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					PCBFABRICATION
                 Nano- And Micro-Filled Conducting
                 Adhesives For Z-axis Interconnects
                                                                       by Rabindra N. Das, John Lauffer, Frank D. Egitto and Voya Markovich,
                                                                                                          Endicott Interconnect Technologies

                 We take a look at micro-filled               these approaches possess inherent       posite structure. Conductive joints
                 epoxy-based conducting adhesives            limitations, for example those re-      can be formed during lamination
                 modified with nanoparticles for z-           lated to drilling and plating of high   using an electrically conductive
                 axis interconnections, especially as        aspect ratio vias, reduced conduct-     adhesive. As a result, one is able to
                 they relate to package level fabri-         ance of narrow circuit lines, and       fabricate structures with vertically-
                 cation, integration, and reliability.       increased cost of fabrication related   terminated vias of arbitrary depth.
                 A variety of conducting adhesives           to additional wiring layers. One        Replacement of conventional plated
                 with particle sizes ranging from            method of extending wiring den-         through holes with vertically-ter-
                 80 nm to 15 µm were incorporated            sity beyond the limits imposed by       minated vias opens up additional
                 as interconnects in printed wir-            these approaches is a strategy that     wiring channels on layers above
                 ing board (PWB) or laminate chip            allows for metal-to-metal z-axis        and below the terminated vias and
                 carrier (LCC) substrates. SEM and           interconnection of subcomposites        eliminates via stubs which cause
                 optical microscopy were used to in-         during lamination to form a com-        reflective signal loss.
                 vestigate the micro-structure, and
                 conducting and sintering mecha-                                                     During the past few years, there has
                 nisms. Volume resistivity of nano-                                                  been increasing interest in using
                 particle-modified adhesives is in the                                                electrically conductive adhesives
                 range of 10-5 to 10-6 ohm-cm. The                                                   as interconnecting materials in the
                 present process allows fabrication                                                  electronics industry. Conductive
                 of z-interconnect conductive joints                                                 adhesives are composites of poly-
                 having diameters in the range of 55-                                                mer resin and conductive fillers.
                 300 microns. There was no delami-                                                   Metal–to-metal bonding between
                 nation of conductive joints after 3X                                                conductive fillers provides electri-
                 IR-reflow (assembly precondition),                                                   cal conductivity, whereas a poly-
                 pressure cooker test (PCT), and sol-                                                mer resin provides better process-
                 der shock. The processes and mate-                                                  ability and mechanical robustness.
                 rials used to achieve smaller feature
                 dimensions, satisfy stringent regis-        Figure 1 – A variety of adhesive-filled microvia structures. Adhesive
                 tration requirements, and achieve           consists of polymer and (A) nano particle, (B) controlled size micro
                 robust electrical interconnections          particle, (C) nano-micro particle mixture, (D) nano tube/wire – micro
                 are discussed.                              particle mixture, (E) micro particle-sheet/flake (2D), and (F) micro/nano
                                                             –low melting point particles
                                                             Figure 2 – Micrographs for the cross-sectional view of adhesives (A)
                                                             Low melting point (LMP) alloys, (B) Silver micro particles, (C) Cu micro
                 The needs of the semiconductor              particles, and (D) mixture of silver nano and micro particles
                 marketplace continue to drive den-
                 sity into semiconductor packages.
                 The high end of this market appears
                 to be standard Application-Specific
                 Integrated Circuits (ASICs), struc-
                 tured ASICs, and Field-Program-
                 mable Gate Arrays (FPGAs). These
                 devices continue to need increasing
                 signal, power, and ground die pads,
                 and a corresponding decrease in pad
                 pitch is required to maintain reason-
                 able die sizes. Traditionally, greater
                 wiring densities are achieved by re-
                 ducing the dimensions of vias, lines,
                 and spaces, increasing the number
                 of wiring layers, and utilising blind
                 and buried vias. However, each of


