Silver-filled Epoxy Research Articles

Preservable superhydrophilicity of thermally cured graphene-nanoplatelets/epoxy nanocomposite coatings

This study aims to preserve the superhydrophility and the ultrafast water permeation property of the thermally cured graphene nanoplatelets (GNPs) coating after epoxy is added to enhance the adhesion of coating on its substrate. Addition of epoxy generally encourages the formation of carbon-carbon chain in the polymerization process during the thermal curing and the GNPs/epoxy nanocomposite becomes hydrophobic. On the other hand, the silver-filled epoxy can effectively suppress the crosslinking of carbon-carbon chain, enabling the thermo-oxidation process to take place where the oxygenated functional groups are attached on the edges of the GNPs. Hence, the use of silver-filled epoxy is capable to preserve the superhydrophilicity of GNPs. A series of comprehensive micro/nanoscopic and spectroscopic characterizations are performed to explain the significant difference in the wettability of the GNPs/epoxy nanocomposites. Molecular dynamics (MD) simulations are employed to elucidate the phenomena in the atomic resolution and nanosecond time scale. The MD results concur well with the characterization results, further verifying the proposed explanation of the interactions of carbon-filled epoxy and silver-filled epoxy with the graphene nanostructure during the thermal curing process. This study provides important insights into the analyses and characterization of the preservable superhydrophilicity of thermally cued GNPs/epoxy nanocomposites.

Effective Method for Wire Bonds Rework Using Conductive Epoxy

The development of wire bonded chips requires us to go through multiple designs to achieve the best packaging performances. Nevertheless, some of the designs may not be tested due to defects preventing the wire bonds to be effectively completed. Unless an effective way to rework such failed assemblies is feasible, the invested resources in the design and manufacture are lost. The MiQro Innovation Collaborative Center, Bromont, QC, Canada, has developed a method to rework such assemblies. In this paper, we present a method where two silver-filled conductive epoxies are used to repair failed gold wire bonds. The mechanical adhesion and electrical properties of such rework assemblies were compared to reference gold wire bonds. The rework method consists of reattaching the sheared end of the wire to the landing pad on which a drop of conductive epoxy is deposited. Each group is at least composed of 40 samples for statistical analysis of the method reproducibility. Experiments were conducted on two identical test chips with gold pads on each substrate. A matrix of 1-mil-diameter gold wire bonds was realized. Then, on each substrate, the first group of wire bonds was used as a reference and the others were sheared at the stitch bond and reworked with two different silver-filled conductive epoxies. The pull tests revealed that all the wires (reference and reworked with epoxy 1 and 2) rupture at the neck of the first ball bond at a similar break load (~12 g). Visual inspection using the cross section of the reworked area showed no visual defect. We could also see that the deposited drop from epoxy 1 is much more likely to make a better electrical connection due to the presence of smaller silver flakes and a better capability to wet both the wire and the pad. The electrical resistance of reference and reworked wire bonds were measured on a two-probe station. On both samples, reworked wire bonds with epoxy 1 have similar resistance (94 $\text{m}\Omega$ ) than the reference (91 $\text{m}\Omega$ ) ones. The ones reworked with epoxy 2 have an electrical resistance (214 $\text{m}\Omega$ ) more than twice than the reference ones. Reliability testing showed increasing electrical resistance throughout 1000 deep thermal cycling cycles for epoxy 2, while epoxy 1 remained stable until 500 cycles. We demonstrated that failed wire bonds could be effectively reattached to the substrate using the appropriate conductive epoxy. This method enables us to reduce development costs by reworking failed considered assemblies to avoid the waste of resources and delays in the run to the market for the new product.

Role of Transparent Die Attach Adhesives for Enhancing Lumen Output of Midpower LED Emitters With Standard MESA Structure

The role of die attach adhesives (DAAs) in influencing the lumen output of midpower (30–150-mA input power) blue and white light emitting diodes (LEDs), which currently dominate the LED backlighting and general solid-state lighting, is studied. It is reported for the first time that a replacement of conventional silver-filled epoxy DAA by an optically transparent DAA can lead to a significant enhancement of light output as high as 13% and 21% increase for midpower blue and white LEDs, respectively. The mechanism for the observed enhancement is discussed in this paper.

