Phenolic Resin Surface Restructuring upon Exposure to Humid Air: A

E-mail: [email protected]; [email protected]. Fax: 734-647-4685., †. Nanjing University. , ‡. University of Michigan. , §. Dow Corning Corporation. ...
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J. Phys. Chem. B 2009, 113, 12944–12951

Phenolic Resin Surface Restructuring upon Exposure to Humid Air: A Sum Frequency Generation Vibrational Spectroscopic Study Xiaolin Lu,†,‡ Jianglong Han,† Nick Shephard,§ Susan Rhodes,§ Alex D. Martin,‡ Dawei Li,†,‡ Gi Xue,*,† and Zhan Chen*,‡ Department of Polymer Science, Nanjing UniVersity, Nanjing, People’s Republic of China, 210093, Department of Chemistry, UniVersity of Michigan, 930 North UniVersity AVenue, Ann Arbor, Michigan 48109, and Specialty Chemicals Business, Materials Science Technology Platform, Dow Corning Corporation, 2200 W. Salzburg Road, Midland, Michigan, 48686 ReceiVed: June 20, 2009; ReVised Manuscript ReceiVed: July 27, 2009

Epoxy and phenolic resins are extensively used for modern microelectronics, for example, as packaging materials. Humidity may greatly alter or degrade their function and application, leading to failure of the device. A nonlinear optical laser technique, sum frequency generation (SFG) vibrational spectroscopy, was used to investigate the molecular surface structures of the epoxy and phenolic resins after exposure to humid air. It was found that the adsorbed water molecules at the phenolic resin surface can induce substantial surface restructuring. The surface phenyl groups were reoriented closer to a perpendicular position to the surface after exposure to humid air from a more parallel position in air. Epoxide group surface restructuring was not observed. 1. Introduction Encapsulating materials are universally used to protect the microelectronic units inside. Upon curing in the packaging process, an encapsulating material positions and stabilizes a microelectronic unit onto the metal leadframe to form an electronic device with an independent function. However, for storage and transportation of the packaging materials a normal shelf life ranging from days to months before curing in the packaging process is needed. During this period, exposure to the atmospheric environment can lead to diffusion of water into the materials.1-3 Although the amount of the absorbed water may not be substantial, as will be shown later, negative effects on the stability and reliability of the electronic unit caused by such absorbed water should not be neglected. The temperature for reflow soldering after packaging has increased greatly because of the prohibition of the use of lead as soldering materials by RoHS.4 Therefore, to avoid delamination of the interface, the interfacial adhesion between the encapsulating material and the metal leadframe must be strong enough to resist the water vapor pressure during the reflow soldering process. In addition, the absorbed water can also reach the interface between the encapsulating material and the metal leadframe. The corrosion of the metal leadframe surface may occur as a result of the chemical reaction involving water at the interface. Both the water vapor pressure and the metal corrosion at the interface at elevated temperatures can destroy the interface between the encapsulating material and the metal leadframe finally leading to the failure of the electronic device. This may be a more pronounced problem with the shrinkage of modern microelectronics, when packaging materials are required to handle the high frequency, power, and operating environment requirements of global interconnect. * To whom correspondence should be addressed. E-mail: zhanc@ umich.edu; [email protected]. Fax: 734-647-4685. † Nanjing University. ‡ University of Michigan. § Dow Corning Corporation.

Extensive research has been performed to study the water absorption to the bulk of epoxy materials,5-16 which are widely used as a main component of packaging or encapsulating materials for microelectronic devices. Many different analytical techniques have been used in such research, including Fourier transform infrared spectroscopy (FTIR),5-11 differential scanning calorimetry (DSC),12,13 dielectric relaxation spectroscopy (DRS),8,14 dynamical mechanical analysis (DMA),14,15 and simulation.16 These studies indicate that at least two kinds of absorbed water molecules exist inside the epoxy bulk: free water molecules and water molecules hydrogen bonded to the polar groups, such as ether, amine, and hydroxyl groups. To the best of our knowledge, no systematic molecular level study has been performed to understand how epoxy or other resin (e.g., phenolic resin) surfaces/interfaces respond to humid air or water moisture. We believe that the studies on such surface/interface structures are important because the epoxy or phenolic resin surface responding to water moisture is the first step of the water diffusion into the material bulk. Also, the interfacial structure of packaging materials composed of epoxy and phenolic resins ultimately determines whether delamination or debonding will occur, leading to the failure of the device. This initial study investigates how water absorption can alter the surface structures of packaging materials. More detailed research about the water effect on the delamination of the buried interface between packaging materials and metal leadframes or dies will be carried out in the future. Recently, a second order nonlinear optical technique, sum frequency generation (SFG) vibrational spectroscopy, has been developed into a powerful tool to study molecular structures at surfaces and interfaces with a submonolayer specificity.17-61 Under the electric dipole approximation, the SFG process is forbidden for centro-symmetric materials, but is allowed at the surfaces and interfaces due to breaking of the inversion symmetry.17-60 Therefore, SFG can be used to selectively investigate molecular surface/interface structures. SFG has been successfully applied to study polymer surface structures and their

