Anhydride-Based Hybrid Adhesive Films for

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Materials and Interfaces

Versatile Epoxy/phenoxy/anhydride-based Hybrid Adhesive Films for Deoxidization and Electrical Interconnection Keon-Soo Jang, Yong-Sung Eom, Kwang-Seong Choi, and Hyun-Cheol Bae Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01142 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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Industrial & Engineering Chemistry Research

Versatile Adhesive

Epoxy/phenoxy/anhydride-based Films

for

Deoxidization

and

Hybrid Electrical

Interconnection Keon-Soo Jang*, Yong-Sung Eom, Kwang-Seong Choi, Hyun-Cheol Bae ICT Materials & Components Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), Daejeon, 34129, South Korea

CORRESPONDING AUTHOR FOOTNOTE *To whom correspondence should be addressed. C.P.: 82-10-4702-5359. Fax: 82-42-860-6469. E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

We manufactured a hybrid adhesive film capable of curable deoxidization by combining thermoplastics and thermosets. The generated acidic moieties on this hybrid adhesive film completely removed the oxide layer of various Sn-based metal solders and Cu, leading to electrical interconnection. The chemical structure, reaction rate and glass transition temperature (Tg) of the films were examined via Fourier transform infrared spectroscopy and differential scanning calorimetry. The initial reaction rate of the hybrid deoxidizing adhesive films increased with increasing concentration of anhydride and phenoxy resin. Meanwhile, the reaction rate at the end for curing completion decreased as a function of anhydride and phenoxy resin. Tg of the films decreased with the increase in anhydride and phenoxy resin concentrations. The fabricated film resulted in complete wetting of the solder/Cu for electrical interconnection between semiconductor chips as a promising electronic application. This hybrid film can be useful for various applications requiring deoxidization and adhesion.

KEYWORDS Adhesive film, Epoxy, phenoxy resin, anhydride, deoxidization (fluxing), electrical interconnection

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INTRODUCTION Thermosetting polymers have been utilized, most notably as structural adhesives and matrices in a myriad of applications such as construction, automotive, aerospace, electronics and biomaterials.1,2 Among thermosets, epoxy resins are considered as the most important class, occupying a predominant position due to their high mechanical robustness, excellent adhesion to diverse substrates, and enhanced chemical and thermal resistance.3,4 New interest in harnessing functional epoxides as supramolecular networks, self-healing polymers, and nanostructured composites has recently surfaced.5–7 These widespread advanced applications of epoxy formulations result from the generated hydroxyl moieties on the backbone of epoxy resins during curing reactions. The hydroxyl units are capable of designing unique architecture, offering strong reversible interaction, chemical reactivity, and functionality.8 This strategic reconfiguration has further been sophisticatedly adapted as adhesives and sealants for high-performance electronics applications, such as highly bendable transparent thin-film transistors9, foldable printed circuit boards10, and flexible microfluidic devices.11 In particular, epoxy adhesives have attracted considerable attention for electrical interconnections between semiconductor chips and substrates such as flip chip bonding due to their superior adhesion to various substrates, caused by the generated hydroxyls.12–14 Sn-based fusible solders, such as Sn/Ag caps on Cu pillars, have been employed for electrical interconnections via melting and wetting of the solders.15–18 However, these Sn-based solders are susceptible to oxidation on the surface, thus requiring a reductant for wettability of the solder, thereby making an electrical path. Without oxidizing capability, the solder balls are surrounded and stuck by the solder oxide layers even above its melting point, causing wetting failure.19,20 However, residual reductants cause corrosion, and the hollow structure without packing materials between electrical paths results in reduced mechanical

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robustness.13,19,20 These drawbacks can be minimized by cleaning and filling vacancy between electrical paths with epoxy resins, which are tedious procedures.13 Motivated by this dilemma, we delicately designed a deoxidizing hybrid adhesive film by combining thermoplastic polymers and thermosetting resins without the use of a low molecular-weight acidic reductant referred to as a deoxidizing (fluxing) agent. Herein, we describe such strategy by utilizing susceptibility of anhydrides to react with yielded hydroxyls on phenoxy and epoxy resins, producing acidic moieties ascribed to deoxidization as shown in Scheme 1. After deoxidization, the molecular structure becomes networked. We also demonstrated simultaneous curable deoxidizing capability via spectroscopic analysis.

