Microelectronics Reliability 53 (2013) 755–760

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Microelectronics Reliability

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Whiskering behaviour of immersion tin surface coating

Balázs Illés a,⇑, Barbara Horváth b,a

a Department of Electronics Technology, Budapest University of Technology and Economics, Budapest, Hungaryb National Institute for Materials Science, Tsukuba, Ibaraki, Japan

a r t i c l e i n f o

Article history:Received 7 September 2012Received in revised form 11 January 2013Accepted 4 February 2013Available online 28 February 2013

0026-2714/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.microrel.2013.02.001

⇑ Corresponding author.E-mail address: billes@ett.bme.hu (B. Illés).

a b s t r a c t

In this paper the whiskering behaviour of immersion tin surface coating was studied in different environ-ments. 2 lm thick immersion tin layer on copper substrate has been tested. Five different environmentalconditions have been applied: a reference (25 �C/50% RH), two elevated temperature tests (50 �C/15% RHand 105 �C/15% RH) and two elevated temperature and humidity tests (40 �C/92% RH and 105 �C/100%RH). The whisker growth was studied by using Scanning Electron Microscopy (SEM). It was observed thatthe immersion tin layer was capable of growing tin whiskers. Most of the detected whiskers were the socalled ‘‘nodule’’ type whiskers, approximately 3–9 lm long with 1–2 lm thickness. The structure of thewhiskers and the tin layer underneath were examined with Focused Ion Beam (FIB) and TransmissionElectron Microscope (TEM) equipped with Energy-dispersive X-ray spectroscopy (EDX) and X-Ray Dif-fraction (XRD) unit. It was found that the temperature induced intermetallic (IMC) layer growth wasthe main stress factor causing the whiskering of the immersion tin coating. In addition under the devel-oped whiskers the IMC layer was found to be uneven. The observed whisker grew from a grain which hasa preferred [01�2] grain orientation for large grains of an immersion tin layer.

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1. Introduction

Tin whiskers are spontaneously growing protrusions from tincoating of few microns in diameter and up to hundreds of micronsin length. Whiskers cause high reliability risks in microelectronicsdue to the possibility of short forming between the pins of finepitch components. Transition to lead-free electronics resulted inusing of immersion tin for surface coating on the copper wiringof printed wiring boards (PWBs). Previously tin–lead alloys wereapplied for surface coating due to their good wetting propertiesand oxidation resistance. In addition, lead was found to be highlyeffective for preventing tin whisker growth.

Current whisker theory postulates that compressive mechanicalstresses cause the whisker growth, such as residual stresses fromthe plating process, mechanically induced stresses; stresses byintermetallic and/or oxide layer growth, and thermal stresses [1].Residual stresses inside the tin plating are caused by factors likethe plating chemistry and technology. The main properties of theplated tin that affect the whiskering ability are: the plating thick-ness and the grain size. Thicker platings require more time forthe intermetallic to migrate up through the grain boundaries creat-ing the full compressive stress cell. It is recommended that the tin-plating thickness should be at least 8 lm to reduce the propensityfor tin whisker growth and to provide a greater incubation time [2].

ll rights reserved.

In the case of smaller grains there are more grain boundaries forgrain boundary diffusion and related growth of intermetallicsand oxides [3], so in this way more stress can be originated fromthese sources.

In the case of immersion tin coatings, the layer thickness is usu-ally between 0.5 and 2.5 lm, thus the relaxation ability of the layeragainst mechanical stresses is very low. However, the grain size ofthe immersion tin layer is relatively large (compared to the grainsize created by electroplating). It is on the same order of magnitudeas the layer thickness. Therefore the probability of grain boundarydiffusion is less than in the case of electroplated tin layers. On theother hand, the amount of the tin in the case of immersion tin lay-ers is much lower than in the case of common electroplated tin lay-ers. However, it has been proved that a relatively thin (�3 lm)electroplated tin layer can also produce whiskers [4]. In addition,some amount of tin transforms into copper–tin intermetallics soonafter the immersion coating process [5]. Consequently, the whis-kering property of the immersion tin layer is very questionablewith this combination of the layer properties.