10.1021/jp9058092 CCC: $40.75  2009 American Chemical Society Published on Web 09/09/2009

Phenolic Resin Surface Restructuring

Figure 1. The chemical structures of the epoxy and phenol resins in this study.

restructuring behaviors in water.17,18,26,27 It has been found that various polymers can exhibit varied responses to water exposure: some polymer surfaces exhibit no surface restructurings, some can have reversible surface side chain reorientations, while others show irreversible surface changes.61 In the current study, we apply SFG to detect the surface structures of various epoxy and phenolic resins before and after exposure to water moisture or humid air. It was found that on the phenolic resin surface substantial restructuring occurred. Through spectral analysis it shows that the orientations of surface functional groups change. 2. Experimental Section 2.1. Materials. A dicyclopentadiene-modified epoxy resin (DCPD, EPICLON HP-7200) was obtained from DIC Corporation, Japan. A biphenyl-type epoxy resin (BPE, CER-3000 L) was acquired from Nippon Kayaku Corporation, Japan. A biphenyl-type phenolic resin (BPP, MEH-7851SS) was received from Meihwa Corporation, Japan. All these three materials were provided by Henkel. Molecular formulas of these materials are shown in Figure 1. Samples for SFG experiment were prepared to 1 wt % in chloroform. Fused silica windows used as substrates for the above materials were purchased from Esco Products, Inc. They were treated sequentially by a sulfuric acid bath saturated with potassium dichromate, a piranha solution bath (a mixed solution with 3:7 volume ratio of 30 wt % H2O2 solution and 98 wt % H2SO4), and air plasma to remove possible surface contamination. Epoxy and phenolic resin films were prepared by spin-coating the 1 wt % chloroform solutions onto the surface cleaned fused silica substrates. Film thickness was controlled by adjusting the spin speed. The FTIR samples were prepared by casting films of DCPD, BPE, and BPP onto potassium bromide substrates from their 5 wt % chloroform solutions. The prepared sample films on the substrates were annealed in an isotemp oven (Fisher Scientific, Inc.) at 90 °C for 1 h.62

J. Phys. Chem. B, Vol. 113, No. 39, 2009 12945 2.2. Ellipsometry, Contact Angle, and FTIR Characterizations. Film thickness of SFG samples was controlled around 50 nm, measured by an ellipsometer (EP3-SW imaging ellipsometer, Nanofilm Technologie, GmbH). Contact angle measurements were performed using a contact angle goniometer (CAM100, KSV Instruments Ltd.). Water and diiodomethane were used as polar and nonpolar solvents, respectively. Each contact angle was measured within 5 s after the liquid drop was placed on the surface. Each droplet of water was around 10 µL and each droplet of diiodomethane was around 2 µL. The built-in software CAM100 was used to capture the images of liquid droplets on the surface and to determine the contact angles. The transmission infrared spectra of the epoxy and phenolic resins were collected using a Fourier transform infrared spectrometer (Spectrum BX, PerkinElmer Inc.). 2.3. SFG Experiment and Data Analysis. SFG theory has been well developed and extensively published in the literature.17,21,22,58-61,63-70 The SFG setup and experimental geometry used in the current investigation have been reported in previous publications.26-38 Briefly, the visible and infrared (IR) input beams overlap spatially and temporally on the polymer surface with the face-down geometry (with the input laser beams going through the fused silica substrate and the resin film to reach the resin/air interface).26,27 The input angles for the visible and infrared beams are 60° and 54°, respectively, and the beam diameters are approximately 500 µm. The pulse energies of the visible and infrared beams were both approximately 80 µJ. SFG spectra were collected with the ssp (s-polarized sum frequency output, s-polarized visible input, and p-polarized IR input) and ppp polarization combinations. Both ssp and ppp SFG spectra were detected between 2700 and 3700 cm-1. The SFG output intensity in the reflection direction can be written as70

I(ω) )

