I have not attempted this sort of thing, but if impedance needs to be maintained, then you must shop for connectors which do so.  Pretty sure I've seen connectors advertised for this, but I've never had a need to use them.

It may be possible, on an edge finger design, on the PCB side, to select one contact, surround with GND on the other contacts, size the signal contact so as to get a 50ohm (or whatever) structure.  The three GND's on the backside could be a continuous contact, no need for separating the copper.  However...  The connector will still not be impedance controlled.  Alternatively, for a differential signal, you could try again to space/alter the contacts to control impedance, but... same problem, the edge finger connector won't be impedance controlled.

HOWEVER: I think you first have to determine if this small discontinuity between boards really matters.  Many systems run perfectly fine with small discontinuities in the path.  When the signal path is less than an eighth wavelength long (for the maximum signal frequency used) (don't neglect to take into account the relative permittivity!), usually one doesn't bother with impedance matching (note, may still have capacitive/inductive issues on the trace, but reflections are not an issue).  [Alternatively, there is a way to go off rise time--which I've forgotten, and can't find at the moment, and I'm not about to calculate and get wrong.  But basically, if the rise time is slower than the time it takes for the signal to traverse the path, then impedance matching is not required.  Ideally, rise time would be slower than the time to traverse and come back, so use twice the path delay.]

Also, if you are dealing with digital signals, you can buffer signals after crossing a discontinuity, to "clean them up".  Alternatively you may use digital transmitters & receivers on both ends, real chips that are meant to deal with non-ideal paths and which will provide a clean signal out of the receiver.  A differential line driver will likely handle most edge connectors and other forms of connectors.

Finally, you may need to match trace lengths on both sides of the connector.  Again, it's about path delay, and how much skew between paths is allowed.

Shawn Upton
Section Head, Test Engineering
Sensors Business Unit
Allegro MicroSystems, LLC
[log in to unmask]
603.626.2429/fax: 603.641.5336


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Lum Wee Mei
Sent: Monday, May 30, 2016 8:38 PM
To: [log in to unmask]
Subject: [TN] Daughter Card and Backplane Impedancne Control Interface

I am not a layout designer and I need some advice on this.

If I have a card cage design where the daughter cards are plugged onto the backplane. How do one manage and ensure there is no impedance control mis-matched between them. What are the things I need to take note of?

Regards,
~wee mei~



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