Introduction to PCB technology backboard design

    Introduction to PCB technology backboard design

    Design If multiple boards need to be connected into a larger system and provide interconnection between them, a backplane may be used to line up and cascade the boards. The back panel is an advanced panel that borrows elements from high speed design, mechanical design, high voltage/high current design and even RF design. These boards are commonly used in mission-critical defense systems, telecommunications systems, and data centers. They adopt their own set of standards that go beyond the reliability requirements of IPC

     Although the backplane follows specific standards not found in many other PCBs, many PCB designers are familiar with the concepts involved in layout and wiring. At first, the large number of connectors and networks and the cramped space on a typical backplane seemed difficult. Nevertheless, a few simple strategies can help you maintain your organizational structure and complete your backplane design while ensuring high reliability. My hope is that you will learn some strategies for implementing the next backplane design in terms of wiring and layout to balance reliability and signal integrity. Without more, let's jump into this rich field of PCB design.

1.Introduction to backplane design
Backboard design, layout and wiring entry need to take a variety of angles. These designs can be difficult because you may find yourself managing thousands of connections on a large board with limited space and layers. In addition, the backplane may actually be involved in powering the sub-cards, and each sub-card may be pulling multiple amperes of current through various high-speed devices. This means that your backplane may need to support about 100 A of current.

    Since the primary function of the backboard is to provide connections between multiple boards in a large system, everything revolves around the connectors you will be using, and these connectors are the starting point of your design. Here are some of the basic tasks involved in backplane design:

    Pin alignment: The first step is to determine the pin alignment on the connector to support the desired routing topology. I'll cover this further below。

    Mechanical Requirements: In addition to the proper placement of the subboard connectors, the guide pins are used to ensure proper fit and structural integrity. The image at the bottom of the list shows a typical guide pin for use with a backplane connector.

   Material selection: for high speed backplanes, this is a key point in the design process. Since the back plane can be very large, any signal that needs to travel across the entire plane can suffer significant losses. Low-loss laminates with tight glass braiding are required to minimize insertion losses on long interconnections。

   Power and grounding strategies: For backplanes that need to provide high power for large quantum boards, you will need power and grounding strategies that help maintain low temperatures. The grounding/power plane arrangement at the different plane layers shall also provide isolation for high-speed signals isolated on the board

   Number of layers: The number of layers required for the backplane will depend on the number of plane layers and the number of signal layers required. The back panel can have up to 24 layers and is a few millimeters thick, which can meet all design requirements。

   The above points are the same as any other considerations in high-speed design. However, once you are working on the backplane, the situation is different because the pin arrangement of the connector limits the wiring. This is an important part of the backplane design and should be carefully planned。

2.It all involves connectors, pin assignments and wiring
Much of the initial design phase will focus on the connectors on the backplane. Selecting connectors, including those used for the backplane, is as much an art as a science, and these connectors will become the primary determinant of signal integrity. Simulation is important to ensure that the signal does not decay excessively at the connector tracking interface.

   Pin arrangement in the connector is also important because it helps to simplify wiring at each layer. In particular, your pin alignment should achieve two goals:

   It should be designed to prevent signals on a given layer from crossing with each other when routed to all connectors on the backplane bus. If you do it right, you might be able to eliminate some of the signal layers.
Ideally, when reaching the pins on each connector, the wiring should run smoothly across the entire (mostly horizontal) backplane。

   It is best done line-by-line, similar to the difference pair routing shown below. Notice how the pins on each connector are staggered in each column, allowing the route in a difference pair to go between the rows of the connector pins. If all the pins are in the same column then I need 2 layers instead of 1 to do the wiring as shown below

   With all of these design requirements in mind, I found it difficult to achieve all of these balances in my first backplane, and we didn't even do the initial component placement. You don't have a lot of freedom in component placement, but as long as the pins are aligned and consistent throughout the connector, you can keep things in order when routing signals through the backplane. Some other tips to help you succeed include

   Minimize the jump through the high speed signal. Each through-hole increases the insertion loss of the interconnection and needs to be minimized
High speed reverse drilling through transition. Reverse drilling increases cost but minimizes short transversal discontinuities on long transmission lines
Don't be afraid to fall. The use of ground backflow helps to isolate the different high-speed trace groups, ensuring a consistent impedance curve, and providing adequate conductors for high return currents.

   Make all unused signal layers flat. If you are supplying power through the back panel, do not worry about dropping additional power panels into the lamination. Distributing the current between the multiple power layers helps keep the PDN cool。

   The backplane design is not for the faint of heart, as it requires multiple specialties to succeed. However, if you have the right team of designers and a set of design tools, you can do most aspects of the design process on one platform. You will be able to complete the backplane design and go through a program ready for production。