become more mature. Regarding signal integrity issues, signal integrity is not someone’s problem. It involves every link in the design chain. Not only system design engineers, hardware engineers, and PCB engineers must consider it, but it cannot even be ignored during manufacturing. To solve the signal integrity problem, we must rely on advanced simulation tools.

Relative to signal integrity, power integrity is a relatively new technology, and it is considered to be one of the biggest challenges in high-speed and high-density PCB design. Power integrity means that in a high-speed system, the PDS power deliver system has different impedance characteristics at different frequencies, so that the voltage between the power layer and the ground layer on the PCB is not the same everywhere on the circuit board. As a result, the power supply is discontinuous, power noise is generated, and the chip cannot work normally; at the same time, due to high-frequency radiation, power integrity problems will also bring EMC/EMI problems. If the power integrity problem cannot be solved well, it will seriously affect the normal operation of the system.

Generally, the power integrity problem is mainly solved through two approaches: optimizing the stack design and layout of the circuit board, and adding decoupling capacitors. When the system frequency is less than 300 ~ 400MHz, the decoupling capacitor can play a role in suppressing frequency, filtering and impedance control. Placing a suitable decoupling capacitor in the right position will help reduce the problem of system power integrity. But when the system frequency is higher, the effect of the decoupling capacitor is small. In this case, only by optimizing the layer spacing design and layout of the circuit board or other methods to reduce power and ground noise (such as appropriate matching to reduce the reflection problem of the power transmission system), etc., can solve the power integrity problem, and at the same time suppress EMC/EMI.

Regarding the relationship between signal integrity and power integrity, “signal integrity is a concept in the time domain and is easier to understand, while power integrity is a concept in the frequency domain, which is more difficult than signal integrity, but in some respects It has similarities with signal integrity. Power integrity requires higher skills for engineers and is a new challenge for high-speed design. It involves not only the board level, but also the chip and package level. It is recommended to do High-speed circuit board design engineers do power integrity on the basis of solving signal integrity.".

"Soften" your design through simulation

Simulation is a test of a virtual prototype that takes all aspects into consideration. As the design becomes more and more complex, it is impossible for engineers to implement every scheme. At this time, they can only use advanced simulation instead of experiment to make judgments.

In today's system design, in addition to the challenges brought about by high-speed and high-density circuit boards, the pressure of rapid product launches makes simulation an indispensable means of system design. The designer hopes to use advanced simulation tools to find problems in the design stage, so as to complete the system design with high efficiency and high quality.

In traditional circuit board design, engineers rarely resort to simulation. More often, it uses reference designs and design guidelines (ie white papers) provided by upstream chip manufacturers to design in combination with the actual experience of engineers, and then test and test the prototypes produced by the design to find out problems and modify the design. This goes over and over again, until the problem is basically solved. Even if the simulation tool is used occasionally to design, it is only limited to the partial circuit. Modifying the circuit means a delay in time. This delay is unacceptable under the pressure of rapid product launches. Especially for large systems, a small modification may require the entire design to be overturned. The loss it brings to manufacturers is immeasurable.

Product quality is difficult to guarantee, the development cycle is uncontrollable, and the over-reliance on the experience of engineers... these factors make it difficult for the above design methods to cope with the challenges brought by the increasingly complex high-speed and high-density PCB design, so advanced simulation must be used. Tools to solve it. "The design schemes given by upstream chip manufacturers are based on their own prototypes, and the products of system manufacturers cannot be exactly the same as those of upstream manufacturers; at the same time, the design requirements of one chip may be contradictory to the other. It must be simulated to determine the design plan.".

In a sense, simulation is to allow software to complete the functional evaluation on the virtual prototype, which can only be completed by testing the physical prototype. It is a more "soft" and more economical solution.

However, the simulation of high-speed and high-density circuit boards is different from traditional simulation. Mentor Graphics technical engineer Yulifu said: "Traditional simulation is done for schematics. It just adds incentives and looks at the output to determine whether the function is correct; while high-speed simulation is based on the premise that the function is correct, depending on the design. What’s the performance? It is not only for the schematic diagram, but also for the PCB design.” Using the simulation tool, you can judge which scheme is closer to the actual demand, and on the basis of meeting the performance requirements, judge which one has the lower cost.

To find a balance between planning and system cost. Yulifu said: "Using simulation tools, you can judge whether the direction of system improvement is correct, point out the direction for the design, increase the success rate of the first board, and make the product go to the market faster. However, no matter how close the simulation result is to the test result, it cannot Replace the actual test system."

Testing is a true judgment of system performance that includes all real environmental factors. However, simulation is a "test" of virtual prototypes. It is aimed at certain specific conditions. There is no tool that can take all real conditions into consideration at the same time. simulation. However, with the development of technology and the continuous improvement of tools, the approximation of simulation results and actual test results is getting higher and higher, and the guiding significance for the design is also increasing, but at the same time, higher requirements are placed on engineers- -Although the tools are getting easier to use, the judgment and improvement methods of simulation results all depend on the engineer's technical level and theoretical foundation.