8π3ω2 sec2 β |χeff(2) | 2I1(ω1) I2(ω2) AT c n1(ω) n1(ω1) n1(ω2) 3

(1) where n1(ωi) is the refractive index of the incident medium at frequency ωi, ω and β are the frequency and the reflection angle of the sum frequency field, respectively. I1(ω1) and I2(ω2) are the intensities of the two input fields with frequencies ω1 and ω2. T is the pulse-width of both input lasers. A is the overlapping cross section of the two input beams at the sample, and χeff(2) is the effective second-order nonlinear optical susceptibility. The different tensor components of χ(2) (χ(2) is the second-order nonlinear optical susceptibility defined in the lab-fixed coordination system) can be measured through collecting SFG spectra using certain different polarization combinations,70 as shown in eq 2 and 3. For example,

χeff,ssp(2) ) Lyy(ω) Lyy(ω1) Lzz(ω2) sin β2χyyz

(2)

(2) χeff,ppp ) -Lxx(ω) Lxx(ω1) Lzz(ω2) cos β cos β1 sin β2χxxz Lxx(ω) Lzz(ω1)Lxx(ω2) cos β sin β1 cos β2χxzx + Lzz(ω) Lxx(ω1) Lxx(ω2) sin β cos β1 cos β2χzxx + Lzz(ω) Lzz(ω1) Lzz(ω2) sin β sin β1 sin β2χzzz (3)

Here χyyz, χxxz, χxzx, χzxx, and χzzz are different components of χ(2) with the lab coordinates chosen such that z is along the interface normal and x is in the input laser incident plane. χeff,ssp(2)

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Figure 2. Humidity absorption curves. “Accelerated” refers to the absorption at 85 °C and a relative humidity (RH) of 85%. “Ambient” refers to the absorption at 22 °C and a RH of 40%.

and χeff,ppp(2) are the components of the effective second-order nonlinear optical susceptibility measured in the experiment by collecting the ssp and ppp SFG spectra. Lii values (i ) x, y, or z) are the Fresnel coefficients, and β1 and β2 are angles between the surface normal and the input visible beam, and the input IR beam, respectively. When the IR frequency is near the vibrational resonance, the effective second-order nonlinear susceptibility, using ssp polarization combination as an example, can be written as eq 4:

χeff.ssp(2) ) χNR +

A

∑ ω2 - ωqq + iΓq

(4)

q

χNR is the nonresonant background. Aq, ωq, and Γq are the strength, resonant frequency, and damping coefficient of the vibrational mode q. An SFG spectrum generated from a surface can then be fitted using eq 5 to obtain parameters of Aq, ωq, and Γq. C is a proportional factor.

|

I(ω) ) C χNR +

A

|

∑ ω2 - ωqq + iΓq 2 q

(5)

3. Results and Discussions 3.1. Water Absorption by the Epoxy and Phenolic Resins. A water absorption experiment was conducted for a packaging material formulation containing DCPD, BPE, and BPP samples in a typical “ambient” environment at 22 °C with 40% humidity and an “accelerated” environment at 85 °C with 85% humidity, for 10 weeks.71 Figure 2 shows the mass change of the formulated material as a function of time measured using a balance. Water absorption indeed occurred, evidenced by the mass increase in both the “ambient” and “accelerated” environments. The mass of the sample in the “accelerated” environment increased greatly over the first 10 days, then reached an equilibrium, with a total mass change of 0.23%. The mass of the sample in the “ambient” environment continued to increase even after 10 weeks. The final mass increase was about 0.06%. The mass change curves shown in Figure 2 are typical for the mass increase of a formulated encapsulating material due to the environmental moisture as a function of time. FTIR spectra of epoxy and phenolic resins were collected when prepared and exposed to humid air for 4 weeks. The resin samples were stored in an isolated glass chamber with a container of water inside the chamber at the room temperature. Comparable to the mass change data, all three resins, DCPD, BPE, and BPP, absorb water molecules after exposure to humid air, shown by the strong O-H stretching signals between 3000 and 3700 cm-1 in Figure 3. This broadband was hardly detected

Figure 3. FTIR spectra collected from epoxy and phenol resins before (lower spectrum in each panel) and after exposure to humid air for 4 weeks (upper spectrum in each panel): (top) DCPD; (middle) BPE; (bottom) BPP.

