Hi folks! Looks like we are on the same page in terms of the solder joint
cross-sectional ideas. I agree with you Werner on the unrealistic
application of accelerated testing results to a degree, with the exception
that Collins has developed a pretty extensive data set on thermal cycle
testing and solder joint thermal fatigue life. We have amassed enough data
with relation to our specific design practices and soldering processes to
have a reasonable confidence in our thermal cycle test results. But as you
point out, there is always the constant battle to understand what failure
mechanisms are at work for a specific set of design constraints and
assembly practices (the introduction of PbFree solder alloys is just scary
when you think about the industry's understanding/application of
accelerated testing). I understood those complications when we conducted
the "dimpling" test and we felt we would learn something from the test.
Ioan - I have seen two very distinct camps of though on solder joint voids:
one camp believes that solder joint voids behave in a classical manner -
the void "blunts" the crack propagation and slows down the growth of the
crack; the second camp believes that the solder joint voids do not
participate in crack propagation in any form. My testing/production
experience with solder joint voids would put me in the second camp. Collins
uses the JSTD-001 voiding criteria - 10%-25% voiding. Again, as Werner's
discussed, large voids or extensive voiding will reduce the solder joint
volume and introduce a solder joint shape that is not a soldersphere. You
may want to get a hold of some of the papers published by Dr. Lee at Indium
Corp. - he has conducted a number of investigations concerning solder joint
voiding and BGA components.
Dave
"Tempea, Ioan" <[log in to unmask]>@ipc.org> on 10/18/2002 07:13:05 AM
Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
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Subject: Re: [TN] Z-axis BGA solder joint verification? Addendum -
Stress Riser Question
Hi again,
to continue Dave's idea, and Werner's statement "there is no question that
SMD SJ fail first", should we worry about voids? Did anybody see BGA joints
failing because voiding?
Thanks,
Ioan
> -----Original Message-----
> From: Werner Engelmaier [SMTP:[log in to unmask]]
> Sent: Thursday, October 17, 2002 1:05 PM
> To: [log in to unmask]
> Subject: Re: [TN] Z-axis BGA solder joint verification? Addendum -
> Stress Riser Question
>
> Hi Dave,
> For once your insight has deserted you.
> You are right, that the failure location is the same whether you have SMD
> or
> NSMD. However, it is not the failure location per se, but the
> time/stress-to-failure that determines whether there is a reliability
> impact.
> And there is no question that SMD SJ fail first, provided of course that
> you
> have good wetting.
> Below s a solder joint where because of misregistration one side is SMD
> and
> the other is NSMD. The consequences of the SMD-stress concentration are
> obvious. Depending on the severity of loading, NSMD SJs will give fatigue
> lives of 1.25 to 3 times longer than SMD SJs, everything else being
equal.
> [Unable to display image]
> As I have indicated in my previous e-mail, I am in agreement with you
> regarding the pad geometry.
> On the other hand, it is not so much "the minimal cross-section area of
> the
> solder joint is the real culprit," as is the difference between the
> cross-section area near the interface relative to the cross-section area
> for
> the rest of the ball. In other words, a column is always better than a
> ball
> of equal height.
> You write: "We have even attempted to force a stress riser generated
crack
> in
> BGA solder joints by "dimpling" the center of the solder balls on a BGAs
> outer row and then
> subjected them to thermal cycling. The solder joints still cracked at the
> solderball/component pad interface!" Unfortunately, because of time/money
> constraints people do "Highly Accelerated Tests." In many cases these
> tests
> are not accelerating the dominant product damage mechanism(s), but create
> earlier failures by making another damage mechanism or loading scheme
> dominant. Also unfortunately, in most cases that does not result in a dif
> ferent failure mode/location. Thus, improper conclusions are drawn
> regarding
> product behavior that are not justified. Your observation is a case in
> point.
> If you do T-cycling [forget about T-shock altogether, and be careful
about
> bend testig] at loading conditions similar to what actually happens with
> product, you will come to different conclusions. I have run T-cycle
> studies
> with more than 100,000 cycles than ran for over 3 years and where not
> everything failed even than. Nowadays nobody can afford to do this--this
> was
> only possible in the Bell Labs which ceased to exist around 1990. But
> having
> to do things 'quick and dirty' [emphasis is on 'dirty'] should not
> encourage
> the industry to draw unwarranted conclusions--thiis can be very
dangerous.
>
> 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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