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Modelling the Influence of Coatings, Potting and Underfills on ThermoMechanical Fatigue of Solder Interconnects in Electronic Packages
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Modelling the Influence of Conformal Coatings on Thermo-Mechanical
Fatigue of Solder Interconnects in Electronic Packages
1,3
1
Maxim Serebreni, 2Ross Wilcoxon, 1F. Patrick McCluskey
Department of Mechanical Engineering, University of Maryland, College Park, MD, USA
2
Rockwell Collins, Cedar Rapids, IA, USA
3
DfR Solutions, 9000 Virginia Manor Road, Suite 290, Beltsville, MD 20705, USA
301-474-0607, 866-247-9457
mserebreni@dfrsolutions.com
Abstract
Protecting electronic components at harsh environments and applications often requires the use of conformal
coatings, underfills or potting compounds. The temperature dependent properties of these materials greatly
depend on their chemical formulation. Conformal coatings used for electronic applications are available in a
variety of materials ranging from silicones, acrylics, polyurethanes, paralyne and epoxies. The glass
transition temperature (Tg) varies for each material and represents a phase change from a hard glassy one to
a soft rubbery state. Temperature fluctuations experienced by electronics can span a wide range that can often
include the Tg of the encapsulant.
In this paper, thermal cycling simulations are performed on QFN and BGA components using different
conformal coating materials. Material characterization is performed to determine the temperature dependent
properties of several conformal coating materials. Results illustrate higher damage accumulation during ramp
down and cold side of thermal cycles. Information obtained in this study is used to develop a mitigation
strategy that enables selection of encapsulants without compromising desired system level encapsulation
method to increase overall board level reliability.
Key words
Conformal coating, solder fatigue, BGA, QFN, Digital Image Correlation, Glass transition temperature
joint and shift the characteristic reliability of components.
Parylene C is a popular coating variant for high reliability
aerospace applications [2]. Although parylene offers
outstanding chemical resistance, dielectric characteristics
and moisture protection it is cost prohibitive in consumer
electronics and requires a specialized application method
using vacuum deposition process. Compared to acrylic and
urethane conformal coatings, parylene possesses lower CTE
and higher melting temperature which provides protection at
higher thermal excursions. Due to the popularity of parylene
coatings in high reliability applications, more extensive
research on thermal cycling reliability is available. Several
investigations into the thermal cycle reliability of
conformally coated components with paralyne C found no
particular decrease in characteristic life of components
except for a slight reduction in early failures for some
components [3]. Investigation into the effect of parylene
coating on thermal fatigue life of solder joints in ceramic
BGA and leadless ceramic chip carriers found an increase in
the thermal fatigue life by a factor of 2X [4]-[5]. The
improvement in fatigue life in CBGA devices is a
characteristic of both the paralyne deposition method and
I. Introduction
Conformal coatings are used in printed circuit board
assemblies to provide protection against airborne
contaminants, moisture and pollutants [1]. Conformal
coatings are available in various chemical formulations
ranging from epoxy, silicone, polyurethane, acrylic to
parylenes. Coatings can be applied using variety of methods
depending on manufacturing practices and protection
requirements. Proper adhesion of the coating is necessary to
properly insulate components from their respective
environment. The interaction of conformal coating with
components under extreme temperatures is often not evident
and potential adverse effects have not been investigated due
to the sheer number of configurations and environmental
stresses that conformally coated electronics experience.
Previous investigations into the effects of conformal coatings
on the reliability of electronic components have illustrated
the impact on solder joint fatigue during thermal cycling.
Due to the large mismatch between the coefficient of thermal
expansion (CTE) of conformal coatings and electronic
components, environmental stresses can develop in solder
1
temperature dependent properties that enable constraining
the CTE mismatch between the CBGA and the PCB resulting
in lower thermal strains and longer fatigue life.
In this paper, the impact of several types of conformal
coatings on QFN and BGA components is assessed using
material characterization and thermal cycling simulations.
Several configurations of conformal coating are simulated
that correspond to spray, brush and dip coated applications.
