Complexity of Problem
Solder joints of electronic assemblies are complex
elements that cannot be studied using the traditional techniques
of structural analysis or fatigue of engineering metals. The acknowledged
complexity of the mechanics of solder joints arises from the following:
- The problem is three-dimensional (3D) with solder joints subjected
to a system of distributed, multiaxial forces and moments exerted
by the interconnected parts. Even when taking advantage of symmetries,
all joints are not equal because of varying distances to the neutral
axis of an assembly and variability in joint geometry and metallurgy.
- A solder joint is a multi-layered, non-homogeneous structure.
Reflowed solder is sandwiched between thin layers of intermetallic
compounds. In the case of SnPb, solder itself is made up of lead-
and tin-rich phases with variations in composition near the intermetallic
layers, e.g., tin-depleted regions on the board side due to the
formation of Cu-Sn intermetallic compounds during
reflow. Moreover, the SnPb microstructure evolves in service.
The microstructure coarsens due to thermally activated grain growth,
a phenomenon that takes place under stress or at constant temperature.
- The mechanical behavior of solder is highly non-linear and temperature
dependent. Solder creeps readily at ambient temperature (and below)
and creep rates increase with temperature.
- Failure of SnPb solder joints is a complex sequence of events
involving microstructural coarsening, matrix creep, grain boundary
sliding, micro-void formation and linking, crack initiation and
crack growth. In the case of SAC and SnAg solder joints, the damage
accumulation process leads to much less coarsening of the microstructure,
if any.
- Most often, electrical opens resulting from solder joint failures
are intermittent and may be difficult to detect accurately. Electrical
continuity may still be maintained when a solder joint is fully
cracked because of contacts between asperities on the opposite
surfaces of the crack. This may result in hard to detect failures
and “No Trouble Found” (NTF) diagnosis during troubleshooting.
In spite of all this, significant progress has been made in the
understanding of SnPb solder joint mechanics, fatigue and failure.
Although the study of SnPb assembly reliability is a semi-empirical
science, a vast body of knowledge, simplified engineering models,
test data and experimental findings has accumulated that provides
useful insight into the mechanical behavior of solder joints of
real assemblies.
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