A method enabling height-control of chips for edge-emitting laser stacking

In this paper we present a stacking method that enables the mounting of edge-emitting laser (EEL) devices at compulsory positions for feeding a coupling waveguide at a low temperature, i.e., 110 °C. Bonding laser chips are integrated to an interposer with embedded waveguides using an electrically conductive adhesive (ECA), i.e., silver-filled epoxy resin that relied on the surface tension phenomenon. A simple dispensing technique based on the capillary effect was used to generate adhesive droplets for bonding purposes. The ability to control the bonding height from 3 to 25 µm was demonstrated. While the post-bond in-plane offsets of the bonding EEL chip are determined by the accuracy of the bonder (i.e., within 2 µm range), the bonding height can be engineered, with a deviation within 1 µm, by balancing the external bonding forces with the surface tension of a certain adhesive volume. The reliability of the bond, i.e., bond strength of 106 gf, and heat dissipation function of the ECA, were also validated. The results obtained in this work imply that the use of this low-temperature, height-controllable approach for the stacking of EEL continues to look highly promising.

Energetic semiconductor bridge device incorporating Al/MoOx multilayer nanofilms and negative temperature coefficient thermistor chip

The design, fabrication, and characterization of an energetic semiconductor bridge device are presented. The device consists of a semiconductor bridge heating element, which has been selectively coated with Al/MoOx multilayer nanofilms to enhance ignition of a conventional pyrotechnics. Integrated negative temperature coefficient thermistor chip provides protection against electromagnetic and electrostatic discharge events. The device was specifically configured to allow ease of interconnection by wire bonds and silver-filled conductive epoxy. Extensive design validation testing was performed. The device has demonstrated low, predictable firing energy and insensitivity. Al/MoOx multilayer nanofilms have no distinct influence on the electrical properties of semiconductor bridge. Nanothermite reaction provides reliable ignition by being able to ignite across a gap.

Silver-filled epoxy composites: effect of hybrid and silane treatment on thermal properties

The combination effects of hybrid nano–micron fillers and filler treatment on the thermal properties of silver-filled epoxy composites are experimentally evaluated. These hybrid composites are fabricated using two different sizes and shapes of silver particles, namely 80 nm with spherical shape and 4–8 μm with flaky shape. In this study, the ratio of silver flakes to silver nanoparticles was varied from 100:0, 75:25, 50:50, 25:75, and 0:100 at a fixed silver loading of 6 vol.%. The silver fillers are treated with 3-aminopropyl triethoxysilane (3AMPTES) at different concentrations of 5, 10, and 30 wt%. The hybrid micro:nano at 50:50 shows the highest storage modulus and the lowest coefficient of thermal expansion (CTE) value compared with other ratios. The silver fillers with 10 wt% of 3AMPTES show improvement in storage modulus, CTE, and thermal stability compared with untreated and further increasing of 3AMPTES at 30 wt% did not show any significant improvement.

A study of single-crystal silicon diodes integrated on flexible substrates using conductive adhesives

In this study, we investigated the possibility of fabricating flexible single-crystal Si-Pd diodes on polymer substrates using silver-filled conductive tape or epoxy. This simple technique mitigates the complexity of process-intensive metal-bonding schemes to achieve backside electrical contacts on semiconductor devices. The performance of such devices is evaluated while bent to a 1.2 cm radius of curvature. The longer-term stability of the devices fabricated is also briefly discussed.

