Forum on reliability for Pb-free electronics Forum on reliability for Pb-free electronics
has three parts: an ‘infant mortality’ branch,
a ‘random’ constant failure rate part, and a
‘wearout’ period.
Integrating the failure rates over time
gives corresponding cumulative failure prob-
abilities as shown in Figure 2.
The ‘infant mortality’ is best taken care
of with appropriate in-house screening pro-
cedures rather than being experienced by
the customer. Its failure rate declines over
time as latent defects resulting from inad-
equate processing fail and are eliminated
from the product population. The failure
rates are typically higher for components
than solder joints; solder joints are typically
good, i.e. properly wetted, or not. At least
that was the case prior to the RoHS Pb-ban.
The ‘random’ constant failure rate por-
tion is only assumed to have a constant fail-
ure rate and represents a grossly averaged
Figure 2. Cumulative failure probabilities corresponding to the reliability ‘bathtub’ curves shown in Figure 1 for
failure probability based on past experience
‘typical’ electronic component without solder attachment and separately for the surface mount solder joints.
with a given or a similar component. It de-
pends among other things on the complex-
cumulative failure probability. The FIT-rate particular for components. He emphasized
ity of the parts and on operating conditions,
approach is not well suited for wearout, risk analysis starting at the planning stages
like temperature and humidity. FIT-values
since this approach is based on the premise of a project, a process much beyond what
are assigned to every part of the system
of a constant failure rate. At Bell Labs, at- had been in MIL-HDBK-217.
presumably based on relevant experience.
tempts were made to handle wearout using Günter Grossmann, EMPA, Switzer-
They are calculated from a number of ac-
iFITs—instantaneous FIT-values at specific land, dealt with the metallurgical aspects of
celeration models, e.g., Arrhenius model
times in the projected life of the system, solder joints and the various behavioral and
for rate-dependent processes with activation
but this approach was found too cumber- reliability modeling efforts for solders.
energies, developed for various processes by
some. Wearout depends on the severity of Max Poech, Fraunhofer ISIT, Germany,
interpolation or extrapolation from some
the operating conditions, whether harsh, illuminated the thermo-mechanical aspects
test condition to the operating environ-
e.g. automotive or military, or benign, e.g. and damage mechanisms of solders and
ment. The assumption being made is that
consumer products, and on the ability of their impact on assembly and aging.
the acceleration model chosen is representa-
the component solder attachment to accom- Werner Engelmaier, Engelmaier
tive of the damage mechanism at work. The
modate cyclic thermal expansion mismatch- Associates, USA, addressed the properties
most well known reliability methodology
es, e.g., rigid leadless solder joints, more and thermo-mechanical reliability of printed
utilizing this approach, MIL-HDBK-217, fell
compliant solder balls or columns, and circuit boards (PCBs), components and
into disfavor because it could not handle
compliant leads like on QFP-components. solder attachments.
wearout.
Unfortunately, more QFN-type components
There is no evidence that this part of
are entering the industry; in many cases, references
the ‘bathtub’ curve actually exists for solder
compliant leads need to be added to them [1] Engelmaier, W., “How to Estimate
joints; if it does, it is at the ‘data-noise’
to achieve the required solder attachment Solder Joint Reliability, Part 1,” Global
level; solder joints are typically assigned low
reliability. SMT & Packaging, Vol. 7, No. 9, Sep-
values, e.g., 0.03 FITs. For systems with large
In the era of the RoHS Pb-ban for elec- tember 2007, pp. 60-64.
numbers of solder joints, even this FIT-rate,
tronic solders, having concerns about the [2] Engelmaier, W., “How to Estimate
seemingly low, may prove too high and un-
long-term reliability of electronic products Solder Joint Reliability, Part 2,” Global
realistic. The FIT-rate approach to reliability is
is wholly appropriate. Because of the signifi- SMT & Packaging, Vol. 7, No. 10, Octo-
attractive because for a complex project, the
cantly higher soldering temperatures required ber 2007, pp. 64-66.
various FIT-values are simply summed up.
for the Pb-free solders, all three regions [3] IPC-D-279, Design Guidelines for Reli-
‘Wearout’ typically plays no significant
shown in the reliability ‘bathtub’ curve are able Surface Mount Technology Printed
part for components since it occurs in time-
affected. The failure rates in ‘infant mortal- Board Assemblies. IPC—Association
frames too long to require consideration.
ity’ are higher, because the Pb-free soldering Connecting Electronics Industries, July
However, ‘wearout’ is the main concern
processes not only increase the possibility of 1996.
for the reliability of solder attachment of
more latent defects affecting the quality of
components. It needs to be addressed by
the assembly, but the very survival through Werner Engelmaier will be giving some of his sol-
a very specific Design-for-Reliability (DfR)
the soldering processes of components and der joint reliability and PCB reliability workshops
(see Refs. 1-3). Wearout is characterized
PCBs is threatened. at the IPC Fall Meeting, September 21-24, 2008
by ever increasing failure rates and, as can
Victor Tiederle of RelnetyX AG, in Schaumburg, IL, the ASOLD-Conference, Oc-
be seen in Figure 2, by a rapidly increasing
Germany, addressed supply chain issues, in
Continued on page 56
www.globalsmt.net Global SMT & Packaging – September 2008 – 45
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