UDK621.3:(53+54+621+66), ISSN0352-9045 Informacije MIDEM 41(2011)3, Ljubljana
A REVIEW ON THERMAL CYCLING AND DROP IMPACT
RELIABILITY OF SOLDER JOINTS IN
ELECTRONIC PACKAGES
Dhafer Abdulameer Shnawah, Mohd Faizul Mohd Sabri, Irfan Anjum Badruddin
Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia
Key words: Soldering, thermal cycling, drop impact, failure mode, material requirement
Abstract: Currently, the trend of miniaturization, light weight, high speed and multifunction are common in electronic assemblies, especially, for the
mobile electronics. One of the most critical aspects of the package reliability is solder joint reliability. So, in that field, thermal cycling and drop/impact
are the primary requirement for solder joint reliability. This paper discusses the reliability of solder joint in term of both temperature cycles load and drop/
impact load from view points of failure mode and relevant material properties. High compliance and high grain-coarsening resistance are identified as
key material properties for high thermal cycling and drop impact reliability respectively. The paper details the requirements solder joints have to meet to
be qualified for the mobile electronics applications. Therefore, this contribution has its value in giving information on suitable material electronic devices
under different loading condition.
Vpliv termičnih in fizičnih obremenitev na zanesljivost
spajkanih spojev v elektronskih vezjih
Kjučne besede: spajkanje, termične in fizične obremenitve, načini odpovedi, lastnosti materiala
Izvleček: Miniaturizacija, zmanjševanje teže, velika hitrost in večopravilnost so trenutni trend lastnosti elektronskih vezij, še posebej namenjenih mobil-
nim napravam. Eden najbolj kritičnih vidikov zanesljivosti elektronskega modula je zanesljivost spajkanega spoja. Le-ta mora biti odporen na termične in
fizične obremenitve. V prispevku obravnavamo zanesljivost spajkanega spoja ter vpliv materialnih lastnosti na vzroke odpovedi po termičnih in udarnih
obremenitvah. Naštejemo vse zahteve, ki jih zanesljiv spoj mora zadovoljevati, da zadosti kvalitetnim kriterijem za uporabo v elektronskih modulih namen-
jenih mobilnim napravam.
1. Introduction
An electronic package integrates metal conductors,
organic/ceramic dielectrics and semiconductors into a
functional device. This variety of materials results in a com-
plex system to build and, increasingly, retain high levels of
reliability. Reliability is influenced by the operation of the
device (e.g., power dissipated, current carried, etc.) and
the environment (e.g., ambient temperature, temperature
changes and imposed mechanical strains) (Frear et al.,
2008). Traditionally, only temperature and power cycling
were of concern for board level reliability, and coefficient of
thermal expansion mismatch between the package and the
board was considered as the primary failure mechanism.
However, due to the proliferation of electronic devices
across market segments, ranging from automotive to small,
hand-held devices; electronic packages experience me-
chanical loading conditions other than just temperature
cycling (Syed et al.). This additional failure mechanism has
their implications on package material selection to design
a robust package meeting reliability requirements for a
particular end use application.
Thermal cycling and mechanical shock are two of major
loads that lead to the failure of board-level solder joints
for portable electronic product. Board-level package is a
multi-material system. These various materials cause the
mismatch of Coefficient of Thermal Expansion (CTE). The
CTE mismatch between PCB (composed of FR4 material
and polymer) and package (composed of substrate, die
and mold cap) results in the thermo-mechanical fatigue
damage of solder joints when the board-level package is
subjected to thermal cycling load. The fatigue crack initi-
ates and propagates through the bulk solder (Zhang et al.,
2009). The increasing occurrence of drop-impact failure
of portable electronics has been traced to the failure of
the solder joints that interconnect the integrated circuit
(IC) components to the printed circuit board (PCB). The
drop-impact of portable electronics leads to bending of the
PCB assembly within the portable electronic device; the
interconnecting solder joints undergo severe deformation
to accommodate the differential bending deformation be-
tween the IC component and the PCB (Wong et al., 2009).
