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'[OT] Survey... What is an acceptable failure rate?'
1999\10\20@174736 by Roger Morella

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The following is a survey that I hope will benefit all of us regarding
the prevailing consensus concerning failure rates and warrenty periods
for industrial and consumer products.  The more people that respond, the
more we will all benefit from the results.

Preface: We have a situation where our customer is asking that we keep
our failure rates at what seems to us to be an unacceptable limit.
While we strive to keep our failure rates as low as possible, there are
practical limits to how low you can go.  This is why we typically give a
1 year warrenty on most of our industrial products.

Questions:
In your opinion, what is an acceptable failure rate for electronic
control assemblies in an industrial product?

What about in consumer products?

What do you feel is a fair warrenty period for industrial products?

What is fair for consumer products?

Has anyone considered an extended warrenty purchase program for their
industrial customers, and if so, how did it work?

Where can we find industry data that addresses these issues?

Thanks for responding

1999\10\21@135212 by Nigel Goodwin

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In message <spam_OUT380E3822.968F48FATakeThisOuTspambuffnet.net>, Roger Morella
<.....rjmKILLspamspam@spam@BUFFNET.NET> writes
>Preface: We have a situation where our customer is asking that we keep
>our failure rates at what seems to us to be an unacceptable limit.
>While we strive to keep our failure rates as low as possible, there are
>practical limits to how low you can go.  This is why we typically give a
>1 year warrenty on most of our industrial products.

How low are they expecting your product to achieve?.

>Questions:
>In your opinion, what is an acceptable failure rate for electronic
>control assemblies in an industrial product?

It all depends on how and why they fail, if the same part keeps failing
it's most probably a design fault and should be corrected. However, if
they all fail in different ways, perhaps it's down to how they are being
used, or the environment they are used in?.

>What about in consumer products?

As above, failure rate should be low if the unit is correctly designed,
and built using decent spec components. One of the most common failures
in TV's (and a recurring one for many, many years!) is resistors in
series/parallel going O/C. Instead of using a resistor large enough to
take the wattage, it's common practice in TV to use 2 or 3 small
resistor in either series or parallel - this is extremely unreliable,
and I would consider this a major design flaw!.

>What do you feel is a fair warrenty period for industrial products?

I don't think the consumer legislation in the UK applies to industrial
products, but I think most still comes with a 12 month guarantee.

>What is fair for consumer products?

In the UK all consumer products have to be guaranteed for 12 months.

--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : nigelgspamKILLspamlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
       | Chesterfield    | Official site for Shin Ki and New Spirit   |
       | England         |                 Ju Jitsu                   |
       \--------------------------------------------------------------/

1999\10\21@141510 by Dave VanHorn

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I've been through this one.
Marketing decides that they NEED an MTBF number to satisfy the customers,
usually because the compettitor has one. They don't want to pay the $$$ to
really do the job (and they have no idea of the size of the job!), but they
want you to "tell them" what the MTBF is.

GAAARRRRGH.

I agree with Nigel, if your defects are scattered, and there aren't a lot of
them, then you're in fine shape. If you see consistent failures in one area
or component, then you need to investigate why.

When I roll out a new product, I insist on seeing all the failed units until
I'm satisfied that they are right.  The repair guys just fix, and they
aren't going to be looking for what caused the failure, that is an
engineer's job.

1999\10\21@163119 by M. Adam Davis

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Roger Morella wrote:
> Questions:
> In your opinion, what is an acceptable failure rate for electronic
> control assemblies in an industrial product?

That depends.  The failure rate for an industrial floor sweeper could be
very high, say 1 out of ten.  This because it is not an object which is
purchased in volume, and it is not an object which will cause the
factory to lose time/money/lives/etc.

A defective laser safety curtain, however, better have failure rates
lower than 1 in every 100.  If someone steps into a machine zone
expecting the machine to go off automatically (besides being called
stupid) they may end up with catastrophic injuries.  The insurance
companies are relentless in finding out who to blame.

> What about in consumer products?

Same deal.  A watch can have 1 in 30 failure rate.  The user takes it
back to the store, it's replaced, and they're all set.  At most they may
have to send it to the manufacturer, but sometimes it's easier and
cheaper to buy a new one.

> What do you feel is a fair warrenty period for industrial products?

