Fatigue Model based on Average Cross-Section Strain of Cu Trace Cyclic Bending. D. Farle/, A. ... in the copper traces, near the connection to the solder pad.
Fatigue Model based on Average Cross-Section Strain of Cu Trace Cyclic Bending
l 2
4 2 3 4 D. Farle/ ,A. Dasgupta ,Y. Zhou ,l.FJ. Caers ,and l.W.C. De Vries Delft University of Technology,Building 36 (EWI),LB 01.480,Delft,the Netherlands
CALCE Electronic Products and Systems Consortium,University of Maryland,College Park,MD 20742 3 Microsoft Research 1 Microsoft Way,Redmond,WA 98052 4 Philips Research,High Tech Campus 7,WDX-3A,Eindhoven,the Netherlands
1.1. Background
Abstract
3D
This study focuses on quasi-static mechanical cycling durability of copper traces on printed wiring assemblies
multitechnology,
multi-functional,
multichip
packaging technology, usually referred to as a "SiP"
PWA specimens populated with Land Grid
(system-in-package), has emerged as one of the leading
Array (LGA) components on copper-defined pads were
methods of miniaturization. These packages can be found
(PWAs).
cycled to failure under zero-to-max, three-point bending.
anywhere
Failure is defmed in terms of electrical opens due to
electronics, to
miniaturization
fatigue damage propagation through the entire cross
Opto-Electromechanical Systems (MOEMS),biochemical
smart
section of the trace. Failure statistics were collected and
'lab-on-a-chip'
failure analysis was conducted to identify fatigue failures
electronic controls [6, 7].
in the copper traces,near the connection to the solder pad. Cyclic bending of this assembly was modeled with 3D, elastic-plastic, finite
deformation
(geometrically
is
needed, from
portable
biomechanical implants, Micro
sensors, and
even
home
appliance
Additionally, these packages
frequently forego leads and even solder balls for further miniaturization.
This generally results in stiffer, more
robust,
interconnects.
solder
Consequently,
the
nonlinear) finite element analysis. Due to the complexity
mechanically weakest point in the chain becomes the
of the geometry, a two-step global-local approach was
traces on the PWA [8-13]. When
used to identify the cyclic strain history and the mean
electronic
stress at the copper trace failure site. A generalized strain-based fatigue model is proposed,
assessing
the
mechanical
assemblies, the
dominant
durability failure
site
of is
generally believed to be in the solder interconnect [12].
to characterize the fatigue durability in terms of the
However, in many package styles, such as BGAs (ball
amplitude of cyclic strain and the cyclic mean of the
grid arrays), LGAs (land grid arrays), MLFs (micro lead
The strain and stress values are
frame) and QFNs (quad flat no-lead), the Cu trace
averaged over the entire cross-section, to be consistent
emanating from the solder pad may be the weakest failure
with the failure criterion defined above.
Average cross
site, especially if the solder joint is copper-defined rather
sectional model constants are iteratively estimated by
than mask-defined, and if there is a sudden neck with
ensuring that they are simultaneously compatible with
sharp re-entrant corners where the copper trace emanates
hydrostatic stress.
both the durability test data and the copper stress-strain
from the solder pad [11, 13, 14]. This is particularly true
curves used in the FEA (finite element analysis).
in situations with cyclic mechanical loading, such as
To
make better use of time, a Response Surface (RS) was
cyclic
created after a study determined certain key constants
drop/shock [5,8,9,11-13,15,16].
varied reasonably linearly with each other from model to The important impact of this study includes insight cycling, a quantitative model to predict its occurrence,
bending, vibration
and
repetitive
1.2. Motivation
model. This RS was used for the actual iteration. into copper trace failures in PWAs under mechanical
quasi-static
This paper investigates the cyclic stress and strain histories in the Cu traces and attempts to quantify the accumulated fatigue damage in terms of these parameters. Prior research by the author had focused on the solder
and validated guidelines to prevent it by design.
interconnects for this package style [17, 18].
1.
present study the focus is on the Cu traces.
Introduction
In the
The literature reveals many examples where the Cu
With the consumer electronics industry constantly seeking out new ways to decrease the size of its products,
trace is the weakest point in the system and the dominant
many methods of miniaturization of electronic circuits are
failure site [10, II].
being explored [1-5]. With new methods, architectures,
been found to be easily preventable by revision to the
and structures come new reliability concerns.
trace design. Studies show that redesigned Cu traces can
While
Additionally, these failures have
reliability and failure studies generally focus on the solder
indeed survive repetitive mechanical loading and that the
joints of electronics, an increase in other failure sites is
failure site shifts elsewhere [10,12,19,20].
being noticed.
