Hi Werner
With NiAu i'm in that --good luck category ;
i have to take chances in that one (pad switching);
the other one :
Would you therefore say that cycling for relatively true life models
(IPC-785)
would be better done on 6 months+ old test vehicles,
or is the initial precycling burn-in as prescribed having sufficient aging
effect,
thus making the results (how much?) realistic ?
Where i'm coming from : I did notice, just as you said, a big difference
between the same batch vehicles (made on same
boards/profile/date/paste/settings)
on comparative shear's :
that is we have parallel shear when boards fresh and when old ; finding big
discrepancies .
We have one QA school here , saying : if the shear values move (same
settings, boards, etc)); process moved ;
using archived samples and reports as comparative records .
Reports on initial fresh data i could live with ; but not shearing old
boards and comparing with fresh, correct ?
I do not particularly agree with shear use as a test method of process
repeatability , as this grain aging would indicate ;
would the "twice the life" relate to something relatively similar as twice
(or so) the shear values ?
Could you shed some light on this topic please .
would have anything on the palladium/likes finishes,
any news/trends on lead-free reliability models ?
Thanks a lot
Paul Klasek
ResMed
-----Original Message-----
From: Werner Engelmaier [mailto:[log in to unmask]]
Sent: Friday, 18 February 2000 11:33
To: [log in to unmask]
Subject: Re: [TN] Requirements regarding solder joint microstructure
In a message dated 02/17/0 14:54:31, [log in to unmask] writes:
>Besides what it written in STD-001B, it might be interesting to add some
>requirements about lead phase size and homogeneity, intermetallics
thickness,
>etc. to get more reliable assemblies.
Hi Christophe,
STD-001B does not address reliability; for reliability information you need
to go to IPC-D-279, Design Guidelines for Reliable Surface Mount Technology
Printed Board Assemblies.
Since the grain structure of solder is unstable and coarsens with time and
temperature, it does not make much sense to somehow specify solder joint
microstructure. True, if you fatigue cycle fine grained solder in an
accelerated test you get about twice the life as compared to coarse grained
solder, but for actual product this is immaterial since within a year's time
the initial fine grained solder is virtually indistinguishable from
initially
coarse grained solder.
Further, I have yet to see solder joint failures as the result of
intermetallic layer being too thick; on the other hand, if you solder to Ni
or Alloy 42, you will have a difficult time determining the IMC layer
thickness.
For the reliability of solder joints, the most important quality aspect is
adequate wetting; on copper the obvious presence of IMC layers is proof of
that, on Ni--good luck. However, in most cases of solder joint failure it is
the result of inadequate solder joint quality, but caused by improper
designs allowing large cyclic thermal expansion mismatches to fatigue the
SJs.
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], Website: www.engelmaier.com
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