refer to the solder connections between a semiconductor package and the
application board on which it is mounted. In unmounted devices, it
may also refer to the package's solder connection features themselves,
solder balls, solder bumps, solder studs, etc,
in the context of their attachment to the package body.
Since solder joints are one of the most fragile elements of a package
(mainly due to the fact that they are small in size and used at high temperatures relative
to their melting points), their reliability is of utmost concern to
assembly engineers. A good understanding of solder joint failures
is essential in the improvement of any package's
joint reliability (SJR).
failures occur for various reasons: 1) poor solder joint design;
2) poor solder joint processing; 3) solder material issues; 4)
excessive stresses applied to the solder joints, etc.. In general,
however, solder joint failures are simply classified in terms of the
nature of the stresses that caused them, as well as the manner in which
the solder joints fail.
joint failures fall under three major categories: 1) tensile
fracture due to stress overloading, which is short-term; 2) creep
failure due to the application of a long-term, permanent load; and 3)
fatigue failure due to the application of cyclical stresses. Of
course, more than one of these stresses can act on a solder joint in a
given situation, so solder joint failure analysis can be challenging at
times. Add to this the fact that solder joint degradation due to
other factors such as corrosion can occur.
Fracture Due to Stress Overloading
fractures attributed to short-term stress overloading are mainly those
experienced by units subjected to gross mishandling
or misprocessing, especially after these units have been mounted on the
application board. The occurrence of mishandling or misprocessing
subjects the parts to thermo-mechanical stress levels that exceed the
fracture strength of the solder joints, resulting in solder joint
examples of events that lead to solder joint fracture due to mechanical
overloading include dropping of the application board (or final product)
to the floor, 'force-fitting' of an improperly loaded application
board into its module or enclosure, high-impact collisions involving the
module containing the application board, and the like. These
incidents subject the solder joints of the device to very high shear
stresses that tend to rip them away from their board.
Failure Due to Creep
that are subjected to permanent mechanical loading degrade over time and
eventually fail. This failure phenomenon is known as creep, and is more pronounced at higher temperatures,
joint failures due to creep at room temperature can also occur.
A warped application board
may not exert any significant stress on the devices mounted on it while
it is in its uninstalled state, especially if the devices were soldered
on the board that way. Once the board is screwed into position in
its final assembly, however, it will be forced to 'straighten out',
causing severe stresses to the solder joints of the mounted devices.
Leaving the board in such a 'stressed' position will put the mounted
devices under constant mechanical loading, which can eventually lead to
Failure Due to Fatigue
Fatigue, or failure
resulting from the application of cyclical stresses, is the third category of solder joint failures.
It is often considered to be the
largest and most critical failure category, since it is encountered in
many different situations that are difficult to control. Solder joint fatigue failure is attributed
primarily to stresses brought about by temperature swings and
mismatches between the coefficients of thermal expansion (CTE's) of the
mounted devices' solder joints and the application board.
Prior to the actual fatigue
fracture, solder joints first undergo cyclic deformation from the cyclic
stresses as the temperature alternates between its high and low values.
Improper design of the solder joint aggravates the effects of this
cyclic deformation, which can occur in large steps (in the order of 1%)
especially in cases of low cycle fatigue. Low cycle fatigue, or
failure from stress cycles that involve long periods wherein the cycle
time is several hours, is a prevalent cause of solder joint failures in
of real-world events that can lead to fatigue failures include: 1)
powering up of an equipment in the day and turning it off at night; 2)
the frequently repeated cycle of driving a car and parking it, with the
application board under the hood; and 3) the orbiting of a satellite
that exposes it to the alternating direct heat heat of the sun and cold
vacuum of space.
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