The strain rate of solder joint under mechanical shock load
(e.g. drop impact) is much higher than that under thermal
cycling load. The strength properties of the bulk solder
will increase with the increasing of strain rate (Wong et al.,
2008b, Zhu et al., 2007). The solder joints have less plastic
deformation due to the higher strain rate under drop load
compared with that under thermal cycling load, so the stress
at the inter-metallic compound (IMC) layers increases and
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D. A. Shnawah, M.F. M. Sabri, I. A. Badruddin:
A Review on Thermal Cycling and Drop Impact Reliability of ... Informacije MIDEM 41(2011)3, str. 186-192
exceeds the fracture strength of IMC. The crack initiates
and propagates along the IMC layer (Mattila and Kivilahti,
2005). The failure mode, and therefore the reliability of
interconnection, relies on the properties of solder matrix.
This paper will discuss the thermal cycling and drop impact
reliability of solder joints in electronic packaging from the
view points of material properties, failure mechanism and
crack propagation.
2. Drop impact reliability
For portable electronic applications, one of the greatest
challenges for the package assembly is to survive a chal-
lenging use environment that includes being dropped,
result at the end in electrical failure (Frear et al., 2008).
‘‘Drop-impact” refers to free fall under gravity followed by
an impact on a target such as the ground. Upon impact, a
fraction of the kinetic energy of portable electronic product
will be converted to sound and frictional heat energy, a
portion to elastic and plastic strain energy in the product
housing, and the rest to elastic and plastic strain energy
of the interior components including printed circuit board
(PCB), integrated circuit (IC) components and intercon-
nects (mainly solder joints) (Wong et al., 2008a). The
literature on drop impact loading of electronic packages
and assemblies is starting to grow (Alajoki et al., 2005, K.
Mishiro, 2002, Mattila, 2005, Tee et al., 2003). The weak
link in the package is the board level solder joint between
the package and the printed circuit board.
The failure mode during drop impact loading is manifested
in interfacial cracking along the solder joint (either on the
package or board side (Suh et al., 2007, Syed et al., 2006)
as shown in Figure 1 (M.P. Renavikar, 2008). In either case,
shock failure is characterized by a lack of solder deforma-
tion and an absence of solder bulk cracking. This is due
to the strain-rate sensitivity of metallic materials. Metallic
materials including solders typically become stronger with
increasing strain rates. Thus, the robustness of a solder
joint is influenced by a complex combination of bulk solder
and inter-metallic properties (Grafe et al.). Ductile failures
through bulk solder typically progress slowly, but crack
through brittle inter-metallic progress much faster (Frear
et al., 2008). The outstanding question is whether “bulk”
properties of solder can be optimized to suppress or delay
this essentially interfacial” crack propagation along the
solder joints. The so-called extrinsic toughening concept
can be invoked to answer the question (Suh et al., 2007).
The extrinsic toughening refers to a toughening mechanism
by reducing effective crack driving force that the crack tip
actually experiences through various energy dissipation
processes without increasing inherent fracture resistance
of the material or interface (Ritchie, 1988). High compliance
(i.e., low elastic modulus) and high plastic energy dissipa-
tion (i.e., low yield strength) ability are identified as key
material properties to be optimized for extrinsic toughening
mechanism (Suh et al., 2007). Hence, solder alloy with low
compliance and high plastic energy can help increase the
drop performance because softer solder joint can help to
absorb more dynamic energy to reduce the dynamic stress
transformed from PCB to IMC/solder interface layer (Che
et al.). There is a high variation in the life of similar solder
joints since the cracks can change from ductile to inter-
metallic due to random variation in the microstructure of
individual joints.
