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Does anyone know if the differential impedance, say of a PCIe bus, can be affected by really cold temperature, like -50 °C? I have a wifi card that does not work from -30 °C and colder. I am wondering if somehow the differential impedance of the bus is changing from the temperature.

This is a 12 layer board, 1/2oz copper, FR-4 and the differential pair traces are on the top later.

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    \$\begingroup\$ Why would you assume that it is the characteristic impedance (not diff.) of the PCIe bus lines? You're just guessing and very likely chasing a "red herring". Would it not be extremely more likely that it is the WiFi card itself having a problem with low temperatures? \$\endgroup\$ Oct 30, 2017 at 15:45
  • \$\begingroup\$ 1) What is the card? 2) how do you know its from temperature and not something else like condensation? \$\endgroup\$
    – Voltage Spike
    Oct 30, 2017 at 15:46
  • \$\begingroup\$ This is an industrial rated card, which means it can handle temperatures down to -55 C. I don't know its from the temperature. I am simply asking if anyone has knowledge of how temperature can effect diff. impedances. \$\endgroup\$
    – Mtk59
    Oct 30, 2017 at 15:50
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    \$\begingroup\$ Although the differential impedance will change with temperature, it will be tiny (the impedance goes down with decreasing temperature). The change is due to the XY CTE of the PCB, but as this is usually of the order of 14ppm for most flavours of FR4, it should not be a noticeable change given that controlled impedances are 10% at best anyway. \$\endgroup\$ Oct 30, 2017 at 16:26

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I would not suspect the impedance was changing but rather the quality of your interconnect + the components on the -55C rated wifi card and whatever is at the other end result in failure. There is no doubt that timing inside the chips will change as temperature goes down. If you have built yourself (or bought yourself) a link (your motherboard). That works at room temperature, the design may not be good enough that combined it works at -55C. I would demand my vendor help read the BER out of their WiFi chip and monitor it as temperature drops.

If you can't get that it would be interesting to look at how the eye changes with a scope (you can keep probe body out of the oven if you're creative.

If you really believe your impedance is changing, and I suspect it is not, you could rent a TDR or a VNA and measure it at different temperatures.

Oh! Another good thing you could try if you can't get any help from the vendor. You could rent the gear to do PCIe compliance testing. There you can definitely put the devices in different modes and generate traffic and see how it responds. The lecroy PERT comes to mind as a good piece of test gear.

Despite how wide spread it is PCIe is still a fickle beast. Oh and it would be worth looking to see if they have passed PCIe compliance (and if they had to play any tricks to do so).

One last piece of advice. If you're running at PCIe 3.0 try to force it to 1.0 and see if it's better. If you're running x4 or x8, try to force it to x1 (either with a piece of kapton tape or your bios).

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At -30C, I would be worried about the physical integrity of the laminate. Other components, mainly the crystal are out of tolerance at such extremes, assuming it is a commercial spec device.

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  • \$\begingroup\$ It is an industrial rated card, rated for -55 C. \$\endgroup\$
    – Mtk59
    Oct 30, 2017 at 15:51
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    \$\begingroup\$ @Micah, in that case (and you trust the vendor not to have stretched the truth about its capabilities), suspect the interface chips in the motherboard it's connected to. \$\endgroup\$
    – The Photon
    Oct 30, 2017 at 16:34
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    \$\begingroup\$ Generally speaking, laminates do not have temperature issues at -30; more probably the parts are not properly rated (a not uncommon problem even on boards nominally rated at -40; -55 is mil spec, not industrial) \$\endgroup\$ Oct 30, 2017 at 19:07

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