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Date: | Tue, 04 Feb 1997 10:53:13 -0500 |
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Hi Gordon,
"1. The process by which reliability distinctions get into
standards.
2. The relative merits of making the distinctions on the
basis of consequences vs. risk of failure."
I'll try to explain from another side of Reliability and
Product Safety.
Reliability -
Bellcore is a private set of standards (in other words they
are not enforced by law) covering telecommunications
equipment. Technical Reference TR-NWT-000332 "Reliability
and Quality Generic Requirements". From this, through
complicated math, failure rates of each component are
used for the calculation of the entire assembly.
FITs "Failure in Time" are used as well as MTBF "Mean Time
Between Failures". A FIT = 1 is one failure in 10^9 hours.
Typically say, a 1/4W carbon composition resistor has a
FIT = 1. MTBF = (10^9 hours)/FITs. Unfortunately, MTBF
doesn't say anything about MTTF "Mean Time to First Failure".
These FIT or MTBF numbers are supplied by either Bellcore
from charts for standard parts (such as the resistor above)
or from the mfr using accelerated tests and calculations.
No where in this document are any calculation based on
any flux residue analysis nor processes related to
boards. The construction of boards as such is covered
in another Bellcore document TR-78 soon to be GR-78.
The purpose of all this number chrunching is to give
the Telco customer (like NYNEX or Ameritech to name
some) a "reasonable" estimate that indicates how
long a device will last. This is driven from the
service provider handling 911 calls for example.
Another would be simply comparison - one piece of
equipment has a one failure in 100,000 hours, another
one has one failure in 500,000 hours.
Safety -
All of the product safety testing I've had done for UL
never addresses board construction, i.e. flux residue, etc.
They do approve clearances between traces and CTI
(Comparative Tracking Index) of the material used.
Failure analysis is done at a first level stage -
something shorts, something opens, something overheats.
It is motivated by "end user" safety.
How all these things get into the standards be they
public standards or private standards is driven basically
by us - the industry. If enough of us get on the
bandwagon, we can pursuade ANSI or law itself to
either rewrite an old or write a new standard.
Think of ANSI as a clearing house for standards. If
enough people have caused a fuss, they are charged
with getting it written. They will turn toward an
established organization for tha actual dirty work.
You will see ANSI standards with a ANSI/xxxx.
The "xxxx" is the actual commitee that performed
the work.
I don't know if I really answered your questions.
Regards, Doug
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The comments and opinions stated herein are mine alone,
and do not reflect those of my employer.
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