bad multilayer ceramic capacitor at 4 Kelvin?

[[email protected]]

I have used multilayer ceramic capacitors for power supply bypassing
for a cryogenic amplifier. Everything worked fine at room temperature,
however as i inserted the circuit in the cryogenic system, after a
while my resistance of bias line to ground dropped to 18 ohms.
I can't even turn the bias to 1 volts (orignally it was 4 volts at room
temperature) due to the current limit of the power supply.
I have 2 guesses, one c'ld be a short of bias line to ground but i have
used magnet wire so this shouldn't be a case, and i tested the
enclosed system was working fine uptill 77 Kelvin.
Second guess, is that i have many bypass capacitors multilayer ceramic
type which cld go bad (I have no electrolytic caps in my circuit).
Does any one know how these caps behave when gone bad or when cooled to
4 Kelvin or so?
Please help.

 

[colin]

I have used multilayer ceramic capacitors for power supply bypassing
for a cryogenic amplifier. Everything worked fine at room temperature,
however as i inserted the circuit in the cryogenic system, after a
while my resistance of bias line to ground dropped to 18 ohms.
I can't even turn the bias to 1 volts (orignally it was 4 volts at room
temperature) due to the current limit of the power supply.
I have 2 guesses, one c'ld be a short of bias line to ground but i have
used magnet wire so this shouldn't be a case, and i tested the
enclosed system was working fine uptill 77 Kelvin.
Second guess, is that i have many bypass capacitors multilayer ceramic
type which cld go bad (I have no electrolytic caps in my circuit).
Does any one know how these caps behave when gone bad or when cooled to
4 Kelvin or so?
Please help.

ouch, thats cold, my only bad experience with ceramic caps is end caps
coming off SMD parts.
does it work again once its warmed up ?

Colin =^.^=

 

[Kiviranta, Mikko]

Second guess, is that i have many bypass capacitors multilayer ceramic
type which cld go bad (I have no electrolytic caps in my circuit).
Does any one know how these caps behave when gone bad or when cooled to
4 Kelvin or so?

My experience is that capacitance of ceramic capacitors with
high-epsilon dielectrics (such as X7R or Z5U) just go down but the
capacitor in general would not fail. Ones with NPO type dielectric seem
to perform quite nicely at liquid He. This is a rule-of-thumb style
observation only: designations such as X7R don't define the chemical
composition, and I find it conceivable that some X7R or other caps might
show increasing capacitance towards cryogenic temperatures even though
the ones I've experimented with do not. I seem to recall that strontium
titanate is an example of an dielectric with such properties.

You can't get high capacitances out of NPO, unfortunately; I once had a
fun hypothesis that whatever the capacitor value is at room temperature,
it will be 100pF at 4.2K . For higher capacitances the Panasonic ECPU
type plastic film caps are available up to 1uF in a 1210-sized case and
they perform fine at 4.2K .

Different thermal expansion coefficients of the capacitor case and
the substrate increase the risk of the capacitor mechanically breaking,
but even then I would expect an open circuit rather than a short. What
sometimes happens is that the thin metal layer (diffusion barrier?)
covering the contacts of an SMD cap gets detached because of the
mechanical stress. Still, off-the-shelf NP0 and ECPU type SMD caps
soldered on ordinary FR4 board seem to tolerate thermal cycling
surprisingly well. I have used those on my own cryogenic amplifier
(which now appears to show 70 pV/rtHz white voltage noise and 1/f corner
approximately at 5 kHz, by the way).

Regards,
Mikko

 

[[email protected]]

ouch, thats cold, my only bad experience with ceramic caps is end caps
coming off SMD parts.
does it work again once its warmed up ?

I can't check it right now, the amplifier is a part of a vacuum system
which has been cooled to 4 Kelvin. I am still trying to see if these is
an implicit short and trying to avoid shutting down of the complete
cryogenic system.

 

[[email protected]]

Thanks Mikko,
I have used the X7R and also as i needed power supply bypassing upto
high frequency (upto 10GHz) i have used RF capacitors from Vishay.
HPC0402A type:
http://www.vishay.com/product?docid=10117&query=HPC
They have very high SRF, and i used 10pF and 1.2pF from them.
Though, just now i have realized they are manufactured on a Silicon
wafer. I know Silicon transistors fail at 4 Kelvin due to carrier
freeze-out but it won't explain the short circuit.
Do you know of any other high frequency good quality capacitors which i
can use for cryogenic appications.

