[time-nuts] aging/failure of un-powered xtal oscillators?

[Didier Juges]

The most common failure modes for electronics are a result of thermal
cycling and elevated average operating temperature. It appears that these
boards have seen neither, so you are on the good side of statistics.

That said, there are a number of failure modes that do not depend highly on
these two factors, and simply because the probability is low that thermal
cycle induced failure would occur, the probability is not zero.

On the other hand, any repair you do on electronic equipment also affects
future reliability. Replacing soldered components on old boards is a tricky
business unless done right. The boards have to be dry and you have to
minimize the stress on the board itself and components nearby when you heat
the board to unsolder a part. In some cases that means putting the board in
an oven to elevate it's temperature such that the thermal shock of the
soldering iron is not so great. That is particularly true if you replace
large components that require a lot of heating.

In my business (military aerospace), we do not routinely replace things
that we do not know with great certainty have to be replaced precisely for
these reasons.

I suggest it would be more effective to develop a screening program to make
sure the boards work (or if they don't, to identify the failure mode)
before attempting to replace any component.



On Fri, May 17, 2013 at 7:07 AM, Grant Hodgson <grant at ghengineering.co.uk>wrote:

> A client company has sourced a quantity of 'New in Box' iSBC series memory
> modules manufactured by Intel in the 1980s for a MULTIBUS based computer
> system. These are still in their original, sealed packaging and have been
> stored (for 25 years) in controlled conditions. These cards are required as
> part of a refresh programme for a mission-critical application (electricity
> generation), which are currently using original Intel cards from the same
> era.
>
> The memory cards use a 64.1kHz oscillator module as the refresh clock for
> the DRAM.  (I suspect that the oscillator module uses either a 2.5MHz or
> more likely a 5MHz crystal whose output is divided down within the module
> by 39 or 78 to 64.1kHz).
>
> As this oscillator is used only for DRAM refresh timing, the accuracy and
> drift are not particularly important.  However, reliability is important -
> i.e. the oscillator must function, even if it is a few ppm off frequency.
>
> AS a matter of precaustion, all the tantalum and electrolytic capacitors
> on the NIB cards are to be replaced as a matter of course, and any socketed
> ICs will probably be removed, cleaned and replaced (to be decided).   There
> are several dozen cards to be replaced.
>
> There is an option to replace the crystal oscillator module, but it is
> uncertain as to whether it makes sense to to the extra work.  The
> replacement oscisllator module will be from a different supplier to the one
> Intel used, and would need some form of qualification.
>
> Is there a known failure mechanism whereby a cystal oscillator module that
> has been sat on a shelf (as part of a complete card) in a benign
> environment develops one or more characteristics that could impact it's
> reliability?
>
> regards
> Grant
>
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