Re: PCB's in liquid nitrogen

On May 30, 11:29 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 29 May 2009 13:01:35 -0700 (PDT), ggher...@xxxxxxxxx wrote:
On May 29, 3:34 pm, Okkim Atnarivik
<Okkim.Atnari...@xxxxxxxxxxxxxxxxxxxxx> wrote:
John Larkin kirjoitti:

On Fri, 29 May 2009 08:42:16 -0700 (PDT), ggher...@xxxxxxxxx wrote:
Does anyone have experience using printed circuit boards at liquid
nitrogen temperatures?  

I don't think you'll have any problems with the board. Wide range temp
cycling can stress surfmount solder joints, so thru-hole parts are
probably safer.

   Large sized SMD components crack sometimes if the card gets thermally
recycled several times. But typically SMDs are no problem at all in
non-commercial experimental-type work. I'd say they are even OK in
commercial work if your circuit is typically just cooled once and
then stays at 77K or 4K for long periods of time. But for commercial
work must experiment with your particular devices because you
are likely to operate them outside the specs anyway. My colleagues
once hung a small circuit from the seconds arm of a wall clock so
that it got dipped into LN dewar and retracted from it a one-minute
cycle time.

   More often there are problems because the frost and condensed
moisture may spoil some components if you just lift the circuit from
LN and let it warm up in open air.

   Regards,
            Mikko

The circuit board will be go through lots of thermal cycling.  It will
be used by students to measure the Johnson noise from resistors as a
function of temperature.  So I expect maximum thermal stress from the
students.

What are you going to use as an amplifier? It will have to be pretty
good to measure Johnson noise with any accuracy.

Some tuned phemt amps can hit noise temps around 40K. Wideband untuned
amps usually run around room temp.

It's easier to go hot than cold!

John- Hide quoted text -

- Show quoted text -

The amplifiers and filters will all live at room temperature.
(Nothing fancy here.)
The filter maximum bandwidth maximum is 100kHz. (A state variable
filter with 1% C’s and 0.1% R’s) The amplifiers bandwidth is up near
1MHz. Except for the first stage where the bandwidth changes with the
source impedance. The electronics seems to be working amazingly
well. (I always worry that two compensating errors are fooling me.)
If you’re interested I could post my latest shot noise data on
Monday.

George Herold
.

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