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E. H. Wonge-mail: eehua@ime.org.sg

R. Rajoo

S. W. Koh

T. B. Lim

Institute of Microelectronics,11 Science Park Road,

Science Park II,Singapore 117685

The Mechanics and Impactof Hygroscopic Swellingof Polymeric Materialsin Electronic PackagingA reliable technique for characterizing the hygroscopic swelling of materials has beendeveloped and used to characterize a number of packaging materials. Using these data,hygroscopic stress modeling were performed. The hygroscopic stress induced throughmoisture conditioning was found to be significant compared to the thermal stress duringsolder reflow. Hygroscopic stress in over-molded wire bond PBGA and molded Flip ChipPBGA was found to be 1.3 times to 1.5 times that of thermal stress. Hygroscopic swellingof the underfill in FCPBGA was found to be the main failure driver during autoclave test.Autoclave performance of FCPBGA package assembled with different underfills and chipswere analyzed. Excellent correlation was found between autoclave performance and thehygroscopic swelling characteristics of the underfills.@DOI: 10.1115/1.1461367#

1 IntroductionPolymeric materials swell upon absorbing moisture. Differen-

tial swelling occurs between the polymeric and nonpolymeric ma-terials, as well as among the polymeric materials constituting theelectronic packages. This differential swelling induces hygro-scopic stress in the package that adds to the thermal stress at highreflow temperature, raising the susceptibility of package to pop-corn cracking@1–5#. This is depicted in Fig. 1. Despite this, hy-groscopic stress has been largely ignored in the analysis of pack-aging stress. This is attributed to the following hindrance:

• Lack of characterization technique• Lack of material hygroscopic swelling characteristics• Unfamiliarity with moisture diffusion and hygroscopic stress

modeling• Under estimation of the magnitude of hygroscopic stress.

These hindrances have been comprehensively resolved and pre-sented in detail in this paper. The significance of hygroscopicstress was demonstrated through:

• Hygrothermal stress analysis of three IC packages of variedconstruction, materials, and size subjected to moisture sensi-tivity test.

• Autoclave test analysis of a flip chip vehicle assembled withdifferent underfills and chips.

2 Hygroscopic Swelling Characterization TechniqueThe technique was developed on two standard thermal analysis

instruments, namely thermal mechanical analyzer~TMA ! andthermal gravitational analyzer~TGA!. The procedure involves:

• Moisture condition, and saturate if possible, two identicalspecimens under the same temperature and humidity for equalduration.

• Desorp moisture from the specimens isothermally in TMAand TGA, respectively; use the same ramp-up rate for the twoinstruments in order to maintain identical moisture distribution inthe specimens.

• Extract dimensional change (DL) and moisture weight loss(DM ) of the specimen at the same desorption interval

• Plot dimensional strain,«5DL/L, versus moisture concen-tration,C5DM /V, whereV is the volume of the specimen

The curve defines the relation between hygroscopic swelling andmoisture content~Fig. 2!. If a linear relation exists between thetwo then a constant of linearity, coefficient of moisture expansion~b!, can be defined mathematically as

b5«

C(1)

where« is the hygroscopic strain andC the moisture concentra-tion. The instantaneous moisture concentration, and hence hygro-scopic strain, will not be uniform throughout the specimen. How-ever, it can be easily verified~Appendix! that the use of uniformstrain («5DL/L) and moisture concentration (C5DM /V) in Eq.~1! is valid so long as~a! the moisture distribution in the twospecimens measured with TMA and TGA, respectively, are iden-tical throughout the experiment; and~b! if a linear relation existsbetween hygroscopic strain and moisture concentration. The coef-ficient of moisture expansion,b, characterized is therefore inde-pendent of specimen thickness. This characterization techniqueoffers the following features:

• Use of standard instruments—encourages and facilitates user-friendly and low-cost implementation

• Continuous measurement—complete and reliable data• High accuracy~microgram in weight and submicrometer in

dimension!—capable of handling small specimen• High temperature characterization—an important requirement

for hygroscopic stress modeling during solder reflow.

3 Hygroscopic Swelling Characteristics of PackagingMaterials

The hygroscopic swelling characteristics of a typical mold com-pound are presented in Fig. 3. Note the linear relation betweenhygroscopic swelling and moisture content, which holds true overa range of temperature. This is generally true for all IC packagingmaterials, including die attach, underfill, and BT substrate. It isthis linearity that endorses the use of a single physical parameter,b, to characterize hygroscopic swelling of materials as described

Contributed by the Electronic and Photonic Packaging Division for publication inthe JOURNAL OF ELECTRONIC PACKAGING. Manuscript received by the EPPD Oc-tober 5, 2000. Associate Editor: S. M. Heinrich.