All thermal cycling simulations are performed for a thermal
profile of -40ºC to 125ºC with 15-minute dwells and 10
ºC/minute ramp rate. Material characterization is performed
to obtain the temperature dependent properties of the
coatings from bulk samples.
Thermal expansion of urethane coatings has been previously
correlated as the cause of solder cracking due to significantly
altering the solder stress environment [6]. The impact of
improper conformal coating using acrylics, urethane, epoxy
and silicone on SnPb BGA components have been previously
investigated and found that heavy coating application can
either decrease or increase thermal fatigue life of solder
joints [7]. Silicone and acrylic conformal coating found to
decrease characteristic life while urethane and epoxy found
to slightly improve characteristic life; however, some first
failures were found to initiate earlier in coated component
compared to the non-coated components. Another study
concluded that a polyurethane conformal coating thickness
of even 10% of the lead thickness is sufficient to stress solder
joints and accelerate fatigue failure of fine-pitch TSOP
components and result in reduction of characteristic life by
as much as 30% compared to a non-coated component [8].
II. Failure Modes of Encapsulated Components
Failure modes of encapsulated electronics have manifested
in the form of delamination or cracking of package internal
interconnects (wire bonds, die etc.) and board level
interconnection such as solder joints and leads. Solder
fatigue is a prevalent failure mechanism that can occurs
under vibrational and thermal loads. Underfills and staking
materials are often used to mitigate the vibrational loads
components experience. In some situation encapsulation of
devices is done using potting or conformal coating to
efficiently mitigate vibration loads. The addition of these
conformal coating although improve vibrational fatigue can
drastically reduce thermal fatigue life. Figure 2 shows a cross
section along the diagonal of a 17mm BGA package after
620 thermal cycles of -55 ºC to 125ºC. Fatigue cracking is
seen to propagate along the board side of the corner most
solder joint. Minimal damage is seen at joints closer toward
the package center. This indicates a classic distance to
neutral effect.
The addition of various conformal coating to electronic
assemblies can largely alter the stress state in solder
interconnects during changes in temperature as the high CTE
and stiffness of the coating acting on components. This
behavior is largely due to the combination of coating
stiffness and coefficient of thermal expansion. Figure 1
summarizes the hardness of different potting compounds
with operating temperatures [9].
Figure 1. Summary of hardness and maximum operating
temperature of commercial potting compounds and glob
tops [9].
Selecting the appropriate encapsulant to provide electrical
insulation and environmental protection without inducing
excessive mechanical stresses is not as simple as
categorization the material based on hardness. Knowledge of
the temperature dependent properties of the material,
environmental/operating temperature range and application
type are needed to be assessed.
Figure 2. Cross section along the package diagonal after
thermal cycling [10].
Figure 3 shows a cross-section and X-ray image of the same
17mm package after 620 thermal cycles that was coated with
heavy application of Humiseal 1B31 conformal coating that
fully underfilled the space between component and PCB.
The difference between the two packages reveal that the
solder joints along the perimeter of the package were more
compressed. Components in Figure 3 exhibited lower
2
characteristic life than non-coated component in Figure 2.
Compared to the control BGA component, the environmental
stress solder joints experience with the acrylic coating has
been significantly altered. The extruded shape of the solder
joints indicates a compressive mean stress during thermal
cycling that resulted in a compressive ratcheting effect. The
stresses imposed on solder joints in electronic components
are caused primarily by temperature cycling aggravated by
differential expansion of entrapped conformal coating [11].
Figure 5 shows the CTE measurements with temperature for
four conformal coating materials obtained from DIC. The
large reduction in CTE measured for the Humiseal 1B31 is
attributed to the reduction of modulus and increased
adhesion of the material at high temperatures. This effect has
been previously observed with TMA measurements as well
for the same material as this material melts after 60ºC.
Figure 5. CTE with temperature measured by DIC
Figure 3. X-ray image and cross-sections along package
diagonal with Humiseal 1B31 after thermal cycling [10].