from the newly prepared resin samples, but the intensity increased greatly after 4 weeks’ exposure to humid air. The sharp vibrational signal centered around 3525 cm-1 for BPP before humid air exposure is due to the O-H stretching mode of the phenol O-H groups. Here both the mass change and FTIR results indicate that epoxy and phenolic resins absorb water. 3.2. Surface Structures of Epoxy and Phenolic Resins. From our previous SFG research, we have shown that SFG signals collected from the experimental geometry used in the current study are mainly contributed from the polymer/air interface, with almost no contribution from the resin/fused silica interface.25 Figure 4 displays SFG spectra of the two epoxy resins collected with the ssp and ppp polarization combinations in the frequency range from 2600 to 3600 cm-1. Strong ssp SFG resonant signals were detected from the DCPD surface in air, dominated by the contributions from C-H stretching vibrations centered at 2864 and 2918 cm-1. Weak resonant signals from aromatic C-H stretching modes of a phenyl ring at around 3000 and 3060 cm-1 were also present. However, no discernible SFG signal was detected in the ppp SFG spectra of DCPD. Since epoxide groups are less hydrophobic than cycloalkyl groups, and air is a hydrophobic medium, cyclo-alkyl groups in DCPD should be present on the surface in air, not the epoxide groups. Therefore, we believe that the ssp SFG signals from methylene groups should be contributed by those in cyclo-alkyl groups, not those from the epoxide groups. The

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Figure 4. The ssp and ppp spectra of DCPD and BPE surfaces.

bridged methylene group inside the cyclohexane ring can be quite ordered and protrude into the air, contributing strong ssp SFG signal. Different from the DCPD surface, no discernible ssp or ppp SFG signal was detected from the BPE surface in air (Figure 4). One of the possible reasons for the absence of SFG signal is because the surface is dominated by the ring structures, with the rings lying down on the surface. Similar to those of DCPD, epoxide groups of BPE like to bury into the material bulk since they have a higher surface free energy than the phenyl groups. SFG spectra have also been collected from the BPP surface in air using ssp and ppp polarization combinations, as shown in Figure 5a. In both spectra, SFG signal contributions from stretching modes of methylene, phenyl, and hydroxyl groups are observed. This indicates that these groups are present on the surface in air and are ordered. The peak assignments and the fitted parameters of these SFG spectra are listed in Table 1. The backbone methylene groups contribute a weak symmetric stretching peak at 2849 cm-1 and a strong peak at 2916 cm-1 in both the ssp and ppp spectra. The strong peak at 2916 cm-1 may come from the asymmetric stretching or/and Fermi resonance of the methylene groups.72 If the peak at 2916 cm-1 in the ssp spectrum is from the asymmetric stretching of methylene group, this asymmetric peak should be much stronger in the ppp spectrum than in the ssp spectrum.72 The experimental data does not support the assignment of asymmetric stretching so the peak at 2916 cm-1 should be from the Fermi resonance with almost no contribution from the asymmetric stretching of methylene groups. The peak at 2916 cm-1 in the ppp spectrum should be assigned to a combination of asymmetric stretching and Fermi resonance of the methylene groups. Since the connected biphenyl backbone has the inversion symmetry, under the electric dipole approximation, it should not contribute substantial SFG signals. Therefore, we believe that the phenyl C-H stretching vibrational peaks located between 3000 and 3100 cm-1 must originate from the end phenol groups. The vibrational peak at around 3540 cm-1 is from the phenol hydroxyl groups, as indicated by the FTIR studies (as shown in Figure 3).

Figure 5. The ssp and ppp spectra of BPP surface: (a) annealed at 90 °C for 1 h; (b) annealed at 90 °C for 1 h, then exposed to air for 1 h; (c) annealed at 90 °C for 1 h, then exposed to air for 1 h, and reannealed at 90 °C for 1 h.

3.3. Surface Restructuring after Adsorption of Water Molecules. The DCPD, BPE, and BPP samples were exposed to the humid air conditions (40% RH at 22 °C) for 1 h. The SFG spectra collected from the DCPD and BPE resins have almost no change in both ssp and ppp polarization combinations (not shown). This shows that the DCPD and BPE surfaces exhibit no structural change or surface structure change not detectable using SFG after exposure to humid air. The data suggests the water molecules mainly interact with the epoxide groups away from the surface. Such interactions do not induce the observable surface restructurings of the epoxy resins, at least in the current experimental time scale. SFG spectra collected from the BPP resin after exposure to humid air for 1 h, however, exhibited substantial changes in both ssp and ppp polarization combinations, indicating that large surface structural changes were induced by humid air (Figure 5a and Figure 5b). These spectra can completely recover after annealing the sample at 90 °C for 1 h, as shown in Figure 5c. This indicates that the surface restructuring of BPP in humid air is reversible. It has been shown that both the epoxy and phenolic resins are materials which can absorb water. Here we can show from their SFG spectra that their water absorption mechanisms may be quite different in the very first step at the resin surface. Since we have not observed surface structural changes for the epoxy resins while absorbing water, we can speculate that the water molecules in humid air may quickly associate with the possible buried epoxide groups and diffuse into the bulk, without substantially affecting the surface