At present, in high-speed PCB simulation, the most unsatisfactory effect is EMC/EMI. This is because for high-speed systems, due to the influence of the via effect, three-dimensional modeling of the system is required to effectively simulate the real environment. However, for a large and complex system such as PCB, it is very difficult to model it in three dimensions. According to Yulifu, at present, the method of expert inspection is mainly adopted, which converts EMC/EMI issues into layout and wiring rules on PCBs in accordance with international general standards.

In addition, in terms of three-dimensional analysis, companies such as Ansoft and Apsim can provide specialized tools and methods, and these tools can be used in conjunction with Cadence and Mentor Graphics system tools.

The choice of efficiency: automatic routing and parallel design

Schematic design is not only about "tracing" the circuit in, but also has many other requirements. Schematic design tools should be able to take these requirements to the next step, supporting automatic routing, functional simulation, and so on.

In order to find a more efficient design path, solve the time pressure of product launch, and quickly bring the product to the market, automatic wiring and concurrent design technology came into being.

"If you can make good use of automatic routing technology, you can reduce drawing time and more than double the design efficiency of PCB." However, if you want to realize automatic routing, you must use the electrified rule manager to integrate system design engineers and hardware design engineers. The design requirements for the circuit are passed to the PCB engineer.

For early simpler systems, the usual practice is for hardware engineers to write down the design requirements one by one and tell the PCB design engineer how to do it. But for complex systems, faced with thousands of connections and countless requirements, hardware engineers cannot record these rules one by one, and PCB design engineers cannot check and implement them one by one. At this time, an electrified rule manager is needed to manage various design requirements. Hardware engineers and PCB design engineers can work together on the basis of the same rule manager.

For automatic wiring technology, "If a company has not mastered the technology well, and the signal integrity problem cannot be solved well, it is recommended not to use automatic wiring. Because if you can't define good rules, you will not be able to drive automatic wiring correctly." No matter how developed the tools are, The computer can not completely replace the human brain behavior, so it is impossible to have 100% automatic wiring. The automatic routing we mentioned above is actually a kind of interactive automatic routing, which requires human participation: some rules before automatic routing need to be further determined manually; after the automatic routing is completed, it needs to be verified and modified by an engineer.

For traditional, relatively low-speed system design, many engineers may have such experience, using Cadence's OrCAD to draw schematics, and then use Mentor's PowerPCB for layout. But this method is no longer suitable in the field of high-speed design. "Data cannot be fully converted between tools of different manufacturers. For example, the traditional method of reading netlists cannot bring some electrical properties and requirements in the schematic diagram to the PCB design, so it is not suitable for high-speed design."

In addition to automatic wiring, parallel design is also an effective way to improve design efficiency for large systems. Concurrent design is collaborative design, which means that a circuit board is divided into several parts, and several people design at the same time. According to Yulif, the current Mentor Graphics tools can already be used in parallel design. If you save the design on one machine, the other machine can see it immediately, and the lines on both sides can be automatically connected together. Can alleviate the task of integration between different designs. Yulif said: "By the end of this year, Mentor Graphics' fully dynamic parallel design tool extremePCB will be available to the market. At that time, engineers will be able to perform fully real-time parallel design just like playing CS on a network. Being seen by the other party in real time can facilitate the cooperation between engineers in different places." For concurrent design, it not only requires good design tools, but also good design methods. Concurrent design should not be divided too finely or too broadly. Two or three people are more reasonable, otherwise the ideas are too scattered, which is not conducive to the design.

Beyond the PCB: System-level considerations for high-speed issues

When the system develops from hundreds of megabytes to tens of gigabytes, chip design, packaging design, and system design can no longer be considered separately. For high-end products, packaging design and system design should be considered when designing chips.

After removing the problems of the software itself, how to streamline the process, reduce the error of the engineer from the process, and enable the engineer to devote more energy to the design, so that the product enters the market as soon as possible, has also become the content that EDA manufacturers are considering.

Generally, the connection line on a system starts from the I/O of the chip (Silicon), passes through the bump and substrate of the package, reaches the pin of the package, and then passes through the PCB to the pin, substrate, bump and pin of another package. Chip I/O. Chips, packaging, and circuit boards are three different areas. Previous engineers would not consider them comprehensively when designing, nor could they know the ideas of other engineers. However, as the design frequency increases, the chip area decreases, and the design cycle shortens, manufacturers should consider package design and PCB design when designing chips, so that the three can be effectively combined. "At this time, no matter from the perspective of signal integrity or the design cycle, we should consider the design of Silicon-Package-Board at the same time, and coordinate the relationship between them. For example, sometimes there will be a lot of The difficult timing problem can be easily solved in the package."