Selecting the Right Connectors in Multiboard Design

When I built some of my first PCBs, I didn’t really pay attention to the specifications on my connectors. It was more about choosing connectors that looked cool, carried the minimum number of wires, and provided some sort of connection to my power supply. As long as it could be easily mounted to the board, I probably used it to the detriment of my board’s performance.

Instead of choosing connectors in multiboard design based on aesthetics, there are plenty of technical points to consider. These points relate to durability, pin count, current rating, and several other performance aspects.

Choosing and Placing Connectors in Multiboard Design

There are plenty of connectors available on the market for routing connections between PCBs in a multiboard system. It cannot be stressed enough that you should always check the specifications of potential connectors against your design requirements. Before shopping for connectors, you should consider the following important points:

• Type of connection: Are you making a board-to-board connection, board-to-cable connection, standardized computer peripheral connection, power/battery connection, or something else?

• Number of required pins: This is important when you consider how data will be routed between boards (serial vs. parallel), as well as the number of ground wires you’ll need to ensure signal integrity.

• Frequency and switching speed limitations: Every connector has impedance, meaning that the resistance encountered by current is a function of frequency. Connectors have frequency limits, and many connectors are specialized for specific applications.

• Current capacity: For connections that supply power, ground, or high current signals between boards, you’ll need to consider the maximum current that the connector can sustain without being damaged. In some cases, the current capacity will depend on the mating between two connectors. A good manufacturer will supply this information for all possible connector combinations in their datasheets. Be careful: manufacturers may supply this information on a per-pin basis!

• Type of connectors: Whether you are using jumpers and shunts, connector contacts, headers, male or female connectors, edge connectors, compression, or PCB spring contacts will determine a lot of your design capacities.

• Operating temperature: The specifications given for connectors are only valid for a certain range of operating temperatures.

• SMT vs. through-hole mounting: How will the connector mount to your board?

• Duty cycle: How many times can the board or cable be placed in the connector before the contacts wear out? This will dictate whether you can use a simple copper connector or a different material that is more durable.

After choosing your connectors, you will need to carefully place and secure them onto your boards. Be careful here when designing the connection between two boards as it is tempting to copy the pin arrangement between them. Instead, the pin layout between two boards should be a mirror image, as shown in the image below. This particular design for assembly issue can be solved with the right component library.

Do you have your pin layout mapped properly?

We all like to think of conductors in a connector as being ideal, but this is simply not the case. A mechanical connection between two elements in a circuit creates some contact resistance. Connectors with larger contact resistance contribute to IR drop.

Contact resistance values for different connectors can range from sub-milliOhm levels to tens of milliOhms. Cheaper connectors, like simple banana connectors, have inconsistent contact resistance, and the manufacturer will typically only supply a maximum resistance value rather than a mean and tolerance.

Whether you’re going for simple or complex connectors, make sure the values are right

Staying Grounded and Synchronized

One of the most important points involved in choosing connectors and cabling for routing between multiple boards is maintaining a consistent ground return path for your signals. The goal here is to supply a return path for signals such that you minimize the loop area for these signals. This minimizes the parasitic inductance associated with the signal, thus reducing susceptibility to EMI.

For single ended signalling, you can provide a return path across a cable by grounding one of the leads in your connection. You should connect ground planes between multiple boards, meaning that your connectors will need to connect directly to the ground plane on each board. This grounded connection will provide a reference for the other signals in your cable. This will minimize the loop area for these signals and help you pass EMC compatibility checks.

Ideally, it is a good idea to use a cable that has sufficient pins to place a ground line between each signal line. This will shield signals in the connection from each other and will minimize the loop area for all signals travelling between boards. Many standard protocols, such as DDR4, include multiple ground connections as part of the specification for this very reason.

For this reason, it may be best to use differential signalling between boards as you will only need to allocate one pin on the connector as a ground plane connection. Note that not all communication protocols will support differential signalling (for example, RS232). However, routing serial data allows you to choose a smaller connector as you need fewer signal lines to transfer data between boards.

Parallel routing for signals on a cable

In some cases, such as parallel data transfer with a large number of bits, this may not be practical as you will use half of your pins just to provide ground connections. You’ll need to consider this when designing a connection between boards in your system.

Routing parallel data at high speed also requires compensating for timing skew when connections between components on multiple boards become very long. Note that some protocols, such as PCIe, do not require exact trace length matching so long as the total connection distance is less than the value stated in the specification.

Selecting and placing the right connectors in multiboard design is easy when you use the right PCB layout and design software with a full suite of design tools. The design features in the OrCAD PCB Designer suite help you design, layout, and route connections between multiple boards in a system. You can search and place the connectors you need quickly and easily.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.