Purpose of tabs on PCB connector jacks? 1

[schaplan]

I've been designing some enclosures over the past few years and have thus far avoided jacks that have the metal spring-tabs on the sides of connectors, preferring plastic ones for a flush finished look.

I'm now presented with a part that has these metal spring tabs and I'm told this is the part we're using. I'm wondering if anyone knows the specific purposes of these tabs? I feel like they inevitably leave something to be desired from an aesthetics perspective, and so they probably have a purpose from a signal integrity perspective.

Here is a link to a part with the tabs:

http://www.digikey.com/product-detail/en/7499211123/732-1862-ND/2056866

Any experience is appreciated. Thanks,

 

[mcgyvr]

I'd think they are to ensure a connection/compliance with any metallic opening it penetrates through..

 

[IRstuff]

The part is described as, "High Performance Low EMI RJ45 Connectors with Integrated Magnetics" and carefuly examination of the photographs of the connector shows that the tabs are contiguous with the metal housing of the connector. As there are no other means of ensuring adequate connection to the chassis EMI shield, these tabs must serve that purpose. This is confirmed by

http://www.digikey.com/product-search/en?vendor=732;site=us;lang=en;Keywords=rj45 magnetics

which shows that housings with these features are described with "shielded, EMI finger"

TTFN
faq731-376

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[itsmoked]

With those finger tabs the box is expected to be metallic and part of the product's radiated noise prevention shielding. If all your prior designs have been plastic this would probably be new to you. Anyway, the entire box's shielding concept is void and null if there are any little air gap slots present in the enclosure. The tabs on the connector are there to mechanically and electrically spring seal the slots formed by the connector and the square penetration hole. That spring connector metal also forms a complete bowl of shielding metal you don't see inside the connector body.

Because of the actual 'mating' of those fingers to your enclosure there will likely be a 'specified' opening that you need to pay attention to. Check the data sheet for that connector for the allowed clearance limits so you don't make the opening too large or small. There may also be some more rigid positioning requirements for the board placement due to all this over your prior designs.

If your EEs are requiring that kind of shielded connector it would alert me to need for 'serious' shielding being required. You should ask them about the frequencies expected and if you're designing in plastic start researching the various ways used to metalize a plastic enclosure for shielding.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[schaplan]

Thanks for the (quick!) feedback everyone. I'll make sure the EE team is aware of the options and requirements associated with these 'EMI fingers'.

Keith - RE: "...the entire box's shielding concept is void and null if there are any little air gap slots present in the enclosure"

Does this mean these features are unusable if the enclosure is air-cooled?

Unfortunately the specification documents do not suggest an opening size for the enclosure. It suggests to me that the fingers may compress all the way to the walls of the jack. I'll see if we can order one and find out.

 

[itsmoked]

"Air cooled" means many many different things. More specific?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[schaplan]

Right. Generally we'll have a fan at an opening intake (~1sq.in), blowing air over heat sinks and components to an outlet opening elsewhere on the enclosure. Will these openings necessarily negate the box's shielding?

 

[VEBill]

There are plenty of avionics and military gear where the airflow is through a tube- or tunnel-like air flow pass-through that is physically inside the chassis, but is effectively outside from the EMI point of view.

The alternative of using screens to shield the air openings is often unworkable due to the Dust and Sand tests.

It really depends on the requirements.

 

[drawoh]

schaplan,

Search for EMI grilles. Tech-Etch, Laird Tech, and some others make them. Some of them filter out dust as well, although you will need a more powerful fan.

--
JHG

 

[itsmoked]

Yep, those are the typical things that are done.

Likely you would prepare to use one of those methods but until you do testing you don't actually do/spend money on any of them as they might not be needed. On the other hand, you don't want to be caught flat-footed with a box that will need to be redesigned or radically modified at the last moment to cover EMI issues.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[schaplan]

I've gained an expanded understanding of enclosure design from your feedback. All of my questions have been answered so far. Thanks guys.