A Multi-Domain Stress Analysis Method For Surface-Mount Solder Joint
S. Ling and A. Dasgupta
Solder joint fatigue failures are a potential reliability hazard in
surface-mount electronic packages under cyclic thermal loading environment.
Proper design and reliability assessment requires accurate modeling of
the stress and strain fields within the solder joint. Some of the
existing closed-form stress analysis models tend to oversimplify the complicated
elastic, plastic and viscoplastic stress state in the solder joint, and
thus fail to give reasonable prediction of the solder joint fatigue endurance.
Extensive finite element analyses require prohibitive investment in terms
of the analysis time and analyst expertise, especially when full scale
elastic, plastic and creep analyses are performed. A generalized
multi-domain approach proposed earlier by the authors is further refined
in this paper to obtain the stress field in J-leaded surface-mount solder
joints, under cyclic thermal loading. The method can also be applied
to other surface-mount lead and solder joint configurations such as gull-wing,
leadless, ball-grid and column-grid joints. The solder domain is
selectively discretized into colonies of nested sub-domains only where
large thermal expansions mismatch. The number of colonies and the
number of sub-domains within each colony can be varied for optimum accuracy.
The Rayleigh-Ritz energy method based on a multi-field displacement assumption
is used to estimate the deformation, strain and stress fields within the
solder domain. In the present paper, the potential energy stored
in the and in the Printed Wiring Board (PWB) segment, both of which are
total potential energy enables modeling of the deformation caused by both
the global and local CTE mismatches. This work represents a significant
improvement over a previous paper presented by the authors, where the global
CTE mismatch was by Rayleigh-Ritz energy scheme and the local CTE mismatch
was estimated by an existing one-dimensional eigen-function analysis fir
interfacial stresses at interfaces of dissimilar materials [Dasgrupta et.
al., 1993]. Moreover, the previous paper could use only one sub-domain
for displacement enhancement. The present model therefore has more
generalized capabilities than the work reported before. Results of
two-dimensional elastic analysis are presented in this paper. Plastic
and creep deformation can be modeled with this scheme by using incremental
load-stepping and/or time-stepping techniques, and will be presented in
a future paper. The final goal is to predict the stress, strain and
strain energy density distributions in the solder domain with good accuracy,
but at a fraction if the computational effort typically required in a full-scale
finite element analysis. The fatigue endurance of the solder joints
can be assessed by combining results from this stress analysis model with
an appropriate damage model.