Quantitative
This study focuses on the occurrence of
of
insights
copper
are
traces
needed
into
so
failures
that
the
fatigue
copper (Cu) trace damage sites just outside the solder
durability
can
joint,and offers up a new formation of fatigue model with
predicted and design guidelines can be developed to
be
model constants for the assessment of the Cu.
prevent these failures.
The existing literature does not
have a consensus of Cu properties for electronics devices [21-24].
978-1-4577-0106-1111/$26. 00 ©20111EEE
-
1110
It is generally accepted that the Ramberg-
-
2011 12th. Int. Conf on Thermal. Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems. EuroSimE 2011
Osgood material model [25] is the best representation to employ:
E- 12: .,0' ,.1 ' " &HDO ,.1 ,, DI-tn,
1
E=
i+ Gzr
(1)
where e is the strain, a is the stress, E is the elastic modulus,
K is
a material
hardening exponent.
coefficient,
and
n
is the .. �
The hardening exponent, n, was
held constant in this study.
Engelmaier [21] and Hong
.1 �--+--1---�-�--��-�-�
[26] show that the sensitivity to the cyclic changes in n is very small compared to that of
K.
Also held constant is
the elastic modulus, E, for the same reason.
Figure
1/4 1/l 1
I
shows a constitutive schematic, pointing out the various monotonic and cyclic values. The decision was made to Engelmaier's work using
E = 82.7GPa [21, 22] has been used most, and falls in the middle of the field as far as the literature goes. Constitutive
-;:-..-::-_ --:;:-;;.
Models
, .'
, ..
Figure 2: [21J Engeimaier's Coffin-Manson plot showing contributions of strains and deviations at extremely low cycles.
aspects of a material model and the differences between use Engelmaier's Ecu and ncu'
..
There are many existing fatigue models that are applied to copper fatigue.
Most prominent is Sines'
model [27], general form seen in Equation
(4).
Sines'
model, however, uses only a linear mean stress effect
Larger K
factor, which doesn't fit well with the observed behavior in this study.
Also well known in the Morrow model, a
form of the Sines model, seen in Equation (5) [28]. This model has a better mean stress effect factor, however it can clearly be seen that if the am value surpasses the a't value, the model mathematically falls apart.
This may
seem physically incorrect, however with the construct of the actual von Mises yield surface it can be seen that if the principal plan is constantly changing with the stress state, a configuration can occur in which the hydrostatic stress (along the hydrostatic axis) could have a magnitude which is greater than the single calculated equivalent stress (or von Mises) stress at the failure surface. Two others widely used are the Smith-Watson-Topper
Figure 1: General stress-strain schematic showing the hysteresis loops of the monotonic model along with the single (dotted) line of the cyclic model, as well as the effects of varying n and K.
(SWT) model [29], Equation (6) and the Walker model [30], Equation (7).
C
1
cycles to failure approach l or, in other words, as the load toward
the
cyclic
fatigue
stress,
a';;
the
k(JH = (Jf Nf b (Ja b tIm = (Jf Nf tIf
-
Engelmaier's work [21] has also showed that as the tends
Neither of these allow for empirical
correction. Clearly a new model was needed.
_
() ()
(4) (5)
corresponding monotonic values to a'; and the fatigue ductility, e'fare lower than expected. Figure 2 shows the fatigue curves tending toward a saturation value before Nt =1. As mentioned previously, the relationship between the cyclic and monotonic values in the constitutive models of Cu is shown in Figure l. Engelmaier's transfer functions between cyclic (with apostrophe) and monotonic (without apostrophe) for strength and ductility are Equations (2) and (3), respectively, which account for the dotted line in Figure 2. The cyclic fatigue constants pair of a'fand e';; must of course lie on the constitutive curve for Cu. Therefore there is a corresponding cyclic Ramberg-Osgood model in Equation are substituted for a and ' (Jf= (J f 2Nf
c.
( t ' ( Cf=C f 2Nft
(I),
K (K')
for the
when a'fand e';;
Experimentation
The copper trace fatigue durability data for this study is taken from experimental work presented elsewhere in the literature by the author [19]. This was work done in conjunction with the author of this study, but performed by co-authors Zhou and de Vries. The test methodology and fatigue results are summarized here for completeness. 2.1.
Specimen & Test Methodology
The package of interest in this study is an RF SIP component for use in portable electronic products.
