The Effect of Improper Conformal Coating on SnPb and Pb

30th SEMI-THERM Symposium

The Effect of Improper Conformal Coating on SnPb and Pb-free BGA Solder Joints during

Thermal Cycling: Experiments and Modeling

Maxim Serebreni1*, Ross Wilcoxon2, Dave Hillman2, Nathan Blattau1, Craig Hillman1

1DfR Solutions

9000 Virginia Manor Rd #290

Beltsville, MD 20705

2Rockwell Collins

400 Collins Road NE

Cedar Rapids, IA 52498 *Maxim Serebreni, [email protected]

Abstract

Application of chip scale packages (CSPs) and bottom

terminated components (BTCs) in harsh use environments

often requires the use of conformal coatings to meet reliability

requirements. In certain coating application methods, the

conformal coating materials can flow underneath the

component and cause solder joint failure during thermal

expansion and contraction of the electronic assembly. In this

study, Ball Grid Array (BGA) components were coated with an

acrylic conformal coating materials using two application

methods and subjected to two thermal cycling profiles to assess

the integrity of SnPb and Pb-free BGA components. To better

understand the observed failure modes, Finite Element

Analysis (FEA) was performed on the conformally coated

BGA packages. Material characterization was performed using

Dynamic Mechanical Analysis (DMA) and Thermal

Mechanical Analysis (TMA) to capture the temperature

dependent properties of the conformal coating to better

correlate simulation and experimental results. Failure modes

were found to greatly depend on the conformal coating material

properties around the glass transition temperature (Tg) rather

than temperature range. Significant difference in the failure

mode was found between the Pb-free and SnPb BGA

components with acrylic conformal coating materials and

temperatures profiles.

Keywords

Conformal Coating, Thermal Cycling, Ball Grid Arrays,

Solder fatigue, Finite Element Modeling

1. Introduction

Conformal coatings are used on Printed Circuit Boards

(PCB) with the intent of providing protection from harsh

environments containing moisture and contamination such as

dust and metallic debris that could cause shorts in electronic

components. In addition, some conformal coatings are

designed to provide thermal insulation, shock vibration

attenuation and electrical insulation for high voltage

components at high altitudes [1]. Conformal coatings are also

being used to help mitigate the risk of tin whiskers on pure tin

surface finishes [2]. These attributes make conformal coatings

especially attractive for high reliability application

environments of avionic and aerospace electronics. Previous

studies have shown that the application of conformal coating to

surface mount resistors and CSPs can reduce the thermal strain

in solder joints and thus extend the thermo-mechanical fatigue

life of components [3-5].

Potential concerns regarding solder joint integrity arise

when the conformal coating material is allowed to flow

underneath the package. Recent investigations have show that

letting conformal coating flow underneath plastic quad-flat no

lead package (PQFN) dramatically reduces the thermo-

mechanical fatigue (TMF) life of the device by changing the

equivalent plastic strain of solder joints from predominantly

shear to an axial loading mode [6]. In such a condition, the

conformal coating expands and contracts in the vertical

direction. As thermal cycling progresses, failure in solder joints

could result from the lifting of the component that causes

excessive tensile and compressive stresses. The amount of axial

stress will greatly depend on the leverage of the conformal

coating on the particular component and the variation of

coefficient of thermal expansion (CTE) and elastic modulus (E)

of the coating with temperature. In addition, conformal coating

materials with a Tg that is within the thermal cycle range could

drastically reduce fatigue life of solder interconnects. As

temperature approaches the materials Tg, large expansion

occurs along with reduction of material stiffness. In cases when

the conformal coating material expansion occurs prior to

adequate softening, large stresses would be applied to solder

joints. This behavior is inherent in thermoset polymers since

materials expansion tends to bedriven by the changes in the free

volume while changes in the modulus tends to be driven by the

increases in movement of the polymer chains. Due to the

intrinsic properties of conformal coating and the large variety

of coating materials used by the electronics industry, the effect

conformal coatings on solder joint TMF life needs to be

investigated.

This research aims to investigate the impact conformal

coatings and their application method can have on Pb-free and

SnPb area array components under thermal cycling condition.