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TABLE 1: Fitting Parameters for the SFG Spectra of the BPP Resina ωi (cm-1)

Γi

assignment

2836 2849 2909 2916 2965 2988 3003 3031 3039 3063 3520 3540

6 6 6 6 6 6 5 6 5 5 30 20

CH2 ss CH2 ss CH2 as or CH2 Fermi CH2 as or CH2 Fermi unassigned unassigned combination ν20b ν7b ν2 OH OH

a

annealed Aq (ssp)

annealed Aq (ppp)

14 ( 1

12 ( 1

35 ( 1

15 ( 1 9(1

16 ( 2 -21 ( 1 -8 ( 1 23 ( 1

-9 ( 1

75 ( 2

45 ( 2

exposed to humid air Aq (ssp)

exposed to humid air Aq (ppp)

13 ( 2

11 ( 2

31 ( 1

20 ( 1

15 ( 2 -13 ( 2 -7 ( 1 44 ( 1 117 ( 3

15 ( 2 -13 ( 1 12 ( 1 77 ( 2

Abbreviations: ss, symmetric stretching; as, anti-symmetric stretching; Fermi, Fermi resonance.

Figure 6. (a) Molecule-fixed coordinate system (a, b, c) of a parasubstituted phenyl group; (b) tilt angle defined as the angle between the c axis and the Z axis in the laboratory fixed coordinate system (X, Y, Z); (c) normal modes of ν2 and ν7b.

dominating hydrophobic methylene groups. This was not the case for the BPP phenolic resin, the surface of which exhibited marked changes. There are some distinct changes in the ssp SFG spectrum collected from the BPP resin surface after exposure to humid air. The intensity of the peak at 3063 cm-1 increased substantially. This peak can be assigned to a ν2 mode (or the total symmetric stretching mode (see Figure 6)) for the phenyl C-H stretching in the phenol group. The signal intensity increase of this peak in the ssp spectrum suggests that the phenyl ring stands up more on the surface or has a more ordered structure. The O-H stretching signal of the hydroxyl group in the phenol group shifted to a lower frequency (from 3540 to 3520 cm-1) and became broader as well. The adsorbed water molecules on the resin surface may more or less contribute to the O-H stretching signal, but this is unlikely for this particular O-H stretching peak at 3520 cm-1. Usually, the adsorbed water signal should be much broader, more typically between 3000 and 3700 cm-1. We suggest the O-H stretching signal peak shift should be mainly due to the hydrogen bonding between water molecules and the hydroxyl group in the phenol group, and the signal is still contributed by the hydroxyl group in the phenol group. It was also found that the peak center of methylene vibrational modes shifted. The methylene symmetric stretching signal changed from 2849 to 2836 cm-1 and the Fermi resonance/ asymmetric stretching from 2916 to 2909 cm-1. We propose that this is also due to the effect of the water adsorption on the surface. Clearly, the exposure to the humid air will induce changes of BPP surface structures. The spectral change of the methylene and phenol groups indicate the orientation and/or order changes of such groups on the surface. The adsorbed water molecules can also undergo hydrogen bonding with the phenol groups. However, such surface restructuring is reversible. After annealing the sample in air at 90 °C for 1 h, the SFG spectra showed

signal recovery. The hydrogen-bonding between the phenol and the water molecules was broken by the thermal energy. Some quantitative structural information of various functional groups such as phenol groups and methylene groups on the BPP surface before and after exposure to humid air can be deduced from SFG spectral fitting and spectral analysis. On the basis of the fitting parameters of the phenyl and methylene C-H stretching signals, we can evaluate phenol and methylene orientations on the surface. The hydroxyl equivalent weight (HEW, the amount of resin in gram containing one mole hydroxyl groups) of the BPP resin is 195. The “n” value (the number of repeating units in each molecule) in the BPP formula is thus calculated to be 1.3. This indicates that most phenol groups are chain end groups. To simplify the discussion, we assume that the hydroxyl group and the methylene group on an end phenol group are at para positions; therefore the end phenol group adopts a C2V symmetry, as shown in Figure 6. It has been shown that for phenol groups with a C2V symmetry, macroscopic second-order nonlinear susceptibility tensor components are related to the molecular hyperpolarizability tensor components through the following equation:40,73,74

