Hi Matthias *et al*,
There are many things to look out if you go to 77K or below.
- LED will have a colour shift due to the change in the band-gap; quite
important for spectroscopy, say.
- You need to compensate for changes in LED forward voltage drop.
- For LN immersed high speed stuff, dielectric constant increases a bit (to
about 1.5)
- Thermal shock is a big issue, make sure not to immerse hot (as in room
temperature) components to LN directly; let them cool down first.
- Make note of CTE of all the materials you use.
- Do not use high Sn lead-free solder below 220K.
- You may need to use indium based solders.
- Avoid cycling high power circuits, if possible; creates efficient
micro-thermal-shocks.
- Measure your capacitors in LN first, tantalum polymers may surprise you
positively (<20% capacitance drop); Try not to use ceramics worse than
NP0/C0G; X7R capacitance is redued 3-fold.
- Measure your resistors in LN first (thin and thick film resistors are
pretty good for LN).
There is more. This is most general. Hope it helps.
Indrek
Indrek Rebane, CTO, Hedgehog OÜ
Riia 185A, 51014 Tartu, Estonia
[log in to unmask], www.hedgehog.ee
tel +372 5698 1845, skype teine_rebane
On 24 November 2015 at 17:02, Matthias Mansfeld <
[log in to unmask]> wrote:
> Hello everybody,
>
> I got a request for a quite simple design (flex or rigid, still
> undefined) containing two dozens THT LEDs, a resistor and a cable.
>
> Application is anywhere at -196°C (I hope not immerged in Liqid
> Nitrogen), and currently I don't know further details like
> temperature change rates, number of "cycles".
>
> I see possible problems with thermal mismatch (220° difference as
> worst case) together with the severe embrittlement.
>
> Any ideas, experiences, advices, application notes, links to special
> design rules etc.? Any help is severely appreciated, as my "designer
> instinct" is maybe enough for normal or hot stuff, but not for this
> application.
>
> Many thanks in advance
> Best wishes
> Matthias Mansfeld
> --
> Matthias Mansfeld Elektronik * Printed Circuit Board Design and
> Assembly
> Neithardtstr. 3, D-85540 Haar, GERMANY
> Phone: +49-89-4620 0937, Fax: +49-89-4620 0938
> Internet: http://www.mansfeld-elektronik.de
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> Fingerprint: 6563 057D E6B8 9105 1CE4 18D0 4056 1F54 8B59 40EF
>
>
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