Experimental procedure follows that of previous study

conducted by authors in which components with only SnPb

solder were used [7]. Results from mechanical characterization

of the acrylic conformal coating were used in finite element

simulation to capture the effect of acrylic conformal coating

and their application method on ball grid (BGA) components

and provide further insight into the associated failure mode in

each of the tested solder alloys under various thermal cycling

ranges.

mailto:[email protected]

Wesling, Test Structures in Thermal Test Chips for Optimum …


2. Experimental Approach

Simulations in the current study were based on

experimental conditions performed in a previous investigation

that included ball grid array (BGA) components, with tin-lead

(63Sn37Pb) solder balls, that were subjected to thermal

cycling. Figure 1 shows an assembled test vehicle from that

study that included 60 components with half of them

conformally coated. In the experimental study of reference [7],

an Anatech event detector system continuously monitored the

continuity of the daisy-chain BGA components to determine

the number of thermal cycles at which solder joint failure

occurred.

2.1 Material Properties

Acrylic conformal coating is widely used in a large variety

of electronic assemblies. To understand the temperature

dependent mechanical behavior of the material bulk samples of

the acrylic conformal coating were prepared and characterized

using TMA and DMA to measure the materials CTE and

modulus as a function of temperature as shown in Figure 1 and

are implemented in FE modeling. Characterization identified

the Tg of the material to be around 15°. The glass transition

temperature corresponds to the midpoint of a region in which

the material’s modulus decreases and material expansion

rapidly increases rather than a single data point in which an

abrupt transition occurs.

Figure 1. Measured material properties

2.2 Test Vehicle

Test vehicles comprised of daisy chained BGA components

with a package size of 17mm x 17mm, 1mm pitch, 256 IO.

Each test board was populated with 60 individual components.

Test boards selected with FR4 material laminate with 0.081

inch thickness and 8 dummy inner layers. BGA components

assembled using 0.005 inch thick stencil. Two conformal

coating configuration were applied to the two halves of each

test vehicle. The left half of each board was coated using

“standard” production spray process and the right half of the

boards coated by a manual process using pneumatic syringe to

completely fill the conformal coating material under each BGA

referred to as “thick” application. This approach represents a

worst-case scenario of conformal coating application that can

occur in dipping method and heavy spray coating applications.

Figure 2. Assembled test vehicle showing two halves of the

board with two conformal application methods [7].

3. Thermal Cycle Testing

Two thermal cycle conditions were used in testing, one

with a temperature range of -55°C to +125°C and the second

with –20°C to +80°C as shown in Figure 7. Both profiles have

a minimum of 15 minute dwell time at each temperature

extreme and a ramp rate of 5-10°C/minute according to IPC-

9701.

Figure 3. Thermal cycles used in experiment and modeling.

4. Test Results

Reliability data of BGA component were analyzed using

regression analysis to determine the Weibull shape factor (β)

and characteristic life (η) for each configuration. The Weibull

function correlates the cumulative failure distribution F(t) to

the number of thermal cycles at which failure occurs shown in

equation (1).

F(t) = 1 − exp (−t

η)β

(1)

The reported characteristic life corresponds to number of cycles

at which 63.2% of the population is expected to have failed.

4.1 Failure Data Analysis

Failure data for each of the component configuration is

analyzed for the two thermal cycles and displayed in Table1.

Cycles to first failure are indicated for configuration in which

only sufficient thermal cycles progressed to cause an “early



failure” event; however, in slots indicated by “N/A”, the

characteristic life occurs past the experimental time frame.

Control samples with SAC305 alloy exhibit shorter fatigue life

than the SnPb components under equivalent thermal profiles.

As standard acrylic conformal coating is introduced,

characteristic life of the two alloys under temperature profile

with -20°C to +80°C falls within a narrower range.

Table 1. Weibull coefficients for experimental data

Alloy

Applicati

on

method

Temp Profile

β η First

failure

SAC305

control 1 7.92 256 173

standard 1 5.74 448 243

Heavy 1 17.09 197 171

control 2 N/A N/A N/A

standard 2 N/A N/A 935

Heavy 2 3.78 191 88

63Sn37Pb

control 1 17 629 514

standard 1 14.6 567 472

Heavy 1 6.14 568 364

control 2 N/A N/A N/A

Standard 2 N/A N/A N/A

Heavy 2 7.21 1234 756

The differences between standard and control samples are not

consistent from the SAC305 to SnPb alloys but are shown to

match under temperature profile 2 with a range of –20°C to

+80°C. With an increase in the thermal cycling range, a larger

discrepancy between the two solder alloy characteristic life is

evident from the cycles to first failure. First failure with

standard conformal coating surpassed the initial 1600 thermal

cycles for the -55°C to 125°C profile for the SnPb alloy with a

much lower first failure observed for the SAC305 alloy.