Temperature Cycling Reliability of High-Temperature Lead-Free Die-Attach Technologies

The demand for electronics capable of operating at temperatures above the traditional 125°C limit continues to increase. Devices based on wide bandgap semiconductors have been demonstrated to operate at temperatures up to 500°C, but packaging remains the major hurdle to product development. Recent regulations, such as RoHS and WEEE, increase the complexity of the packaging task by prohibiting the use of certain materials, such as lead, in electronic products. Traditionally, lead has been widely used in high-temperature solder attach. In this paper, a series of Pb-free die-attach technologies have been identified as possible alternatives to Pb-based ones for high-temperature applications. This paper describes the fabrication sequence for each system and assesses their long-term reliability using accelerated thermal cycling and physics-of-failure modeling. The reliability of the lead-rich alloy was confirmed during this investigation, while early failures of the silver-filled epoxy demonstrated their inability to survive high temperatures. An empirical damage model was developed for the silver nanoparticle paste based on fatigue-induced failures. Encouraging reliability data have been presented for the gold-tin solid-liquid interdiffusion system where bond quality was demonstrated to be a critical factor in its failure mode and mechanism.

Silver-filled electrically conductive epoxy and silver nitrate-plated textile composite materials for EMC

A study on the effect of silver-filled electrical epoxy materials and the effect of electroless silver nitrate-plated woven fabrics on electromagnetic shielding effectiveness of various woven fabrics has been reported. Polyester, cotton, polyester/viscose blend, and polyester/cotton blend fabrics were treated with silver-filled electrical epoxy by pad-cure method. The polyester fabrics were treated with various levels of silver nitrate concentration, temperature, and time to analyze EMSE. The electromagnetic shielding effectiveness of these silver-filled electrical epoxy-coated fabrics and electroless silver nitrate-plated woven fabrics has been measured in the frequency range 20—18,000 MHz using network analyzer equipment with MIL-STD 285 and also the morphological structures of the silver-filled electrical epoxy-coated woven materials and electroless silver nitrate-plated woven fabrics were analyzed using scanning electron microscope. It is observed that the silver-filled electrical epoxy cotton fabric has better electromagnetic shielding effectiveness than the polyester/cotton blend, polyester/viscose blend, and polyester fabric in the frequency range 20—18,000 MHz. It is also observed that the silver-filled electrical epoxy-coated cotton fabric has maximum shielding effectiveness (50—69 dB) in the frequency range 60—15,000 MHz. It is observed that the silver nitrate concentration and temperature have a significant influence on electromagnetic shielding effectiveness of silver nitrate-plated fabrics. It is also observed that the maximum EMSE (45—55 dB) was obtained in the frequency range 180—8000 MHz.

A portable device for temperature control along microchannels

For many physical, chemical and biological measurements, temperature is a crucial parameter to control. In particular, the recent development of microreactors and chip-based technologies requires integrated thermostatic systems. However, the requirements of disposability and visual inspection of a device under a microscope cannot accommodate equipment such as external heaters. By exploiting a silver-filled epoxy that can be injected and solidified in a microfluidic chip, we demonstrate a simple and inexpensive design of a conductive path, which allows heating by the Joule effect of both sides of a microchannel. In addition to permitting the maintenance of a constant temperature along the channel walls, our method can control the temperature gradient across the channel, thus enabling non-equilibrium studies in a microfluidic geometry.

Bonding stress and reliability of high power GaAs-based lasers

GaAs-based lasers were bonded to oxygen-free high-conductivity (OFHC) copper heat sinks using a eutectic PbSn solder or a silver-filled conductive epoxy, and life tested. Epoxy-bonded devices were observed to have a larger failure rate on life test than solder-bonded devices. Bonding stress, as measured by the degree of polarization (DOP) of photoluminescence, was found to be the largest in epoxy-bonded devices. As well, the type of heat sink and bonding adhesive affected the stress in the laser material, with bonding stress increasing when there was a larger mismatch of coefficients of thermal expansion between the laser material, adhesive, and heat sink. Results suggested that heat sink material and bonding adhesive contribute to stress within the laser material and the resulting performance of the device.