It is widely thought that the immersion tin coatings are totallywhisker-free as was advertised during the initial lead-free transac-tion step. Nevertheless this is not true; immersion tin coatings canalso develop whiskers [6]. However, the whiskering intensity isprobably less than in the case of electroplated tin coatings. Thereare only limited numbers of examples in the literature for theexamination of the whiskering behaviour of immersion tin layers.In most of the existing publications, the whisker phenomenon isonly a ‘‘side issue’’ of the layer characterisation [7].

756 B. Illés, B. Horváth / Microelectronics Reliability 53 (2013) 755–760

Gorbunova and Glazunova [8] reported that the practice ofdepositing chemical (immersion) tin coating onto a prior coatingof tin–lead alloy is extremely prone to whisker formations andthe whiskers reach the lengths that are observed on galvanicdeposits. Chen et al. [5] have observed tin whiskers on immersiontin coatings around holes of the circuit boards after 15 days ofroom temperature storage and they have found that the drivingforce of the whiskering is the Sn–Cu IMC layer formation. Theyhave also proved that thicker immersion tin finish produces moreand longer whiskers. Although thicker immersion tin layer can pre-serve solderability for longer time, however the risk of whiskeringis also larger. In the case of immersion tin coatings, Lamprecht andHutchinson [9] have found that the intermetallic layer growth pro-duces internal compressive stress in the boundary tin layer. If thisstress is not relieved, the potential of whisker growth exists.

Whiskering can be accelerated with elevated environmentalconditions. Whisker growth has been studied under dry conditions,such as 50 �C/50% RH (Relative Humidity) [10] or 50–105 �C/15%RH [11] and high humidity conditions such as 60–85 �C/85–95%RH [12,13] or 105 �C/100% RH [14]. The main difference betweenthe dry and high humidity conditions is the level of corrosion onthe tin coating. According to the whisker literature, presence ofthe tin oxide layer is necessary for whisker growth, due to theblocking effect of the oxide layer against the relaxation of mechan-ical stresses within the tin coating [5,14]. In high humidity condi-tion the corrosion of the tin layer is much more rapid andaggressive than in the case of low humidity condition. The local-ized corrosion can produce further mechanical stress against thetin coating, which can also trigger the whisker growth [14].

Besides the results of the limited amount of researches in thistopic, there are a lot of unclear questions about tin whiskers grownfrom immersion tin layers. The aims of this research were the fol-lowing: investigate the whiskering behaviour of immersion tincoatings in various dry and high humidity conditions; study the ef-fect of corrosion and high temperature on the developed whiskers;find relationship between the grain orientation and the whiskeringof the immersion tin layer.

2. Experimental procedures

Immersion tin coating was deposited on 30 lm thick copperpads by replacement reaction. The copper pads were positionedon FR4 substrates with 10 � 10 mm2 dimensions. The substrateswere immersed into acidic etching solution for 40 s to remove oxi-des and contaminations; then they were washed with 40 �C warmdistilled water. The components of the immersion bath were thefollowing: methane sulfonic acid (500 g/L), thiourea (120 g/L) (ascomplexation) sodium hypophosphite (30 g/L) (to promote thereduction reaction), and tin methane sulfonate (25 g/L) (for sourceof Sn2+). The deposition time was 15 min, and the bath tempera-ture was 60 �C. After the coating process, the substrate was washedwith 40 �C warm distilled water and then dried in room tempera-ture. Before the tests, the samples were stored in vacuum foil.

Five different test circumstances have been applied to acceler-ate the whisker appearance and investigate the effect of the differ-ent temperature and humidity levels on the formation of whiskers:

1. R: room environment, 25 �C/50% RH2. D1: dry test 1, 50 �C/15% RH3. D2: dry test 2, 105 �C/15% RH4. W1: wet test 1,40 �C/92% RH5. W2: wet test 2, 105 �C/100% RH

Five samples were used in each test, so the total number of testsamples was 25. The tests were 350 h long. In the case of the

elevated tests, the tin coating was consumed by the intermetallic(IMC) layer growth after 350 h.