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in Eq. ~1!. The coefficient of moisture expansion, CME~b!, ofcommon packaging materials have been characterized and pre-sented in Fig. 4. Observations:

1 CME of packaging materials differs significantly which is theorigin of hygroscopic mismatch and stress in IC packaging.

2 CME of packaging materials generally increases with tem-perature.

4 Moisture Diffusion and Hygro-Mechanical ModelingKnowledge of moisture distribution within the package is

needed for hygroscopic stress modeling. This can be best donethrough moisture diffusion modeling. Most commercial FE soft-ware is not equipped with moisture diffusion modeling capability.But with an appropriate thermal-moisture analogy, moisture diffu-sion can be modeled using the thermal diffusion function of thesoftware. The analogous technique for a homogeneous materialsystem@6# has recently been extended to a multi-material system@7# so as to enable modeling of advanced packages. The FEAimplementation scheme is presented in Table 1, where wetness isdefined asW5C/C sat,C is the moisture concentration,C sat isthe saturated moisture concentration, andD the moisture diffusioncoefficient. An example of FEA moisture diffusion modeling of aPBGA package is illustrated in Fig. 5.

With the knowledge of moisture distribution in the package, aswell as the hygroscopic swelling characteristic of the materials,hygroscopic stress in the package can be readily computedthrough hygro-mechanical modeling. A simple thermal-hygro

Fig. 1 Hygroscopic swelling raises package stress duringsolder reflow

Fig. 2 Setup and technique for characterization of hygro-scopic swelling property of material

Fig. 3 Linear relation between swelling and moisturecontent—demonstrated with a typical mold compound

Fig. 4 CME of packaging materials as a function oftemperature

Table 1 FEA thermal-moisture analogy for moisture diffusionmodeling of multi-material system

Fig. 5 Moisture distribution in PBGA package „quarter model …

after 168 hr at 85°C Õ85%RH

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analogy has been developed~Table 2! so that hygro-mechanicalmodeling can be performed using the thermo-mechanical functionof commercial FEA.

5 Significance of Hygroscopic StressThe significance of hygroscopic stress on the moisture sensitiv-

ity and autoclave performance of electronic packages has beencomprehensively studied.

Moisture Sensitivity Performance During Solder Reflow.The moisture sensitivity performance of three types of packages—MQFP 160, wire bonded PBGA, and flip chip PBGA—were ana-lyzed. The packages were subjected to moisture sensitivity test atJEDEC moisture sensitivity level 2 and 3 to induce failure. Thecritical sites where failure first initiated where identified throughdetail failure analysis~Fig. 6!; these are:

• MQFP: The corners of die pad to mold compound interface• Wire bonded PBGA: the Au-plated power and ground rings

with mold compound interface• Flip chip PBGA: the corners of die to underfill interface

The thermal and hygroscopic stresses at the critical sites ofthese packages have been evaluated and the in-plane shear stressis tabulated in Table 3. The hygroscopic stress has been normal-ized with thermal stress for relative comparison. Thermal stress

was evaluated from molding temperature 175°C to reflow tem-perature 220°C. Hygroscopic stress was evaluated after moistureconditioned at moisture sensitivity~M.S.! Level 2 ~85°C/60%RH/168 hr! and Level 3~30°C/60%RH/192 hr!, respectively. Note:

~a! Thermal stress is dominant in the leaded package with hy-groscopic stress constituting about 15% of the system~hygrother-mal! stress at M.S. Level 3 moisture precondition.

~b! Hygroscopic stress, on the other hand, is clearly dominantin the molded laminate substrate packages—30% to 50% higherthan thermal stress at M.S. Level 3 moisture precondition; andmore than double that of thermal stress when moisture precondi-tioned to M.S. Level 2.

Typical thermal and hygroscopic stress as well as deformationcontours of wire bonded PBGA package are depicted in Fig. 7. Itis important that hygroscopic stress be evaluated, together withthermal stress, when analyzing the moisture sensitivity perfor-mance of the packages.