|

| |(

χyyz,ν7b ) χyyz,ν2

)(

βcaa,ν7b 2(〈cos 3θ〉 - 〈cos θ〉) βaac,ν2 (7 + 2r)〈cos θ〉 + (1 - 2r)〈cos 3θ〉

)|

(6)

where βcaa,ν7b, βaac,ν2, and βccc,ν2 are the molecular hypolarizability tensor components for vibration modes ν7b and ν2, and r is the ratio of βccc,ν2/βaac,ν2 for the ν2 vibration; θ is the tilt angle defined as the angle between the surface normal and the principal axis of the C2V symmetry. We used the bond additivity approach to evaluate the values of βcaa,ν7b/βaac,ν2 and r values of approximately 0.47 and 0.69, respectively, which are similar to those reported in the literature.74 These values were used to determine the orientation angles of the phenol groups before and after exposure to humid air (water adsorption to the phenolic resin surface). It should be noted that the bracket indicates the averaged orientation angle, since the surface phenol groups may not adopt the same tilt angle (or a δ-distribution of the orientation angle). Generally, a Gaussian distribution function can be used to describe the average tilt angle and its distribution,

[

f(θ) ) C exp -

(θ - θ0)2 2σ2

]

where θ0 is the averaged tilt angle, σ is the angle distribution, and C is the normalization constant.22,75 Therefore we have 〈cos θ〉 ) ∫cos θ · f(θ) · sin θ dθ and 〈cos 3θ〉 ) ∫cos 3θ · f(θ) · sin

Phenolic Resin Surface Restructuring

Figure 7. For a phenol group, the curves of the calculated χyyz,ν7b/ χyyz,ν2 as a function of the averaged tilt angle θ0 for several different tilt angle distibutions (0°, 12°, 25°, 40°) when a Gaussian distribution function is assumed. The experimental values are indicated by the two solid straight lines. The upper line is from the sample before exposure to humid air. The lower line is from the sample after exposure to humid air.

Figure 8. A schematic representation shows that the phenol group stands up more after exposure to humid air.

θ dθ. Figure 7 shows the calculated |χyyz,ν7b/χyyz.ν2| value as a function of the averaged tilt angle θ0 for different tilt angle distributions (0°, 5°, 12°, 15°, 20°, 25°, 30°, 35°, 40°) when a Gaussian distribution function is assumed to describe the orientation distribution. The values indicated by the two straight lines in Figure 7 are the experimental data deduced from the BPP surface spectra. The value of 0.35, indicated by the upper line, is obtained from the BPP surface before exposure to humid air. According to Figure 6, the possible phenol orientation on the BPP surface before water adsorption is between the two extremes of a tilt angle of 70° with a δ-distribution and a tilt angle of 90° with a distribution width of 12°. After the water adsorption to the BPP surface, the value was measured to be 0.16, indicated by the lower line in Figure 7. This shows that the possible phenyl orientation is between the two extremes of a tilt angle of 37° with a δ-distribution and a tilt angle of 0° with a distribution width of 40°. It is evident that the phenol aromatic ring tilts more toward the surface normal after the water molecules are adsorbed to the surface (Figure 8). Before exposure to the humid air, the phenol groups more or less lie down on the surface, with large orientation angles. Hydrogen bonds may form between hydroxyl groups of these phenol groups, which make such a structure more stable. After the surface was exposed to humid air, water molecules were adsorbed on the surface and the phenol hydroxyl groups can form hydrogen bonds with water molecules on top of the surface, enabling the phenol aromatic ring to stand up more on the surface. After annealing the sample again, SFG spectra recovered. This demonstrated that the BPP surface structural change is a reversible process. After the water molecules desorbed from the BPP surface, the phenol aromatic ring lies down more toward the surface again, and perhaps hydrogen bonds are formed again between hydroxyl groups of the phenols. Figure 7 indicates that the largest possible tilt angle distribution for the phenyl groups on the phenolic resin surface after water adsorption is much broader than that before water adsorption. This may suggest that the water molecules can interact with the phenol molecules

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Figure 9. Calculated χppp/χssp ratio (upper curve) and intensity ratio (lower curve) between the signals in ppp and ssp spectra for the ν2 mode.

