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How to design connector pin arrangement for PCB?

2021-12-22

How to design connector pin arrangement for PCB?

Introduction to connector pin design

There are several important points to consider in connector pin design: what is the connection length? Is it a short board to board connection or a long cable to drive the signal? What frequency (for analog signals) or what is the signal rise time (for digital signals)? Routing differential pairs or single ended signals? Whether a specific impedance target needs to be achieved, or whether the connection is short enough that the impedance does not matter
Wiring on the connector is not much different from wiring on the PCB. Signal integrity problems, impedance mismatch problems and signal loss problems may be encountered, which is the same in the case of high-speed PCB.
No pin arrangement has been designed, or you want to know why your favorite development board needs to use a specific pin arrangement, which helps to decompose some specific situations. By interpreting the effects of different frequencies and signal types, some good design practices for determining connector pin arrangement are analyzed.

DC connection

Between two PCB boards or through cable wiring, the main consideration is the total current carried. Pins and similar lightweight connectors can accommodate the maximum current per pin (typically about 1 a). If more current needs to be carried for a given voltage, the voltage needs to be bridged across multiple pins. Another thing you can do is route multiple voltages on a single connector, the same method used in desktop power supplies.
The connector pin arrangement of the DC system shall transmit the grounding signal along the interconnection. Please pay attention to this grounding signal, because this is the on-board reference plane, which needs to carry the return current, so the number of wiring and wires should be adjusted accordingly. It is not recommended to attempt to bridge two different grounds on two different circuit boards using GND connections, especially when they are located on different grid circuits and separated by a certain distance. A short circuit may occur between two points carrying a large amount of current, melting the cable. This is due to the natural DC grounding offset between different points in the power grid.

Low frequency / low speed

Low frequency and low speed are relative: what matters is the length of the connection and whether impedance is required. For low-speed digital buses in the range of 5-10 ns, as long as the connection is short enough and at least 1 GND line is included in the connector pin distribution, there may be no need to worry about crosstalk or reflection. If you pull the power supply into the connector pin, be sure to follow the same rules as the DC connector.
On high pin count row pins or other connectors with long pin lines, when some signals are far away from the grounding pin, they will become EMI sources. Similarly, these signals are easier to receive crosstalk, especially when ribbon cables or other flat cables are used. The following example uses a 14 pin connector with ground interleaving between some IOS. By placing GND between pin groups, GND will provide shielding against noise and help prevent EMI. If desired, this example can be used with long connectors. For board to board connection, it is absolutely possible to remove some GND pins, and from the perspective of noise, it should still be no problem because the distance is very short.

High frequency / high speed

For high speed / high frequency signals, things similar to the above pin arrangement can still be used, but differential pairs are usually used. Pairs of ground pins are preferably provided to prevent crosstalk between differential pairs. More ground pins are more advantageous because they provide more shielding and help minimize any impedance mismatch that may occur. For high frequencies, such as in the GHz range, a simple needle arrangement will no longer (or at least should not) be required. Coaxial (u.fl) connectors will be the best choice for RF signals, while other signals and power supplies can be routed through their own connectors.

Select connector

Find connectors that can meet the required current, frequency / bandwidth, specific signal standards or all the above requirements. There are many connectors available on the market.

PCB connector pin arrangement

When arranging connector pin assignments for components, pay attention to any shields and pins! You will be surprised that the custom pin reverses so frequently between two covered connectors that it cannot be corrected on the finished board; You will have to rebuild the cable. This is one of the things that may be helpful to put the connector in front of you to ensure that you correctly define the connector pin arrangement.
Once the PCB has selected the connector and designed the connector leads, you can start building schematic diagrams using PCB design programs such as CircuitMaker. You can create custom schematic symbols for their connector pin assignments, or you can find standard connections from the built-in parts database