At present, there are signs that the frequency of printed circuit board design is increasing. With the increasing data rate, the bandwidth required for data transmission also makes the upper limit of signal frequency reach 1GHz or even higher. Although this high-frequency signal technology is far beyond the scope of millimeter wave technology (30GHz), it does involve RF and low-end microwave technology.
The engineering design method of RF must be able to deal with the strong electromagnetic field effect that usually occurs at the higher frequency band. These electromagnetic fields can induce signals on adjacent signal lines or PCB lines, which will cause annoying crosstalk (interference and total noise) and damage the system performance. Return loss is mainly caused by impedance mismatch, which has the same influence on signals as additive noise and interference.
High return loss has two negative effects: 1. The signal reflected back to the signal source will increase the system noise, making it more difficult for the receiver to distinguish the noise from the signal; 2. Any reflected signal will basically degrade the signal quality, because the shape of the input signal has changed.
Although the digital system only processes 1 and 0 signals and has very good fault tolerance, the harmonics generated when the high-speed pulse rises will lead to the weaker signal with higher frequency. Although forward error correction technology can eliminate some negative effects, part of the bandwidth of the system is used to transmit redundant data, which leads to the degradation of system performance. A better solution is to make the RF effect contribute to the integrity of the signal, rather than damage it. It is suggested that the total return loss at the highest frequency (usually the poor data point) of the digital system is -25dB, which is equivalent to VSWR of 1.1.
The goal of PCB design is smaller, faster and lower cost. For RF PCB, high-speed signal sometimes limits the miniaturization of PCB design. At present, the main way to solve the crosstalk problem is to manage the ground plane, space the wires and reduce the stud capacitance. The main method to reduce the return loss is impedance matching. This method includes effective management of insulating materials and isolation of active signal lines and ground lines, especially between the signal lines and the ground where the state jumps.
As the interconnection point is the weakest link in the circuit chain, in RF design, the electromagnetic property of the interconnection point is the main problem faced by engineering design. It is necessary to inspect each interconnection point and solve the existing problems. The interconnection of circuit boards in the system includes chip-to-circuit board, internal interconnection of PCB and signal input/output between PCB and external devices.
Interconnection between chips and PCB boards
Pentium IV and high-speed chips with a large number of I/O interconnection points have come out. As far as the chip itself is concerned, its performance is reliable, and its processing rate has reached 1GHz. At the recent GHz interconnection seminar, the most exciting thing is that the method to deal with the increasing number and frequency of I/O has been widely known. The main problem of chip-PCB interconnection is that the high interconnection density will cause the basic structure of PCB material to become a factor limiting the increase of interconnection density. At the meeting, an innovative solution was put forward, that is, the local wireless transmitter inside the chip was used to transmit data to the adjacent circuit board.
Whether this scheme is effective or not, it is clear to the participants that IC design technology is far ahead of PCB design technology in terms of high-frequency applications.
PCB interconnection
The skills and methods of designing high-frequency PCB are as follows:
1. 45 angle should be adopted at the corner of the transmission line to reduce the return loss;
2. High-performance insulated circuit boards with strictly controlled insulation constant values at different levels should be adopted. This method is beneficial to effectively manage the electromagnetic field between the insulating material and the adjacent wiring.
3. It is necessary to improve PCB design specifications related to high-precision etching. It should be considered that the total line width error is +/-0.0007 inch, the undercut and cross-section of the wiring shape should be managed, and the plating conditions of the wiring sidewall should be specified. It is very important to manage the geometric shape of wiring (wires) and the coating surface as a whole to solve the skin effect related to microwave frequency and realize these specifications.
4. There is tap inductance in the protruding lead, so avoid using components with leads. In high frequency environment, it is best to use surface mount components.
5. For signal vias, it is necessary to avoid using pth process on sensitive boards, because this process will lead to lead inductance at the vias. For example, when a via hole on a 20-layer board is used to connect 1 to 3 layers, the lead inductance can affect 4 to 19 layers.
6. Provide a rich grounding layer. Molded holes should be used to connect these grounding layers to prevent the influence of 3D electromagnetic field on the circuit board.
7. To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating. This kind of plating surface can provide better skin effect for high frequency current. In addition, this kind of highly solderable coating requires less leads, which helps to reduce environmental pollution.
8. The solder resist layer can prevent the flow of solder paste. However, due to the uncertainty of thickness and the unknown of insulation performance, the whole board surface is covered with solder resist material, which will lead to a great change of electromagnetic energy in microstrip design. Generally, solder dam is used as solder resist.
If you are not familiar with these methods, you can consult an experienced design engineer who has been engaged in the design of military microwave circuit boards. You can also discuss with them the price range you can afford. For example, copper-backed coplanar microstrip design is more economical than stripline design, so you can discuss it with them to get better suggestions. Good engineers may not be used to thinking about cost, but their suggestions are also quite helpful. It will be a long-term work to train young engineers who are unfamiliar with RF effects and lack of experience in dealing with them.
In addition, other solutions can be adopted, such as improving the computer model to have the ability to deal with RF effects.
PCB interconnection with external devices
Now we can think that we have solved all the signal management problems on the board and the interconnection of discrete components. So how to solve the signal input/output problem from the circuit board to the wires connecting the remote devices? Trompeter Electronics, the innovator of coaxial cable technology, is working to solve this problem and has made some important progress (Figure 3).
In addition, look at the electromagnetic field given in Figure 4. In this case, we manage the conversion between microstrip and coaxial cable. In coaxial cable, the ground layers are circularly interwoven and evenly spaced. In microstrip, the ground layer is below the active line. This introduces some edge effects, which need to be understood, predicted and considered in design. Of course, this mismatch will also lead to return loss, and it must be minimized to avoid noise and signal interference.
The management of internal impedance of circuit board is not a negligible problem in circuit board design. The impedance starts from the surface of the circuit board, then passes through a solder joint to the connector, and finally ends at the coaxial cable. As impedance varies with frequency, the higher the frequency, the more difficult it is to manage impedance. The problem of using higher frequency to transmit signals in broadband seems to be the main problem in circuit board design.
