As already outlined in Part 1 (featured in the October issue of Tech Briefs), the main function of a connector is to enable the transfer of electrical signals.
So far we have covered:
- Connection type and PCB mounting style;
- Electrical properties and EMC;
- Mechanical and Basic Environmental Considerations
Design for Manufacture and Servicing
Another role for these components is to allow electronic designs to be modular in nature, so that servicing during their lifetime becomes easier. In order to make assembly during manufacture and subsequent servicing as easy as possible, there are some considerations to be factored in when selecting a connector.
Mis-mating a connector can, at best, prevent the system from working and could potentially lead to irreparable damage. While circuit protection mechanisms can be employed, they add size, weight and cost to the system. Many interconnection solutions include some form of polarization, such as a blanked pin, or a more sophisticated feature molded into the housing.
In certain circumstances (such as pushing one PCB down onto another), so-called ‘blind mating’ is required. This means that the connector halves are pushed together without the ability to see them mate. In this situation, misalignment can result in damage to the connector. However, alignment features including plastic shrouds on headers or alignment posts can ensure that a perfect blind mate occurs every time, removing the risk of connector impairment. An alternate approach to blind mating two PCBs together is to use spring pins and contact pads, where there is a far greater tolerance to minor misalignment during assembly.
In general, if hazardous voltages are to be exerted onto a connector exposed to the outside world, then, for safety reasons, a socket should be incorporated instead of an exposed pin. Safety can be enhanced further by using “scoop-proof” sockets, where the female receptacles are housed deeper within the socket, thereby removing any likelihood of high-voltage contacts being touched accidentally by a user.
In summary, if your module is likely to be connected and unconnected more than once in its lifetime, or access at the time of manufacture is an issue, consider:
- Polarization features;
- Features to assist with blind mating or wide mating mis-alignment;
- Sockets with scoop-proof design for live assemblies.
Another aspect affecting manufacture is that of assembly tooling. Certain types of connector require hand assembly tools for low volume R&D/prototyping work and then more sophisticated automated versions for use in a large scale production context. Having the appropriate tooling available is key to ensuring a reliable end product, so that connectors are assembled correctly in line with manufacturers guidelines. An alternative approach here, which is especially suitable for short-run projects or custom designs, is to source cable assemblies from the connector manufacturer, thereby avoiding the expense and inconvenience of tooling.
Physical Space Issues
As already mentioned, space (both footprint/real estate and above-board space) is a key challenge for almost all design engineers these days. In response to this, connector manufacturers now offer high-density connectors that pack a huge number of contacts into a small space.
Each situation is unique, and thought will need to be given to various practical issues:
- Existing available space on the PCB;
- Space and access to mate and unmate the connector;
- The height above the PCB needed for the connector pair;
Whether the space is enough for the type of connector needed (due to the previous considerations of current rating, number of connections and any mechanical mating assistance). In addition, engineers must take care not to impede any airflow paths that could cause thermal issues to arise.
CAD Models and Samples
Many connector manufacturers apply 3D models in .stp and .igs (STEP and IGES) formats that are compatible with almost all 3D design software. By downloading these models and including them in their designs, engineers can quickly and easily identify any potential difficulties before committing to physical hardware. Evaluation samples can also often be obtained, to give engineers a feel for the actual products.
Standards and Certifications
Standards are important for gaining necessary approvals for products and, while some standards or legislations (such as RoHS and REACH) apply to almost all applications and markets, other industry-specific standards can have significant bearing on the choice of connector selected.
EU RoHS Directive
Originally became law in 2006 (2002/95/EC), effectively banning a total of six substances. This was further extended in 2015 by amendments to RoHS 2 (2011/65/EU), with four more plastic-based substances added to the list. While RoHS does not strictly apply to connectors as they are considered to be components with no inherent function and are not CE marked, they are inevitably used in products that have to meet RoHS requirements. Therefore, most manufacturers certify conformity of their connectors, thus making the task of end product certification much easier to accomplish.
EU REACH Directive
EC1907/2006 currently (as of Jan 2018) requires declaration of 181 substances of very high concern (SVHC), although these substances are infrequently used in connectors and unlikely to be of concern during the selection process. The same applies to the REACH ‘Restriction List’ of 64 substances (Annex XVII).
Conflict Minerals Legislation
Conflict minerals legislation seeks to eliminate the use of minerals sourced from mines in war-torn geographic regions. The main legislation for the industry came into force as part of the Dodd-Frank Wall Street Reform and Consumer Protection Act of 2009 in the United States, which requires OEMS that report to the US Securities and Exchange Commission (SEC) to also report on their use of four minerals and whether they were conflict-free.
- Tantalum and Tungsten are very rarely found in connectors, if at all, and should not be of concern.
- Tin and Gold are used extensively and, when selecting connectors, assurances should be sought from the manufacturer that the constituent materials come from non- conflict sources.
Similar to RoHS, many reputable connector manufacturers are not reporting to the SEC themselves, but make this data available proactively to support their customers.
Beyond the materials-related standards and directives, there are many other product- and industry-specific standards that require products (including connectors) to be tested and certified for compliance. These are particularly common in industries such as military, aerospace and medical, with regard to safety, but also exist in communications to ensure correct interoperation of devices from different manufacturers.
This concludes the second part of this white paper. In the third and final part, the task of selecting an appropriate connector supplier will be discussed.