blister testing for adhesion strength measurement of kenny...

Kenny MahanDepartment of Mechanical Engineering,

University of Maryland,

College Park, MD 20742

David RosenDepartment of Mechanical Engineering,



Bongtae Han1Fellow ASME

Department of Mechanical Engineering,



e-mail: [email protected]

Blister Testing for AdhesionStrength Measurement ofPolymer Films Subjected toEnvironmental ConditionsA blister testing procedure combined with a numerical procedure can be used effectivelyfor characterizing the adhesion strength. A challenge arises when it is implemented aftersubjecting samples to various environmental conditions. The concept of pseudopropertyis introduced to cope with the problem associated with property changes during environ-mental testing. The pseudoproperty set is determined directly from a deflection versuspressure curve obtained from a typical blister test. A classical energy balance approachis followed to evaluate the energy release rate from the critical pressure and pseudoprop-erty set. The proposed approach is carried out for an epoxy/copper interface after sub-jecting samples to full moisture saturation and a high temperature storage condition. Inspite of significant change in property, the energy release rates are calculated accuratelywithout extra tests for property measurements. [DOI: 10.1115/1.4034454]

Keywords: blister test, adhesion strength, numerical method, environmental testing,polymers

1 Introduction

In microelectronics, there are many interfaces that cometogether to create a product. One such interface is found betweenflexible polymer films and rigid substrates. Whenever two materi-als are structurally bonded together, a potential reliability issue iscreated that must be monitored. If the adhesion strength at theinterface is weak, delamination can occur, leading to failure of thedevice. Therefore, it is imperative to assess the reliability of theinterface under the same conditions the device will be subjectedto in order to avoid failure under device operating conditions.

The reliability of the interface can be quantified by experimen-tally measuring the critical energy release rate of the interface.The energy release rate, G, is an indication of the adhesionstrength of the interface since it quantifies the energy required tocreate new surface area for interfacial crack propagation [1]. Acommon adhesion strength testing method that is effective for thistype of flexible/rigid interface is the blister test [2–11]. Previouschallenges with this technique involving creation of a well-defined blister area have been addressed, leading to more consist-ent blister test results [12].

Analytical solutions for the blister test exist when testing undertwo specials cases—(1) the bending dominant case where the filmthickness is much smaller than the blister deflection and (2) thestretching dominant case where the film is assumed to be a thinelastic membrane, the blister forms a spherical cap, and the deflec-tion is much smaller than the blister radius. For these cases, theenergy release rate is determined only from the maximum pres-sure obtained from the testing. For the cases where the approxima-tions are no longer valid, numerical methods must be used tocalculate the energy release rate, and thus, the material propertiesof the film must be known. However, challenges exist when thematerial properties of the film (Young’s modulus, E, and Pois-son’s ratio, �) are not known prior to testing.

It is well known that long-term exposure to high temperature andmoisture conditions, as well as the presence of moisture at the inter-face, can lead to a decrease in the adhesion strength of an interface[13–16]. To fully characterize the reliability of the interface, testingis typically performed at as-manufactured and environmentallydegraded conditions such as thermal aging, moisture absorption, etc.However, any degradation or change in the polymer during environ-mental conditioning can affect the structural properties, Young’smodulus, E, and Poisson’s ratio, �, of the polymer [17,18]. In addi-tion, polymers are known to have rate-dependent properties[19–21]. Due to the prescribed rate and/or the degradation fromenvironmental aging, the structural properties of the polymer canpotentially shift the deformation mode of the film to some mixtureof stretching and bending deformation modes, which adds to theuncertainty in using analytical solutions for long-term degradationtesting. Without knowledge of the appropriate testing domain and/orthe structural properties of the polymer at the time of testing, itbecomes difficult to determine the energy release rate accurately.

