| Created: 5/21/95 |
Updated: 4/18/97 |
Fatigue Properties of Solder (Solder Joint Thermal Fatigue Life)
Objective
Develop an empirical design equation to
predict the solder joint fatigue life of gull
wing surface mount leads.
The generic stress model will be obtained
based on an analytic stress analysis and
design of experiments technique. This model
includes the damage contribution from both
the local and the global CTE mismatch.
Stresses obtained from this model will be
used to determine the partitioned elastic
energy, plastic work, and creep work so that
an energy partitioning technique can be used
to assess the thermal fatigue life.
Background
When subjected to temperature cycling, the
solder joint attaching a leaded surface mount
component to a printed wiring board (PWB)
undergoes high stresses and strains due to
mismatches in thermal expansion between the
surface mount component itself, its lead, the
solder, and the PWB. The most obvious
source of these stresses and strains is the
global coefficient of thermal expansion (CTE)
mismatch between the PWB and the
component body or chip carrier. In addition
to this global CTE mismatch, there are local
CTE mismatches between the lead and solder
and the solder and PWB. These CTE
mismatch stresses are dependent on the exact
geometry of the lead end and solder, as well
as on the exact nature of the applied loading.
Approach
- Determine stresses due to the local and
global CTE mismatches separately, then
combine them to determine the stress state
for the actual problem.
- Use the physics-of-failure approach
combined with a full factorial design of
experiments to determine the dominant
parameters that influence the reliability of a
solder joint. The purpose of the design of
experiments is to determine the magnitude
of influence of the dominant variables so
that an analytic equation can be determined.
- Develop a design equation for partitioned
elastic energy, plastic work and creep work.
Work Accomplished
- For gull wing leads, an analytic model has
been developed that predicts the elastic
energy and plastic work in a solder joint
while incorporating the variables of lead
material, solder joint height, package and
board material, dwell time, maximum
temperature, minimum temperature, lead
stiffness, and mismatch type.
- The second phase, of the study modeling
the creep energy, has been completed.
Creep behavior has been modeled using a
simple structure consisting of two rods, one
of a viscoplastic material and the other of a
linear elastic material, in series and
constrained between fixed supports. The
analytically obtained hysteresis curve has
been found to be very close to the one
obtained from FEA simulation.
- Once all the partitioned energies are
obtained, the analytically obtained fatigue
life agrees favorable with the FEA
simulations. This analytic model will help
design engineers easily assess fatigue life
of gull wing packages without having to
conduct detailed FEA simulations.
