Electronics industry trends develop and change, technologies emerge and improve, and new applications bring new requirements and challenges. While this obviously has an impact on the electronics, it also has a significant impact on connector technology needed to support it.
Leading connector manufacturers are constantly developing their product offerings to meet the increasingly exacting requirements of the industry. Some of the areas being addressed include reductions in size, weight, and power (SWaP), and the ramping up of data throughput as well as greater ruggedness and reliability.
While wireless technology becomes more prevalent, connector use is on the rise. In some applications such as power transfer, connectors are the only effective option. As all devices become smaller, and many take a modular approach to design in order to bring products to market faster, multiple printed circuit boards (PCBs) are the norm, requiring an ever-increasing number of connectors to be employed. Yet, as connector manufacturers release new products to the market, the choice becomes wider and seemingly more complex.
By following a few simple steps, selecting the optimal connector can be easily accomplished. This article looks at the factors that need to be considered when choosing a connector solution and concludes with a specifying connectors checklist that summarizes the key factors identified.
The first step is to think about what is being connected. Is the requirement to mate two (or more) PCBs together, or is a cable involved — either board-to-board, board-to-cable, or cable-to-cable? Answering this will define the types of connectors required.
The manufacturing process is also a consideration. In the past when there was less choice of connectors available, it was far too common to see a pin-through connector on an otherwise exclusively surface-mount board. While this is a valid solution from a technical perspective, adding a pin-through connector (or any other component) to a surface-mount PCB means adding another step in the production process, thereby adding cost to the finished assembly. Another important point from the manufacturing perspective is to ensure that any connector selected is compatible with the company's automated optical inspection (AOI) equipment, thereby eliminating an expensive manual inspection stage.
If multiple connectors such as pin headers and sockets are to be used to connect two boards, then tolerances become a consideration. The placement on the PCB and the tolerances of the manufacturing process need to ensure that the connector alignment is always within tolerance, to guarantee the PCBs will mate properly without putting undue strain on the solder joints. For this reason, many connector manufacturers will recommend specific land patterns for PCB pads so that solder surface tension contributes to better positioning during reflow.
So, consider surface-mount versus through-board PC tail — what is the main type on your PCB?
Also consider gull-wing or outward J-lead tails (easy to visually inspect) versus BGA or inward J-lead (not easy).
The principal purpose of a connector is to move electrical signals from one place to another, so considering the electrical needs of the application is crucial to defining the connector. Starting with basic direct current (DC) parameters, knowing the currents and voltages defines many of the necessary properties. The voltages present will affect the choice of material; if high voltages are present, then an insulator material with strong insulation properties could be required. An alternative approach is to select a connector with wide pin spacing, but this can lead to a larger-than-necessary connector that may not be acceptable, depending on the space constraints within the application.
Current defines the contacts or pins on the connector. Not only must they be able to handle the steady-state current, but they must cope with any surges that may occur. In some cases, multiple pins may be connected together to handle high-power signals such as power feeds and ground connections. An alternative approach (if the use of separate connectors for power and signals is not possible due to limited space) is to specify a modern connector type using mixed technology that incorporates dedicated pins for power and signal traces.
While the pins may be able to handle the currents present, the small levels of resistance inherent in the pins may create heat in very high-current applications. These could potentially cause safety issues or simply require additional cooling (which adds cost and consumes space). Although this section covers electrical requirements, space is already becoming a consideration in these specifications and needs to be considered in parallel:
Space for a connector large enough to handle the required current,
Space for enough contacts or additional connectors, and
Space to allow for cooling or additional cooling elements.
As system speeds increase, crosstalk between multiple signals is often a problem for designers who need to wring as much performance as possible out of a system. Crosstalk occurs when there is coupling (inductive or capacitive) between the different signal lines, thereby impacting the overall signal integrity. While a lot of attention is paid to this during PCB layout, a poorly selected connector can undo all of that good work. If this is likely to be a concern, then selecting a connector with good anti-crosstalk properties is essential.
As connectors are often the entry or exit point for electrical signals from a product or system, they are also potential entry or exit points for unwanted electromagnetic interference (EMI) radiation that can impact the system itself or other systems close by. Shields can be designed and manufactured to address this, but consideration should also be given to connectors with built-in shielding or off-the-shelf shield sizes from the connector supplier.
Another consideration related to electrical performance of the system is sequencing — whether the ground, power, or a particular signal (such as an enable line) should mate before other signals. This could be achieved by specifying a connector that has some pins longer than others so they mate first and de-mate last.
Mechanical and Environmental Considerations
Connectors can often be subject to mechanical stresses during normal operation, especially if the application in which they are involved requires regular plugging/unplugging of cables. For many applications, especially harsh environments (such as defense, industrial, aerospace, or motorsport), the whole assembly, including the connectors, will inevitably be subject to shock and vibration.
In applications where considerable mechanical stresses are likely, it is good practice to select a connector type that does not rely solely on the solder connections for mechanical fixing to the PCB. This is especially true with surface-mount devices (SMDs). Fortunately, many connectors incorporate features to improve mechanical rigidity, including bosses and threaded inserts, allowing them to be fixed securely without needing to add a gluing process in production.
Obviously, the connector has to be suited to the environment in which it is to be located. At a basic level, this will mean checking the operating temperature range supported (including any thermal rise due to currents being taken into account). Other factors, such as high humidity or any aggressive substances, may necessitate a special connector that is approved for such an environment.
Connecting a cable to a PCB in a rugged environment may require additional features to make a more reliable solution:
Strain relief through latches, jackscrews, board, or panel mount studs;
Hoods or backshells for mechanical protection, electrical shielding, and usability (easy to handle);
Retaining walls to allow backpotting with epoxy resin around the rear of the cable connector; and
Other environmental protection, such as sealing, dust caps, or designed against other ingress.
The number of times the connector will be mated/unmated is another key consideration. While many connectors that are integral to the design will only be mated once, user-accessible connectors could go through many thousands of mating cycles, especially in applications such as charging portable equipment. As such, the plating material on the contacts — both the actual material and the thickness of the plating — must be suited to the application. In very demanding applications, such as space or medical technology, there are other subtle but important considerations; for example, when a connector is used in a vacuum, outgassing from the insulator material can have long-term implications for the surrounding equipment, and lower outgassing properties are therefore more desirable.