In this paper, a “pseudo” properties-based hybrid approach isintroduced to extend the applicability of blister testing to the caseswhere the appropriate testing domain and/or the structural propertiesare not available. A typical numerical procedure that takes a classi-cal energy balance approach is implemented to evaluate the energyrelease rate using the pseudoproperties determined from thedeflection–pressure data. The analytical solutions for the bendingdominant case and the stretching dominant case are reviewed first inSec. 2. A numerical procedure to calculate energy release rate isdescribed in Sec. 3. Section 4 describes how the pseudopropertiesare determined from the pressure–deflection curve and also provesthat the pseudoproperties uniquely determine the energy releasedrate. Section 5 describes the sample preparation, experimental setup,and results of testing and Sec. 6 concludes the paper.

2 Background: Review of Analytical Solutions

A schematic of a blister test sample with a predefined area isillustrated in Fig. 1. In a typical blister test experiment, pressureloading is applied through the hole in the substrate layer to loadthe film layer. Pressure is increased monotonically causing thefilm to inflate like a blister of radius a, until the film delaminates

1Corresponding author.Contributed by the Electronic and Photonic Packaging Division of ASME for

publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received March 3,2016; final manuscript received August 14, 2016; published online September 2,2016. Assoc. Editor: Satish Chaparala.

Journal of Electronic Packaging DECEMBER 2016, Vol. 138 / 041003-1Copyright VC 2016 by ASME

Downloaded From: http://electronicpackaging.asmedigitalcollection.asme.org/ on 09/04/2016 Terms of Use: http://www.asme.org/about-asme/terms-of-use

along the interface at a critical pressure, Pcrit, and critical blisterheight, wcrit.

Closed-form analytical solutions for the energy release rateexist for particular cases, such as when the polymer film is rela-tively stiff and under mainly bending loading [4] or very flexibleand under mainly stretching [6] loading.

For the bending dominant case, the film deforms mainly inbending and can be regarded as a flexible circular plate with abuilt-in edge constraint. Considering a small blister height (w �a), Timoshenko’s beam theory yields [4]

w ¼ 3Pa4 1� �2ð Þ16Et3

(1)

Then, by applying an energy balance method at the instant thefilm delaminates from the substrate, the energy release rate, G, isexpressed as

Gbend ¼3 1� �2ð Þa4

32Et3� P2Crit (2)

Similarly, an analytical expression exists for the stretchingdominant case. For this case, assumptions are made such that thefilm is a thin elastic membrane, the blister forms a spherical cap,and the deflection, w, is much smaller than the blister radius, a.Under these assumptions, the pressure–deflection relationshipis [6]

w ¼ 3Pa4 1� �ð Þ8Et

� �13

(3)

and then by applying an energy balance method, the energyrelease rate, G, is expressed as

Gstretching ¼5

4

3

8

� �1=3" #C1

P4Crita4 1� �ð ÞEt

� �13

(4)

where C1 is a numerical constant, 0.518, to accommodate the vol-ume difference of the spherical cap due to the clamped end condi-tion of the blister [12].

Alternately, the energy release rate can be expressed just by thecritical pressure, the critical blister deflection, and a constantvalue for both the bending dominant and stretching dominant ana-lytical expressions

Gbend ¼Pcritwcrit

2(5)

and

Gstretching ¼5C1

4Pcritwcrit (6)

For the cases of intermediate film thickness and/or large deflec-tion, a mixture model was proposed, where a correction factorwas prescribed to account for the presence of both modes [4,22].This approach is limited to only very specific material interfaces.

In general, numerical approaches can be employed for the caseswhere the above analytical solutions are not applicable. Section 3is devoted to the numerical procedure to determine the energyrelease rate.

3 Numerical Procedure to Calculate Energy ReleaseRate

A numerical model can be developed to recreate the stress/strain state in the film at the point of delamination. Then, by usinga simple energy balance method, the energy release rate, G, canbe extracted from the model. Although the numerical procedure isto be used for general cases, one of the two ideal cases (pure-stretching) described in Sec. 2 is also used to confirm the accuracyof the numerical procedure.

3.1 Finite Element Analysis (FEA) Model Setup. Due tothe simplistic geometry of the blister test, a two-dimensional (2D)axisymmetric model can be established using commercial FEAsoftware. Material properties of the film and substrate layer shouldalso be known and input into the model. Boundary conditions forthis model are shown in Fig. 2. In addition to the axisymmetricboundary conditions prescribed at the center, the ux displacementis constrained at the bottom corner of the substrate.