Scheme 1. Schematic reaction mechanism of curable deoxidizing hybrid adhesive films

EXPERIMENTAL Bisphenol A type epoxy monomer with the epoxy equivalent weight (EEW) of 187 g/eq and phenoxy resin (molecular weight: 50,000 – 60,000 g/mol) were obtained from Kukdo Chemicals Co. in South Korea. Tetrahydrophthalic anhydride (Sigma-Aldrich) was used as a curing agent.

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Tetrahydrofuran (THF, Sigma-Aldrich) and methyl ethyl ketone (MEK, Sigma-Aldrich) were utilized to dissolve the curable materials and the phenoxy resin matrix. Sn19–Bi27–In54 (Sn/Bi27/In54), Sn42–Bi58 (Sn/Bi58) and Sn96.5–Ag3.0–Cu0.5 (SAC305) with melting temperatures of 86, 139 and 219 °C were used, resepectively, to probe the deoxidizing capability of hybrid adhesive films. The epoxy equivalence weight ratios to the curing agent were 1:1, 1:1.5, and 1:2, which were abbreviated as DA10, DA15, and DA20, respectively. The weight ratios between the entire thermosetting resin mixture and the phenoxy resin were 100:40, 100:80, and 100:120, denoted by P40, P80, and P120, respectively. The final mixture was dissolved and mixed in THF or MEK at room temperature for 1 h by using a vortex mixer. Subsequently, the final solution was poured onto a releasing film. Film thickness was tunable from 30 to 200 µm via thickness-controlling tools. Information on the chemical reaction and thermal properties of the hybrid adhesive film was examined by differential scanning calorimetry (DSC, TA Ins. Model Q20). Approximately 5.0 mg of the deoxidizing film in hermetic aluminum pan was measured at a heating rate of 10 °C/min under nitrogen atmosphere. To investigate the deoxidizing (fluxing) capability of the films for removing the solder oxide layer, three different solder balls, namely, Sn/In27/Bi54, Sn/Bi58 and SAC305 with a diameter of ca. 500 m were utilized. Three balls of each solder were placed between an Au plate and the film. Wetting test was conducted in a surface mounting technology (SMT, Sanyo Model SK5000) reflow chamber under nitrogen atmosphere below the oxygen concentration of 1,050 ppm.

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Given the 219 °C melting point of SAC305, the chamber temperature was increased from room temperature to 240 °C at various heating rates of 10 to 60 °C/sec and held at 240 °C for 5 min. UV-visible spectroscopy (Perkins Elmer Lambda 750) was performed, in the wavelength range from 400 to 800 nm at a resolution of 5 nm to quantify film transparency. Fourier transform infrared (FTIR) spectra of the films on NaCl pallet were recorded using a Nicolet 6700 FTIR spectrometer. The number of scans for each spectrum was 16. Chemorheological properties were investigated using a torsional parallel plate rheometer (HAAKE MARS III, Thermo Scientific Inc.). The viscosity and shear modulus of the films were measured under dynamic conditions at a frequency of 1 Hz and a heating rate of 10 °C/min. The proper condition of controlled-strain (displacement) mode was determined to be  = 0.01. The shear modulus at room temperature was converted to Young’s modulus by using Poisson’s ratio of 0.33 based on the equation: G = Y/2(1 + ν) where G, Y and ν are the shear modulus, Young’s modulus and Poisson’s ratio, respectively. To examine the oxidizing (fluxing) capability in an electronic application, an ultrahigh accuracy flip-chip die bonder (M9, Laurier) was employed to attach a SAC305 solder-capped Cu pillar bump and a copper-plated pad on a printed circuit board (PCB) treated with an organic solderability preservative (OSP). The hybrid deoxidizing adhesive film was placed between the chip and the substrate and heated at 80 °C/min. The diameter and height of the solder-capped Cu pillar bump were ca. 80 and 60 m, respectively. The height of the solder cap and the Cu pillar was 20 and 40 m, respectively. The pitch was 140 m and the number of bumps was 2,500.

