|EPSI INC. - Electronics Packaging Solutions International Inc.
P.O. Box 1522, Montclair, NJ 07042 USA, tel. & fax: +1 (973)746-3796
|ABSTRACT (PAPER GIVEN AT SMTA INTERNATIONAL CONFERENCE, OCT. 8, 2009, SAN DIEGO, CA):
"CLOSED-FORM, STRAIN-ENERGY BASED ACCELERATION FACTORS FOR THERMAL CYCLING OF LEAD-FREE ASSEMBLIES"
by Jean-Paul Clech
EPSI Inc., Montclair, NJ, USA, Email: email@example.com
Gregory Henshall and Jian Miremadi
Hewlett-Packard Co., Palo Alto, CA, USA, Emails: firstname.lastname@example.org, email@example.com
A closed-form Acceleration Factor (AF) model has been developed for solder joint reliability under thermal
cycling conditions. The proposed AF formulation relies on a strain-energy failure criterion and an approximation
of stress/strain hysteresis loop areas. The AFs are functions of temperature swings, temperature extremes, and
hot and cold dwell times. The AF formulation accounts explicitly for creep of solder during hot and cold dwell
periods. As a result of solder creep / stress relaxation during dwell times, the AF model includes the two material
constants of a power-law creep rate equation, namely, a stress exponent and a creep activation energy. The
component / assembly dependence of AFs, which can vary by a factor of 2.36 times or more according to
experimental data and FEA modeling, is also accounted for via a single component correction factor. The paper
presents the model formulation, its application to SAC305, SAC387/396/405 and SnPb assemblies, and discusses
the type of data that is needed to extend the model to other lead-free solders or mixed assemblies. The
advantages and limitations of this physically-based, closed-form model compared to other approaches in the
reliability engineer’s toolbox are also discussed.
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