All Things Connectors Part 2: The Mechanical Side in Your PCB

If you read part 1 of our series on connectors, then you know about the set of design, then you learned about some of the major specifications that will drive component selection. As we saw in that post, there is a set of mechanical considerations that could be the major factors influencing selection of connectors. Mechanical aspects include more than just the overall size of the connector, and the mechanical characteristics of a connector can influence everything from reliability to user experience.

In this section of our connectors series, we’ll look deeper at the mechanical aspects of connector selection. Specifically: when should you worry about different mechanical specifications? How do you account for placement in the PCB layout? What can you do to help prevent interference? We’ll look at these questions and much more in this article.

Mechanical Aspects of Connector Selection

The major mechanical aspects of connectors that are important for selection and system design include the following:

• Vibration and shock resistance
• Wire gauge size for leads
• Mounting style (SMD vs. through-hole, or panel mount)
• Connector mating orientation (vertical or horizontal)
• Keying mechanism and retention/pulling resistance
• Materials

For a PCB designer, the first points that often come up after choosing between a standard interface connector or custom connector is mounting style. However, the other aspects above need to be considered, typically whenever the design will be implemented for anything other than consumer products. Industrial-grade products, as well as other high-reliability products, will place a greater focus on these other aspects of connector design.

Vibration Resistance

When connectors are assembled and mated with their cable/mating component, the assembly of these components can be damaged or disconnected by vibration. More rigid connector materials with stronger retention force will tend to have higher vibration resistance up to higher frequencies. These connectors should be selected based on the deployment environment, so some knowledge of the vibrations that may be encountered during operation is needed. There is also the need to consider PCB mounting; through-hole connectors will have greater vibrational resistance than surface-mount connectors.

Mounting Style

Some connectors will be available as either through-hole or surface-mount components. When there is no room on the PCB, and some greater mechanical strength is required, panel-mounting is one option that is available for some connector styles.

Consolidated connectors like this will be panel-mounted before connecting to the PCB.

If a panel-mount connector is used, it will then attach to the PCB through a set of flying leads that can be soldered to through-holes, or possibly with an intermediate cable. The other option is to connect back to the PCB with a flex ribbon. In either case, the system will tend to have much higher reliability, and this option will be a requirement for some systems that require bulkier connectors.

From the above points, we should realize that mechanical design of connectors brings many tradeoffs. Rugged construction often translates into larger form factors, costs, and weight, as well as requiring through-hole mounting in the PCB. Lower profile connectors tend to have weaker pulling resistance, keying and retention, and vibrational resistance, as well as requiring surface-mount placement in the PCB layout.

Keying and Retention

This is another aspect of connector body and assembly design. Some connectors will need to be keyed so that they cannot be reversed during mating. A familiar example should be in shrouded pin headers; if the connector pinout is not reversible, the cable assembly will have a notch that slides into the shroud to prevent rotation.

Retention is not directly related to keying, but it does depend on the design of the connector body and mating component assembly (either cable or the mating connector directly). Keying is sometimes part of the retention mechanism, such as in some circular connectors. Another example is the three-pin through-hole plastic connector shown below; the shroud will have a snap-in keying mechanism that also provides a very strong retention force.

Hard plastic header with keying and retention built into the shroud.

Finally, an important part of keying and retention is the materials used to build the connector. Stronger materials like stainless steel that have a built-in retention mechanism will have much higher retention force than plastics. However, not all connectors will be available in these stronger materials, so custom connectors might be needed if a high retention force is needed.

Connectors in Your PCB Layout

Once connectors are selected in terms of electrical and mechanical specifications, what should a designer do to ensure the connectors will work in the PCB layout? The concept of “will work” is defined from three perspectives:

• Fit to the enclosure
• Mounting in the PCBA
• Preventing interference

It’s a mechanical designer’s job to design an enclosure to accept connectors of a given form factor and mounting style. Once the enclosure is designed and the mechanical constraints on the PCB are determined, these constraints need to be communicated to the PCB designer who will work on the layout.

To prevent mechanical interference between connector bodies and other components, the mechanical designer should define a 2D keepout region where components will not be placed, or where component height is restricted. A keepout is simply a layer definition in PCB design software; mechanical layers can also be used as keepout layers just by giving them an appropriate name. The designer can program the design rules to flag a DRC error if components are accidentally placed in the keepout.

There is a keepout around the connectors along the top edge of this PCB.

Keepouts are important for the mechanical back-check portion of the design. Once the layout is finished it will need to be checked against mechanical constraints and keepouts to ensure there is no mechanical interference. This is one of the final steps in the PCB design process before transitioning a design into manufacturing.

When you need to select and place connectors, as well as simulate an entire assembly for thermal and mechanical reliability, use the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity.

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