Solder Joint Reliability,
is the ability of solder joints to remain in conformance
to their visual/mechanical and electrical
specifications over a given period of time, under a specified set of
operating conditions. SJR is often expressed as a probability value at a
given confidence level, and is therefore discussed in the context of a
given population of devices.
SJR is a measure of the likelihood that a solder joint will not fail
throughout its intended operating life, subject to the various thermo-mechanical
stresses that it encounters in the course of its operation.
reliability has two aspects - 1) component-level solder joint
reliability; and 2) board-level solder joint reliability.
deals basically with the reliability of solder
joints within the package structure itself prior to board mounting, and
is primarily assessed by
performing reliability tests on unmounted parts. Board-level SJR deals
with the reliability of the solder joints of a package after it has been
mounted on a board or substrate, encompassing both the solder-to-package
and solder-to-board interfaces.
Board-level SJR is more representative
of the reliability of a package operating in the field, but requires a
more complex system of assessment (since board-level reliability testing
is more difficult to implement). As a prudent approach, most
companies perform both component- and board-level SJR testing before
releasing a product with previously uncharacterized solder joint
importance of solder joint reliability became more emphasized in recent
years as a result of two factors: 1) the shift of the semiconductor
2) the emergence of fine-pitched surface-mount packages that employ
hundreds of solder joints for electrical connection.
The shift to lead-free
solder is in response to the industry's initiative to make its
operations more environment-friendly by eliminating the use of materials
that cause damage to the environment, which in this case is the
lead-containing Pb/Sn solder used for lead finish.
are willing to make the transition to Pb-free solders, but are concerned
with the reliability implications of such a transition. Thus,
companies are ensuring that all the necessary modeling and reliability tests are
conducted on their products before these are released with new solder
joint material. Industry-standard reliability tests are performed to generate
reliability data proving that the new Pb-free solders
meet the solder joint reliability requirements of the industry prior to
three major mechanisms of solder joint failure, although these often
interplay with each other simultaneously. These are: 1)
or fracture due to mechanical overloading; 2)
or damage caused by a long-lasting permanent load or stress; and 3)
or damage caused by cyclical loads or stresses. Thus, solder joint
reliability studies must take these mechanisms into consideration.
earlier, one way to analyze solder joint reliability is to perform
solder joint modeling, or analysis of solder joint strengths and
weaknesses using computer models. This is usually done through
finite element analysis.
Solder joint modeling must be performed at different levels to take into
consideration all aspects of solder joint reliability. Experts
perform solder joint modeling at board level, component level, joint
level, and microstructure level. The use of modeling at the design
stage of a solder joint system to make it inherently reliable by design
is known as
'Design-for-Reliability', or DFR.
modeling, solder joint reliability is also assessed through reliability
consists of subjecting representative samples bearing the solder joint
of interest to industry-standard reliability tests so that: 1) factors
that cause or accelerate the various solder joint failure mechanisms
will be uncovered and understood; and 2) actual reliability data may be
generated for further analysis.
Fatigue, or failure due to
loading with cyclical stresses, is a major concern in solder joint
cycling, or TC, which accelerates fatigue failures, is
therefore an important ingredient of any component-level or board-level solder joint reliability
program. Since bulk of real-life solder joint failures are caused
by the mismatch between the coefficients of thermal expansion between
the component and the substrate, board level thermal cycling in air has become
an industry standard for assessing solder joint reliability.
temperature cycling, however, is considered by many as a conservative
test, since the device package and the board or substrate on which it is
mounted reach almost the same peak temperatures under this test
(isothermal system). In real-world applications,
however, the temperature experienced by the package may be significantly
higher than that of the board because of the heat contributed by the
operating device's junction temperature. To
simulate this aspect, many companies employ power
cycling for SJR testing.
reliability test that subjects the device to alternating 'power on' and
'power off' conditions, while keeping the ambient temperature in
temperature cycling, other reliability tests performed by companies to
assess component-level SJR include pressure cooker testing
shock testing, vibration testing, and high-temperature storage testing.
Board-level SJR tests, aside from board-level
include board-level high-temp operating life testing
(HTOL) and temp-humidity-bias testing
(THB), as well as
innovative tests such as the board drop test and the board bend test.
Just like SJR, the
of units being shipped to customers is also a challenge to
manage. Since visual
inspection is not highly effective in weeding out devices with potential
solder joint issues, new technologies have emerged to help companies
increase the solder joint quality of devices and assemblies being
shipped to customers, an example of which is automated x-ray inspection
systems (AXI). Electrical testing at different temperatures or
mechanical loading conditions to check for increased interconnection
resistances is also employed.
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