The samples were checked before the tests and then after every50 h from the beginning with a FEI Inspect S50 Scanning ElectronMicroscope (SEM), (applied Acc. Voltage 20 kV). During the statis-tical evaluations, the axial length of a whisker was measured be-tween the surface and the tip of the whisker (according to theJESD201 Standard). The measurement accuracy for the whiskerlength was about ±5%. It mainly depends on the magnification ratioof the given micrograph. The same 2100� magnification was ap-plied for the whisker density calculations. The unit for the whiskerdensity is whiskers/2500 lm2. The average whisker length anddensity was calculated by lognormal distribution from 20 mea-surement results from each test panel, so all statistical results isdetermined by 100 measurement points.

During metallurgical evaluation the cross-sections of the layersand whiskers were developed using a JEM-9320FIB Focused IonBeam (FIB) (with a Ga Ion Source and Acc. Voltage of 30 kV) andobserved with a FIB Scanning Ion Microscope Image (FIB–SIM).The samples were coated with approximately 50 nm carbon layerfor surface protection before the FIB examination. The cross-sec-tional images were observed with a 45� tilt angle. The cross-sectionof the whisker and the layer underneath were also observed by aJEM-2100F-2 200 kV Field Emission Gun Transmission ElectronMicroscope (TEM) and by Energy-dispersive X-ray spectroscopy(EDX) analysis in order to identify the layer elements and interme-tallic types. The electron diffraction maps (crystal orientationstudy) were also performed on the X-Ray Diffraction (XRD) unitof the JEM-2100F-2 TEM.

3. Results

The layer structure of the samples was examined before thebeginning of tests by cross sectioning. It was found that the tincoating was continuously developed and its thickness was�2 lm on average. However during the SEM analysis of thecross-sections, it was observed that a high amount (400–500 nm,20–25% in thickness ratio) of the tin layer was consumed by IMCformation. According to the literature, the main compound of thisIMC layer is Cu6Sn5 [5]. The significant IMC formation – directlyfrom the beginning of the immersion tin deposition – generatescompression force in the tin layer which can induce whiskergrowth. The surface of the samples was also examined by SEM,7 days after the deposition directly before the environment tests.Interestingly, some whiskers were found on the samples with nod-ule type morphology. Their size was typically 1 lm in diameterand 3–5 lm in length (Fig. 1). This rapid whisker formation atroom temperature has also observed by Chen et al. [5].

During the evaluation of the statistical results the time scale ofthe tests was divided into three periods according to the change ofthe whisker densities: Part 1 (P1): 0–150 h; Part 2 (P2): 150–250 hand Part 3 (P3): 250–350 h. Fig. 2 presents the average whiskerdensities measured on the samples. In Fig. 2 (and also in Fig. 3)the ‘‘AVD’’ means the Average Deviation, what is the average valueof the standard deviation of the measured points in a given curve.In period P1 the number of newly developed whiskers was verylow in all test conditions. In period P2 moderate whiskering (2–2.5 pcs./2500 lm2) has started in the case of D2 and W2 testswhere the temperature was above 100 �C. This tendency grew inperiod P3, where the number of whiskers reached 8–10 pieces/2500 lm2. In the case of the W2 test (105 �C/100% RH), the densityeven reached 11 pcs./2500 lm2. Contrarily in the case of R, D1 andW1 tests where the temperature was below 50 �C, the number ofwhiskers changed slightly during the tests, the values were be-tween 1 and 1.8 pcs./2500 lm2.

Table 1IMC layer and tin coating thickness ratios.

Sample type IMC layer/Sn coating (%)

After deposition 20/80R (350 h) 35/65D1 (350 h) 45/55D2 (350 h) 95/5W1 (350 h) 40/60W2 (350 h) 95/5

2 mµ

Fig. 1. Whisker on the immersion tin coating, 7 days after the deposition.

0 50 100 150 200 250 300 3500

1

2

3

4

5

6

7

8

9

10

11

Time [h]

Aver

age

whi

sker

den

sity

[pcs

./250

0m

]µ R (AVD 0.3)±

D1 (AVD 0.4)±(AVD )D2 2.1±

W1 (AVD 0.2)±W2 (AVD )±2.6

P1

P2 P3

Fig. 2. Average whisker densities.

0 50 100 150 200 250 300 350

4,0

4,5

5,0

5,5

6,0

6,5

7,0

7,5

8,0

Time [h]

R (AVD 1.6)±D1 (AVD 1.7)±

(AVD )D2 1.2±W1 (AVD 0.9)±W2 (AVD )±02.2

Aver

age

whi

sker

leng

th [

m]

µ

P1

P2 P3

Fig. 3. Average whisker lengths.

