[SI-LIST] Re: Voltage rating of a Ceramic capacitor

Since I have received a flood of emails both through the list and directly, I 
will try to respond to all of you in a single email to the list concerning 
your questions on my post.  If I miss anything please email me directly.

First, yes, NPO has a higher failure rate than X7R.  This is because there 
are two principle failure modes in X7R and only one in NPO.  Yes, I said this 
correctly.  The failure mode is due to complex processes but in X7R they are 
shared so the overall stress is less while all of the stress in one mode 
causes early failure in NPO.  The current leakage rate increases in NPO by a 
power of 2.46 while X7R increases by a power of only 1.42 when aged under 
heated bias.  This leads to failure rates several orders of magnitude higher 
for NPO caps at low voltages.  The failure rate is even higher than NPO for 
Z5U or Y5V caps since one of the failure modes is now again under principle 
stress.  In HALT testing, X7R caps will reach a 30 percent failure rate in 
about 170 hours and Z5U will reach the same failure rate in only 45 hours.  
So for highest reliability use X7R followed by NPO and lowest reliability 
option would be the Z5U or Y5V caps.  I will send a pdf version of a 
technical paper on the HALT screening test I developed for zero failure high 
reliability designs to anyone who requests it.  (This will probably elicit 
another flood of emails so be patient if it does not arrive tomorrow.)

Second, why I don't recommend Z5U caps.  Beyond the reliability discussion 
above, the principle reason that should concern EMI and SI designers is the 
very nonlinear voltage to capacitance response at low voltage.  I have seen 
some transients and switching effects become magnified because of this effect 
and lead to a closure of eye-diagrams.  The ferromagnetic aging effect is 
another problem.  Manufacturers will sometimes reduce the aging effect by 
heating Z5U and Y5V caps above their Curie temperature and then cool them 
under bias to reduce the aging effect while they sit on the shelf.  But the 
effect returns when you expose them again to unbiased heat above the Curie 
temperature (soldering, reflow, etc.).

The out of circuit HALT tester I designed uses a machined Teflon block bolted 
to an aluminum base.  The Teflon block was machined with channels about 75 
mils wide and 150 mils deep.  At one end of and common to all the channels 
was a nickel/gold plated copper metal strip which was the circuit ground.  At 
the other end of the channel was a spring loaded push pin which was also 
nickel/gold plated.  It is a simple operation to load caps by retracting the 
push pin, dropping the cap into the channel and releasing the push pin 
against the cap to hold it in and make electrical contact during heated bias. 
 Please be aware that a failing cap will usually short and explode during 
HALT testing.  If you protect each cap with a fuse then keep it outside the 
oven -- high temp fuses are expensive.  You can eliminate the fuse if you use 
a high voltage-high current power supply.  Please be careful with this test!

Next, You will almost always achieve your best results at high frequency by 
paralleling several X7R/NPO caps and using gnd/pwr plane capacitance to reach 
the capacitance, impedance and ESR characteristics you need.  There are 
papers already written on these procedures so I will not go further with this.

I should mention, however, that I have seen people suggesting that you should 
use Z5U and Y5V because they have a lower inductance.  Yes, it is often true 
that X7R caps will have lower inductance.  The higher dielectric constant of 
Z5U and Y5V caps allows the manufacturer to get the same capacitance with 
fewer plates in the same size package which will lead to lower inductance.  
But the other problems remain!  The solution to this is to use low voltage 
X7R ceramic caps that are designed to be thinner with wider/lower resistance 
plates.  The thinner the cap the lower (in general) the inductance.  This 
should make perfect sense to those of us who are microwave designers using 
single layer caps.  There are a whole series of (and for some reason more 
expensive) low inductance MLC's on the market that use this principle.

I have seen very few books on these subjects (maybe I should write one).  The 
best information comes from technical papers written for passive symposiums.

Lastly, my original post was not meant to give those designers who use Z5U 
and Y5V caps indigestion.  If you find that these high dielectric value caps 
work in your design, then congratulations!  If you get a large number of them 
back in a year or two after the caps have aged or failed in or next to a hot 
processor then you have been forewarned.

Harold L. Snyder, Jr.
Scientist & Consultant







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