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The cross-section of the SMT-processed solders on the PCB and the flip-chip bonded films in an epoxy holder was mechanically ground by a polishing machine (Bestpol P201 model, Ssaul Bestech, South Korea) equipped with a water cleaning system. The polished samples were observed by optical microscopy (OM Microphot-FXA, Nikon) equipped with a digital camera. The light source was a 12V–100W halogen lamp (Philips 7724) controlled by the BioRad MRC-600 computer software program, COMOS® (BIO-RAD Lab.). Images photographed by a 40× objective lens were observed on a color monitor.

RESULTS AND DISCUSSION

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Figure 1. Experimental scheme (a). Solder particles of Sn/In27/Bi54 (b,c) and Sn/Bi58 (d,e) embedded in DA15P120 films, and Sn/Bi58 in DGEBA (f,g). (b) 85 °C, (c) 90 °C, (d,f) 135 °C, and (e,g) 145 °C. The insets in b and c show cross sectional OM images of unwetted and wetted solders on Au plates, respectively. The inset in d shows the fabricated hybrid film. The reaction mechanism among an anhydride, and phenoxy and epoxy resins is complicated due to several competing reactions. The anhydride reacts with hydroxyls on both phenoxy and epoxy resins, producing half-esters consisting of a carboxylic acid, which can react with oxirane, yielding another hydroxyl. Acidic moieties can be generated by the reaction between the hydroxyl and another anhydride as described above or another epoxy to form an ether linkage,

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which is activated by the acid. The generated acids may not only participate in the epoxy reaction but also remove the oxide layers of metal solder particles, inducing the solder wetting on a substrate. Figures 1b–e monitor the wettability of Sn/In27/Bi54 and Sn/Bi58 incorporated in the DA15P120 film on Au substrates at a heating rate of 10 °C/min. The Sn/In27/Bi54 and the Sn/Bi58 solder particles were wetted on Au substrates above the melting temperatures of each solder in Figures 1c, e, respectively, probably due to the generated acidic moieties derived from chemical reactions. The wettability of the Sn/Bi58 solder on gold substrates was observed before and after its melting temperature via the cross-sectional image of the samples as shown in insets of Figures 1b, c, respectively. Without the produced acidic moieties, Sn/Bi58 solders remained unwetted in the pristine DGEBA resin (Figures 1f, g) because the solder oxides still remained on the surface.21

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Figure 2. Solder particles of SAC305 embedded in DA15P120 film on Au substrates at 210 °C (a,c,e) and 230 °C (b,d,f) at a heating rate of 30 °C/min (a,b), 40 °C/min (c,d), and 50 °C/min (e,f).

The SAC305 is the most widely used solder for electrical interconnections for various electronic applications17,18. This eutectic solder shows a relatively high melting point of 219 °C. Thus, it requires a latent cure mechanism to prevent the film gelation. When the film gelation occurs prior to the melting temperature of solder, the solder ball will remain unwetted even above the melting point. The flowable film resins (above 100 °C) constantly move into and out of the interface between solder balls and a substrate due to the reaction-triggered turbulence of resins (sometimes small movement). The turbulence effect disappears after the film gelation due

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to increased viscosity of the hybrid film, thereby hindering the solder wetting. Wetting can also be achieved by simply tailoring the heating rate, particularly by elevating it for the processing temperature to rapidly reach the melting point of the solder prior to gelation. As seen in Figure 2, the heating rate was controlled from 10 to 50 °C/min. Up to the heating rate of 30 °C/min, wetting was not observed in SAC305 solder particles embedded in the DA15P120 film onto the Au pad. The heating rate faster than 40 °C/min delayed the cure substantially enough to decrease the conversion of the curable film at a certain chamber temperature at which the solder balls were melted. Then, the wetting of SAC305 on the Au pad was induced above its melting point due to the absence of solder oxide layers. Instead of heating rate tuning, the composition can be modified to shift the reaction peak temperature toward high temperature.