The model must consider large deformations during the analy-sis due to the nature of the test. The pressure is increased in stepto the critical pressure using a constant increment. The pressureand deflection data of the center of the blister are recorded at eachstep to form a numerical pressure versus deflection curve.

3.2 Energy Release Rate Calculation. To numerically eval-uate the energy release rate, G, the energy balance method isemployed at the critical moment where the blister delaminates.Two cases are examined: the moment before delamination whenthe blister radius is still a and the moment after an infinitesimaldelamination of Da when the blister radius is aþDa. The energybalance of interest is the difference between these two key statesat constant pressure, Pcrit, i.e.,

DW ¼ DW1 þ DW2 (7)

where the change in work input into the system, DW, is evaluatedas

DW ¼ PCritDV ¼ PCritðVjaþDa � VjaÞ (8)

where V is the volume of the blister.The energy dissipated when the crack propagates, DW1, is eval-

uated as

DW1 ¼ 2pGDa (9)

Fig. 1 Schematic illustration of a blister test

Fig. 2 Schematic of boundary conditions applied in axisym-metric 2D model of blister test

041003-2 / Vol. 138, DECEMBER 2016 Transactions of the ASME


and the change in energy stored in the film layer, DW2, is eval-uated as

DW2 ¼ð

rdejaþDa �ð

rdeja (10)

Combining Eqs. (8)–(10) into Eq. (7), the energy release rate canbe expressed as

G ¼Pcrit VjaþDa � Va

� �� W2

aþDa�W2a

� �2paDa

!(11)

To demonstrate this procedure, a polymer film with structuralproperties of E¼ 2.5 GPa and �¼ 0.4 was analyzed. For the pur-pose illustration, a thin blister (hbend¼ 100 lm) with a blisterradius of a¼ 5 mm was considered, which fell in the pure-stretching domain. The deflection versus pressure curve obtainedby the FEA model is plotted in Fig. 3. Equation (3) was also usedto plot the analytical stretching relationship and there is very goodagreement. The slightly overestimated deflection of the analyticalsolution is attributed to the fact that it neglects stress resultingfrom bending.

The stored energy in the film can be directly determined fromthe FEA results. For the volume calculation, a shell integrationmethod can be employed to evaluate the volume as

V ¼ 2pðaradius

0

rwðrÞdr (12)

where w(r) is the displacement of the film layer along the radius.For the stretching case, the blister profile from the FEA analysisbefore and after delamination is illustrated in Fig. 4 (Pcrit¼ 10 -psi). The blister profile is then imported into mathematical soft-ware (MATLAB) and the shell integration is carried out using atrapezoidal integration method to obtain the total blister volumefor both before, Va, and after, VaþDa, delamination.

The procedure continues to calculate the critical energy releaserate. Using the stress state at the critical pressure, the storedenergy, W2ja, is determined. After all variables are found, theenergy release rate is evaluated for a particular Da using Eq. (11).This procedure can be repeated over several Da step sizes andthen by taking the limit of G with respect to Da, the energy release

rate can be determined. By evaluating G as Da approaches zero,the error associated with the arbitrary step size is eliminated, thusallowing for an accurate determination of the energy release rateat the instance of delamination.

This step is shown in Fig. 5 for the pure stretching case, wherethe step size is 25 lm. According to the analytical Eq. (4),GAnalytical¼ 15.1 J/m2. Following the numerical procedure, theenergy release rate is evaluated as GStretch¼ 14.8 J/m2. As men-tioned earlier, the analytical solution slightly overestimates Gsince it neglects stress resulting from bending. It is clear from theresults that there is good agreement with the numerical methodand analytical approaches when within the appropriate domain.

