2. Experimental Procedure Figure 1:

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1 Influence Of Carbon, Metal-Coated Polymer, and Nano Powders On Sintering, and Electrical Performance of Nano-Micro-Filled Conducting dhesives For Z-xis Interconnections Rabindra N. Das, Frank D. Egitto, John M. Lauffer, Mark D. Poliks and Voya R. Markovich Endicott Interconnect Technologies, Inc., 1093 Clark Street, Endicott, New York, Telephone No: bstract This paper discusses micro-filled epoxy-based conducting adhesives modified with nanoparticles, carbon, and metalcoated polymer fillers for z-axis interconnections. variety of conducting adhesives with particle sizes ranging from 5 nm to 15 µm were incorporated as interconnects in printed wiring board (PW) or laminate chip carrier (LCC) substrates. Scanning electron microscopy (SEM) and optical microscopy were used to investigate the micro-structure, and conducting and sintering mechanisms. Sheet resistance of g, carbon, and metal-coated polymer filler was low. mong all, metalcoated polymer showed the highest resistance. Sheet resistance decreased with increasing curing temperature. Drop in resistance for carbon-doped samples was 90% from 200 o C to 275 o C. It was found that with increasing curing temperature, the resistance of the conducting paste decreased due to sintering of metal particles. Sintering temperature and corresponding grain growth of nano-micro adhesive was further evaluated using different size nano particles, and shows optimum sintering at 240 o C. dhesives formulated with highly filled silver nano-micro particles exhibited a Z- axis coefficient of thermal expansion (CTE) of 17 ppm/ o C, and as high as 41 ppm/ o C for carbon doped, highly filled silver nano-micro systems. Similarly, Z-CTE of highly filled metal-coated polymer fillers was 28 ppm/ o C. s a case study, a variety of z-axis interconnect constructions for a flip-chip plastic ball grid array package, as well as for PW were fabricated and evaluated at both the subcomposite and composite levels to understand and reduce paste-to-package CTE mismatch. Several conductive adhesives were used in the z-axis interconnect constructions for LCC and PW. The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55 to 300 µm. The processes and materials used to achieve smaller feature dimensions, satisfy stringent registration requirements, and achieve robust electrical interconnections are discussed. 1. Introduction The demand for high-performance, lightweight, portable computing power is driving the industry toward miniaturization at a rate not seen before. Electronic packaging is evolving to meet the demands of higher functionality in ever smaller packages. To accomplish this, new packaging needs to be able to integrate more dies with greater function, higher I/O counts, smaller pitches, and greater heat densities, while being pushed into smaller and smaller footprints. One packaging strategy devised to help meet these demands allows for metal-to-metal z-axis electrical interconnection of subcomposites during lamination to form a composite structure. Conductive joints are formed during lamination using an electrically conductive adhesive (EC). s a result, one is able to fabricate structures with verticallyterminated vias of arbitrary depth. Replacement of conventional plated through holes (PTHs) with verticallyterminated vias opens up additional wiring channels on layers above and below these vertical interconnections, and eliminates via stubs which cause reflective signal loss. Vertically terminated vias facilitate a more space-efficient package redesign. Conductive adhesives should meet the following requirements in order to achieve electrical and mechanical performance goals in an organic substrate: bility to fill small diameter holes Low Z-joint resistance Good mechanical strength Interconnect coefficient of thermal expansion (CTE) tuned to match the CTE of the substrate Low temperature sintering of the EC Typically, adhesives formulated using controlled-sized micro particles have been used to fill small diameter holes for Z-interconnect applications. During the past few years, there has been increasing interest in using electrically conductive adhesives as