Electronic Design - SI analysis in PCB design backplane connector and process

1. Circuit board backplane connector design

PCB backplane connector (Backplane Connector) is a type of connector commonly used in large-scale communication equipment, ultra-high-performance servers and supercomputers, industrial computers, and high-end storage devices. Its main function is to connect the daughter card and the backplane. A 90° vertical structure is formed between the single board and the backplane to transmit high-speed differential signals (Differencial Signal) or single-end signals (Single end Signal) and large currents.

The 2mm HM connector is a head-to-tail splicing design. There are different types such as A, B, C, etc. The two function blocks in the A type have the function of guiding and positioning (positioning with the connector on the board), which can prevent Insert the wrong direction. Type B has no positioning function at all, and Type C, as the splicing end, has partial positioning functions, as shown in the figure below. In the use of a connector splicing group or a single connector, the problem of connector positioning must be considered. 2mm connectors have two types: inter-column shielding and shell shielding. In actual use of the connector, the selection should be considered based on the arrangement of the ground pin signal and the shielding requirements; from the perspective of EMC, it is best to choose a shielded shell. In addition, there are HS3 connectors specially designed for high-speed signal transmission. The shielding between the pins and signals has been considered in the design of the connectors. The crosstalk generated by the connectors during high-speed signal transmission is small, and the usage rate of the signal pins is also higher, but the price is More expensive.

In principle, the model of the connector is the model of the transmission line, but the signal transmission does not refer to the ground plane, and the return path is formed through the ground pin. There must be many signal lines sharing a ground loop, so the conduction interference caused by the crosstalk of the connector must be Pay attention to.

For connector pin signal arrangement, first determine the signal distribution, and reasonably allocate the signal, power, and ground pin positions and numbers. The principle is to reduce crosstalk, reduce radiation, and ensure the ground loop. It is best to have a return path near each signal pin. The key is The signal line is separated from other signals by the ground pin. Taking into account the electrical plug-in, for the 2mm HM connector, the ground pin is longer than the power pin, and the longer pin is allocated as the ground and power connection pin. It is recommended to use the ground pin and signal The needles are arranged in a plum blossom pattern, staggered according to the high-speed signal and ground needle positions to reduce crosstalk.

Second, the SI analysis in the PCB design process

PCB signal integrity is not a new phenomenon, but it did not receive much attention in the early days of the digital field. With the development of information technology and the advent of the Internet era, people need to communicate through various high-speed digital communication/computing systems. In this huge market, signal integrity analysis plays an increasingly critical role in ensuring the reliable operation of these electronic product systems. Without pre-SI guidance, the prototype may always be on the test bench. Without SI verification after wiring, the product may go wrong in the application. SI analysis runs through the entire process of high-speed design and is closely integrated with each design step. Generally speaking, SI analysis has two states: analysis before wiring and analysis after wiring.

Before PCB routing, SI analysis can be used to select I/O technology, clock distribution, chip package type, device type, layer stack, pin assignment, network topology, termination strategy, etc. SI analysis comprehensively considers a variety of design parameters, and the resulting scheme is used as a device

The layout and routing instructions ensure the signal integrity of the physical layout. It will follow the noise and timing requirements. SI analysis will reduce repeated design and placement/wiring rework, thereby reducing design cycles.

After PCB routing, SI analysis can verify the correctness of the SI design guidelines and design constraints. It will check SI conflicts in the current design, such as reflected noise, ringing, crosstalk, and ground bounce. At the same time, it reveals the SI problem that was ignored before wiring, because the analysis after wiring is based on realizing physical layout data rather than predicted data or models. In short, it can get more accurate simulation results.

If SI analysis is followed completely during the entire PCB design process, a reliable high-performance system can be quickly realized. In the past, the physical designs produced by layout engineers were merely mechanical layouts for mechanical production, and hardly involved any signal integrity design. With the rapid development of electronic systems, system engineers in charge of developing hardware gradually have to consider signal integrity design, such as formulating design rules and wiring restrictions. Usually, their knowledge in this area comes from the experience accumulated by previous product designers, so they do not understand the nature of SI problems.

Facing this kind of challenge, professional SI engineers are needed to join. When considering the use of new processes, such as new devices or new chip packaging or board production processes, SI engineers will analyze the electrical characteristics of the technology from the SI aspect, and then simulate through SI modeling and simulation software to formulate wiring guidelines . These SI tools should be accurate enough to build template-level interconnects, such as vias, traces, and planar stacks. At the same time, it must have sufficient simulation speed to analyze what if the drive/load model is selected and the termination strategy. Finally, SI engineers will formulate a series of design rules and pass them to design engineers and wiring engineers. Then the design engineer (in charge of the overall system design) needs to ensure that the design rules are fully implemented. After the preliminary wiring and layout of the board are completed, a partial analysis of the key network can be performed, and post-wiring verification can also be performed. The SI analysis process involves many related networks, so the simulation speed must be fast, even if it may not reach the accuracy expected by SI engineers. Once wired

PCB engineers get the SI layout and routing rules, they can generate an optimized physical design based on these constraints, and will provide a report on SI conflicts in the routing system. For these conflicts, wiring engineers will work with design engineers and system engineers to solve these SI problems.