Elastic modulus was obtained using TA Instruments RSA3
Dynamic Mechanical Analysis (DMA) system. Thin film
specimens were prepared from bulk samples of cast
conformal coatings. Pre-strain setting was adjusted for each
material as to prevent overstraining the material during the
large transition in temperatures. Most samples fractured past
their measured Tg. Modulus values past this point were
interpolated up to the maximum temperature used for
simulation. All the conformal coating values exhibit an
elastic modulus below 1 MPa past their respective T g. The
elastic modulus can be calculated from the loss (𝐸 ′′ ) and
storage (𝐸 ′ ) moduli using equation 1.
III. Material Characterization
Measurement of conformal coatings CTE was performed
using commercial Digital Image Correlation (DIC) system
by Correlated Solutions that was fitted with a thermal
chamber shown in Figure 4. DIC offers direct measurement
of surface strains and is especially suitable for non-contact
full field measurements.
2
2
1
𝐸 = (𝐸 ′ + 𝐸 ′′ )2
Eq. 1
The storage modulus represents the elastic portion and is
proportional to the stored energy and the loss modulus is
proportional to the dissipated energy. Conathane CE1155
exhibits the longest transition region with highest T g out of
the four-conformal coating. Conathane CE1155 exhibits
three distinct regions where some glassy, rubbery and fluid
regions are evident. The glass transition region is a kinetic
process which depends on both the measurements method
and data evaluation procedure [12]. This is evident in the
difference between the data obtained through DIC and DMA
listed in Table 1.
Figure 4. DIC system with thermal chamber
Values for the Tg obtained using DIC are found to be
consistently lower than values obtained using DMA.
Changes in the CTE in polymers tend to be driven by changes
in the free volume while change in the modulus tend to be
driven by increase in translational and rotational movement
Thermal expansion measurements using DIC enable
simultaneous testing of multiple materials reducing test time
compared to Thermo-Mechanical Analysis (TMA) which is
limited to measurements of individual material samples.
3
of the polymer chain. Change in CTE tends to initiate before
a significant decrease in the modulus since lower levels of
energy are required to increase the free volume compared to
increase motion along polymer chain.
made uneven to illustrate the unpredictable nature of
conformal coating flow during heavy application. A 30 µm
thick layer of solder with two elements through thickness
was used for volume weighted average of creep strain energy
density.
Figure 6. Elastic modulus obtained using DMA.
Table 1. Summary of CTE, modulus and Tg values for
conformal coating.
Figure 7. Cross-section view of QFN and BGA packages
with three conformal coating configurations.
Linear elastic material properties used in simulations of the
QFN and BGA packages are shown in Table 2. Tabulated
temperature dependent elastic properties of the conformal
coating were taken from material characterization section.
IV. Finite Element Analysis
Quarter-symmetric models of a 6x6mm QFN package and a
17mm BGA component were modeled in Abaqus 6.14 as
shown in Figure 7. Control conditions refers to component
assembled to the board with no coating applied. CC1 refers
to components with conformal coating applied to the
package only. CC2 refers to condition in which some
conformal coating has bridged between the package edge and
the PCB. CC3 refers to condition in which the coating
material fully encapsulates the component both above and
below. Conformal coating thickness of 150 µm was chosen
for both QFN and BGA packages. In CC2 coating
configuration a 100 µm thick gap of the coating was modeled
to bridge between the component and the board. A 62 µm
solder joint height was chosen for the QFN package and a
BGA ball height of 335 µm. Corner solder joint were meshed
with 1703 elements in QFN package and 3148 in BGA
package with hexahedral C3D8R element. In all simulations,
the conformal coating was decoupled from the solder joints
as tied contacts between dissimilar conditions with large
CTE mismatch can cause stress singularities to concentrate
at their interface [13]. The gaps between solder joints that the
conformal coating bridges in the QFN components were
Table 2. Linear elastic material properties for simulation
Creep constitutive model used for SAC305 is shown in
equation 2 with constants listed in Table 3.