interconnecting materials in the electronics industry [1,2]. Conductive adhesives are composites of polymer resin and conductive fillers. Metal to-metal bonding between conductive fillers provides electrical conductivity [3-6], whereas a polymer resin provides better processability and mechanical robustness [7]. Conductive adhesives usually have excess filler loading that weaken the overall mechanical strength. Therefore, reliability of the conductive joint formed between the conductive adhesive and the metal surface to which it is mated is of prime importance. Conductive adhesives can have broad particle size distributions. Larger particles can be a problem when filling smaller holes (e.g., diameter of 60 µm or less), resulting in voids. Several nanoand micro-filled adhesives have been reported for advanced packaging applications [8-12]. lthough several composites have been available, the authors believe that there is potential scope for improvement of the existing materials, such that flexible and reliable materials can be developed for z-axis interconnections. In the present study, micro-filled adhesives were modified to improve overall electrical performance. The first objective of this study is to investigate the effect of nanoparticle addition to microcomposites and reduce sintering temperature. Nanoparticles of silver were chosen because of their higher electrical conductivity and chemical stability. Nanoparticles were mixed with microparticles to improve the

2 sintering behavior of the adhesives. second objective is to use a metal-coated polymer and carbon fillers. ddition of carbon will control the overall CTE without compromising electrical conductivity. Metal-coated polymers inter-diffuse during processing and produce a metallic network. This work also deals with CTE differences between the adhesives and the substrates to which they are mated. This paper presents a CTE assessment of different materials stacks in an effort to determine the most suitable conductive adhesive for a particular package. variety of adhesive joints were tested in a z-axis interconnect construction for a laminate chip carrier (LCC) and printed wiring board (PW). The structure employs an electrically conductive medium to interconnect thin cores (subcomposites). The cores are built in parallel, aligned, and laminated to form a composite. 2. Experimental Procedure variety of nano-micro silver, carbon, and metal-coated polymer fillers and their dispersion into epoxy resin were investigated in order to achieve uniform mixing in the adhesive. In a typical procedure, epoxy-based conductive adhesives were prepared by mixing appropriate amounts of the conducting filler powders and epoxy resin in an organic solvent to form a paste like composition. For conductivity measurements, a thin film of this paste was deposited on a non-conducting substrate and cured at different temperatures ranging from 100 o C to 375 o C. C For fabrication of a high-density LCC, a 0S/1P joining core such as that described in reference [8] was constructed using a copper power plane, 35 µm thick, sandwiched between layers of a dielectric material composed of silicafilled polymer. Through holes in the joining cores, formed by laser or mechanical drilling, and having diameters ranging from 50 µm to 250 µm, were filled with an electrically conductive adhesive optimized to suit the various hole diameters. The adhesive-filled joining cores were cured and cross sectioned to evaluate hole fill quality. dhesives were characterized by scanning electron microscopy (SEM) and optical microscopy to ascertain particle dispersion and interconnection mechanism. Keithley micro-ohmmeter was used for electrical characterization. Room temperature (25 o C) viscosity was measured using a MELVERN C-VOR Rheometer in oscillation mode using 50 Pa stress at 1 Hz. Heats of reaction of adhesives were studied using a differential scanning calorimeter (DSC). Practical adhesion (90 degree peel test) and tensile strength were measured using an Instron (Model 1122) and MTS tensile tester, respectively. D Figure 1: Micrographs for the top views ( and ) and cross-sectional views (C and D) of metal-coated polymer based adhesives (through hole diameter ~150 microns).