That depends on the expected lifecycle of the product.  If I sell
mechanisms to control traffic lights, an expected 14 yr. life cycle, I
should probably cover at least 10% of its life in a warranty.  But then
you really need to get to the base of the problem and say:
How much can I charge for the product?
What are the MTBFs for the parts I'm using?
What are the warranties I'm given from my part sources?
Are the people who assemble my boards/parts giving me any warranty?
If my MTBF is 1 year, and my customer wants a two year warranty, will
they also accept a 2.5x cost increase?
Does my customer really want a MTBF, or is it just so they can fill out
all the forms they need to fill out?  (remember - MTBF is an estimate
based on statistics which has little legal binding)

> What is fair for consumer products?

90 days parts/labor
1 year parts

> Has anyone considered an extended warrenty purchase program for their
> industrial customers, and if so, how did it work?

It appears to me that industrial people don't want 'intangible
accessories' such as extended warranties.
"Either the part is going to work for two years, or it's not.  The
warranty period won't change that, and chances are in 19 months when it
breaks, I won't be responsible for it - but I am responsible for the
budget report at the end of the month..."

If you can justify extending the warranty to two years for x dollars,
roll the cost in, and tell them it comes with a 2 year warranty.  Every
extra item on the invoice is questioned by the boss and the
accountants.  If it's not a necessary, tangible part, they look at it as
superfluous.

> Where can we find industry data that addresses these issues?

It varies so much from product to product and situation to situation.
the best you can do it find others who are doing the same thing your
are, and see what they are doing.  If they offer a 2yr warranty, you
offer a 3. etc, etc, etc.

> Thanks for responding

-Adam

1999\10\21@164751 by Michael Lee

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----- Original Message -----
From: Roger Morella <.....rjmKILLspamspam.....BUFFNET.NET>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Wednesday, October 20, 1999 10:46 PM
Subject: [OT] Survey... What is an acceptable failure rate?


{Quote hidden}

Statistically, the MTBF *must* increase with system complexity.  Say for
example when manufacturing, a solder joint fails 1 in 100,000 times.
Is this acceptable? Most, including me, would be happy with this figure.
If your product contains 1,000 solder joints, 1 in every 100 units will be a
failure straight off the assembly line. Thats just the soldering now
consider
resistors,capacitors,transistors,ics,pcbs,assembly errors ....
These probabilities are cumulative, and in complex systems it is not unknown
for the system to be more likely *not* to work.

I beleive it was motorola who introduced the 6-sigma reliability concept,
which is the latest management buzz in my own company.  However, it has its
basis in hard mathematics, not bullshit - which is more often the case.

So, in summary, it depends on complexity and design.

> What about in consumer products?

I refer you to the answer above.

> What do you feel is a fair warrenty period for industrial products?

Failures once off the steep initial part of the 'bathtub' reliability curve
are rare.  Ask your customer if he would be prepared to pay for a 100%
burn-in
screening of the product prior to delivery.  The complexity and design will
determine the burn-in required.  You might want to ask searching questions
as to environmental & vibrational testing.  Mentioning money always makes
'em uncomfortable.

> What is fair for consumer products?

A 12 month warranty appear to be the norm.

> Has anyone considered an extended warrenty purchase program for their
> industrial customers, and if so, how did it work?

N/a

> Where can we find industry data that addresses these issues?

Try a reliability consultant.

> Thanks for responding
>

1999\10\21@192145 by Roland Andrag

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On the same sort of topic, has anyone seen a book or web page devoted to
making a circuit design bulletproof? I would love to hear a few tips from
anyone willing to share!

Cheers
Roland

1999\10\22@002530 by Russell McMahon

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>On the same sort of topic, has anyone seen a book or web page devoted to
>making a circuit design bulletproof? I would love to hear a few tips from
>anyone willing to share!


As it seems to be pertinent I'll crosspost something I just sent to the
ARocket list - the subject is the survivability of electronics etc under
impact. Has some relevance to current topic.


     Russell McMahon
_____________________________

>From another world - http://www.easttimor.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))


=================================================



Electronics, properly designed, can survive decelerations of upwards of
1000g.
Dave Hall notes that the military fire ordnance containing GPS units inside
at closer to 10,000g.
Nothing sensible will survive a full accelerated descent but if you assume
maximum terminal velocities in the 100 - 150 mph range (a less than fully
streamlined body falling vertically) then the degree of crumple afforded by
your late lamented rocket plus some purposeful internal decelerating
material should get accelerations down into the above range.

Getting even overall deceleration of the subcomponents  in your equipment is
vital. What is required is either encapsulating or encasing in a homogeneous
"stiff" medium which is able to transfer the force applied to the outside OR
having individual parts able to handle the deceleration by themselves
(usually harder).