𝐻1
𝜀̇𝑐𝑟 = 𝐴1 (𝑠𝑖𝑛ℎ𝛼𝜎)𝑛 exp (− )
Eq. 2
𝑘𝑇
Where equation 𝜀̇𝑐𝑟 is the steady state creep strain rate, k is
4
Boltzmann’s constant, T is the absolute temperature and 𝜎 is
the applied stress. The constants 𝐴1 , 𝛼, n and 𝐻1 , which is
the apparent activation energy.
corner joints. The uneven bridging section of conformal
coating are shown to cause addition bending and result in
decrease of damage in one of the corner joints and increase
damage in joints located along the package center.
Table 3. Constants for equation 2 [14]
The three configurations of conformal coatings in the QFN
package are shown in Figure 8 with the component
suppressed. Slots were made to accommodate solder joints
as conformal coatings are applied post reflow.
Figure 9. Accumulated creep strain energy density in QFN
solder joints with Conathane CE1155 conformal coating.
Figure 10 demonstrates the stress contours of the corner joint
in QFN package with Conathane CE1155 CC3. The highest
stresses are found to concentrate at the package side of the
solder joint. Stress concentration along the joint toe initiates
fatigue cracks that eventually propagate through the fillet
resulting in an open circuit. To demonstrate the difference in
stress states between a non-coated and coated QFN package
the element indicated in Figure 10 is selected for analysis.
Figure 8. QFN conformal coating configurations
The size of the gap that is available for the conformal coating
to flow underneath is significantly larger for BGA packages
compared to QFN components due to the large thermal pad
at the center of the component preventing complete
underfilling. Therefore, it is expected that the influence of
the same coating and application methods will result in
different behavior between dissimilar package styles.
Thermal cycling simulations illustrate the creep strain energy
density for QFN device with Conathane CE1155 in Figure 9.
Accumulated creep strain energy density is used as a damage
indicator as creep deformation accounts for the dominant
inelastic strain deformation. The results of the creep strain
energy density contours in Figure 9 indicate that highest
damage for CC2 configuration. A shift in distance to neutral
effect with CC2 is also evident compared to CC1 and CC3
that indicate progressive increase of damage toward the
Figure 10. Corner solder joint in QFN with Conathane
CE1155.
Axial stress and strain from the element centroid are plotted
to generate a hysteresis loop during the first three thermal
cycles as shown in Figure 11. The hysteresis loop indicated
by the red lines for coated component is significantly larger
in size compared to one in control package. This indicates
that a significant portion of the increased strain energy
5
density is generated by the axial loading. It was found that
the shear loading component in the QFN components was
not changed with the addition of conformal coating. This
observation deviates from previous simulation results that
indicate having conformal coat flow underneath a QFN
device significantly drops fatigue life by enhancing shear
loads in solder joint with no particular mention to the
modification of the axial loading components [15].
The influence of Acrylic and Paralyne conformal coating on
BGA reliability during thermal cycling have found no
significant change on characteristic life [16]. This is due to
proper spray application that prevented bridging of the
coating between components and boards. On the contrary,
studies on the effect of heavy application of conformal
coating on thermal fatigue of QFN component indicated that
Polyurethane based conformal coating reduced fatigue life
by a factor of 3X while Acrylic conformal coating had no
significant influence [17]. The observed reduction in fatigue
life is dependent on the amount of coating that ingresses
underneath the component and the temperature dependent
properties of the coating at a given thermal cycle. Figure 13
illustrates this effectively by comparing the accumulated
strain energy density in BGA package with coatings.
Figure 11. Stress-Strain hysteresis loop comparison
between control and CC3 with Conathane CE1155.
Figure 12 shows the volume weighted accumulated creep
strain energy density for the simulated QFN package with
different conformal coatings and configurations. CC1
conditions indicates similar values to the non-coated control
configuration in grey column. CC2 indicates a 3.3X increase
in strain energy density over control condition. Since a linear
relationship between cycles to failure and strain energy
density is assumed an equivalent reduction in fatigue life can
be expected. CC3 condition for the Hysol PC12-007M and
Humiseal 1B31 indicates lower damage compared to CC2
except for Conathane CE1155 that remains consistent with
CC2 values. These findings demonstrate that the application
method along with the conformal coating condition is
responsible for fatigue life reduction.