                 OnBoard Technology October 2008 - page 14                                                      www.Onboard-Technology.com
                                                                                                                           PCBFABRICATION
                                                                                 conductivities of sub-micrometer
                                                                                 sized nickel particles used for elec-
                                                                                 trically conductive adhesive. Inoue
                                                                                 et al investigated the variations in
                                                                                 electrical properties of a typical iso-
                                                                                 tropic conductive adhesive (ICA)
                                                                                 made with an epoxy-based binder
                                                                                 that are caused by differences in the
                                                                                 curing conditions. Coughlan et al
                                                                                 described electrical and mechani-
                                                                                 cal analysis of conductive adhesives
                                                                                 where the main properties of joint
                                                                                 resistance and adhesive strength
                                                                                 were examined before and after dif-
                                                                                 ferent environmental treatments.
                                                                                 Fu described cluster effects of nano
                                                                                 fillers in conductive adhesives. San-
                                                                                 cakter et al reported pressure-de-
                                                                                 pendent conduction behaviour with
                                                                                 particles of different sizes, shapes,
                                                                                 and types. The effects of external
                                                                                 pressure on the filler resistance
                                                                                 were measured. Jiang et al reported
                                                                                 on surface functionalised nano sil-
                                                                                 ver-filled conductive adhesives. Li
                                                                                 reported that self-assembled mon-
                                                                                 olayers (SAMs) protected silver
                                                                                 nano-particle-based conductive ad-
Figure 3 – SEM micrographs for the polymer nano-micro-composite filled            hesives. Although several compos-
silver based conducting adhesives; (A) un-sintered at 200°C, (B)-(D)             ites are available for the advance of
sintered at (275 +10)°C, (E) un-sintered at 300°C, and (F) sintered micro-       semiconductor technology, there is
composites at 365°C                                                              potential scope for improvement of
                                                                                 the existing materials, so that low
Conductive adhesives usually have        for these mixed-sized silver parti-     processing temperature, flexible,
excess filler loading that weaken         cle-filled conductive adhesives. Goh     reliable processes and material can
the overall mechanical strength.         et al mentioned the effect of an-       be developed for Z-axis intercon-
Therefore, reliability of the conduc-    nealing on the morphologies and         nections. Furthermore, all studies
tive joint formed between the con-
ductive adhesive and the metal sur-      Figure 4 – Parallel lamination of subcomposites (cores) to form laminate
face to which it is mated is of prime    chip carrier having four signal wiring planes with a stripline transmission
importance. Conductive adhesives         line structure
can have broad particle size distri-
butions. Larger particles can be a
problem when filling smaller holes
(e.g., diameter of 60 µm or less), re-
sulting in voids. Several nano-and
micro-filled adhesives have been
reported for advanced packaging
applications. For example, Xiao et
al describes epoxy or silicone based
conductive adhesive joints and their
thermal and mechanical stabilities.
Jeong et al reported the effect of
curing behaviours, solvent evapora-
tion and shrink, on conductivity of
adhesives. They also described con-
ductivity of micro filled adhesives
upon addition of nanoparticles. Lee
reported on the addition of nano-
sized silver particles to micro-sized
flakes, and the effect on resistivity


www.Onboard-Technology.com                                                     OnBoard Technology October 2008 - page 15
PCBFABRICATION




                 Figure 5 - SEM micrographs of adhesive-filled joining core; (A) 55 micron hole diameter, and (B) higher
                 magnification

                 have described materials prop-              tion; (2) fabrication of z-intercon-      Adhesives were characterised by
                 erty and reliability assessment at a        nect, and (3) reliability of the inter-   Scanning Electron Microscopy
                 macroscopic level but have never            connect package.                          (SEM) and optical microscopy to
                 described device level fabrication,                                                   ascertain particle dispersion and
                 integration and reliability issues.                                                   interconnection mechanism. A
                 Conductive adhesives without de-            Experimental procedure                    Keithley micro-ohmmeter was used
                 vice level integration will be of less                                                for electrical measurements.
                 importance for Z-axis intercon-             A variety of silver, copper, and low
                 nects.                                      melting point (LMP)-based nano
                                                             and micro particles and their dis-        Nano-micro and micro filled
                 The objective of this present study is      persion into epoxy resin were inves-      conductive adhesives
                 to investigate the effect of nanopar-       tigated in order to achieve uniform
                 ticle addition to microcomposites.          mixing in the adhesive. In a typical      Nanoparticle generally refers to the
                 Nanoparticles of silver were chosen         procedure, epoxy-based conductive         class of ultra fine metal particles
                 because of their higher electrical          adhesives were prepared by mixing         with a physical structure or crystal-
                 conductivity and chemical stability.        appropriate amounts of the con-           line form that measures less than
                 Nanoparticles were mixed with mi-           ducting filler powders and epoxy           100 nanometers (nm) in size. They
                 croparticles to improve the sinter-         resin in an organic solvent. For          can be 3D (block), 2D (plate), 1D
                 ing behaviour of the adhesives. The         conductivity measurements, a thin         (tube or wire) structures. In gen-
                 paper presents a reliability assess-        film of this paste was deposited on        eral, nanoparticle-filled conductive
                 ment of nanocomposite joints con-           a substrate and cured at different        adhesives are defined as containing
                 ducted by testing samples exposed           temperatures ranging from 150°C           at least some percentage of nanos-
                 to pressure cooker tests (PCT), IR-         to 365°C. For reliability assess-         tructures (1D, 2D, and/or 3D) that
                 reflow, and solder shock. The work           ments, two paste films were lami-          enhance the overall electrical con-
                 was extended to the development             nated together.                           ductivity or sintering behaviour of
                 of a z-axis interconnect construc-                                                    the adhesives. Figure 1 represents a
                 tion for a laminate chip carrier and        In the fabrication of a high-density      theoretical comparative model for a
                 printed wiring board (PWB). The             laminate chip carrier, a joining core     variety of possible structures based
                 structure employs an electrically           consisting of a single metal refer-       on powder filling a microvia. In
                 conductive medium to intercon-              ence plane and no circuit traces          this instance, the volume of the mi-
                 nect thin cores (subcomposites).            for signal transmission (0S/1P) was       crovia is constant for all six cases.
                 The cores are processed in paral-           constructed using a copper power          Conductivity is achieved through
                 lel, aligned, and laminated to form         plane, 35 µm thick, sandwiched be-        metal-metal bonding. Increasing
                 a composite. The net effect is a            tween layers of a dielectric material     the number density of particles
                 composite laminate having vertical          composed of silica-filled allylated        increases the probability of metal-
                 interconnections with small diam-           polyphenylene ether (APPE) poly-          metal contact. Each contact spot
                 eter holes that can terminate arbi-         mer. Through holes in the joining         possesses a contact resistance. For
                 trarily at any layer within the cross       cores, formed by laser drilling, and      microparticles, the number den-
                 section of the package. There is no         having diameters ranging from             sity of particles will be much less
                 requirement for PTHs to be formed           50 to 75 µm, were filled with an           than for nanoparticles. Therefore,
                 at the composite level. This effort is      optimised electrically conductive         microparticle-filled vias will tend
                 an integrated approach centring on          adhesive. The adhesive-filled join-        to have a lower contact resistance,
                 three interrelated fronts: (1) mate-        ing cores were cured and cross sec-       although the probability of parti-
                 rials development and characterisa-         tioned to evaluate hole fill quality.      cle-particle contact will be less. In