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Figure 3. FTIR spectra for DA10P40 (a), DA15P40 (b), and DA20P40 (c): Spotted (····), dashed (----) and solid (━) lines indicate anhydride, carboxylic acid, and epoxide, respectively

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Spectroscopic analysis such as FT-IR and nuclear magnetic resonance (NMR) has been widely employed to detect units for carboxylic acid, epoxide, and anhydride22,23. The generated acidic moieties, and the consumed anhydride and epoxide were monitored at different ratios of the films with increasing temperature via FT-IR (Figure 3). The spectra for DA10P40, DA15P40 and DA20P40 are detailed in Figures 3a–c, respectively. For all samples, the peak areas for anhydride and epoxide contributable to 1790 and 911 cm-1 were reduced as a temperature function. With increasing anhydride concentration, the anhydride and epoxide units were consumed at a high rate. For DA10P40, the peaks for the anhydride and the epoxide moieties almost disappeared at 220 and 260 °C, respectively. Meanwhile, the peaks for DA20P40 were all consumed at 160 °C. The peak assigned to acidic units at 1716 cm-1 completely disappeared around 240 °C. The additional quantity of anhydride may accelerate the reaction in the presence of phenoxy resins. The acidic units associated with 1716 cm-1 were activated even at room temperature because the components were homogeneously dissolved and mixed in a solvent during the film fabrication, allowing certain reactions between anhydride and the phenoxy and epoxy resins.

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Temperature ( C) Figure 4. Differential scanning calorimetry curves (a) and viscosity (b) of DA10P40, DA15P40 and DA20P40 To confirm the spectroscopic results, DSC was employed to assess the reaction as a function of anhydride concentration from DA10P40 to DA20P40. DSC graphs along with the wetting tests also provide information on correlation between the wetting temperature and the reaction peak temperature (exothermic peak temperature in a DSC curve). The exothermic peak temperatures for all films were observed at ca. 230 °C, which contradicts the FT-IR results, that

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is, the epoxide and anhydride units for DA20P40 were consumed faster than those for DA10P40. However, Figure 4 shows that the initial reaction temperature for DA20P40 begins at lower temperatures than that for DA10P40. The slope at the reaction end for DA20P40 is also sharper than that for others. The endothermic peaks around 150-170 °C may be caused by stress relaxation. Given that anhydride units were opened by hydroxyls in phenoxy and epoxy resins, special architecture may be formed due to intermolecular interactions such as hydrogen bonding, which possibly influence the stress relaxation. With increasing anhydride concentration, surplus acids impede the interactions, releasing the stress relaxation. In another aspect, the stress relaxation effect was minimized by compensating the exothermic curve for the initial reaction with the endothermic curve ascribed to the stress relaxation. Based on the FT-IR and DSC results, it is believed that the reaction rate between anhydride, hydroxyl and epoxide increased with the increase in anhydride content. However, the entire crosslink network was formed at a similar temperature (ca. 270 °C) for all films at a heating rate of 10 oC/min.

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Figure 5. FTIR spectra for DA15P40 (a), DA15P80 (b) and DA15P120 (c): Spotted (····), dashed (----) and solid (━) lines indicate anhydride, carboxylic acid and epoxide, respectively.

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For investigating the effect of phenoxy resins on the reactions, the films including different compositions (DA15P40, DA15P80, and DA15P120) were monitored by FTIR as a function of temperature as shown in Figure 5. With increasing temperature, the anhydride and epoxide units of DA15P40 were gradually consumed and completely disappeared at 180 °C whereas those of DA15P120 disappeared at 100 °C. Similar to Figure 3, the acidic moieties as a deoxidizing unit were pre-activated at room temperature due to the opening of anhydride by hydroxyls in a solution mixture during film manufacture.