Fig. 1: SAC405 solders joint failure in shock conditions
(M.P. Renavikar, 2008)
However, the strain rate experienced by solders joint or the
boards during drop/shock testing is estimated to be102/
sec., which belongs to dynamic-to-impact loading condi-
tion. Under these conditions the behavior of the metallic
material is denominated by elasticity (Suh et al., 2007).
In other words, plasticity is suppressed under these high
strain rates; therefore, elastic compliance is becoming
a key material property for drop impact performance. A
high compliance solder is expected to be favorable for
drop impact performance because it tends to lower stress
transfer to vulnerable joint region (Garner, 2009, Kim et
al., 2007). Figure 2 is a schematic diagram showing two
different hypothetical solder joint behaviors during drop
Fig. 2: Schematic stress-strain behavior of solder joint
with two hypothetical alloys with different
compliances. Note high compliance alloy (alloy
2) has lower stress under the same board
displacement or strain (Suh et al., 2007)
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D. A. Shnawah, M.F. M. Sabri, I. A. Badruddin:
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testing. Alloy 2 has higher elastic compliance than alloy 1
and as a result, the stress at the solder joint of alloy 2 is
lower than that of alloy 1 at the same board deflection (and
therefore the same strain). Therefore, the solder joint with
alloy 2 takes longer board deflection (or strain) to reach the
same critical stress of the joint than the solder joint with
alloy 1. In other words, a solder alloy with higher elastic
compliance is expected to exhibit longer critical strain to
failure (i.e., higher drop resistance) than a solder alloy with
lower elastic compliance (Suh et al., 2007).
One of concerns for industry to address solder intercon-
nect reliability under mechanical drop impact is the test
methods for qualifying designs/materials and for quality
assurance during manufacturing (Newman, 2005, Seah et
al., 2006, Wong et al., 2005). Classic mechanical solder
joint tests like shock, vibration and drop, result at the end
mainly in electrical pass/fail information. More essential for
solder joint characterization are test methods that provide
more detailed information on the solder joint failure mode
occurred. Fast Solder Ball Shear Test (see Figure 3) is
recommended to address solder interconnect reliability
(Grafe et al.). The interdependence between the various
strength characteristics of a solder ball interconnect is de-
picted in Figure 4. A ductile/elastic solder alloy (1) is able
to withstand higher strain rates compared to a stiffer solder
alloy (2) before reaching the IMC fracture limit. The ability
of a solder joint to deform in its bulk before IMC fracture is
basically measured with FBST. The output from the FBST
involves two basic parameters: (1) Energy before peak
force (mJ) and (2) Fracture mode occurred in testing. The
energy before peak is the area below the force plot till the
peak force (see Figure 5), which directly correlates to the
type of fracture which has happened and hence is useful
for solder joint characterization (Grafe et al.).
Fig. 3: Fast ball shear test arrangement (Grafe et al.)
The failure of board-level solder interconnects in drop
tend to be more extensive in ball grid array BGA packages
than land grid array because the joint is thicker and more
dynamic strain is imposed (Frear et al., 2008). The drop
impact failure behavior of the BGA joints was classified into
three types in terms of the crack initiating points (see Figure
6); a crack initiating in the IMC layers (CI), a crack initiating
in solder balls (CS), and a failure occurring as a result of
large ductile deformation of solder balls (DD). For these
three types of the failure, the corresponding three types of
practical failure situations can be considered (Tsukamoto
et al., 2010). The CI-failure can occur in the practical situa-
tion that the BGA joints are subjected to high speed impact
loadings such as drop conditions. The CS-failure can occur
in the case that some objects bump into the solder balls in
the packages. The DD-failure can occur in the case that
the large shear deformation of solder parts occurs under
low displacement- rate conditions (Tsukamoto et al., 2010).
Fig. 6: Failure model of BGA joints subjected to shear
loading (Tsukamoto et al., 2010)
Fig. 4: Joint strength vs. strain rate (Grafe et al.)