in different ways. As we mentioned above, the O-H stretching signal for BPP surface after exposure to humid air is much broader. Similar orientation analysis can be done for methylene groups on the BPP surface. We found that different from the phenol aromatic ring case, the surface orientation of methylene groups only slightly changed after exposure to humid air, which will not be discussed in detail here. 3.4. Weak Signals in the ppp Spectra. Figure 5 indicated that the ppp SFG signals collected from the BPP surface were much weaker than the ssp signals, especially in the phenyl vibrational range (3000-3100 cm-1). This can be interpreted based on the evaluation of the ν2 mode, a total symmetric stretching vibrational mode in the ppp spectrum. For the ν2 mode at an azimuthal surface, there are only two nonzero hyperpolarizability components which have the relation of βccc,ν2 ) 0.69βaac,ν2 according to the bond additivity approach. Also, based on the transformation provided by Hirose, C. et al., with assumptions of an azimuthal surface layer and freely twisting phenyl groups, we have

χxxz ) Nsβaac(0.52〈cos θ〉 - 0.02〈cos 3θ〉)

(7)

χxzx ) Nsβaac(0.02〈cos θ〉 - 0.02〈cos 3θ〉)

(8)

χzxx ) Nsβaac(0.02〈cos θ〉 - 0.02〈cos 3θ〉)

(9)

χzzz ) Nsβaac(0.39〈cos θ〉 - 0.05〈cos 3θ〉)

(10)

Further, applying the Fresnel coefficients using our “facedown” geometry, assuming the refractive indices of the surface layer being 1.2 without considering the dispersive effect, i.e., n′(ω) ) n′(ω1) ) n′(ω2) ) 1.2, we have the following equation

χppp 0.03〈cos θ〉 - 0.03〈cos 3θ〉 ) χssp 0.34〈cos θ〉 - 0.01〈cos 3θ〉

(11)

Now the ratio of χppp/χssp can be evaluated, assuming a δ-distribution of phenol ring for simplification. As shown in Figure 9, the ratio of χppp/χssp is small. Since the ratio of the ppp and ssp signal intensities is square of the above χppp/χssp, which is even smaller, it is reasonable to detect much weaker ppp signal from the BPP surface. 3.5. Contact Angle Measurement Results. Surface contact angles of the epoxy resin DCPD surface and phenolic resin BPP surface have been measured using water and diiodomethane. Contact angles were also measured on the surface of a 1:1

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TABLE 2: Contact Angle Measurement Results sample name

4. Conclusion

CA (degree) H 2O

DCPD BPP DCPD + BPP CH2I2 DCPD BPP DCPD + BPP

71 ( 3 68 ( 2 70 ( 4 7(2 29 ( 3 9(2

TABLE 3: Surface Free Energies Calculated Based on the Geometric Mean Method γd

γp

γtotal

γDCPD (mN · m-1)

46

5

51

γBPP (mN · m-1)

40

9

49

γDCPD+BPP (mN · m-1)

45

6

51

weight blend of DCPD and BPP. The results are summarized in Table 2. It was found that the water contact angle on phenolic resin BPP surface is slightly lower than that on the DCPD surface, showing that the BPP surface is slightly hydrophilic, while the DCPD surface is slightly hydrophobic. According to the SFG studies discussed above, this might be because on the DCPD surface in air, methylene groups are dominated, whereas on the BPP surface, in addition to the surface methylene groups, other groups including hydroxyl groups, which are more hydrophilic, are also present on the surface. The contact angle of the DCPD and BPP blend is reasonably in between, as a result of comparable interactions with water for both resin surfaces. The measured contact angles on the DCPD and BPP surfaces using diiodomethane are very different. The angle on the DCPD surface is much smaller, indicating that it is more compatible with diiodomethane. Since diiodomethane is a nonpolar molecule, and the surface dominating methylene groups for DCPD is also nonpolar, they can interact with each other favorably, resulting in a small contact angle. For BPP, SFG studies indicate that some hydroxyl groups are present on the surface; they can form hydrogen bonding with adjacent surface hydroxyl groups. Such hydroxyl groups are polar groups, resulting in a larger contact angle of diiodomethane on the BPP surface than that on the DCPD surface. The diiodomethane contact angle on the 1:1 DCPD and BPP blend surface is similar to that on the DCPD surface, confirming that the DCPD surface is more hydrophobic than the BPP surface. The contact angle measurements can be correlated to the SFG data well. SFG studies have shown that the DPCD surface is dominated by the hydrophobic and nonpolar methylene groups, while the BPP surface has the presence of polar and hydrophilic hydroxyl groups. Therefore, SFG can provide the molecular-level interpretation for contact angle measurement results. From the contact angles measured using water and diiodomethane, surface energies of DCPD and BPP can be calculated using the geometric mean method,76 as shown in Table 2. It is very clear that the surface free energy of DCPD has a higher dispersive (nonpolar) part and the surface free energy of BPP has a higher polar part, which matches well with the SFG results.

