[SI-LIST] Re: TDR's and ESD Protection
- From: "Loyer, Jeff W" <jeff.w.loyer@xxxxxxxxx>
- To: "'JSAWDY@xxxxxxxxxxx'" <JSAWDY@xxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Tue, 26 Mar 2002 15:45:50 -0800
Here's a possibility based on (possibly) related experience. Note that I'm
not an ESD expert.
While in the Fab (many years ago), we noticed enormous amounts of static
building up on the "boats" that held the wafers (and the wafers themselves).
A large part of the blame for the static build-up was attributed to the fact
that they were constantly under HEPA filters - large amounts of airflow.
You experience a similar phenomena when you get out of your car after
driving fast on dry days. We had to go to more conductive boats to solve
the problem.
I wonder if the static isn't being built up on the center conductor of your
cables while they're sitting around and their first opportunity for
discharge is when you touch the poor TDR head to them. Your wrist straps
and conductive mats wouldn't help this. There are simple meters available
to measure static build-up, and should detect it if this is the culprit. To
remedy this, I would suggest shorting each cable's center and outer
conductors together before testing.
I believe Polar Instruments TDRs are less susceptible to ESD damage, since
they can afford to put ESD protection on their heads (their rise-times are
accordingly slower). The problem is that I don't believe their instruments
measure differential impedance/length.
Jeff Loyer
-----Original Message-----
From: JOHN SAWDY [mailto:JSAWDY@xxxxxxxxxxx]
Sent: Tuesday, March 26, 2002 1:32 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR's and ESD Protection
We are experiencing a high failure rate on our Tektronix SD-24 TDR heads
when we test cable assemblies. Our cables assemblies sometimes are 40 to 80
feet in length and consist of multiple coax and twinax cables within each
assembly. We typically use the TDR to measure differential electrical
length and skew of the finished assembly.
We use wrist straps and conductive mats on the benches. We short all pins
to ground before we connect to the TDR to try to remove any residual
charges.
I am wondering if anyone else has some experience with this and if they
might have a solution.
I have also heard that "mechanical noise" generated by movement of the cable
may be a culprit but I am not sure of the magnitude of such noise. Can
anyone confirm that this is a possibility?
Does anyone have any experience to suggest that one brand may be more robust
then another or that one model of head may be more robust.
Looking forward to your responses,
John
John F. Sawdy, Senior Applications Engineer
Meritec, a division of Associated Enterprises
voice mail: 440-354-3148 x 267
fax: 440 -354-0687
mailto:jsawdy@xxxxxxxxxxx
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