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February 2002

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Thu, 21 Feb 2002 09:12:43 +0800
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Guy,

The received wisdom I have from our design and engineering team is that
components stored undisturbed for more than 5 years may (and I stress
'may') have a significantly degraded reliability/functionality.

You talked of instruments having been serviced for 50+ years, and that was
the point I was trying to make - if equipment is in service (being used or
'exercised') on a regular basis, it tends to keep going, but degrade if
just left under 'normal' conditions. Consider a car that is left unused for
any length of time. My experience is that they never quite function
properly straight away if you then try to use it. On the other hand, a car
in regular use and properly maintained can go on for years and years.

Genny's situation is a judgement call, based on experience and risk
assesment. I agree that solderability will be the main issue, and that if
properly stored, 5 years is a conservative estimate of a component's
lifespan. Depends on the application. In my line of military avionics, we
don't like to take too many chances on mission critical equipment, and
component obsolescence/lifetime buys is a major issue with us -
increasingly so as technological changes advance at a seemingly exponential
rate. We're also increasingly having to use industrial grade parts instead
of MIL parts as availability of MIL parts diminishes. Additonal screening
to MIL levels is time consuming and expensive, so we have to minimise risks
in other ways. One of them is to limit the time for which components can be
stored. Regular exercise (power up, stimulation or whatever is appropriate)
helps to keep the parts in shape.

Peter




                    Guy Ramsey
                    <gramsey@ACIU        To:     [log in to unmask]
                    SA.ORG>              cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST
                    Sent by:             Aero/ST Group)
                    TechNet              Subject:     Re: [TN] How old is old?
                    <[log in to unmask]
                    ORG>


                    02/20/02
                    09:59 PM
                    Please
                    respond to
                    "TechNet
                    E-Mail
                    Forum.";
                    Please
                    respond to
                    Guy Ramsey






I was surprised by Peter's response to your post. I wonder what he means by
exercised.

I have work experience with manufactures of electronic instruments that are
serviced for 50+ years (Been in business since just before W.W.II. In the
early seventies they transitioned from analog to digital 27 V NMOS logic
devices. They have components in storage from that time frame. There are
occasional problems with lead frames that have failed (rusted under the
gold) and occasional bad lots of die (didn't test everything before it was
placed in storage). But, mostly, they are able to service and guarantee
their work using solid state devices that were manufactured over 25 years
ago. They use (stored in their own vaults or purchased through brokers)
components that have been out of production for five to ten years; these
would include Crowbar ICs, Switch-mode regulator ICs, Bucket-Brigade ICs,
OP
AMPs, Voltage Regulators and Transistors. I cannot recall getting bad (non
functional) parts but we did have our soldering problems.

Occasionally, there were differences in the performance of components that
required changes to passive component values in some designs. We believed
this was due to variation that was present at the time the components were
made rather than related to changes over time.

Relays or other Electro-mechanical devices are a concern (reed switches,
motors, valves, solenoids). We had problems with them. But it would be hard
to place a shelf life on them, are they hermetically sealed (many were to
allow chemical cleaning)? Most of the problems I experienced were with open
parts. Today many relays are open assuming no-clean processing.

Electrolytic capacitors, carbon resistors are a concern but carbon
resistors
are really a thing of the past. SMT Electrolytic cans, five years would be
a
very conservative limit if you are talking about function.

Solderabiltiy is your main concern Genny.

Guy Ramsey
Senior Technician / Instructor
E-Mail:  [log in to unmask] <mailto:[log in to unmask]>
Ph:  (610) 362-1200 x107
Fax: (610) 362-1290



> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Genny Gibbard
> Sent: Tuesday, February 19, 2002 10:19 AM
> To: [log in to unmask]
> Subject: [TN] How old is old?
>
>
> One of the purchasers has asked if they can purchase some parts
> with a 2000
> date code on them for 'a really great deal!'  The part is an 8 lead uSOIC
> package
> At what point do you say they are too old?
> I would appreciate info specific to this package or, just in general,
> guidelines for chip or standard SM components.  We do not use
> BGA's yet.  We
> aren't trying to do lead free.  We generally spec 60/40 HASL finish on
our
> boards.
> Thanks for your time.
>
> Genny Gibbard (mailto:[log in to unmask])
> Product Transition and Support
>
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