Figure 4 represents the cumulative failure distribution for

the SAC305 components with thick and standard coating for

the two temperature profiles. It is evident that the application

method impacts fatigue life significantly more than the

temperature range. A similar trend is evident for the SnPb

components.

Figure 4. Cumulative failure distribution for the Pb-free

acrylic samples with thick and standard coating

Figure 5 displays the cumulative failure distribution for the

SnPb components performed in the previous study along with

the standard acrylic coating performed in the current study.

Failure rates for the standard and thick coating under profile 1

show close resemblance to that of the SAC305 components

with earlier failures in the thick coated components. Initial

failures of the standard coated components are displayed for

profile 2 and illustrate the drastic increase in fatigue life with

standard coating between the two solder alloys and temperature

profiles.

Figure 5. Cumulative failure distribution for the SnPb acrylic

samples with thick and standard coating from previous study

[7]

4.2 Cross-sectional Failure Analysis

Metallographic cross section assessment performed on

both Pb-free and SnPb BGA components post thermal cycling

to reveal damage and crack location in solder jonits with and

without conformal coating.

Figure 6. Pb-free control sample showing cracking along

bottom pad.

Control components with Pb-free solder show cracking along

the solder joint/board pad interface as shown in Figure 6;

however; SnPb control components exhibit cracks along the

upper solder joints/component pad interface. This initial

difference in failure site between the control samples indicates



that additional factors such as component warping, pitch size,

ball height, local CTE mismatch, package configuration and

intermetallic layer all affect fatigue life of components and are

inherent factors in each solder alloy and component selection.

Figure 7. Failure of Pb-free BGA with acrylic conformal

coating (-55 to 125C) a) standard b) heavy coating application

Cross-sections of failed Pb-free solder joints are shown in

Figure 7 for both the standard and thick coated components.

Standard Pb-free components exhibit similar cracking location

as those in the control components with corner joints cracking

before joints at the center due to distance to neutral point effect.

Since a similar characteristic life between the control and

standard coated components was observed, the failure location

was as expected to be comparable. Introduction of the thick

conformal coating was found to have a different failure mode

than the standard coating. Since the acrylic conformal coating

represents a much larger area than the effective area of the

solder joints between the PCB and substrate, the mechanical

strains placed on the solder joints were imposed by the thermal

expansion of the conformal coating. Figure 8b) shows severe

plastic deformation in the Pb-free solder joints and no evident

distance to neutral effect. Similar cracking at the board pad is

observed for the -20°C to 80°C thermal cycle with less plastic

deformation as in Figure 7. Large compressive strains caused

solder joint to be extruded outwards. This failure mode is also

attributed to the lack of adhesion and interaction between the

conformal coating and solder alloy during the low and high

temperature extremes. At the high temperature dwell, solder

joints creep and conformal coating softens and eliminates any

possible adhesion between solder and the acrylic material. At

the cold temperature, extreme, the solder is compresses and

squeezed outward in the normal direction to the applied load.

The lack of hydrostatic stress on solder joints during low

temperature dwell implies that no physical constraint is being

placed on the solder and allows for the deformation to occur.

This solder/conformal coating interaction is greatly dependent

on the conformal coating CTE and modulus at the dwell

temperature. It is important to note that the cross-section shown

in Figure 7 is that after 1000 thermal cycles, and electrical

failure was detected in a fraction of the number of cycles took

to severely deform the Pb-free solder alloy to the one shown.

Failure analysis of the Pb-free solder joints exhibits similar

failure location on the upper solder/component pad interface as

the control components. Figure 8 displays SnPb BGA with

standard acrylic coating post 1600 thermal cycles with slight

cracking and grain coarsening under temperature profile 2.

