How to Select a Connector
Engineers have a love/hate relationship with connectors. With so many varieties and manufacturers to choose from, it’s not easy to make a selection when you’re designing a new product. We’ve put together this rough guide to help you choose a connector series.
Is the connector a sole-source series?
A sole-source part is a part that it is only available through one manufacturer. This is not necessarily a bad thing, but do your research. Check the electronics distribution networks to make sure the part you’ve selected is stockpiled in reasonable quantities. You don’t want to hold up a shipment to your customers because of limited part availability.
Also be wary of how recently the connector series came to market. A connector that has been around longer has had time to be adopted by an industry, and is less likely to be discontinued. Many connectors are developed for specific industry applications, but knowing that a series is widely used will give you confidence that you can depend on it for years to come.
Signal Integrity and Impedance Matching
Does your design include differential pairs or single-ended signals that must be impedance-controlled? If so, you’ll want a connector with pins that are matched to the impedance of your signals. Each discontinuity along your signal transmission will degrade signal quality, and a connector already presents a discontinuity where it transitions from connector to cable. Optimize your signal quality by ensuring the connector series includes pins which are matched to the desired impedance of your signals. Connectors designed to support differential pairs often include nearby ground pins to provide a uniform and continuous ground path adjacent to the signal for the distance that it travels through the connector.
This 3D PCB rendering utilizes a combination of surface-mount and through-hole connectors.
Current Capacity
The current carrying capacity of a connector’s mated pair depends on the headers installed on the PCB at both ends of the cable, the cable housing, the wire, and the wire crimps at each end. Will the voltage drop across the cable length affect your application? If you’re supplying power across the connector, does the per-pin current capacity meet the load at the other end?
Genderless Connectors
There are a number of genderless connector series on the market. With no specific connector intended to be the plug or receptacle, there is no way to accidentally design a PCB for the wrong mating connector. (While this is true, don’t forget to review the component’s orientation on the PCB!)
Another benefit of genderless connectors is that you only need to order and stock one model number — not two.
Latching, Locking, and High Retention Connectors
Connectors with a latching/locking feature usually have a clip on the wire housing or the header. When the cable plug is inserted into the header, it will lock into place and maintain connectivity under demanding environmental conditions.
As an alternative to latching or locking connectors, some connectors are built with higher degrees of retention. The higher the retention, the less likely the cable will become disconnected once the product is in the field.
Surface-Mount, Through-Hole, or Both?
Surface-mount connectors are a great way to save valuable routing space on a PCB. But they come at the expense of being placed with a pick and place machine. If the rest of the devices on your PCB are soldered by hand, a through-hole connector might be the better way to go. Some connector series offer only surface-mount or through-hole connector varieties — but not both. If you can’t make a choice, ask yourself if there’s ever the possibility of your board needing to interface with components that are solely surface-mount or solely-through-hole. A connector series which includes headers of both varieties will provide maximum flexibility for the lifetime of your product family.
Contact Material
While different conductive materials have varying levels of resistance and corrosiveness, it’s more likely that you’ll need to consider the contact material’s thermal properties and gold plating thickness when selecting a connector.
Lead-free solder melts at a higher temperature than leaded solder, which means the connector’s contact must be able to withstand these temperatures. Manufacturers will generally indicate whether or not the product is RoHS compliant, which is indicative of its contacts supporting lead-free solder.
For gold contacts, manufacturers will often offer connector variations with different thicknesses of gold plating. Generally speaking, the more gold on a contact the more connection cycles the contact will permit. Ask yourself: Will the end user be regularly interfacing with the connector? Or is the connector rarely used and fully contained within an enclosure?
Mating Connectors
It’s easy to have tunnel vision when you’re designing a board, and to only see the space you are working with. Don’t forget that for every connector, there’s another connector that must mate to it. Some connector series have small headers that take up little space on the PCB, but larger mating connectors that plug into (or around) the headers. If a header is placed too close to adjacent components on the PCB, the nearby components may interfere with the mating connector and prevent insertion. Latching connectors with large shrouds are usually susceptible to this issue — Watch out for them!
Stand-off Height
Some series of connectors are built specifically for use with stackable form factors, or more generally for mating one PCB with another. When selecting a connector for stackable PCBs, review it carefully to determine that the available combinations of your connector and its mate are available at the required surface-to-surface stacking heights. If there are any tall components on your PCB (transformers, heatsinks, or modules), verify that the mating height of the connector series will provide sufficient clearance for your parts.
Pin Pitch
Be conscious of the pin pitch when selecting a surface mount connector. A small connector might sound like a space-saving opportunity, but this could come at the expense of pins that are tightly packed into a small area. Make sure the pad-to-pad clearance of the connector isn’t so tight that it will prevent the PCB manufacturer from meeting spec — or your PCB cost will increase. Similarly, a tight pin pitch isn’t worth it if your CM is spending precious man hours removing shorts from the pins.
I’ve seen cases where a common pin pitch allows one PCB footprint to support multiple series of connectors. This kind of scenario can allow for future flexibility as well as product customization.
Connector Length
If a PCB has a warped surface, surface mount connectors of longer length are more likely to experience solder issues than their shorter counterparts. Any slight bow or twist in the PCB, and a long connector may come out of the pick-and-place machine with insufficiently soldered pins that will require touch-up.
Are pre-assembled cables available for the connector series?
Pre-assembled cables are not for everyone. But in some applications, they can be a convenient way to speed up the assembly process, reducing the overall assembly time and associated costs. Connector manufacturers such as Molex, Samtec, and TE Connectivity each offer off-the-shelf cable assemblies.
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There are many facets to connector selection, and each engineer will have their success stories and probably at least one or two horror stories. What is most important to remember is that a connector series which appears to solves your biggest issue today may create unexpected problems tomorrow. Overlooking any detail may adversely affect the electrical properties of your design, but could also impact procurement, the assembly process, and even your customer.
The Engineering Design Group has a background designing PCBs for military and aerospace applications. We understand how to select connectors that meet demanding environmental conditions, longevity, and manufacturability. Contact us today, and let’s make sure the connectors on your PCB will meet your needs today, tomorrow, and into the future.