B. Illés, B. Horváth / Microelectronics Reliability 53 (2013) 755–760 757

Fig. 3 shows the average whisker lengths measured on the sam-ples. In period P1, in the case of lower temperature tests (R, D1,W1), significant length increase was observed on the whisker

which had formed directly after the immersion tin deposition.However, these length increases have saturated at the end of P1(at 150 h) and only 0.5–1 lm further increase was observed in per-iod P2 and P3. The maximum length of the detected whiskers was9.1, 9.7 and 6.8 lm in the case of tests R, D1 and W1, respectively.Contrarily, in the case of D2 and W2 tests where the temperaturewas above 100 �C, the length of the previously developed whiskerschanged slightly in period P1, where only 0.5–1 lm increase wasfound. However, on these samples a larger length increase hasstarted at the beginning of period P2, mainly in the case of theW2 test. Length results of the D2 test reached the average lengthlevel of the R, D1 and W1 tests at 350 h, till the average whiskerlengths of W2 test have exceeded it. The maximum length of thedetected whiskers was 8.8 and 11.9 lm in the case of tests D2and W2, respectively. The length increase had exponential satura-tion shape during the lower temperature tests and had the satura-tion at the end of period P1, while the length increase was linearduring the higher temperature tests and has only started at theend of period P1.

The diameter of the developed whisker was between 0.8 and2 lm however it has not shown any relationship with the differenttest conditions. Thickness ratio of the IMC layer and the tin coatingwas examined by SEM during the aging. It was observed that in thecase of the D2 and W2 of tests, the tin coating was almost com-pletely consumed by the IMC layer after 350 h (Table 1). In theother tests the increase was also significant. Therefore the testswere stopped at 350 h.

Only nodule type whiskers were found during the morphologi-cal evaluation of the whiskers. The typical diameter of the whiskerswas 0.8–1.2 lm. Fig. 4 shows the longest detected whisker with�15 lm length. In Fig. 5 another typical nodule whisker is pre-sented. The corrosion effects of test W1 and W2 were also exam-ined. Corrosion spots are localized change from the silver-coloredtin surface finish appearing as non-reflective dark spots in an opti-cal microscope, or as dark grey areas in case of Scanning ElectronMicroscope Backscattered Electron image mode (SEM–BSE). Onlysome trace of corrosion was observable on the samples. This wasprobably caused by the short test duration.

Metallurgical evaluations were done to understand the mecha-nism of whisker growth on immersion tin coatings and to comparethe metallurgical particularities with the observations in the caseof electroplated tin coatings. Firstly, cross-sections were preparedfrom the whiskers and the layer underneath by FIB. In Fig. 6across-section of a whisker can be seen. The whisker faced towardsthe ion beam, which destroyed most of the whisker body duringcross-sectioning. Consuming of the immersion tin layer by theIMC growth is also observable in this micrograph.

The cross-sections were also studied by higher resolution TEMin order to examine the grain structure and by EDX analysis in or-der to identify the layer elements and intermetallic compositions.In (Fig. 7a) the TEM micrograph of the sample (presented inFig. 6) can be seen with the element map of the layer (Fig. 7b).Compared to the state in Fig. 6, the TEM measurements have beencarried out 1 lm deeper in the layer. The grain size of the

5 mµ

Fig. 4. The longest detected whisker (D1 test, 50 h).

2 mµ

Fig. 5. A typical whisker on the immersion tin coating (D2 test, 50 h).

IMC layer

Cu base

Sn coating

Whisker

2 mµ

Fig. 6. Cross-section of a whisker (D2 test, 350 h).

758 B. Illés, B. Horváth / Microelectronics Reliability 53 (2013) 755–760

immersion tin layer is relatively large: 1–2 lm as was expected –sometimes it was larger than the thickness of the coating. Thewhisker can be defined according to the border of the tin coatingwhich is visible in (Fig. 7a). The IMC layer is uneven under thewhisker and a large IMC apex is wedged into the immersion tinlayer (Fig. 7b). During the analyses of the cross-sections, this phe-nomenon was usually observed however in different extent. Theaccurate composition of the IMC layer was measured at five points(marked with a–e in Fig. 7a). According to the results (Table 2) thecomposition of the IMC layer is mainly Cu6Sn5.