4 Energy Release Rate With Pseudoproperties

Let us consider an intermediate case where the film thickness is250 lm. The deflection versus pressure curve is plotted for this

Fig. 3 Deflection versus pressure curve for an ideal stretchingcase; finite element method results and analytical solutions arecompared

Fig. 4 Blister profile before and after a delamination ofDa 5 75 lm obtained from the FEA analysis: the case of Fig. 3with Pcrit 5 10 psi

Fig. 5 G versus Da plot for numerically determined energyrelease rate for the case shown in Fig. 4

Journal of Electronic Packaging DECEMBER 2016, Vol. 138 / 041003-3


case against both analytical solutions in Fig. 6. It is clear that test-ing of this film falls outside either of the analytical solutions’domains. Since this case is no longer in the realm of the analyticalsolutions, numerical methods must be used to calculate the energyrelease rate, and thus, the material properties of the film must beknown. The concept of pseudomaterial properties is proposed tohandle the unknown material properties.

A load-blister height curve is obtained from a typical blisterexperiment. It is important to note that numerous sets of E and �can produce the same curve. This is illustrated in Fig. 7. Each ofthese curves was created by making an ad hoc assumption for �and using a regression analysis to establish the correspondingmodulus (this set is referred to as “pseudoproperty pair”) that pro-duced the same deflection versus pressure curve. The pseudoprop-erty pairs used for the analysis are shown in Table 1. It is clear

that each of the pseudoproperty pair (E, �) recreates the samecurve and accurately describes the critical state where delamina-tion occurs.

Using Pcrit¼ 25 psi, the energy release rate was calculated fromthe numerical procedure presented in Sec. 3. The resulting energyrelease rate for the reference case (E¼ 2.5 GPa and �¼ 0.4) wasG¼ 48.1 J/m2. The energy release rates for all pseudopropertypairs were also calculated and the values are summarized in Table1. It is clear from the table that any pseudoproperty pairs that aredetermined from the deflection versus pressure curve by makingan ad hoc assumption of E or � can be used to determine G withvirtually no effect on the energy release rate.

This finding is not intuitive but can be understood from Eqs. (5)and (6). This finding, i.e., G can be determined numerically with-out prior knowledge of the film’s material properties, enables theblister testing to accommodate any changes in the material proper-ties after environmental conditioning where the structural proper-ties of the polymer are subject to change and degrade betweensample sets.

The cases that were examined in this paper had a small changein Poisson’s ratio, which seems reasonable based on the dataavailable in the literature. Further investigations are recommendedif a case with dramatic changes in Poisson’s ratio is to be studied.

5 Application

Using the numerical procedure with the pseudoproperties, theadhesion strength was assessed for an interface of a commonepoxy used in electronic packages and a copper substrate.

5.1 Sample Preparation. For testing, a rigid copper substratewith 12 3-mm-diameter test holes (Fig. 8(a)) was used in thisapplication. Each copper substrate was subjected to successivegrinding using a rotational grinding machine set at 150 rpm. Thesubstrates were grinded using 240, 400, 600, and 800 grit paperfor 5 mins each. This process helped ensure a uniform surfaceroughness on the order of �6.5 lm while removing anyimpurities.

The radius dependency of the energy release rate is obvious.An accurate well-defined predefined area is critical to the samplerepeatability [12]. To establish the extent of the predefined area, astencil piece composed of flat sheet metal with 12 3/8 in. diameterholes was placed on top of the substrate so that the stencil holeswere concentric with the pressure holes. Using a residue-free tape,the stencil piece was taped along its edges to the substrate, as seenin Fig. 8(b). The exposed substrate areas became the predefinedareas for 12 blisters. The pressure holes were then temporarilyblocked with a residue-free tape. The stencil prevented the tapecontact outside the predefined area. This approach greatly reducedthe possibility of introducing impurities to the interface of interestduring plug creation.