3 Results and Discussion The formation of a conduction path was observed by SEM images. Figures 1 to D shows cross sections and top views of a metal-coated polymer based adhesive. Metal coating produces a continuous metallic network. In the adhesive, the average metal-coated filler diameter is in the range of 8 µm. Filler loading was high, adjacent particles united mutually, and necking phenomena between fillers occurred; namely, a conduction path was achieved [3]. similar result was observed when metal-coated particles were replaced by silver and carbon particles. variety of carbon-based silver-filled adhesives with a mixture of nano and micro particles were studied. In nano-micro mixtures, nano particles occupy interstitial positions to improve particle-particle contact for conductivity. For the silver nanoparticles (~5 nm size), the fillers can self sinter and make a continuous conduction path. high surface area of silver nanoparticles necessitates an excess amount of solvent in order to make high loading silver paste. Figure 3: SEM micrographs for the nano-micro filled silvercarbon based conducting adhesives; () sintered at 250 C, and () sintered at (275+10) C. C Figure 2: SEM micrographs for the nano-micro filled silverbased conducting adhesives; () un-sintered at C, () un-sintered at C, and (D) sintered at C. It is well known that change in grain size has a direct impact on the electronic properties of a system. In view of this, a systematic investigation of electrical resistance behavior of silver and silver-carbon nanocomposites has been carried out, and the results of such an investigation are presented. Figure 2 shows SEM images of the specimens collected from nanocomposites with different sintering temperature, from lower temperature (Figure 2) to higher (Figure 2C). s can be seen, the main components are a mixture of nanoparticles and microparticles. The nanoparticles may contact with the adjacent ones, but the nano aggregation lengths are short, less than 10-fold of the microparticle diameter on average (Figure 2). s the sintering temperature increases, particle diffusion becomes more and more obvious. The aggregation length becomes much longer, resulting in the formation of one-dimensional jointed particle assemblies developing into a smooth continuous network (Figure 2C). Conductivity measurements show that the resistance drops 50-90% from 200 C to 275 C. In contrast, the silver-carbon nano-micro systems show a much different morphology as can be seen in Figures 3 and 3. They do not follow the same sintering mechanism as observed for the nanocomposite shown in Figures 2 and 2C, where nanoparticles sinter, grow larger particles, and eventually diffuse with microparticles to form a continuous network. For the silver-carbon system, nanoparticles sinter

4 first and form a continuous network (Figure 3), but microparticles maintain their identity, as if they didn t sinter with temperature. t higher sintering temperature (275 C), micro particles start to diffuse with the sintered nanoparticles and form a continuous smooth surface. Resistance (milliohm) Temperature ( 0 C) silver-1 10 % C 20% C 35%C silver-2 Figure 4 : Resistance as a function of temperature for carbonbased and silver-based nano-micro pastes (same size: 3 long, 1/3 wide) Conducting carbon and metal-coated polymers are typically low density systems. The reliability of the joint formed between the conductive adhesive and the metal pad to which it is mated when forming the z-interconnect structure is of utmost importance. ddition of low density carbon or metal-coated polymer will reduce the interfacial problem. variety of carbon, silver and metal-coated polymers blended with appropriate polymers and cured at ~200 o C to 275 o C for 2 hours, showed low volume resistivity, in the range of 10 4 to 10 6 ohm-cm, which is similar to that of micro-filled adhesives. Volume resistivity decreases with increasing curing temperature due to sintering of metal particles. There is a significant resistivity drop with increasing curing temperature from 200 o C to 275 o C. Figure 4 shows resistance of carbon-based and pure silver-based nano-micro conducting adhesives as a function of curing temperature. Nano-micro adhesives modified with carbon particles showed similar resistance as nano-micro filled adhesives when cured at 275 C. Conductivity measurements show that the 10-20% carbon containing adhesive showed a 90% resistance drop when cured at 275 C instead of 200 C. Resistance decreases with increasing curing temperature due to sintering of metal particles. Figure 5 shows grain growth for different nanoparticles when cured at 100 C to 275 C. For the pure 5 nm silver system, grain size increases with increasing curing temperature from 100 o C to 150 o C, and eventually converts into a smooth, continuous structure. Here, if the surface as viewed in the micrograph is visually smooth (no grain), a 10 µm grain size is assigned for purposes of plotting the data. Grain growth is more complicated for nano-micro systems. Sintering behaviors of adhesives having 5 µm particles modified by addition of (5 +30) nm, or 15 nm, or 30 nm particles were completely different from each other, but all required 240 o C for complete sintering. lthough pure 5 or 15 nm particles sinter at 200 o C, in nano-micro systems diffusion is different. Here nanoparticle to microparticle diffusion will be more important than nano-nano diffusion. Nanoparticle size up to around 30 nm is sufficient for complete diffusion at around 240 o C. ut larger particles, such as the 80 nm particle system, require at least 275 o C for complete diffusion. Table 1 summarizes sintering temperatures, and shows micrographic views of nano, micro and nano-micro systems. Figure 5 reveals a sintering temperature plot for 5 nm particles to 5 µm particles. s expected, ECs with 5 µm particles require a higher temperature for sintering. This kind of plot helps to understand particle size requirements for nano-micro systems. Grain size (micron) nm 5+30 nm 15 nm 30 nm 80 nm Temperature ( 0 C) Figure 5: () Grain growth as a function of curing temperature for nano and nano-micro systems. (5 nm = pure silver nanoparticle, all other systems are nano-micro, where nanoparticles were mixed with 5 µm particles), and () sintering temperature as a function of particle size (inset: enlarged data for particle sizes below 100 nm).