Figure 8. SnPb BGA with Acrylic standard coating showing

cracking in upper left size of solder joint at 1600 cycles.

Cross-sections of the SnPb components reveals that the

magnitude of damage closely correlate to the characteristic life

variation found in the BGA solder joints under both thermal

cycles. A different failure mechanism with the thick acrylic

coating between the SnPb and the Pb-free solder joints was

observed along with a phenomenon that explains the difference

in cycles to failure primarily between the thick coated

components. Figure 9 displays the SnPb solder joints with thick

acrylic coating post thermal cycling for a) corner joint and b)

joints at the center of the row for the -20°C to 80°C thermal

cycles. Cracking along the diagonal of the SnPb solder is found

along with grain coarsening at the crack area and similar

extrusion as seen in the Pb-free components. Whoever; unlike

the Pb-free components, electrical failures in the SnPb

components were detected as cracking along the diagonal of the

solder joints propagated prior to the severe compressive

deformation took place. Unlike the Pb-free solder, distance to

neutral point in the thick coated SnPb components remains



dominant and is evident from the lack of damage in solder

joints at the center of the row with progressively lower damage

in solder joints closer to the neutral point of the component.

This phenomenon can be attributed to the lower stiffness of the

SnPb solder joints which allow for more components warpage

and load sharing between neighboring solder joints.

Figure 9. SnPb BGA with thick conformal coating failed at

1055 thermal cycles (-20 to 80C) a) Corner joint b) joints in

middle of row.

Variation in the characteristic life between the Pb-free and

SnPb components is also strongly dependent on the local CTE

mismatch between solder alloy and component. Due to a

different microstructure and mechanical properties, the stress

distribution in SnPb corner joints is subjected under lower axial

strain but with larger shear strains than the Pb-free solder. In

conjunction with the axial compression due to the thick acrylic

coating, solder joints are placed under mixed-mode thermo-

mechanical loading which progresses cracking along the

diagonal of the solder joint.

In a few instances, no failure was recorded in SnPb

components with thick acrylic coating. Figure 10 displays

corner and middle row SnPb components with thick coating

cycles from -20°C to 80°C for 1600 cycles with not an evident

change in resistance. Both locations along the row show

severely compressed solder joints with cracks running along

the diagonal of the component. It is believed that in this

instance the large compressive stresses caused crack closure to

overate the rate of crack propagation and avoid an open to

occur. This process is not fully explained and its exact

contribution to the observed increase in cycles to failure in

SnPb components is not yet known.

Figure 10. SnPb BGA with thick acrylic coating survived

1600 cycles a) corner joint b) joint in middle row

5. Finite Element Modeling

BGA package is modeled using commercial finite element

software ABAQUS 16.4. Global/local modeling approach is

used to create a quarter symmetric finite element model of the

BGA package and has proven to provide accurate results in

modeling solder in electronic packaging [9]. Copper pads on

both the PCB and substrate are modeled without solder mask.

Plugs of fine mesh were created to model corner solder joints

and plugs with coarse mesh used to model the rest of the solder

joints as shown in Figure 11.

Figure 11. Local model (a) joint with coarse mesh (b) jonit

with fine mesh



To capture creep and plasticity deformation of solder joints,

Schubert’s constitutive model based on a hyperbolic sine

function was implemented for both Pb-free and SnPb solder

shown in equation (1) [8]. Where 𝜀̇𝑐𝑟 is the steady state creep

strain rate, 𝐴1 constant, 𝐻1 is the apparent activation energy, k

is the Boltzmann’s constant, T is the absolute temperature, 𝜎 is

the applied stress, 𝛼 and n prescribe the power law relationship

between creep strain rate and applied stress.

𝜀̇𝑐𝑟 = 𝐴1(𝑠𝑖𝑛ℎ𝛼𝜎)𝑛 exp (−

𝐻1

𝑘𝑇) (2)

Solder A1 α n H1 k

SAC305 277984 0.02447 6.41 54041 8.314

63Sn37Pb 23343483 0.06699 3.3 67515 8.13

Modeling of the components with standard conformal coating

was omitted from the analysis since failure rates of standard

coated components matches those with the control components

in both solder alloys and thermal cycles. Figure 12 shows the

global view of the quarter symmetric BGA model.