The grain orientation of the whisker and the tin layer under-neath was measured in ten points by TEM–XRD method in orderto study the relation between the whisker growth and the grainorientations in the tin layer. According to the measurements twograins were detected, the whisker grain (at measurement points1–7) and another grain next to the IMC apex (at measurementpoints 8–10). The orientation of the whisker grain is [01�2](Fig. 7c) while the orientation of the other grain next to the IMCapex is totally different [20�3] (Fig. 7d).

4. Discussion

By comparing the results of the different environmental tests, itwas concluded that the test temperature was the main factor thataffects the whiskering behaviour of the immersion tin layer. Theresults of the tests with the same (or similar) temperature levelhave showed similar tendencies despite of the very different rela-tive humidity levels. In our previous studies it was found that thecorrosive climate can also be the trigger of the whisker growth[14]. During this study no significant trace of corrosion was ob-served on the samples. Therefore, the corrosion of the tin layer asan effecting factor is excluded in the case of immersion tin coat-ings. The reason is probably that the corrosion did not have enoughtime to develop, the IMC formation has consumed the thin tin coat-ing much faster. In our previous study about underplated tin layers[14], significant corrosion traces was observed after 800 h and atleast 1500 h in 105 �C/100% RH condition and 4000 h in 40 �C/92% RH condition was necessary for corrosion induced whiskering.

The test temperature affects mainly on the growth of the IMClayer which is probably the main source of the mechanical stressesin the case of immersion tin layers. During the IMC layer growth,the copper atoms diffuse into the tin grains and form intermetalliccompounds within the grain boundaries. The mostly detected IMCcompound was Cu6Sn5 (Table 2) which has lower density (8.27 g/cm3) than copper (8.96 g/cm3). This causes volume expansion atthe IMC layer [18]. In the case of the immersion tin layers the effectof this volume expansion is more serious than in the case of elec-troplated tin layers. Hence the thickness ratio of the IMC layer inthe coating was 20% after the deposition and reached the 40–95%during the elevated tests (Table 1). In the case of the electroplatedtin layers this thickness ratio is usually under 10% after the depo-sition and does not reach 30% during the elevated environmentaltests (assumed a typical 10 lm thick coating).

The surface oxide layer of the tin coating (which always exists,only the thickness is different) blocks the volume expansion of thecoating. This generates compressive stresses in the tin coating to-wards the vertical direction of the Cu and Sn interface and thestress increases with the number of diffusing atoms. The weakerpoints of the surface oxide layer can relieve the internal compres-sive stress by driving tin atoms out of the tin coating as tin whis-kers [14]. The IMC layer growth proceeds much faster on highertemperatures (according to the Arrhenius equation) due to thehigher probability of two atoms collusion. This results in larger ki-netic energies of the atoms, which decrease the activation energyof the reaction. Therefore, at 105 �C temperature the IMC formationis much faster than at 50 �C or at room temperature. For this rea-

112

200

312020

312332

Z=[0 1 -2] Z=[2 0 -3]

Meas. 1-7 Meas. 8-10

2 mµ Cu

Sn

Border of IMC

•1•2

•3•4

•5•6

•7 •8

•9•10

(a) (b)

•a

Whisker

•c

•d•e Wedged

IMC apex

(c) (d)

Border of tin coating

Border oftin coating

•b

Fig. 7. (a) TEM micrograph of a whisker and the layer underneath; (b) EDX element map of the cross-section; (c) TEM–XRD image of measurement points 1–7; (d) TEM–XRDimage of measurement points 8–10.

Table 2Composition of the IMC layer.

Meas. Atom (%) Cu/Sn Composition

a 48.70/44.87 Cu6Sn5

b 48.68/47.76 Cu6Sn5

c 78.84/19.77 Cu3Snd 47.07/43.87 Cu6Sn5

e 51.79/43.98 Cu6Sn5

B. Illés, B. Horváth / Microelectronics Reliability 53 (2013) 755–760 759

son, the compressive stress developed in the immersion tin coatingis larger at higher temperatures and this explains the largeramount of longer whiskers (mainly in the case of W2 test). How-ever at elevated temperatures, generally about 2/3 of the absolutemelting temperature (T/Tmp P 0.7 or �85 �C), bulk diffusion beginsto dominate which results in a more regular and continuous inter-metallic layer than on lower temperature [19].