A schematic of the remaining sample preparation procedure isshown in Fig. 9. Before creating the predefined area, the pressurehole must first be temporarily plugged. Silicon rubber was chosenfor the temporary plug because of its known low adhesion prop-erty, which ensured that it did not adhere to the tape, blister mate-rial, or copper. A commercially available silicon rubber (GERTV615A) was prepared according to the manufacturer’s specifi-cation and then spun in a centrifuge for 3–5 mins to remove anygas introduced by mixing. The compound was then carefully

Fig. 6 Deflection versus pressure curve for a blister test speci-men with an intermediate thickness

Fig. 7 Deflection versus pressure curve for intermediate casewith several pseudomaterial property pairs to fit experimentalcurve

Table 1 Consequence of ad hoc Poisson’s ratio selection

Poisson’sratio

Pseudo-E(GPa)

Energy releaserate (J/m2)

Percenterror (%)

0.35 2.6 48.3 0.40.4 2.5 48.1 0.00.45 2.4 48.1 �0.1



poured into each pressure hole, ensuring that no gas bubbles formin the process. Meanwhile, 12 1.5-mm-diameter dowel pins wereallowed to soak in a silicon primer solution (GE SS4120) for5 mins, then allowed to dry for 5 mins. By doing so, the dowelpins adhered to silicon rubber. A pin was carefully inserted intoeach pressure hole, serving as a convenient way to remove the sili-con plugs later in the process. The system was allowed to cure for24 h at room temperature.

After a full cure, the tape covering the pressure holes was care-fully removed. A small amount of methanol was applied with acotton swab inside the stencil to remove any possibility of con-tamination from the tape’s adhesive material. A thin coat of a dryfilm release agent (Sprayon MR311) was then applied inside thestencil, thus defining a low adhesion predefined area for each blis-ter specimen. The use of a wet release agent, such as silicon oil,proved to be unsuccessful due to transport and mixing issues withthe blister material.

To further establish the integrity of the predefined area, a �10-nm thick layer of aluminum was deposited on top of the dry filmrelease agent via the use of a vacuum chamber (Denton DV-502A). The completed predefined area is composed of the releaseagent and aluminum layers. While the release agent created thelow adhesion surface, the thin aluminum layer provided a clearboundary that prevented the epoxy from adhering directly to thepredefined area. The stencil was then removed from the substrate.

To create the film layer, a shim of 300 lm was first createdaround the edge of the substrate using the tape. The two-partepoxy was prepared according to the manufacturer’s recommen-dation, spun in the centrifuge for roughly 2 mins to remove anyvoids, poured onto the substrate, and allowed to spread evenly viagravity. The samples were then allowed to cure at room tempera-ture for 48 h. After the full cure was achieved, the silicon plugswere removed by pulling the dowel pins. Finally, the film layerwas sectioned into individual specimens on top of the substrate toensure that delamination from one specimen did not affect theothers during testing.

5.2 Experimental Setup and Testing Procedure. Duringtesting, the pressure, center blister deflection, and blister profile

were captured and integrated into a LABVIEW program to synchron-ize data collection. A schematic of the experimental setup isshown in Fig. 10. Once the specimen is secured over the pressureport, the three-axis translation stage can also be used to ensureaccurate alignment of the laser above the center of the blister tocapture the maximum deflection during each test.

The pressure regulator (Tescom ER3000) applied pressure at arate of 1 psi/s while the laser triangulation sensor (MTI MicroTrakII) and camera (Sentech MC133-USB) recorded the center blisterheight and blister profile, respectively. The pressure regulatorallowed for a resolution of 0.1 psi, and the laser displacement sen-sor had a resolution of 1.25 lm. For each specimen, the pressurewas increased monotonically while the blister continued to grow.Once the blister reached a critical pressure, the film delaminatedfrom the substrate. The critical delamination was captured both ina sudden increase in deflection at PCrit on the deflection versuspressure curve, and visually through the camera profile.

Several representative deflection versus pressure curves for theas-is case are shown in Fig. 11. An overhead image of the

Fig. 8 Copper substrates for blister test samples (a) beforeand (b) after attaching the stencil

Fig. 9 Schematic of sample preparation procedure: (a) creat-ing temporary plug, (b) creating the predefined area, (c) curingthe adhesive, and (d) the final sample ready for testing



predefined area of a specimen can be seen in Fig. 12(a). When thepressure reached a critical pressure, the film delaminated from thesubstrate and it was recorded with the camera (Fig. 12(b)). Thismoment of delamination captured in the video matched the criticalpressure recorded in the deflection versus pressure graph for thespecimen, which confirmed the validity of the experimental data.