THERMAL BEHAVIOUR OF SILVER-FILLED EPOXY ADHESIVES Technological implications in microelectronics

One-component silver-filled epoxy adhesives electrically conductive with low extractable ionic content have been used in microelectronics for attach applications. For the specific application described here there was evidence showing that adhesive polymerisation was not complete with negative consequences for the electronic devices. The present study aimed at understanding the causes of such disadvantages and setting up the best conditions to make the epoxy adhesive perfectly suitable for the required applications. The work was carried out mainly by means of differential scanning calorimetry (DSC) and with the help the dynamic mechanical thermal analysis (DMTA) technique. The study succeeded in determining that the technological problems were due to incomplete polymerisation that caused solvent and monomer retention. These residues produced a leakage of the insulating properties of some capacitors present in the devices jeopardising the sensor working. Moreover, the study also showed that it was possible to avoid these difficulties adjusting the polymerisation temperature to 130°C and respecting curing time of 3 h: with these new curing conditions excellent results were achieved.

Characterization of electroplated bismuth-tin alloys for electrically conducting materials

Electrically conducting adhesive materials are promising alternatives for lead (Pb)-containing solders in microelectronic applications. However, most common silver-filled epoxy materials have various limitations to meet the requirements of the solder joints yet. To overcome these limitations, several new formulations have been developed recently. Among them, a new high conductivity Pb-free, conducting adhesive developed for low temperature applications has been previously reported. This conducting adhesive contains a conducting copper filler powder coated with a low melting point metal or alloy, such as Sn or BiSn alloys. The low melting point layer serves as a joining material among the filler particles as well as to the substrate. In this paper, characterization of electroplated BiSn alloys on a Cu substrate is reported for their microstructure, electrical properties, oxidation behavior, and others. The experimental results have provided a better understanding of the joining mechanism of the newly developed Pb-free conductive adhesive materials.

Development of conducting adhesive materials for microelectronic applications

Electrically and/or thermally conducting adhesive materials are classified into two categories depending on their conduction modes: isotropic and anisotropic materials. Silver-particle filled epoxy is the most common example of the class of isotropic materials which are conductive in all directions. This material has been long used in the electronic applications as a die-bonding material, where its good thermal conduction rather than its electrical conduction property is utilized. The silver-filled epoxy material has several limitations for high performance electrical interconnections, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, corrosion issue due to silver migration, difficulty in rework, and so forth. The anisotropic conducting material provides electrical and/or thermal conduction only in one direction. An anisotropic conducting film (ACF) is used for interconnecting TAB mounted chips to a liquid crystal display panel, where fine pitch interconnection and low temperature assembly are required. In this paper, a brief review of the state-of-art conducting adhesive technology is provided. Subsequently, development of new conducting adhesive materials is presented for several different applications, which include high temperature materials for ceramic substrates, and low temperature materials for organic substrates.

Fracture behavior of isotropically conductive adhesives

The fracture behavior of several commercial, silver-filled epoxies was studied using a combination of fracture mechanics, surface science, and microscopy. Three-point bend tests revealed that the bulk fracture toughness of the silver-filled epoxies fell within a narrow range 1.1-1.3 MPa-m/sup 0.5/. Both electron and optical microscopy studies indicated that crack path deflection due to the silver-particles was the primary micromechanical deformation mechanism. Surprisingly, the interfacial fracture energies between the epoxies and a copper surface ranged from 50 to 900 J/m/sup 2/. Contact angle measurements on the cured epoxies indicated that some epoxy surfaces are more active than others. However, the correlation between thermodynamic work of adhesion and fracture energy is rather weak and suggests only a modest trend. In summary, although the use of contact angles/surface energies to predict adhesion is promising, much more effort is required to make it a reliable screening tool. Fortunately, the use of interfacial fracture mechanics can detect differences in adhesive strength, and should allow packaging engineers to select die attach adhesives with improved adhesion.