I have used those on my own cryogenic amplifier

(which now appears to show 70 pV/rtHz white voltage noise and 1/f corner
approximately at 5 kHz, by the way).

I would really like to have a look at ur amplifier, if you want to
share. Is it published, already?

 

[Robert Baer]

I have used multilayer ceramic capacitors for power supply bypassing
for a cryogenic amplifier. Everything worked fine at room temperature,
however as i inserted the circuit in the cryogenic system, after a
while my resistance of bias line to ground dropped to 18 ohms.
I can't even turn the bias to 1 volts (orignally it was 4 volts at room
temperature) due to the current limit of the power supply.
I have 2 guesses, one c'ld be a short of bias line to ground but i have
used magnet wire so this shouldn't be a case, and i tested the
enclosed system was working fine uptill 77 Kelvin.
Second guess, is that i have many bypass capacitors multilayer ceramic
type which cld go bad (I have no electrolytic caps in my circuit).
Does any one know how these caps behave when gone bad or when cooled to
4 Kelvin or so?
Please help.

Many possible things could be happening.
First, warm them back up to verify proper operation.
Second, wire up one capacitor, feeding the 2 leads to the outside
world and measure the capacitance as a function of temperature.
There is a good possibility that this will also show the same abrupt
transition to a short circuit near/at 77K.
If warming the capacitor back up maked the cap good again, then the
ceramic might be changing to a superconductor below 77K.
Depending on how much capacitance you need in various parts of the
circuit, you might try mica capacitors or mylar capacitors.

 

[Tim Williams]

If warming the capacitor back up maked the cap good again, then the
ceramic might be changing to a superconductor below 77K.

Eh? I know (I think I know..) perovskite(sp) superconductors are relatively
similar in crystal structure, but I never heard of titanates
superconducting.

Tim

 

[[email protected]]

Many possible things could be happening.

First, warm them back up to verify proper operation.

I did something similar. I slowly increased my VDD from 0 in very small
steps. At first, the current drawn was high (say 0.05 Amps), however
slowly it decreased to 0.01Amps. And basically with time i could put
VDD = 2.2 volts and VSS = -1 volts, with Out(+) and (-) at 1 volts.
This implying that there is around 1.2 mA of current in each half of
the circuit and seems i am also getting some amplification out of it.
But i want to do some more tests to confirm. The power supply still
says 10 mA of current being drawn i want to know where it is going.
We have spent something around US$2000 to cool this whole system and if
i want to change something on the amplifier that money goes in drain.
Thats why i am hoping to avoid this. Right now i only have access to
VDD, VSS, GND and Out+,- pins which are on a connector outside the
vacuum system.

Second, wire up one capacitor, feeding the 2 leads to the outside
world and measure the capacitance as a function of temperature.
There is a good possibility that this will also show the same abrupt
transition to a short circuit near/at 77K.
If warming the capacitor back up maked the cap good again, then the
ceramic might be changing to a superconductor below 77K.

I am going to try this, the first thing in the morning. Hopefully, this
is the case.

Depending on how much capacitance you need in various parts of the
circuit, you might try mica capacitors or mylar capacitors.

If you w'ld look at my circuit Bob, i need a range of capacitors for
bypassing the power rails, 1.2pF to something 1 uF. the pFcapacitors
have to be of high frequency types with SRF in GHz regime. I am trying
to find some which are non-silicon.

 

[Terry Given]

Eh? I know (I think I know..) perovskite(sp) superconductors are relatively
similar in crystal structure, but I never heard of titanates
superconducting.

Tim

besides, 18 Ohms is a pretty shitty superconductor.

Cheers
Terry

 

[[email protected]]

besides, 18 Ohms is a pretty shitty superconductor.

Cheers
Terry

The 18 ohms i think is coming from the magnet wire (Thin 36 Gauge
phosphorus bronze wire from lakeshore), that piece of crap had 6 ohm
from the PC board to the connector at room temperature (almost 5ft
long). So i don't wonder if cooled to 4 K it would show that value of
resistance. Infact they do quote 8 ohm per meter at 4 Kelvins:
http://www.lakeshore.com/temp/acc/am_wirets.html

So i think the capacitor has become superconducting on the PCB and when
i measure resistance between the pins VDD and GND on the connector all
i get is the resistance of the 2 connecting wires.

