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Subject:
From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Mon, 7 Jul 2008 14:02:39 -0500
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Absolutely correct on all 3 points, on all 4 differences, and both
suggestions.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: Friday, July 04, 2008 9:33 PM
To: [log in to unmask]
Subject: Re: [TN] [LF] [TN] SN100 for Reflow Application

Hi Amol,
First, I would suggest you use a different lab in the future, or get
them educated. The statements: " The presence of intermetallic at the
interface shows that a "Black Pad" condition is not present." is pure
nonsense.
Essentially, but not exclusivley, 'Black Pad' requires that IMCs are
formed-th at enriches the P-content at the Ni/IMC interface.
Second, the fact that you have 3 different failure modes-pad cratering,
brittle interfacial separation, & BGA-side cracks, occuring, sometimes
in combination, shows that you do not have a significant weak link. Some
of the RoHS-capable PCB base materials are more brittle with lower
cohesive strength, but seeing multiple failure modes indicates that is
not the issue.
Third, you do not tell me what other processes follow reflow-however,
whatever it is puts significantly higher stresses on the SAC-SJs than on
the SnPb-SJs.
CONCLUSION: The root cause is obvious-it is the switch from SnPb to SAC
and ALL that entails.
CRITICAL DIFFERENCES: A) SAC is about 40% stiffer than SnPb=>higher
stresses;
B) delta-T to Tg of SAC Solidus v. SnPb Solidus is more than
double=>higher stresses;
C) delta-T to Tg of SAC Soldering T's v. SnPb Soldering T's is more than
50% higher causing more warpage=>higher stresses;
D) SAC creep rates are about 50x slower than SnPb=>higher stresses.

I would suggest attending my new workshop Pb-Free Soldering Processes-
Survival, Quality, Reliability.

Werner
Future workshops:
Pb-Free Soldering Processes-Survival, Quality, Reliability, July 17/18,
Thal, Switzerland Pb-Free Soldering Processes-Survival, Quality,
Reliability, August 18, Orlando Reliability Issues with Lead-Free
Soldering Processes, September 22, Schaumburg Failure Mode and Root
Cause Analyses Reliability (Fatigue, Brittle Fracture, ENIG), September
22, Schaumburg Solder Joint Reliability: Parts 1 & 4, Oct. 9, Moscow,
Russia Solder Joint Reliability: Parts 1 & 4, Oct. 17, Timisoara,
Rumania Solder Joint Reliability: Parts 1 & 4, Oct. 22, Tallinn, Estonia



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