This research has shown that both the epoxy and phenolic resins can absorb water by measuring mass change and FTIR. Surface structures of the epoxy and phenolic resins in air and upon exposure to humid air (after adsorption of the water molecules to the surfaces) were studied by sum frequency generation (SFG) vibrational spectroscopy. No evidence from the SFG spectra shows that the surface structure changes for the epoxy resins, which may be due to the rigid surface structure of epoxy resins. A substantial surface restructuring of the phenolic resin was observed. It was found that the adsorbed water molecules at the phenolic resin surface can induce the phenol orientation change from “parallel-like” to “perpendicularlike” orientation through hydrogen bonding. Contact angle measurements show that the epoxy resin surface is more hydrophobic and nonpolar in comparison to the phenolic resin surface. This can be interpreted at the molecular level using SFG results. This research demonstrates that SFG can probe molecular structures of surfaces of packaging materials. In the real packaging process, the resin materials will be processed under much higher temperatures and buried interfaces need to be probed to understand the performance (including failure) of resins. Studies of buried interfaces involving resin materials under high temperatures will be probed using SFG in the future. Acknowledgment. This work is supported by NSF (CHE0449469), SRC (P10419), Dow Corning Corporation, University of Michigan, Nanjing University, and Henkel. Daiwei Li is supported by a CSC fellowship. Supporting Information Available: More details regarding the hyperpolarizability tensor component deduced from the bond additivity approach, the Fresnel coefficient calculations and the geometric mean method used for surface free energy calculation. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Morgan, R. J.; O’Neal, J. E.; Fanter, D. L. J. Mater. Sci. 1980, 15, 751–764. (2) Lin, R.; Blackshear, E.; Serisky, P. Moisture induced package cracking in plastic encapsulated surface mount components during solder reflow process. Proc. 26th Rel. Phys. Symp. 1988, 83–89. (3) Kinjo, N.; Ogata, M.; Nishi, K.; Kaneda, A. Speciality Polymers/ Polymer Physics; Springer-Verlag: Heidelberg, Germany, 1989; Vol. 88, p 1. (4) RoHS is the abbreviation of the following documentation: The restriction of the use of certain hazardous substances in electrical and electronic equipment. Off. J. Eur. Union 2003, 46, L37, 19. (5) Ngono, Y.; Marechal, Y.; Mermilliod, N. J. Phys. Chem. B 1999, 103, 4979–4985. (6) Musto, P.; Ragosta, G.; Mascia, L. Chem. Mater. 2000, 12, 1331– 1341. (7) Liu, M.; Wu, P.; Ding, Y.; Chen, G.; Li, S. Macromolecules 2002, 35, 5500–5507. (8) Mijovic, J.; Zhang, H. Macromolecules 2003, 36, 1279–1288. (9) Li, L.; Liu, M.; Li, S. J. Phys. Chem. B 2004, 108, 4601–4606. (10) Li, L.; Zhang, S.; Chen, Y.; Liu, M.; Ding, Y.; Luo, X.; Pu, Z.; Zhou, F.; Li, S. Chem. Mater. 2005, 17, 839–845. (11) Cotugno, S.; Mensitieri, G.; Musto, P.; Sanguigno, L. Macromolecules 2005, 38, 801–811. (12) Zhou, J.; Lucas, J. P. Polymer 1999, 40, 5505–5512. (13) Zhou, J.; Lucas, J. P. Polymer 1999, 40, 5513–5522. (14) Pethrick, R. A.; Hollins, E. A.; McEwan, I.; MacKinnon, A. J.; Hayward, D.; Cannon, L. A.; Jenkins, S. D.; McGrail, P. T. Macromolecules 1996, 29, 5208–5214. (15) Colombinia, D.; Martinez-Vegab, J. J.; Merle, G. Polymer 2002, 43, 4479–4485. (16) Mijovic, J.; Zhang, H. J. Phys. Chem. B 2004, 108, 2557–2563. (17) Chen, Z.; Shen, Y. R.; Somorjai, G. A. Annu. ReV. Phys. Chem. 2002, 53, 437–465.

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