                 OnBoard Technology October 2008 - page 16                                                       www.Onboard-Technology.com
                                                                                                                            PCBFABRICATION
the case of a nano-micro mixture,        from nanoomposites with different        tion suggests that the sintering
the micro-scale particles could          sintering temperature, from lower        mechanisms are different for the
maintain a low contact resistance,       temperature (Figure 3A) to higher        nanocomposites synthesised in the
whereas nano-scale particles can         (Figure 3E). As can be seen, the         two different mixtures. Based upon
increase number of particle con-         main components are a mixture of         the morphologies observed above,
tacts. Nano- and microparticle mix-      nanoparticles and microparticles.        we suggest a sintering mechanism
tures could be nanoparticle-micro-       The nanoparticles may contact with       for the nanocomposites at low tem-
particle, nanoplate (2D)-micropar-       the adjacent ones, but the nano ag-      perature as follows.
ticle, nanotube (1D)-microparticle,      gregation lengths are short, less
or any combination of these three        than 10-fold of the microparticle        In the high-concentration region,
cases. Another possibility is use of     diameter on average (Figure 3A). As      nanoparticles are highly reactive
low melting point (LMP) filler. The       the sintering temperature increas-       due to immediate particle to par-
LMP filler melts and reduces inter-       es, particle diffusion becomes more      ticle contact. Moreover, the diffu-
particle resistance. Hence, conduc-      and more obvious. The aggregation        sion (sintering) of nanoparticles
tive adhesives can be categorised as     length becomes much longer, re-          should be higher than that of the
nano, micro, nano-micro, or LMP          sulting in the formation of one-di-      corresponding bulk solid. With the
based systems.                           mensional jointed particle assem-        increase of size, the particles need
                                         blies developing into a smooth con-      higher temperature for diffusion to
Figure 2A shows a cross section of       tinuous network (Figures 3B-D).          make a uniform metallic network.
a LMP-based adhesive. LMP melts          Conductivity measurements show           Figure 3F shows sintering at 365
and produces a continuous metal-         that the resistance drops 30-50%         oC for a microcomposites where
lic network. In the silver adhesive,     from 200°C to 265°C. In contrast,        minimum particle size in the range
the average filler diameter is in the     the nanocomposites synthesised           of 5 microns. However, in the low-
range of 5 µm. Filler loading was        with a nano-micro mixture show a         concentration region (metal con-
high and adjacent particles united       much different morphology as can         centration approximately 84%), the
mutually and necking phenomena           be seen in Figures 3E. The nanopar-      polymer plays an important role. In
between fillers occurred; namely, a       ticles are less (low concentration       this region, the amount of polymer
conduction path was achieved, as         approximately 84% metal). They           is sufficient to prevent metallic dif-
shown in Figure 2B. A similar result     are not following the same sinter-       fusion/sintering (Figure 3E) even
was observed when silver particles       ing mechanism as observed for the        for 80 nm particle.
were replaced by 4 µm Cu particles       nanocomposite shown in Figures
(Figure 2C). A variety of silver filled   3B-D. Instead, most of the particles
adhesives with a mixture of nano         maintain their identity, as if they      Core fabrication
and micro particles were studied.        didn’t sinter with temperature.
In nano-micro mixtures, nano par-        Figure 3 shows nanocomposites            Nanocomposites were used for hole
ticles occupy interstitial positions     sintered at lower temperature and        fill applications to fabricate z-axis
to improve particle-particle contact     higher temperature. The observa-         interconnections in laminates.
for conductivity. For the silver nano
particles (~80 nm size), the fillers      Figure 6 – Photograph of nanocomposite filled z-interconnect chip
can self sinter and make a con-          carriers with vias having 55 micron diameter shown in cross section
tinuous conduction path. A high
surface area of silver nanoparticles
needs an excess amount of solvent
in order to make high loading silver
paste. Figure 1D represents micro
structures of nano-micro silver
filled adhesives.