Figure 13. Accumulated creep strain energy density for
BGA with three coating and application methods.
It is evident that the influence of CC2 on BGA is not as
significant as in the QFN component. Conathane CE1155 is
shown to be most damaging similar to the QFN package with
CC3 with the Humiseal 1B31 and Hysol coatings resulting
in equivalent damage across configurations. A more
progressive increase in strain energy occurs for the BGA
package from CC1 through CC3 that is proportional to the
leverage coating has between components and boards.
V. Discussion of Results
The hysteresis loops calculated for the corner joint in the
BGA package are shown in Figure 14. Average of the stress
and strain was taken for the same volume of solder used for
volume weighted average of the creep strain energy density.
The average stress values that generate the hysteresis loops
for the BGA components with CC3 are used to qualitatively
illustrate the change in stress state experienced by the solder
with the different conformal coatings. The axial hysteresis
response between the Humiseal and Hysol coatings is found
to be almost identical. This observation correlates with the
trend in accumulated creep strain energy density shown in
Figure 13. A unique behavior in an increasing compressive
mean stress in solder is found for all three coatings. The size
of the hysteresis loop is driven by coating type based on
Figure 12. Accumulated creep strain energy density for
QFN with three coatings and applications.
6
highest temperature dependent modulus. Larger axial loads
are transferred to solder joints with CC3 coatings that posses
the larger elastic modulus for the longer duration of the
thermal cycle.
Below this region, high compressive loads occur and can
potentially cause large plastic deformation in joints and
result in accelerated failure. The worst possible
environmental stresses occur for coated component when
thermal cycle crosses the coatings T g as shown in this study.
Therefore, components that are coated via heavy application
of heavy conformal coating should utilize coating with Tg
that is outside the expected operating temperature range.
Addition alternative is to underfill components with a
dedicated underfill materials that have low CTE and high
stiffness to constrain thermal expansion and prevent flow of
conformal coating underneath component.
Figure 14. Average axial stress and strain for BGA corner
joint with three conformal coatings with CC3 application.
The increase in axial stress component can be associated
with the Tg of the coating materials as shown in Figure 15. A
peak in the tensile stress occurs as the coating expands across
the Tg. The highest peak tensile stress is evident for the
coating that maintains the highest elastic modulus for the
longest temperature duration across the T g.
Figure 16. Optimal low environmental stress operating
temperature region with Conathane CE1155 coated
components.
III. Conclusion
Based on simulation results and material characterization,
the following conclusions can be made:
1. The addition of conformal coating that bridges the gap
between component and PCB enhances the axial loads
and not the shear loads introduced to solder joints as
previously reported.
2. Urethane conformal coating is found to be most
damaging compared to the Acrylic and Epoxy materials
especially for QFN components when allowed to flow
underneath the part.
3. The increase in damage with conformal coatings is
attributed to the added tensile and compressive stresses
due to coating expansion and contraction across the
glass transition temperature.
4. Conathane CE1155 is found to be more damaging due
to higher stiffness across longer temperature range
compared to acrylic and epoxy.
Figure 15. Average axial stress with time for BGA corner
joint with three conformal coatings with CC3 application.
Figure 16 illustrates the stresses and strains as function of
temperature for conformally coated component with
Conathane CE1155. The stress-strain relationship with
temperature is nonlinear and highly depends on the material
properties across the Tg. The lowest stress region identified
for this configuration is shown by the bounding box for a
narrow temperature range of 42 ºC to 85ºC. This region is
above the Tg of the conformal coating where the material is
soft enough to counteract the high CTE coating expansion.
The bounding box indicates a region that would experience
a similar stress state to that of a non-coated component.
Acknowledgment
The authors would like to thank Dr. Craig Hillman and Dr.
Nathan Blattau for support of this research and Dave Hillman
of Rockwell Collins for supplying materials for
characterizations.
7
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