[SI-LIST] Re: connector design question
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: "'Jim Nadolny'" <jim.nadolny@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 25 Feb 2008 17:07:31 -0600
Jim-
The internal self inductance of the lead will increase, due to the high
permeability of the iron, which in turn will increase the loop inductance.
But it turns out to not be much of an issue above 1 MHz. There is another
problem to watch out for, though.
The skin depth scales inversely with the sqrt of the permeability. A high
permeability means a thinner skin depth at lower frequency than in copper.
This means at about 1 MHz, the current is mostly on the outer surface and
there are no internal magnetic field lines in the high permeability
material, so no impact on the loop inductance.
I gave a presentation about 15 years ago on the loop inductance of alloy 42
lead frames vs copper lead frames and showed at 100 kHz, the loop inductance
of the alloy 42 (high permeability) was about 5x that of the copper lead
frame, but by 10 MHz, the loop inductance of the alloy 42 had dropped to the
same as the copper lead frame.
However, the problem with the high permeability material is the skin depth
is so small. This means the series resistance of your lead can potentially
by much higher than an equivalent copper lead, by 10-100x.
The plating will help to buffer this problem, which is what self equalizing
cables do, as you point out. The trick that used to be used on alloy 42 lead
frames was to over plate the lead frame with 80 microinches of silver. This
provided a wire bondable surface on one end, a solderable surface on the
other and gave good DC and low frequency resistance.
You'll want to put in the numbers to see what thickness plating you need to
achieve your low frequency resistance specs. Of course, it its short enough,
it may not matter.
--eric
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Dr. Eric Bogatin, President
Bogatin Enterprises, LLC
Setting the Standard for Signal Integrity Training
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jim Nadolny
Sent: Monday, February 25, 2008 4:20 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] connector design question
Greetings,
I'm designing a high speed digital connector and to meet some mechanical
requirements I am looking at a pin made of an iron alloy with gold
plating. The gold plating will get me the required contact physics
properties for durability, contact resistance, etc. but I got to
thinking about the iron alloy base material.
The self partial inductance of the contact will increase due to the
permeability but the loop inductance won't change (compared to a non
ferrous base material). And the capacitive coupling should not be
affected at all as that is driven by the potential distribution in the
pin field. In short, I don't think the SI characteristics of the
connector are affected much by the use of a ferrous base material in the
signal pin.
I am not looking at this as a self equalized connector - the length is
only 0.5". And it's not an RF connector where intermod distortion is an
issue. That said - I figured I would get the collective input from the
group, I could use some unbiased feedback here...
Thanks in advance.
Jim Nadolny
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