 

[[email protected]]

Many possible things could be happening.
First, warm them back up to verify proper operation.

[snip]

We have spent something around US$2000 to cool this whole system and if
i want to change something on the amplifier that money goes in drain.
Thats why i am hoping to avoid this. Right now i only have access to
VDD, VSS, GND and Out+,- pins which are on a connector outside the
vacuum system.

Two ideas:
1) Get some capacitor candidates and cool ferociously, external to
the cyro setup, to see what they do.
2) Time-domain reflectometry of the cryo setup would tell you the
location and nature of the short, if it exists. If you don't have a
TDR, just make one--a simple rig suffices.

James Arthur

 

[Kiviranta, Mikko]

I have used the X7R and

The capacitance of X7Rs I've fiddled with fall to 1/10 when cooled to
4.2K. NP0s keep their capcitance better. But of course this depends on
the actual chemical composition of the dielectric.

also as i needed power supply bypassing upto
high frequency (upto 10GHz) i have used RF capacitors from Vishay.
HPC0402A type:

I've been happy with small-value NP0s in a 0402 case, which can also
have quite high SRFs. I have no experience on Si caps but would expect
them to be OK.

I would really like to have a look at ur amplifier, if you want to
share. Is it published, already?

I presented it in the WOLTE-7 workshop, a preprint is found in
http://www.24.fi/kiviranta/xeus/

Regards,
Mikko

 

[joseph2k]

I have used multilayer ceramic capacitors for power supply bypassing
for a cryogenic amplifier. Everything worked fine at room temperature,
however as i inserted the circuit in the cryogenic system, after a
while my resistance of bias line to ground dropped to 18 ohms.
I can't even turn the bias to 1 volts (orignally it was 4 volts at room
temperature) due to the current limit of the power supply.
I have 2 guesses, one c'ld be a short of bias line to ground but i have
used magnet wire so this shouldn't be a case, and i tested the
enclosed system was working fine uptill 77 Kelvin.
Second guess, is that i have many bypass capacitors multilayer ceramic
type which cld go bad (I have no electrolytic caps in my circuit).
Does any one know how these caps behave when gone bad or when cooled to
4 Kelvin or so?
Please help.

Everything i know about multilayer ceramic capacitors tells me that they
will not fail in this way. All the failure modes that i know of are high
temperature types. Unfortunately i have scant data at 77K and none below.

 

[colin]

The 18 ohms i think is coming from the magnet wire (Thin 36 Gauge
phosphorus bronze wire from lakeshore), that piece of crap had 6 ohm
from the PC board to the connector at room temperature (almost 5ft
long). So i don't wonder if cooled to 4 K it would show that value of
resistance. Infact they do quote 8 ohm per meter at 4 Kelvins:
http://www.lakeshore.com/temp/acc/am_wirets.html

So i think the capacitor has become superconducting on the PCB and when
i measure resistance between the pins VDD and GND on the connector all
i get is the resistance of the 2 connecting wires.

could you acheive anything usefull by pulling up the out+/- pins
externally via 1k resistors to VDD,
or maybe more to compensate for short, ?
or has it already warmed upto room temp by now ?

Colin =^.^=

 

[Mr. J D]

I have used multilayer ceramic capacitors for power supply bypassing
for a cryogenic amplifier. Everything worked fine at room temperature,
however as i inserted the circuit in the cryogenic system, after a
while my resistance of bias line to ground dropped to 18 ohms.
I can't even turn the bias to 1 volts (orignally it was 4 volts at room
temperature) due to the current limit of the power supply.
I have 2 guesses, one c'ld be a short of bias line to ground but i have
used magnet wire so this shouldn't be a case, and i tested the
enclosed system was working fine uptill 77 Kelvin.
Second guess, is that i have many bypass capacitors multilayer ceramic
type which cld go bad (I have no electrolytic caps in my circuit).
Does any one know how these caps behave when gone bad or when cooled to
4 Kelvin or so?
Please help.

Its common sense that as an object goes towards 0 Kelvin, the brownian
motion of its atoms decrease dramatically. If anything, I would assume
a decrease in capacity for a capacitor at temps of 77 Kelvin and lower.
As the brownian motion slows down, so does the number of collisions,
decreasing electron transfers.