Autoclave Performance of Flip Chip Assembly. The nor-malized autoclave test~121°C/100%RH! life of a flip chip PBGApackage with 3 different UBM~under bump metallization! sys-tems, assembled with and without underfill, is tabulated in Table 4@8#. While the presence of underfill can enhance moderately theautoclave life of the flip chip package for some UBM system, itcan do much more harm for other UBM systems. In the case of

Table 2 FEA thermal-hygro analogy for hygro-mechanicalmodeling

Fig. 6 Critical sites of three types of packages

Table 3 Normalized thermal stress versus hygroscopic stress„in-plane shear stress …

Fig. 7 Hygrothermal stresses and displacement contours ofwire bonded PBGA package during moisture sensitivity test

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UBM-B, the presence of underfill has drastically reduced the au-toclave life to 30% that without underfill. This has been attributedto the hygroscopic swelling of the underfill resulting in tensileloading on the UBM as well as the solder bump~Fig. 8!.

A quantitative comparison of the impacts of hygroscopic swell-ing of six underfills on flip chip package’s autoclave life is tabu-lated in Table 5. Note:

~a! The autoclave life correlates very well with the transversehygroscopic strain induced by the underfill—high hygroscopicstrain corresponds with low autoclave life. Both underfill C and Dwith the worse autoclave performance experienced the largest hy-groscopic swelling.

~b! Thermal transverse strain is compressive in nature and itsmagnitude is an order lower than that of the hygroscopic strain—hygroscopic strain, not thermal strain, is the dominant failuredriver

These experimental as well as analysis results underline the im-portance of underfill hygroscopic swelling on autoclave perfor-mance of flip chip assemblies.

6 Discussion

Mechanics of Hygroscopic Swelling. Hygroscopic swellingis similar to thermal expansion in that both are physical phenom-ena involving molecules equilibrating at a larger distance apart asa result of temperature increase and moisture absorption, respec-tively. In both cases, the magnitude of expansion is solely gov-erned and can be fully described by a single physical property—coefficient of thermal expansion and coefficient of moistureexpansion, respectively. This similarity between thermal expan-sion and hygroscopic swelling forms the basic for the hygro-mechanical modeling technique described in Section 3.

An alternative postulation based on solid mechanics has beenproposed@9#. Swelling was attributed to the vapor pressure withinthe microvoids of the polymer resisted only by the modulus of the

polymer. The vapor pressure increases by 550 times~from ther-modynamics! while the modulus of packaging materials generallydecrease by 10 times as temperature is raised from 30°–220°C.Hence, the solid mechanics approach would suggest a three tofour order of increase in swelling correspond to the raising oftemperature. Actual CME measurement, on the other hand,showed increase of less than an order~Fig. 4!.

Failure Mechanism in Autoclave Test. Underfill in flip chipassemblies undergoing an autoclave test plays both positive andnegative roles.

• Positive role: It lowers the corrosion rate by impeding mois-ture diffusion toward the UBM system, the critical site.

• Negative role: it raises the transverse stress acting to separatethe UBM through hygroscopic swelling.

It is clear from the test results that the negative role of underfillout-weighs its positive role in many instances.

The strong presence of hygroscopic swelling induced stress hasintroduced a new failure mechanism—stress corrosion cracking@10#; that is, cracking in the presence of stress and corrosionagents. This new failure mechanism deviates from the originalintent of the autoclave test@11#. Similar observation was alsoobserved in the 85°C/85%RH damp-heat test@12#. A review of thefeasibility of these tests for accelerated corrosion testing of flipchip assembly with underfill is suggested.

7 ConclusionsHygroscopic stress is a significant failure driver in IC packages

that has been ignored for too long. The impacts of hygroscopicstress have been convincingly demonstrated through:

~a! moisture sensitivity analysis of 3 packages—the magnitudeof hygroscopic stress in molded wire bonded PBGA and moldedflip chip PBGA was found to be significantly higher than that ofthermal stress.

~b! autoclave analysis of flip chip PBGA with 3 UBM systemsand 6 underfills—the autoclave life of the flip chip package can beseverely degraded by hygroscopic swelling of the underfill.

The procedures for hygroscopic stress modeling have beencomprehensively established and presented. These include hygro-scopic characterization technique, hygroscopic swelling character-istics of packaging materials, moisture diffusion and hygro-mechanical modeling techniques.

AcknowledgmentThis work is the result of a project initiated by the IME Elec-

tronic Packaging Research Consortium 1999, the members ofwhich were Advanced Micro Devices~Singapore! Pte Ltd, AgilentTechnologies Singapore Pte Ltd, Gul Technologies Singapore Ltd,

Table 5Table 4 Normalized autoclave performance of different UBM,with and without underfill

Fig. 8 Hygroscopic swelling induced tensile failure in auto-clave testing of flip chip PBGA package

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Hitachi Chemical Asia-Pacific Pte Ltd, Infineon Technologies~Asia Pacific! Pte Ltd, Institute of Microelectronics, Johnson Mat-they ~Singapore! Pte Ltd, Lucent Technologies MicroelectronicsPte Ltd, Philips Singapore Pte Ltd, STMicroelectronics Pte Ltd,ST Assembly Test Services Pte Ltd, Sumitomo Bakelite SingaporePte Ltd, and Micron Semiconductor Asia Pte Ltd.

