[SI-LIST] Production testing of loaded PCB's power supply impedance
- From: Peter Baxter <peter.baxter@xxxxxxxxxxxxx>
- To: si-list <si-list@xxxxxxxxxxxxx>
- Date: Mon, 16 Jul 2001 20:32:35 +1000
Hi,
There has been a lot of talk about obtaining a good low-inductance,
power system impedance on PCBs.
I am curious as to whether anybody actually does production line testing
of their loaded PCB's power system impedance (with the SMPS
disconnected). That is, both the PCB planes, as well all of the correct
value bypass capacitors loaded (with all other components installed as
well).
I've asked people how do they know that all bypass capacitors were
installed on their PCBs, and how do they know that the loaded capacitors
are the correct value? Most people have told me that they don't know.
Unless a capacitor is installed on a coupon, and the coupon's power
system impedance specially measured, no one checks it.
I was thinking about this, in relation to some boards that I had
designed. While the impedance/frequency area of interest would be
between 1MHz and 500MHz (testing for a flat impedance line), I couldn't
imagine a practical test at anything above 1MHz. However, a 1MHz test
may still give an indication as whether things are basically correct.
Someone suggested that you couldn't tell through an impedance
measurement, whether a capacitor was missing from the board. They
thought that this sort of test wouldn't be repetitive, and would be more
trouble than its worth. Fair enough.
My thought was a four terminal impedance measurement at 1MHz. The
"impedance under test" is ground referenced (as one terminal of the
power system's "impedance/capacitor" would be attached to the test
system's ground), so a single ended ac current source would have to be
used. The excitation voltage would have to be kept low in order not to
switch on any logic IC's ESD diodes. I'm thinking of impedances around
30 milli-ohms and capacitances of 30nf.
It has only been a curiosity in my mind, however I am wonder whether
others have consider it too.
Regards,
Peter Baxter
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