Sintering

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 investiga-
tion of electrical resistance behav-
iour of silver nanocomposites has
been carried out, and the results
of such an investigation are pre-
sented here. Figure 3 shows SEM
images of the specimens collected


www.Onboard-Technology.com                                                      OnBoard Technology October 2008 - page 17
PCBFABRICATION
                 Conductive joints were formed               on either side of the 0S/1P core.      tive joints in the test vehicle were
                 during composite lamination us-             Two signal layers are added to the     further examined by IR-reflow (3X,
                 ing electrically conductive nano-           composite structure each time one      225°C), PCT and solder shock. No
                 composites. Z-axis interconnection          adds an additional 2S/1P core and      intrinsic failure mechanisms were
                 was achieved using joining cores.           an additional 0S/1P core. A struc-     observed. There was no cracking
                 Through holes in the joining cores,         ture with four signal layers com-      or delamination at the paste joints.
                 formed by laser or mechanical               posed of five subcomposites (two        Conductive joints are stable even
                 drilling and having diameters rang-         2S/1P cores and three 0S/1P cores)     after multiple IR-reflow (3X), and
                 ing from 50 µm to about 300 µm,             is shown schematically in Figure 4.    PCT followed by a 15 seconds sol-
                 were filled with an nanocomposite            Although this particular construc-     der dip.
                 based electrically conductive ad-           tion comprises alternating 2S/1P
                 hesive. The adhesive-filled joining          and 0S/1P cores, it is possible to
                 cores were laminated with circui-           place multiple 0S/1P cores adjacent    Conclusions
                 tised subcomposites to produce a            to each other in the stack.
                 composite structure. Lamination                                                    A variety of micro-filled conduct-
                 was used to cure the adhesive in            Figures 5 shows SEM micrographs        ing adhesives modified with nano
                 the composite and provide Z-inter-          of a joining core having paste-filled   particles were used for a z-axis in-
                 connection between the circuitised          holes with a diameter of 55 µm as      terconnection applications. High
                 subcomposites. A variety of joining         a typical representative example. A    aspect ratio, small diameter holes
                 core structures such as 0S/1P (P=           photograph of a composite laminate     anywhere in the range of 55 to 300
                 Power, S= signal), 0S/2P, etc. were         structure is shown in cross section    microns were successfully filled.
                 used for hole fill applications. The         in Figure 6. Proper preparation of     Addition of nanoparticles reduces
                 cores can be structured to contain          the subcomposites is crucial to ob-    sintering temperatures of micro-
                 a variety of arrangements of signal,        taining robust, reliable joining be-   filled conducting adhesives. Excess
                 voltage, and ground planes. In ad-          tween dielectric layers and between    polymer (16% or higher) based
                 dition, signal, voltage, and ground         the conductive paste and the op-       adhesives were less sensitive to
                 features can reside on the same             posing copper pad. Sufficient flow       sintering. Conductive joints were
                 plane.                                      of the dielectric materials must be    stable after 3x IR-reflow, PCT, and
                                                             achieved during lamination to al-      solder shock. The nanocomposite-
                 By alternating 2S/1P and 0S/1P              low for complete encapsulation of      filled joining cores were laminated
                 cores in the lay-up prior to lami-          circuitised features and achieve       with circuitised subcomposites to
                 nation, the conductive nanocom-             good dielectric-to-dielectric bond-    provide stable, reliable z-intercon-
                 posite electrically connects copper         ing. Package level and sub-com-        nections among the circuitised
                 pads on the 2S/1P cores that reside         posite level reliability of conduc-    subcomposites.


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                 OnBoard Technology October 2008 - page 18                                                     www.Onboard-Technology.com

				
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