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Fatigue and creep wearout failures in electronics—a historical retrospective
Werner Engelmaier
“We have talked about reliability in academic terms as if it had only some abstract meaning:
however, both fatigue and creep have had world-changing political and economic impact.”
Fatigue and creep wearout
failures in electronics—a
historical retrospective
In my past columns on reliability I always the horizontal axis. For many steels, the those early days no engineer would have
concentrated on essentially pure technical fatigue life increases near-asymptotically even dreamed of designing any structural
contents. This column is somewhat of a with decreasing stress amplitude until it member to be stressed beyond its yield
deviation from that past. becomes essentially stress-independent point for static loads, let alone for fatigue.
With the introduction of reciprocating leading to the concept of a so-called As the development of non-ferrous
and rotating machinery during the ‘fatigue limit’ or ‘endurance limit’— metals progressed, the use of aluminum
Industrial Revolution in the early 1800’s, ‘Dauerfestigkeit’ in German, a stress level and copper alloys as structural members
the fatigue of metals became a recognized below which failure will not occur. also increased. These metals, unlike the
engineering problem. The first beginnings At the time of Wöhler, the primary steels, do not have a totally elastic regime
of systematic fatigue testing date to A. structural metal was steel with its well- in a stress-strain diagram. Initially on
Wöhler from 1858 to 1870; working with defined elastic loading range nicely loading, the deformations are primarily,
railroad axles, he developed a graphical delimited by a natural yield point. The but not entirely, elastic.
representation of the fatigue results that stress-strain diagram for steel shows a Because non-ferrous metals do not
has become known as the Wöhler or S-N totally elastic increase in stress during exhibit the pronounced demarcation of
(actually S-logN) stress-life curve. Figure 1 loading until the yield point is reached; a yield point between elastic and plastic
shows the S-N curve for a low-carbon steel at this point most steels yield by plastic strains, a yield strength has to be defined.
The S–N curve is historically plotted deformation with a slight stress drop. In fact, different yield strength definitions
with a linear stress axis and a logarithmic Further loading will again cause the stress have been developed for different material
scale for the cycles-to-failure; because to increase while the metal continues to classes
1
to suit the specific behavior of
of Wöhler’s original graphs, it has yield plastically. When the ultimate tensile these materials.
become customary in the fatigue field strength is reached, the stresses typically As a consequence, non-ferrous metals
to plot the dependent variable, Nf, on will decline before fracture occurs. In typically do not exhibit an “endurance
limit.” The non-elastic portion of the
deformations and strains increases
with increasing stresses until the plastic
strains are the primary component of the
deformations.
Thus, fatigue of these metals is better
described in a strain-life curve, known as
the Manson-Coffin plot shown in Figure 2
for some electrodeposited copper.
For electronic packaging and
interconnection technology, steel plays
essentially no role, whereas copper is an
absolutely essential metal.
Another metal category, soft solders,
is also of vital importance for electronic
packaging and interconnection technology.
These metals behave even less so than
steels, because they creep readily at the
Figure 1. Wöhler or S-N curve for low-carbon steel.
temperatures at which they are used.
38 – Global SMT & Packaging – September 2009 www.globalsmt.net
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