Since the typical Poisson’s ratio for this type of polymer is 0.4,it was used as an ad hoc assumption for the pseudomaterial prop-erties. The 12 as-is samples tested were then modeled in FEA andwent through a nonlinear optimization method to determine a cor-responding pseudomodulus for each sample. For as-is samples,the average pseudomodulus was 3.58 GPa with a standard devia-tion of 1.12 GPa. The average energy release rate was 25.0 J/m2

with a standard deviation of 4.37 J/m2.

5.3 Results of Environmental Testing. The importance ofthis approach becomes more apparent when evaluating the effectof environmental conditions. In addition to as-is testing, sampleswere subjected to two environmental testing conditions: moisturedegradation and thermal aging. The moisture degradation sampleset consisted of six specimens and was subjecting to a 100% rela-tive humidity environment at room temperature for a duration of 7

days. The thermal aging sample set consisted of ten specimensand was subjected to a high temperature storage condition(125 �C) over a 7-day period.

Figure 13 shows a representative pressure versus deflectioncurve and Table 2 lists the statistical information for each sampleset tested. Additionally, the average and range of the energyrelease rate and pseudomodulus are shown in Figs. 14 and 15,respectively, for each sample set.

As expected, the deflection versus pressure curve showed areduction in the critical pressure for the moisture degraded case.Compared to the average as-is pseudomodulus, the average mois-ture degraded sample pseudomodulus stayed relatively constant at3.78 GPa. However, the consequence of subjecting samples to thismoisture environment was a 57% reduction in the energy releaserate from as-is values.

As seen in the deflection versus pressure curve for the thermalaging case, there was a drastic change in the film’s response to theblister test, which was an indication of changes to the structuralmaterial properties of the film layer. While the energy release rateincreased by 13% due to additional curing at the higher

Fig. 10 Schematic diagram of the blister test setup

Fig. 11 Representative deflection versus pressure curves foran epoxy/copper interface at as-is testing conditions

Fig. 12 A typical blister sample (a) before and (b) after criticaldelamination event



temperature, the average pseudomodulus of the thermally agedspecimen increased by over an order of magnitude to 46.1 GPa.

Of most importance is how this sudden change in polymerbehavior did not greatly affect the energy release rate of the

interface (Fig. 14). Failure to consider changes in the structuralmaterial properties of the film in energy release rate calculationswould result in erroneous data. In the thermally aged case if theoriginal as-is material properties were used to evaluate G thenthere would be a gross over-estimation of the energy release ratethat would not capture the actual adhesion strength of the inter-face. Misuse of environmentally aged data can lead to a misrepre-sentation of the actual adhesion strength of a critical interface—potentially leading to unintended failure.

6 Conclusion

An alternative numerical/experimental blister test procedurewas implemented to characterize the adhesion strength of an inter-face when the film’s structural properties were unknown. The pro-cedure was based on the fact that the energy release rate can becalculated from a set of modulus and Poisson’s ratio values thatcan be determined from the experimental pressure versus deflec-tion curve of the blister test. An updated specimen preparationmethod was presented and implemented, which accurately definedthe blister radius without adversely affecting the substrate surface.The proposed approach was carried out for an epoxy/copper inter-face after subjecting samples to full moisture saturation and ahigh temperature storage condition. Adhesion strength of the inter-face for as-is (G¼ 25.0 J/m2), thermally aged (G¼ 28.2 J/m2), andmoisture degraded (G¼ 10.7 J/m2) samples was presented. Theadhesion strength decreased significantly after moisture degrada-tion, while the thermal aging condition caused only a slightincrease of the adhesion strength in spite of a significant change inmodulus. This approach was proven most useful when investigat-ing the effect of environmental conditions on the adhesionstrength since the structural properties of a polymer film candegrade over time leading to added uncertainty in the originalproperties of the film.

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Fig. 14 Energy release rate comparison of three conditions:(a) as-is, (b) after thermal aging, and (c) after moisturedegradation

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blister testing for adhesion strength measurement of kenny...

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