Appendix: Proof of Independence of CharacterizedCME on Specimen Thickness

The following work proves that despite the nonuniformity ofinstantaneous moisture concentration and hygroscopic strain inthe test specimen, the coefficient of moisture expansion, CME,obtained using average moisture concentration and hygroscopicstrain reflects the true material property that is independent of thespecimen thickness.

• Test Specimen: For ease of illustration, though not necessary,the test specimen is assumed to have uniform cross-section withthicknessH

• Moisture distribution: Assumed instantaneous moisture con-centration along the thickness as CIF~z!

• Hygroscopic strain: Assumed hygroscopic strain as a un-known function of moisture concentration; i.e.,«5b* C, then in-stantaneous strain along thickness is given as«5b* f (z)

• In the characterization experiment, the coefficient of moistureexpansion~CME! was obtained using Eq.~1! based on averagemoisture concentration and hygroscopic strain

CME5«ave

Cave5

*0H«dz

*0HCdz

5*0

Hb* f ~z!dz

*0Hf ~z!dz

(A1)

For all packaging materials characterized in the experiments, thehygroscopic strain was found to be linearly proportional to themoisture concentration, Eq.~A1! becomes

CME5«ave

Cave5b (A2)

References@1# Tanaka, N., Kitano, M., Kumazawa, T., and Nishimura, A., 1997, ‘‘Evaluation

of Interface Delamination in IC Packages by Considering Swelling of theMolding Compound Due to Moisture Absorption,’’ 47th ECTC, pp. 84–90.

@2# Wong, E. H., Chan, K. C., Tee, T. Y., and Rajoo, R., 1999, ‘‘ComprehensiveTreatment of Moisture Induced Failure in IC Packaging,’’ 3rd IEMT, pp. 176–181.

@3# Nguyen, L. T., Chen, K. L., and Schaefer, J., 1995, ‘‘A New Criterion forPackage Integrity Under Solder Reflow Conditions,’’ 45th ECTC, pp. 478–490.

@4# Liu, S., and Mei, Y., 1995, ‘‘Behavior of Delaminated Plastic IC PackagesSubjected to Encapsulation Cooling, Moisture Absorption, and Wave Solder-ing,’’ IEEE CPMT Part A., Vol. 18, No. 3, pp. 634–645.

@5# Lin, T. Y., and Tay, A. O., 1997, ‘‘Dynamics of Moisture Diffusion, Hygro-thermal Stresses and Delamination in Plastic IC Packages,’’ ASME Advancesin Electronic Packaging, EEP-Vol. 19-1, pp. 1429–1436.

@6# Crank, J., and Park, G. S., 1956,The Mathematics of Diffusion, Oxford Uni-versity Press.

@7# Wong, E. H., Teo, Y. C., and Lim, T. B., 1998, ‘‘Moisture Diffusion and VaporPressure Modeling of IC Packaging,’’ 48th ECTC, pp. 1372–1378.

@8# Teo, P. S., Huang, Y. W., Tung, C. H., Marks, M. R., and Lim, T. B., 2000,‘‘Investigation of Under Bump Metallization Systems for Flip–Chip Assem-blies,’’ 50th ECTC, pp. 33–39.

@9# Tee, T. Y., Fan, X. J., and Lim, T. B., 1999, ‘‘Modeling of Whole Field VaporPressure During Reflow for Flip Chip BGA and Wire Bond PGA packages,’’Int. W/S on Electronic Materials & Packaging.

@10# Hertzberg, R. W., 1996,Deformation and Fracture Mechanics of EngineeringMaterials, Wiley, New York.

@11# Huang, Y. W., Teo, K. W., Chua, K. L., Yang, M. W. R., and Ferng, W., 1999,‘‘The Effects of Underfill on the Pressure Cooker Test Performance of FlipChip on Board Assembly,’’ Proc. InterPack, pp. 1121–1127.

@12# Caers, J. F. M., Oesterholt, R., Bressers, R. J. L., Mouthaan, T. J., and Verweij,J. F., 1999, ‘‘Reliability of Flip Chip on Board—First Order Model for theEffect on Contact Integrity of Moisture Penetration in the Underfill,’’ 48thECTC, pp. 867–871.

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