Figure 12. Global model without underfill

Material properties used in the analysis are presented in Table

2. These values are a culmination of both published and

measured quantities and are assumed to be linear elastic except

solder alloy

Table 2. Material Properties

Local BGA plugs and a single quarter symmetric model were

merged and tie constrains generated between plug surfaces and

global model. To avoid incorrect results in the thick coated

model, a 20 µm gap was placed between solder and conformal

coating as shown in Figure 13 to avoid potential over

constraining of the solder joint during thermal cycling.

Figure 13. FE model structure with conformal coating

5.1 Pb-free BGA

Figure 14 shows the Von Mises stress contour plot for the

corner and die shadow joints at the beginning of the hot dwell

period. Stresses do not correspond to the directionality of

the load but imply that the location of maximum stress is

found at the solder/copper pad interface along with a slight

Figure 14. Von Mises stress distribution in SAC305 BGA

with thick conformal coating at 125°C.

Material Elastic

Modulus

(GPa)

Poisson’s

ratio

CTE

(ppm/ºc)

PCB 33.9 0.13 17

Substrate 28 0.13 13

Copper 118.5 0.326 16.7

Silicon 130 0.28 2.6

SAC305 20

63Sn37Pb 24



distance to neutral point effect at the corner joint which is not

presented in joints along the die shadow and is characteristic of

predominantly axial loading. The location of maximum stress

changes from the upper to lower interface during high and low

dwell periods, respectively. To illustrate the effect of the thick

conformal coating on the stress-strain state of the solder, a 25

µm layer of elements directly above the board copper pad is

volume averaged. Figure 15 illustrates the average maximum

principal strain for the Pb-free BGA for temperature profile 1

with and without conformal coating. largest average strain

value in BGA solder joints occurs at the center and graduate

decreases toward the solder/copper interface while average

Figure 15. Maximum principal strain with time and

temperature with and without conformal coating for Pb-free

BGA under temperature profile 1.

stress values are largest at the interface and decrease toward the

center of the joint. Since solder displacement is driven by

thermal expansion of the conformal coating, solder joints are

placed under displacement control loading conditions. With the

thick conformal coating, larger axial strains accumulate during

cold temperature dwell. This values occurs at a point at which

the acrylic material still maintains rigidity with high modulus.

More damage is accumulating at the start of the glass transition

temperature region rather than after the glass transition. At the

onset of the transition region, the elastic modulus of the acrylic

has not sufficiently decreased to allow even for a small increase

in the CTE.

To illustrate the difference in compressive loading caused

by conformal coating contraction during cold temperature

dwell the average axial strains are compared. Figure 16

illustrated the axial strains with time for the Pb-free BGA with

and without conformal coating for the same layer of elements.

It can be seen that the first high temperature dwell, both the

control and thick coated joints reach an equivalent state;

however, thick coated joints reach a much larger compressive

strain than the control. This cause a shift from positive to

negative axial mean strain which correlates to the failure mode

observed in cross-sectional analysis.

At the start of the subsequent thermal cycle, the strain state

at the interface of the Pb-free solder joint is under compressive

loading. The existence of a compressive preload has been

previously shown to contribute to larger accumulation of

plastic work per thermal cycle [10]. In this experimental

condition, the compressive preload occurs at the cold

temperature dwell and continues up to the glass temperature at

which the acrylic material softens. A similar trend is observed

at both temperature profiles with higher strain obtained for

profile with higher temperature extremes.

Figure 16. Axial stress distribution with time for Pb-free

corner BGA under temperature profile 1 with and without

conformal coating.

5.2 SnPb BGA

The stress-strain behavior of the SnPb solder joints at the

interface is similar to that observed in Pb-free components only

with noticale difference in magnitude. Figure 17 illustrates the

maximum principal strains at the corner SnPb BGA with and

without conformal coating at the high temperature dwell.

Figure 17. SnPb Max. principal strain at 125°C for the corner

BGA (a) with conformal coating (b) without conformal

coating.

Temperature

No coat

With coat

No coat

with coat



The strain distribution confirms the dominance of distance to

neutral point effect in SnPb components. Figure 18 illustrates

the same SnPb components during the cold temperature dwell

in which a noticeable difference is observed due to the thick

conformal coating.

