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April 1998

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Subject:
From:
Paul Klasek <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 16 Apr 1998 14:19:29 +1000
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Hi Werner

On the purely technical point of view you may be (usually are) right ;
on the practical it worked slightly differently :
1
The reduction of life has been calculated and accepted ,
on base of major reduction of early failures , influencing as you can
imagine the warranties ( 1-2-3 years ) .
Therefore , commercially , little emphasis is on the longevity of the
product ;
technology and depreciation laws currently being changed to reflect the
realistic pace do also encourage this short term focus.
Even simple burn in gives results , and the say 10% cycling did screen
early faults quite effectively , once the short occurred and interrupted
the continuity (switched tested unit off) ; it's been established .
Obviously without this closed mode of observation ; the "visual"
inspection is more than less useless ; as you said .

It is likely that due to the commercial,  perhaps disputable ethics of
the "designed" life character, not many folks will be responding .

2
When we did on the telecomm products the vibration tests, in various
designed frequencies , simulating rail transport ( droning ), etc., we
found that more than the joints we tested componentry, = the components
, still very much mixed even today , failed long before we got to see
anything on the joints .
It has been good indicator of what should be tied down , or poured over
with epoxy ;
but poor indicator of the whole board early faults status .
The mix is just too diverse , unless you make SIMM cards only , and
cap's notoriously disintegrate for all different reasons than joint far
too quickly .
Or, take a MQFP  , you can vibrate all the legs to oblivion ; with one
dry not being fazed at all ; the body will support it ,
while cycled under load it will show up .

To sum it up, for example on that IPC-SM-785 you helped us to get going
;
if on page 20 you take the telecomm category on leadless with 4700
cycles and establish whatever part of infant mortality (pages 6-7)
eradication on precycling you sacrifice from the length of the 7 years
life ; considering you'll have likely new telephone within 5 years ;
you are still ahead , happy OEM , with less warranty problems for you
AND the customer .

Thanks for all the help Werner

paul ; ResMed ;


>----------
>From:  Engelmaier[SMTP:[log in to unmask]]
>Sent:  Thursday, 16 April 1998 10:48
>To:    [log in to unmask]
>Subject:       Re: [TN] temp cycling of soldered assemblies
>
>Hi Sheila,
>You did not say what kind of temperature cycling nor for how many cycles this
>"temperature cycling after solder rework" would involve. But from a purely
>technical point of view, temperature cycling of product to be shipped to the
>customer is silly. It is not a very effective ESS techique for full
>assemblies, because if it were carried out to be effective it would do more
>damage to good solder joints than finding bad ones. Any temperature cycling
>of
>full assemblies will accumulate some fatigue damage and therefore reduce the
>life.  Whether this is significant or not will depend on the severity and
>number of temperature cycles as well as the design parameters of the
>assembly.
>To find bad-improperly wetted, other so-called defects rarely make a
>difference-solder joints one would be well advised to use vibration,
>preferably at -40 degrees C. Good solder joints are not damaged, bad ones
>fall
>apart.
>
>Werner Engelmaier
>Engelmaier Associates, L.C.
>Electronic Packaging, Interconnection and Reliability Consulting
>7 Jasmine Run
>Ormond Beach, FL  32174  USA
>Phone: 904-437-8747, Fax: 904-437-8737
>E-mail: [log in to unmask]
>
>
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