5 Table 1 : Sintering temperature for different nano and nano-micro systems. The micrograph in the table shows how a small grains structure is eventually transferred to a no-grain, continuous structure. Carbon-based adhesives sometimes show some level of flexibility. In general adhesives having high strength are generally high modulus adhesives with limited flexibility. Such adhesives having a high modulus create a high stress condition in the Z-interconnect joints due to extreme differences in the coefficient of thermal expansion (CTE) between the dielectric and the Cu pad. The result of the high stress is cracking/delamination during assembly reflow. Carbon-based adhesives with appropriate composition show some level of flexibility. variety of carbon-based and silicone based, blended, low stress adhesives were formulated. These adhesives offer low stress and moderate adhesion strength. Figure 6 shows some bendable adhesives. These adhesives show low resistance, in the range of 20 milliohms. Figure 6: Carbon-silver based (black) and silicone-silver based adhesive.

6 Filler loading (vol.%) Figure 6: CTE versus loading. CTE (ppm/ 0 C) conducting adhesives. Low Tg epoxy-based adhesives have very high Z-axis CTE, in the range of 226 ppm/ o C, and may not be good for a Z-joint. Low Tg epoxy-based conductive adhesive joints will experience high stress during assembly reflow. CTE of the subcomposites and conductive adhesives is crucial to obtaining robust, reliable joining between dielectric layers, and between the conductive paste and the opposing copper pad. CTE of the dielectric and paste materials must be close to each other to survive assembly reflow, especially high temperature lead free reflow. In the present paper, we systematically investigate package CTE for various constructions. Figure 6C: Z-axis CTE (lpha) calculated from the graph, for high Tg and low Tg epoxy-based conductive adhesives. This work also deals with Z-expansion issues between adhesives and the substrates in which they are incorporated. The effect of carbon on the CTE of adhesive joints was measured using a Q200TEM (Texas Instruments). Highly filled (~70V%) nano-micro-filled adhesive has Z-CTE of 18 ppm/ o C. ddition of polymer enhances the Z-CTE. For example, 50 vol% silver system has CTE of 41 ppm/ o C. Most of the highly filled carbon-doped samples have CTEs in the range of ppm/ o C depending on composition. highly filled metal-coated polymer system also has a low Z-CTE of 28 ppm/ o C. highly filled silver-based system has the lowest CTE for the systems investigated. s silver is replaced with lower density particles such as carbon or metal-coated polymer, the CTE of the paste increases. Figure 6 shows the variation of CTE for adhesives with varying filler content. CTE decreases with increasing filler content. dhesives fabricated from 40-55% v/v metal epoxy nanocomposites showed z-axis CTE in the range of ppm/ o C. Thermal expansion of adhesives fabricated from ~70% v/v nanocomposite showed CTE of about 18 ppm/ o C. The glass transition temperature (Tg) is an important parameter for conducting adhesives. It indicates the temperature at which the resin matrix is transformed into a soft elastic state. Figure 6C shows Z-axis CTE of low and high Tg epoxy-based Figure 7: Conducting adhesive filled PTH before () and after () thermal cycle. Conductive adhesive filled plated through holes (PTH) were investigated (see Figure 7). For high Z-CTE conducting adhesives, PTH as well as conductive adhesive will expand during thermal processing, stressing, and assembly reflow, resulting in bulging or cracks, and delamination in the joints (Figure 7). The root of this problem is the extreme Z-axis CTE mismatch between the copper plating and the conducting adhesives. This mismatch is especially extreme at assembly temperatures above the glass transition temperature (Tg) of the dielectric layers, where conducting adhesive Z-axis CTE approaches ppm/ o C, compared to 15 ppm/ o C for copper. For these kinds of structures, it is desirable to use low CTE, high Tg conducting adhesives. The Z-CTE of conducing adhesive can be reduced by adding excess filler. Conducting adhesive consisting of 60V% or higher filler are preferred as PTH fill materials to reduce Z-CTE mismatch. s a case study, mixed dielectric was used for composite structures. y alternating 2S/1P and 0S/1P cores in the lay-up prior to lamination, the conductive paste electrically connects