Fig. 5: FBST force vs. displacement (Grafe et al.)
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3. Thermal cycling reliability
Thermo mechanical fatigue occurs when materials with
different CTEs are joined and used in an environment that
experiences cyclic temperature fluctuations resulting in
imposed cycling strain. Thermo mechanical fatigue is a
major deformation mechanism concern for solder joint in
electronic packages (Frear et al., 2008). The type and
magnitude of strains in solder joints under conditions of
thermo mechanical fatigue is often quite complex. For
surface mount applications, the strain is nominally in shear
as shown in Figure 7. However, tensile and mixed-mode
strains can occur due to bending of the chip- carrier or
board as shown in Figure 8 (Abtew and Selvaduray, 2000,
Frear, 1991, Frear et al., 1989).
Fig. 7: solder joint subjected to shear strain during
thermal cycling due to CTE mismatch (Abtew
and Selvaduray, 2000)
Fig. 8: solder joint subjected to tensile loading due to
substrate flexing (Abtew and Selvaduray, 2000)
The combination of strain and temperature during thermo
mechanical fatigue has a large effect on the microstruc-
ture, and micro structural evolution of solder joints (Frear,
1991, Frear et al., 1989). Strain concentration enhances
diffusion leading to micro structural coarsening at elevated
temperatures (Abell and Shen, 2002). It has been observed
that typically only a fraction of the solder joint cross–sec-
tion actually participates in cyclic deformation because
strain distribution inside solder joints is seldom uniform.
Deformation of the most highly strained areas of solder
joints leads to localized deformation. The recrystallization
or grain coarsening takes place first in the regions where
the microstructure is most heavily deformed plastically and
then gradually expands. Failure eventually occurs due to
cracks that form in the coarsened regions of a joint. The
thermal anisotropy of the recrystallized grains enhances the
nucleation of micro cracks along their boundaries (Mattila,
2005). The failure mechanism under thermal cycling has
been widely studied by many researchers (Hirano et al.,
2001, Lee et al., 2002, Sohn, 2002). It was observed
that cracks always take place inside the matrix of solder
along or close to intermetallic layers closely parallel to
the direction of imposed shear strain as shown in Figure
9. The propagation path of the crack shown in Figure 9a
is enclosed entirely within the recrystallized region of the
interconnection shown in Figure Figure 9b (Mattila et al.).
The propagation of cracks, and therefore the reliability of
interconnection, relies on the properties of solder matrix.
The solder alloy with low strength facilitate plastic deforma-
tion of the solder alloy by external stress of solder joint and
cracks are generated and grow more easily within the solder
and shows poor fatigue resistance (Che et al.).
(a)
(b)
Fig. 9: Thermal cycle results (-40/125 OC) of
SAC305 solder bump for a CSP BGA
(Mattila et al.)
Standardized accelerated thermal cycling tests (ATC) are
commonly used to evaluate the thermo mechanical reli-
ability of electronic assemblies (Laurila et al., 2007, Li et
190
al., 2009, Zhang et al., 2005). During ATC, assemblies are
uniformly heated up and cooled down in order to induce
thermo mechanical strains and stresses in interconnections
and interfaces of the assemblies. The main processing unit
of the contemporary handheld multimedia smart Nokia N95
phone (see Figure 10), the Application engine (AE), was
chosen for thermo mechanical reliability characterization
under accelerated thermal cycling test. The AE component
is a stacked-die BGA package-on-package design. The
structure of the component is shown in Figure 11. A polar-
ized image of the critical interconnection cross-section after
testing is shown on the left side of Figure 12. The image
shows that a crack has initiated and propagated through the
interconnection close to the intermetallic compound layer
on the PWB side of the interconnection. The image also
shows recrystallization of the bulk solder. The calculated
von Mises stress contour map of the critical interconnection
cross-section at the time of the peak stress (at the end of
the ATC-40oC low-temperature dwell) is shown on the right
side of Figure 12. It can be seen from Figure 12 that the
contour map agrees well with the observed crack location
(Karppinen et al., 2010).