It
contains multiple wirebonded dice, flip-chip dice, and
1/4
(2)
Nf =lY2
(3)
Nf=
2.
ceramic SMD passives, all on a common substrate. The wirebonded dice are attached to the laminate board with die attach.
There are no die stacks in this SiP.
The
-21102011 12th. Int. Conf on Thermal, Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems, EuroSimE 2011
package is fairly stiff because of the many components molded together on a multi-layer substrate and has an LGA architecture, which includes interconnection pads along the perimeter, and a large thermal land in the center. A representation of the PWA specimen used in this study can be seen in Figure 3, shown in a standard 3-point flexure setup.
These are single-sided PWAs with daisy
chained LGA components grouped into 4 columns of 3 components each, with adequate space between columns to accommodate the center roller without causing any damage.
These
PWAs
are
JEDEC
(Joint
Electron
Devices Engineering Council) standard bend test boards. The overall board thickness is 1.56mm, which is a 7-layer layup pattern with O.035mm thick metallization layers and 0.475mm thick dielectric layers. 45flm
thick
and
120flm
wide.
Figure 4: An image of a dye-stained test board with
The Cu traces are The
major
insets showing the position of land #1 and the trace
board
attached to it.
dimensions are: 48mm wide by 130mm long, with a 90mm
wide by 40mm
functional
daisy-chained
long bend test area. components
are
The test setup and equipment is a custom 3-point bend
Non
used,
with
setup.
The board is clamped at the center roller, in an
corresponding daisy chains in the PWB, to facilitate
attempt to maintain symmetry.
electrical failure monitoring.
allow in-plane sliding, to prevent excessive membrane strains in the test specimen.
The two outer rollers
The center roller imposes
out-of-plane cyclic movement from zero to max.
Since
the loading is zero-to-max (R=O) and not completely reversed (R=-1), tests are conducted with PWAs facing both directions in the fixture. interconnects
experience
Thus, in some tests the
convex
curvature
(tensile
loading) and in some they experience concave curvature (compressive loading). The
next
steps
include:
(a)
overstress
tests
to
determine the destruct limits of the test specimen; (b) failure analysis to determine the dominant failure modes in the overstress tests; (c) selection of AST (accelerated stress test) levels that are below the destruct limits, do not cause failure mechanism shifting, but are sufficiently severe to produce timely failures;
and (d) tests and
analysis to characterize the specimen response to the
Inner Roller
entire range of excitations anticipated in the accelerated
Figure 3: '3-point' test board & bending moment diagram. Components within each group (inner & outer rows) are initially expected to experience similar strain distribution.
stress test. To characterize the specimen for mechanical cycling, a
populated
loading,
The geometry and loading of this specimen are clearly
test
board
was
subjected
to
overstress
instrumented with strategically placed strain
gauges (Figure 5) to record the resulting strain levels
symmetric about the center roller. Three point bending is
(Figure 6).
known to generate a linear gradient of bending moment
showed that a deflection amplitude of about 3mm was to
The overstress tests for the components
and curvature along the length of the specimen, as shown
sufficient
schematically in Figure 3. Consequently, the stress levels
mechanical cycles.
fail
interconnects
within
the
first
few
are highest in the interconnects of the inner columns,
were therefore kept under 3mm. The overstress tests for
The amplitudes for cyclic testing
followed by those of the outer columns. All of the Cu
the board also showed that the force-deflection curve
traces emanating from the lands have been angled so as
remained linear within the 3mm deflection envelope.
not to be perpendicular to the bend direction, except for
Finally, the board was flexed to each of the load levels
those attached to land number one, as shown in Figure 4.
meant to be used in the AST, and strain histories were
Although the components are located symmetrically with
recorded (Figure 6).
are
Cyclic fatigue testing was performed next. Eight test
asymmetric since they are located at the top right corner
boards, populated with 12 components each (schematic
respect
to
the
center
roller,
Land
1
locations
of each component (as shown later in Figure 7). Thus the
seen in Figure 5), were subjected to 3 different load
local curvature at Land 1 is different for each column,
levels, or "I) values", to generate sufficient data points on
generating 4 different loading levels in each bend test.
the S-N curve. The zero-to-max tests were conducted by
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3 / 10
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2011 12th. Int. Conf on Thermal. Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems. EuroSimE 2011
loading both in the concave (compressive) and convex (tensile) configurations.
A "failure" was recorded when
the component registered resistance above the failure
traces emanating from the corner solder pads, just at the edge of the solder mask where the solder joint ends, as shown in Figure 7.
threshold for 10 cycles within 10% of the first failed cycled.
Meaning, for example, if the first failure was at
1,000 cycles, the tenth must be