7 copper pads on the 2S/1P cores that reside on either side of the 0S/1P core. Two signal layers are added to the composite structure each time one adds an additional 2S/1P core and an additional 0S/1P core. structure with four signal layers composed of five subcomposites (two 2S/1P cores and three 0S/1P cores) is shown schematically in Figure 8. photograph of a composite laminate structure is shown in cross section in Figure 8. Theoretically the Z-CTE of this stack will be around 33 ppm/ºc. Optimized metal-epoxy adhesives with Z-CTE around 40 ppm/ºc were used for hole fill applications to fabricate Z-axis interconnections. s we introduced low Z-CTE dielectric materials, the predicted CTE will be higher than lower CTE dielectric (dielectric 2) but lower than higher CTE dielectric (dielectric1). The mixeddielectric composite has Z-CTE of 41 ppm/ºc. Z- CTE was measured at the region having the maximum density of pastefilled vias. Here, the measured Z-CTE is higher than predicted. This may be due to the fact that paste volume, paste CTE, and Cu-dielectric interaction were not considered in the calculation. Figure 8: Schematic () of LCC having four signal wiring planes with a stripline transmission line structure, and photograph () of actual z-interconnect LCC with vias having 55 μm diameter shown in cross section. The Z-CTE of glass cloth-reinforced dielectric composites is more complicated. Figure 9 shows various multilayer substrates consisting of Cu and modified glass-reinforced epoxy. Substrate thickness varies from 552 µm to 1656 µm. The 552 µm thin substrate has a Z-CTE of 69 ppm/ºc. The 1102 µm and 1656 µm thick laminates have Z-CTEs of 68 ppm/ºc and 81 ppm/ºc, respectively. s the laminate thickness of the core material increases from 552 µm to1655 µm, the Z-CTE increases to 81 ppm/ºc. This illustrates that the Z-CTE of a thick core is more sensitive to Cu dielectric interaction. Overall, the Z-CTE of laminate is more likely to follow the epoxy Z-CTE. Conducting adhesive with Z-CTE ~70 ppm/ºc will require more polymer, and may not be suitable for achieving high conductivity. In this case, low Z- CTE paste, close to the CTE of Cu, will be favorable. Figure 10 shows joint structures with low CTE paste as a typical representative example. Conclusions variety of micro-filled conducting adhesives modified with nano particles, carbon, and metal-coated polymer fillers were used for z-axis interconnection applications. ddition of nanoparticles reduces sintering temperatures of micro-filled conducting adhesives. Conducting adhesives having only nanoparticles will sinter at significantly lower temperature than nano-micro systems. Nanoparticles in the range of 5-30 nm size diffuse with micro particles at relatively lower temperature than 80 nm particles. ECs having particles ranging from 5-30 nm in the nano-micro system follow similar growth kinetics. Carbon, silicone and blended systems produce low stress, bendable joints. ll high Tg adhesives maintained high electrical conductivity and low Z-CTE. Carbon doping levels of 10% (wt/wt) or less are suitable for joining cores. Investigation of a variety of dielectric constructions was used to optimize package CTE. Overall, Z- CTE of paste in the range of 40 ppm/ºc or less is suitable for filled dielectrics. For glass cloth-reinforced dielectric, a Z- CTE of paste close to that of Cu is favorable. Conductive adhesives having Tg above reflow temperature and low Z- axis CTE are the most suitable for obtaining low stress Z- joints. C Figure 9: Multilayer substrates with glass cloth-reinforced pre-preg () 1656 µm thick, () 1102 µm thick, and (C) 552 µm thick.

