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

[Bob Camp]

Hi

There are some pretty good papers on system reliability as parts age. Most of what I have read assumes the system is operating, and that failed parts get replaced. In the sense of those papers, crystal oscillators are not like fan bearings. There is no guarantee of an ultimate end of life wear out. 

Bob

On May 17, 2013, at 9:09 AM, Azelio Boriani <azelio.boriani at screen.it> wrote:

> Usually aged crystals are reliable.
> 
> On Fri, May 17, 2013 at 2:07 PM, 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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