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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