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TechNet E-Mail Forum <[log in to unmask]>
Date:
Thu, 17 Nov 2005 22:46:40 +0100
Reply-To:
TechNet E-Mail Forum <[log in to unmask]>, Ingemar Hernefjord <[log in to unmask]>
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From:
Ingemar Hernefjord <[log in to unmask]>
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Still  awaiting for an answer from the experts: can we neglect the
varying distribution of lead when soldering BGAs and other parts? If
not, how do we handle the situation on the floor. Production people want
know what to do when they do sample metallurgical investigations and see
Lead distributed many a way. See example sent to Steve, one and the same
BGA. Bright spots is Lead.
Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Ingemar Hernefjord
Skickat: den 17 november 2005 03:07
Till: [log in to unmask]
Ämne: [TN] SV: [TN] SV: [TN] [LF] Reliability prediction of LF soldering

Bingo jk,
That's what I mean with reality. Scientist being, as you are with your
background, you have to admit, however, that Arrhenius law is adaptable
on hot solder joint as well as cold, but it's a tough job to 'estimate'
all the thousand individuals because of the different life they live.

I would like to pay attention to my own central question which was
indeed a super'estimation': can we omit the various lead distribution in
BGA solder joints (and others as well)? I did not contribute with a
solution myself, because I wanted to wake up the dragons to see what
treasures they were lying on.

Our company has been working for years with the announced topic, and in
cooperation with other companies. All do the same. If you do your
'estimations' without too many of my reality factors, your MTBF
calculations give an interesting answer. As the result belongs to our
company, I can't give you the exact figure, but I can go so far as to
say this:
MTBF for LF BGA solder joints is : MTBF(LF)=MTBF(Sn/Pb) times a
multiplicand that is less than 1.0. All is confirmed by numerous tests.

Upon this number, you may build your up-and-down pyramid by putting in
all the real life factors that I mentioned. We are working on that, but
it seems to be a long working day before we have nice and reliable (!)
model.

Inge

-----Ursprungligt meddelande-----
Från: Joyce Koo [mailto:[log in to unmask]] 
Skickat: den 16 november 2005 23:34
Till: TechNet E-Mail Forum; Ingemar Hernefjord
Ämne: RE: [TN] SV: [TN] [LF] Reliability prediction of LF soldering

"Some GaAs chips are so hot
that you burn your fingers when you touch them,"...

Laptop by the name of it should be able to put on your lap... how many
of
lap top is cool enough to sit on your lap?  By the way, I couldn't
understand why the laptop guys always ad that "ultra-light" "compact",
meanwhile you need one of "cooling pad" to operate these ultra stuff...
http://www.maplin.co.uk/Module.aspx?ModuleNo=45656&&source=14&doy=search

As for reliability, heat dissipation sure get rid of all the MTBF
prediction
(how many of prediction based on the hot location and cool location in
the
same unit?)..don't get me started..
                       jk

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Ingemar Hernefjord
Sent: Wednesday, November 16, 2005 4:21 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] [LF] Reliability prediction of LF soldering


Exactly. A processor chip can go really hot, while other chips are
biased in a way  that they have nearly no heat dissipation at all. The
boards general temperature is another factor, starting a board at 30,000
feet or at Saudi ground is two extremes, that affect the calculations.
And the placement on the boards, we have boards mounted on a cooler
waffle, big differences in BGA temperatures. Some GaAs chips are so hot
that you burn your fingers when you touch them, others are working once
in a month etc. Some balls carry a lot of current, others just pA.
Center of superBGA differs from perimetrically placements. Warpage from
the boards have an impact. Vibration and other mechanical injections
have an impact. And so on. I'm not doing MTBF predictions myself, but I
realise that we must have to do with very complex models to be able to
predict the real life for a given number of components on a specific
board. I still believe in making tempcycling, switched life test and
life tests to failure and other practics which I call hand-on. I'm
probably rigid, conservative and too old to learn new things...

Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Ahne Oosterhof
Skickat: den 16 november 2005 21:48
Till: [log in to unmask]
Ämne: Re: [TN] [LF] Reliability prediction of LF soldering

>From my "engineering" experiences:

I used the "scientific" method to calculate an MTBF before finishing a
paper
design. Then changed the design if the number ended up too far out of
the
desired ballpark.
The changes could be made (I thought) because for most components the
demonstrated MTBF depended a lot on how they are being used. A 1W
transistor
dissipating 0.1W lives longer than one running at 1.0W. (Kind of like
comparing one solder joint on a 0402 resistor in a desktop modem (super
benign environment) to one on the corner of a BGA located close to the
shear
edge of a board that is used in a portable computer by a forester in the
Pacific NW in the middle of winter (throw it, shake it, cool it, etc.).
After building engineering samples of the design I required life cylce
tests
to find a 'demonstrated MTBF', a number that would go in the spec
document
for customers to see.

This discussion is really "fun".
Ahne.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar Hernefjord
(KC/EMW)
Sent: Wednesday, November 16, 2005 10:25
To: [log in to unmask]
Subject: Re: [TN] [LF] Reliability prediction of LF soldering

Agree, wasn't an answer on my question. We have complicated situation
with
mixed technology, and we have few persons with a deep understanding in
the
mystery of metallurgy and kinetics and so on. Perhaps someone else can
give
a tip. The scientific reports are sometimes too academic, and my job is
to
'strip' the report and make it understandable for those who work on the
'floor' (old fashioned expression, guess we are all standing on floor,
except for the high bosses, who are in the blue). So, again, can you
omit
the variations of lead distribution or not? It's very important that I
give
the soldering responible people a correct information.
Inge

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of John Burke
Sent: den 16 november 2005 19:09
To: [log in to unmask]
Subject: Re: [TN] [LF] Reliability prediction of LF soldering


Sharp words for Technet..........................8-)

John

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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Whittaker, Dewey
(AZ75)
Sent: Wednesday, November 16, 2005 10:04 AM
To: [log in to unmask]
Subject: Re: [TN] [LF] Reliability prediction of LF soldering


At least if he did that he could take it for granite that he was working
with cutting edge technology.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Wednesday, November 16, 2005 10:59 AM
To: [log in to unmask]
Subject: Re: [TN] [LF] Reliability prediction of LF soldering

Hi Ingemar,
Well, tell us--what choice do you have? Are you a 'scientist' or an
'engineer'? Shall we stop making product? Go back to the stone
age--assuming
of course that the quartz is uniformly distributed in the blade of your
stone axe?
Since when do we live in a perfect world?

Werner

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