Nevertheless it was observed during the analyses of the whiskercross-sections that the IMC layer was usually more uneven underthe developed whiskers than at the surrounding area (like inFig. 7a). This observation is conventional in all whisker studies.At the uneven areas the active surface of the IMC layer – wherethe IMC grains can develop the compressive force against the tingrains – is larger than in the case of an even IMC layer. In additionthe geometric characteristic at the IMC layer can also concentratethe compressive stress [18]. These effects can produce larger com-pressive stress against the neighbouring tin grains than an evenIMC layer would underneath them. Similar results were presentedin the case of a thin (2–3 lm) electroplated tin coating by Kim et al.[15]; however Zhang et al. [20] have found that in the case ofimmersion tin layers the minimized whisker tendency is associ-ated with an even growth of IMC layer but the whisker growth pro-pensity cannot relate directly to the IMC surface roughness.Therefore our assumption is that this phenomenon could have

importance for the whisker growth from immersion tin coatings,however further investigations are necessary to prove it. In a thick-er electroplated tin coating such effect is less serious due to the lar-ger relaxation ability of the layer.

The focus of this study was also to compare the orientation ofthe whisker grain and the surrounding grains. Previously Choiet al. [16] has found that the orientation of the whisker grain[210] differs from the surrounding grains and also from the pre-ferred orientation of tin grains [321] in the electroplated coating.In our case it was observed that the orientation of the whiskergrain and the other surrounding grains was also different, sincethe orientation of the whisker grain was [01�2] while the orienta-tion of the surrounding grain was [20�3]. However according toKim et al. [17], this observed [01�2] whisker grain orientation isalso a preferred orientation of the tin grains in the immersioncoating.

The phenomenon that the amount and mainly the length of thewhiskers did not change in the case of D2 and W2 tests in period P1can be explained by the relaxation effect of the higher temperatureon the tin layer. As it was discussed in the introduction section, thethin immersion tin layer (0.5–2 lm) results in a very low relaxingability of the layer. However the higher temperature (at D2 and W2tests) could decrease the compressive stress by the IMC growth atthe beginning of the tests until around 150–200 h. In the case ofthe tests at a lower temperature (R, D1 and W1) the relaxation le-vel had to be much lower than the compressive stress level of theIMC growth in period P1. Therefore, the length of the whiskerswhich have developed after the deposition could increase,although the number of the whiskers did not grow during the P1phase in the case of R, D1 and W1 tests. In period P2 and P3 somefurther whiskers have also developed in the lower temperaturetests like in the higher temperature tests, but they could notchange the saturation characteristic of the length increase.

760 B. Illés, B. Horváth / Microelectronics Reliability 53 (2013) 755–760

5. Conclusions

The whiskering behaviour of immersion tin coating has beeninvestigated in different environmental conditions. It has been pro-ven that the immersion tin layer also has the ability to grow tinwhiskers. In ambient room temperature condition, the first whis-kers were detected after 7 days of the deposition process. Themajority of the whiskers and the longest whiskers were observedin the case of high temperature (>100 �C) tests. However at thebeginning of the tests the higher temperature had some relaxationeffect on the tin layer which could delay the whisker growth. Rel-ative humidity of the tests has not got effect on the whiskergrowth. The corrosion did not have enough time to develop, theIMC layer formation consumed the thin tin coating much faster.Therefore, it has been concluded that main compressive stress fac-tor for immersion tin coatings was the temperature induced IMClayer growth. In the case of the high temperature (>100 �C) teststhe consuming of the tin layer by the IMC layer reached the 95%.Under the developed whiskers the IMC layer was usually foundto be more uneven than at the surrounding area. This can producelarger compressive stress against the neighbouring tin grains thanan even IMC layer could produce under them. The orientation ofthe whisker grain and the neighbouring grains was found to be dif-ferent compared to each other. The observed whisker grew from agrain which has a preferred grain orientation [01�2] for the grainsin the immersion tin layer.

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