8 Figure 11: Z-interconnect joint structure. cknowledgments The authors acknowledge the valuable contributions of S. Hurban, G. Kohut, and D. Thorne. 9. Das R. N., Egitto F. D., Markovich V. R., Nano- and Micro-Filled Conducting dhesives for Z-axis Interconnections: New direction for high-density, highspeed, organic microelectronics packaging Circuit World 2008,34(1), Rowlands M. and Das R.N., Manufacture and Characterization of a Novel Flip-Chip Package Zinterconnect Stack-up with RF Structures IMPS (International Microelectronics and dvanced Packaging Society) 40th International Symposium on Microelectronics (November11-15, 2007)pp Rowlands R. and Das R.N., Electrical Performance of an Organic, Z-interconnect, Flip-Chip Substrate 57 th Electronic Components and Technology Conference proceedings (May 29 June1, 2007)pp Kevin Knadle, Reliability and Failure Mechanisms of Laminate Substrates in a Pb-free World IPC pex 2009 (submitted). References 1. Liu, J., 1999, Conductive dhesives for Electronics Packaging, (ritish Isles: Electrochemical Publications Ltd, 1999), pp Liu, J., Rorgren, R., and Ljungkrona, L., 1995, High Volume Electronics Manufacturing Using Conductive dhesives for Surface Mounting, J. Surf. Mount Technol., Vol. 8, No2 (1995), pp Ye, L., Lai, Z., Liu, J., and Tholen,., 1999, Effect of g Particle Size on Electrical Conductivity of Isotropically Conductive dhesives, IEEE Trans. Electron, Packag., Manuf., Vol. 22,(1999), pp Yasuda, K., Kim, J. M., Rito, M., and Fujimoto, K., Joining Mechanism and Joint Property by Polymer dhesive with Low Melting lloy Filler, Int. Conf. on Electron. Packag., (2003),pp Yasuda, K., Kim, J. M., Yasuda, M., and Fujimoto, K., New Process of Self-Organized Interconnection in Packaging by Conductive dhesive with Low Melting Point Filler, Int. Conf. on Solid State Devices and Materials,(2003), pp Yasuda, K., Kim, J. M., and Fujimoto, K., dhesive Joining Process and Joint Property with Low Melting Point Filler, 3rd Int. IEEE Conf. on Polymer and dhesives in Microelec. and Photon., (2003), pp Yao, Q., and Qu, J., 2002, Interfacial versus Cohesive Failure on Polymer- Metal Interfaces in Electronic Packaging Effects of Interface Roughness, SME J. Electron. Packag., Vol.124 (2002), pp Egitto, F.D., Krasniak, S.R., lackwell, K.J., and Rosser, S.G., 2005, Z-xis Interconnection for Enhanced Wiring in Organic Laminate Electronic Packages, Proceedings Fifty-Fifth Electronic Components and Technology Conference, May 31 to June 3, 2005, Lake uena Vista, FL (IEEE, Piscataway, NJ, US),(2005), pp