Fig. 10: Construction of the device: (1) lower enclosure,
(2) main board and (3) display assembly. The
Application engine (AE) component is marked
for closer examination (Karppinen et al., 2010)
Fig. 11: Application engine component package
(Karppinen et al., 2010)
4. Consecutive Multiple Loadings
Since portable electronics are often dropped after working
for a period of time, usually interconnections are subjected
to consecutive thermo-mechanical and mechanical load-
ings. The thermal cycling before drop test can introduce
two different changes to the microstructure of intercon-
nections: 1) The thermal mechanical strain and elevated
temperature induces recrystallization in highly deformed
region and fatigue cracks along large angle grain boundar-
ies are developed; 2) The thickness of inter-metallic layers
increases and the interfacial structure evolves with time.
The first change weakens the mechanical properties of bulk
solder under drop impact loading so that the cracking oc-
curs partly through bulk solder (Mattila and Kivilahti, 2006).
Without temperature variation, the isothermal annealing has
only the aging effects to interfacial microstructure. Since
the effective time for inter-metallic growth is approximately
only the total time of the upper soak stages in thermal cy-
cling (Xu et al., 2005), inter-metallic layers have much more
time to grow during isothermal annealing. If copper UBM
is used on the component side, the formations of Cu6Sn5
and Cu3Sn follow the typical growth kinetics (Mei et al.,
1992, Paul, 2004, Rönkä et al., 1998). Given adequate
time, the formation of “Kirkendall void” in the Cu3Sn layer
(Zeng et al., 2005) is much more severe during isothermal
annealing and the rupture of inter-metallic layer becomes
the primary failure mechanism, which degrades the drop
loading reliability significantly (Mattila and Kivilahti, 2006).
5. Conclusion
Elevated operating temperatures can degrade/change
the materials properties/ performance and the reliability
of the solder joint.
The dominant failure mode under thermal cycling load is
recrystallization-assisted crack nucleation and propagation.
Hence, solder joint with a good fatigue resistance can be
expected as result of inhibiting recrystallization.
High strength solder joint can exhibit a good fatigue re-
sistance due to suppressing plastic deformation during
thermal cycling loading.
The good drop performance can be attributed to extrinsic
toughening mechanisms through high bulk compliance and
high plastic energy dissipation during crack propagation.
Softer solder joint can help to absorb more dynamic energy
during drop impact loading to reduce the dynamic stress
transformed from PCB to IMC/solder interface layer.
Fig. 12: Left: The polarized light cross-sectional image of
the critical interconnection in ATC. Right:
The calculated FEA stress contour map of the
critical interconnection at the time of peak
stress (end of low-temperature dwell at 40oC)
(Karppinen et al., 2010)
D. A. Shnawah, M.F. M. Sabri, I. A. Badruddin:
A Review on Thermal Cycling and Drop Impact Reliability of ...Informacije MIDEM 41(2011)3, str. 186-192
191
Acknowledgement
The authors would like to acknowledge the financial sup-
port provided by the Institute of Research Management
and Consultancy, University of Malaya (UM) under the IPPP
Fund Project No.: PS117/2010B
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Dhafer Abdulameer Shnawah*, Mohd Faizul Mohd
Sabri, Irfan Anjum Badruddin
Department of Mechanical Engineering, University of
Malaya, 50603 Kuala Lumpur, Malaysi
*Author correspondent/ Corresponding Author, Tel:
+60 162375504, e-mail: dhafer_eng@yahoo.com
Prispelo: 17.11.2010 Sprejeto: 23.08.2011
D. A. Shnawah, M.F. M. Sabri, I. A. Badruddin:
A Review on Thermal Cycling and Drop Impact Reliability of ...Informacije MIDEM 41(2011)3, str. 186-192