Figure 18. SnPb BGA Min. principal strains at -55°C for corner

joint (a) with conformal coating (b) without conformal coating.

Larger strains are concentrated along the diagonal of the joint

with thick coating compared to a more uniform distribution of

the control component. This simulation assists in correlating

the failure model observed in cross-sectional analysis to the

temperature dependent material behavior of the acrylic coating.

6. Conclusions

This study resulted in identifying the effect of conformal

coating materials on solder joint fatigue life in Pb-free and

SnPb BGA packages. Experimental testing of acrylic

conformal coating materials with various temperature

dependent CTE and E exhibited failure modes ranging from

fatigue to overstress in Pb-free and SnPb solder joints. Finite

element simulation proved to be correlate well with associated

stress-strain state to the observed failure mechanism. An

accurate characterization of the conformal coating temperature

dependent properties has shown that the glass transition

temperature of the conformal coatings is a critical factor

affecting fatigue life. Thermal cycling profiles which crosses

the glass transition temperature of the material proven to be

more damaging than the temperature range with thick

conformal coating application. SnPb BGA components have

proven to be more robust to acrylic conformal coating under

both temperature extremes and application method. This results

is supported by the mechanical behavior inherent to SnPb

solder and the package type used in this study. Additional

experimentation is required to fully investigate the influence of

conformal coating BGA components by altering the conformal

coating materials and package type along with thermal cycling

conditions.

Acknowledgments

The authors would like to thank Gil Sharon for his fruitful

discussion of the modeling results as well as technical staff of

DfR Solutions and Rockwell Collins for assisting with sample

preparation, test setup and material characterization.

References

1. Qi, Haiyu, Michael Osterman, and Michael Pecht. "Design

of experiments for board-level solder joint reliability of

PBGA package under various manufacturing and multiple

environmental loading conditions." Electronics Packaging

Manufacturing, IEEE Transactions on 32.1 (2009): 32-40.

2. Woodrow, Tom and Ledbury, Eugene, "Evaluation of

Conformal Coatings as a Tin Whisker Mitigation

Strategy", IPC/JEDEC 8th International Conference on

Lead-Free Electronic Components and Assemblies, San

Jose, CA, April 18-20, 2005

3. Kokko, Kati, Laura Frisk, and Pekka Heino. "Thermal

cycling of flip chips on FR-4 and PI substrates with

parylene C coating." Soldering & Surface Mount

Technology 22.3 (2010): 42-48.

4. Norman, Jacov, Shlomo Segman, Pei-haw Tsao, and

Arkady Voloshin. "Effect of Conformal Coating on Low

Temperature Induced Strain in Electronic

Packages."Proceedings of the 1992 Joint ASME/JSME

Conference on Electronic Packaging 2 (1992): 797-802.

5. Tong, Ho-Ming, et al. "Effects of parylene coating on the

thermal fatigue life of solder joints in ceramic

packages." Components, Hybrids, and Manufacturing

Technology, IEEE Transactions on 16.5 (1993): 571-576.

6. Vianco, Paul, and Michael K. Neilsen. "Thermal

Mechanical Fatigue of a 56 I/O Plastic Quad-Flat Nolead

(PQFN) Package." SMTA International Conference

Proceedings (2015): 85-94.

7. Wilcoxon, Ross, Dave Hillman, Doug Pauls, and Dan

White. "The Impact of Improper Conformal Coating

Processes on BGA Solder Joint Integrity." SMTA

International Conference Proceedings (2015): 870-882

8. Schubert, A., et al. "Fatigue life models for SnAgCu and

SnPb solder joints evaluated by experiments and

simulation." Electronic Components and Technology

Conference, 2003. Proceedings. 53rd. IEEE, 2003.

9. Fan, Xuejun, Min Pei, and Pardeep K. Bhatti. "Effect of

finite element modeling techniques on solder joint fatigue

life prediction of flip-chip BGA packages." 56th

Electronic Components and Technology Conference 2006.

IEEE, 2006.

10. Yu, Da, Hohyung Lee, and Seungbae Park. "Reliability

assessment of preloaded solder joint under thermal

cycling." Journal of Electronic Packaging 134.4 (2012):

041008.

Documents

The Effect of Improper Conformal Coating on SnPb and Pb