What does it take to limit deceleration to 1000g at 100mph>
Lets see, swapping to pretending units.
100mph ~= 45m/s say 50 m/s
9 ~= 10 m/s/s
Deceleration Distance = Vel^2/20/g's
= 2500/20/1000 = 0.125m

ie if you can decelerate LINEARLY over 0.125m = 5 inches from 100 mph
deceleration will be around 1000g.

Stopping time = Vel/Accn = 5mS at 1000g!

G force goes up as the square of the impact speed
G force goes down in inverse proportion to stopping distance.

To put some perspective on what 1000g means -

A typical resistor (SFR16x) weighs about 0.1 gram (1/300 oz) and small
capacitors etc well under 1g (1/30 oz).

A 10g (1/3 oz) component will weigh 10Kg during this (brief :-)) period.
It's not hard to imagine designing so that eg resistors will withstand this
sort of force - imagine trying to pull through hole resistors off a circuit
board by hand given a good grip
.
A 100g (3 oz) module will weigh 100kG during this period - imagine a
heavyish person standing on your equipment with just the ball of their
foot - a not inconceivable design objective.

Translating this into people terms is not so attractive - a 100kG person
will weigh 100 tons - attempting to survive the imposition, however
temporary, of an evenly applied 100 ton weight is not an attractive thought.

The main tricks, after building it properly, are:

   Achieving linear deceleration.
   Getting the direction right - if the design is for a lawn dart but it
turns sideways just before impact and decelerates at 90 degrees to the
design direction, failure is "more likely" :-)

I build talking communicators which  are often used by children. It is not
unknown for these to be thrown off tables etc - often without 0.125m being
available for deceleration :-) Actual stopping distance is ill defined but
on a concrete floor is only the pcb mounting flexure distance. "Sensible"
construction ensures that these (usually) survive such treatment.


regards




     Russell McMahon


From: We <rrgpspamspam_OUTthefaultline.net>

Brings up another topic: Survivability. Have not heard that in the civilian
world of rocketry, yet. In the event of a land dart or core sample recovery,
there are things you can do to ensure greater (not certain!) survivability
of
expensive electronics and other  items of sentimental or economic interest.
Sectional density is one key; any item withing the rocket with a high
sectional
density (read longer than wider and heavy) becomes a spear on impact and
will do
just that to anything  in the rocket both  forward and aft of the object.
Most
altimeters are not made to be potted (cast in  a strong matrix to hold parts
in
place) but could be placed in machined aluminum thick walled containers.
Kind of
odd for an HPR vehicle, not so for an Experimental one and 180 bucks is not
trivial.
A friend called just last night to tell me about an impact in which his
allthread in his nose cone penetrated completely through the accordion
pleated
kevlar webbing in his recovery system! Hey, that stuff is supposed to be
bullet
proof!
Food for thought and I know the great thinkers on the list will have many
more
suggestions.
Bill Colburn

1999\10\22@053929 by ruben

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Date sent:              Thu, 21 Oct 1999 16:29:33 -0400
Send reply to:          pic microcontroller discussion list <@spam@PICLISTKILLspamspamMITVMA.MIT.
EDU>
From:                   "M. Adam Davis" <KILLspamadavisKILLspamspamUBASICS.COM>
Subject:                Re: [OT] Survey... What is an acceptable failure rate?
To:                     RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU


M. Adam Davis wrote:

{Quote hidden}

A safety device, like a laser safety curtain, has to be designed so a
(single) fault in any component (or program) related to the safety must
be detected in a 'safe' way. If the fault isn't detected and not leading
to a failure, the device still has to be safe with up to 3 undetected
consecutive failures (like bit faults in program memory, faulty resistor
or capacitor values, shorts in ic's or on circuit boards, etc...).
Detected here means that the fault should be detected by the operator
either directly or within one working cycle of the device (ie until the
next time it is to be reset) in a way that doesn't affect the safety, most
usually by not resetting and actuating it's output contacts.

This is the general idea with safety related electronics but there are
different categories with different 'degrees' of inbuilt safety.

One common way to design safety electronic devices is to have a dual channel
system where each channel does the same work and then compares each others
result and output contacts before actuating its own output contact. This, of
course, means more components and a theoretically higher MTBF, which is also
an aspect of safety devices because if it fails to reset a lot because of
internal or external faults it is a higher chance that the operator tampers
with the device (ie shorts the output contacts) resulting in no safety
at all.


==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +4640142078
FAX INT +4640947388
spamBeGonerubenspamBeGonespam2.sbbs.se
==============================

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