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Figure 6. Differential scanning calorimetry curves (a) and viscosity (b) of DA15P40, DA15P80 and DA15P120 The spectroscopic information for DA15P40, DA15P80 and DA15P120 was correlated with the DSC study in Figure 6. As the concentration of the phenoxy resins was increased, the exothermic peak attributable to curing broadened. The incorporation of high phenoxy concentrations decreased the endothermic peak area around 150 °C associated with stress relaxation resulting from the unusual film architecture. Small exothermic shoulder peaks were observed below 100 °C, which may be assigned to the reactions between hydroxyls and anhydrides. Nucleophilic attack at carbonyl groups on anhydride occurs by the hydroxyl units. Under basic conditions such as pyridine, the production of esters and carboxylic moieties can be accelerated.24

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Figure 7. Glass transition temperatures of hybrid adhesives with various compositions

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The mechanical properties of adhesives for electronic devices are of considerable importance. The glass transition temperature of thermoset polymers is a representative of mechanical robustness. Figure 7 highlights the glass transition temperatures of polymeric films consisting of various compositions. The glass transition temperature of the films was reduced with increasing anhydride content and increasing phenoxy concentration. The reduction in glass transition temperature was believed to be primarily due to inadequate stoichiometric ratios.

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Wavennumber (cm ) Figure 8. FTIR spectra for DA15P120 ranging from 3150 to 2000 cm-1: Dashed rectangle indicates hydroxyls Figure 8 monitors the interplay between consumption and generation of hydroxyls for DA15P120 as a function of temperature. Initially, the hydroxyls on the phenoxy resins show the broad peaks between 3100 and 2800 cm-1 at room temperature. The associated peaks were gradually reduced up to 140 °C due to the reaction between anhydrides and the hydroxyls on the phenoxy resins. Beyond 140 °C, the peaks became increasingly large and sharp with increasing

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temperature owing to the generated hydroxyls on epoxy resins as the reaction progresses and finally ends. Above 220 °C, the peaks rarely changed, suggesting the reaction completion.

50 µm

Figure 9. Optical microscopic cross-sectional image of flip-chip bonded DA15P120 film perpendicular to Cu-plated substrate. White arrow indicates solder joint between Cu pillar and Cu-plated substrate. Inset shows the fabricated hybrid adhesive film on a device prior to wetting test

One promising application of deoxidizing capability of the adhesive is for electrical interconnections between chips and substrates.13,14 To examine the deoxidizing capability of the hybrid film for metal oxide layers, a device/substrate designed with Sn/Ag solder caps and Cu pillars was used for the electrical interconnection test, in particular, flip-chip bonding. The wetting of flip-chip bonded samples has been routinely investigated by optical microscopy. The cross sectional filp-chip bonded DA15P120 shows the complete wetting between the Sn/Ag caps and Cu pillars in Figure 9, indicating the deoxidizing (fluxing) capability of the hybrid film for

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the Sn-based solders and Cu. Due to the good wettability, the wetted solder on the sidewall of Cu pillar was partially observed. This finding can be solved by controlling the reactivity and the viscosity of the adhesive film, which is currently under scrutiny. CONCLUSION The combination of phenoxy resin and epoxy–anhydride resin allowed for the fabrication of the hybrid deoxidizing adhesive film. The deoxidization was observed in various Sn-based metal solders was observed without additional deoxidizing agents. The reaction rate of the films was increased whereas the glass transition temperature of the films was decreased with the increase in concentration of either anhydride or phenoxy resin. The hydroxyl groups of DA10P40 were gradually reduced up to 140 °C; beyond this temperature, additional hydroxyl groups were produced. The deoxidizing capability of the film allowed for complete wetting of the solder/Cu for electrical interconnection between semiconductor chips, thus offereing a promising electronic application. These simultaneous deoxidizing and curing capabilities render the hybrid adhesive film ideal for use in high performance electronic applications that require deoxidization and adhesion.

ACKNOWLEDGEMENT This work was supported by Nano-Convergence Foundation (www.nanotech2020.org)) funded by the Ministry of Science, ICT and Future Planning (MSIP, Korea) & the Ministry of Trade, Industry and Energy (MOTIE, Korea) [Commercialization of 100Gbps optical receiver and transmitter modules based on nano Ag-coated Cu paste], and the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation 21



Program. No.10082367, [Eco-friendly Interconnection Paste and Process with Laser Bonding for Flexible LED Module]. This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Trade, Industry and Energy [17PB2200], and Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(MSIP) (No. 2017-0-01281, One-stop Multi-functional Interconnection Material for Flip-chip bonding of ICT semiconductor device).

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Anhydride-Based Hybrid Adhesive Films for

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