Development of high conductivity lead (Pb)-free conducting adhesives

Electrically conducting adhesive technology is one of the alternatives being actively investigated for the possibility of replacing the solder interconnection technology used for microelectronics applications. An isotropically conducting adhesive consists of metallic filler particles dispersed in the matrix of a polymer resin. Silver-filled epoxy resin is commonly used for thermal conduction in die attach applications. Silver particles can provide electrical and/or thermal conduction, while epoxy provides adhesive bonding of the components to a substrate. This material has several limitations when it is considered as a replacement for solder interconnections, such as low electrical conductivity, low joint strength, increase in contact resistance upon thermal cycling, lack of reworkability, and silver migration. In order to overcome these limitations, a new formulation is proposed based on alternative Pb-free conducting filler powder and tailored polymer resins. The conducting filler particles are coated with low melting point, nontoxic metals which can be fused to achieve metallurgical bonding between adjacent particles as well as to a substrate. This new conductive adhesive material has shown improved electrical and mechanical properties over the existing silver-filled epoxy materials.

Aging effects on the electrical properties of silver-filled epoxy adhesives

Electrically conductive adhesives, such as silver-filled epoxies, are used in many applications where standard soldering or direct metal-to-metal connections are not feasible. Anomalous behavior at these connections has been observed as the development of increasing resistance over time in the absence of mechanical failure between the adhesive and substrate. The results of aging of electrically conductive adhesive joints to aluminum- and gold-metallized substrates show that little change occurs at low humidity under an ambient flow of nitrogen. With increasing humidity and temperature, there is a measurable increase in resistance over time, particularly in the case of aluminum-metallized substrates. Secondary ion mass spectrometry (SIMS) was used to investigate interfacial phenomena that may be responsible for increasing resistance observed in the aluminum-metallized samples exposed to accelerated aging. Preliminary results from this technique indicate the presence of an oxide layer between the adhesive and the metallization layer that is consistent with increasing resistance measured for samples exposed to accelerated aging. The results also support the conclusion that deterioration at the adhesive-aluminum interface, and not degradation in the bulk adhesive, is the primary cause of increasing resistance.

Attenuation in silver-filled conductive epoxy interconnects

The attenuation of microstrip transmission lines fabricated with a new type of multiphase epoxy adhesive, called area bonding conductive (ABC) adhesive, has been measured from 45 MHz to 10 GHz. This ABC adhesive consists of a silver flake fill in a polymer matrix, and allows for both thermal and electrical conductivity in addition to increased strength. The board processing details are discussed along with the screen printing procedure. A technique for extracting the attenuation and sheet resistivity is presented which is based on network analyzer measurements of the S/sub 21/ scattering parameter. The performance for three different concentrations of silver flake in the conductive epoxy is compared with copper microstrip. There is a significant decrease in attenuation and sheet resistance as the concentration of silver increases, but in all cases the attenuation is a well-behaved function of frequency. Also, it is shown that microstrip transmission lines fabricated using stencil printing perform better at high frequencies than those made from screen printing.

Composite material behavior under applied electric field: material switches to a high-resistive state

Current-voltage characteristics of the electrically conductive silver-filled epoxy Ablefilm ECF-563 preform switches to a high-resistive state under low bias voltage. The observed phenomena is argued to be an intrinsic property of electrically conducting composite materials caused by strong localized centers that introduce space charge.

Electrically conductive adhesives for surface mount solder replacement

Electrically conductive adhesives offer a lead free, relatively low temperature process for attachment of surface mount components to printed circuit boards (PCB's). The National Center for Manufacturing Sciences (NCMS) Conductive Adhesives project has defined a requirement for an isotropically conductive adhesive (ICA). In this study, 25 commercially available silver-filled epoxies were evaluated for joint resistance stability on copper and tin/lead surface finishes after 500 h of 85/spl deg/C/85% RH aging. Mechanical properties were evaluated through lap shear and impact testing. This testing led to a set of requirements for a surface mount adhesive including a volume resistivity of 1 m/spl Omega/-cm, less than a 20% shift in resistance after aging, and the ability to pass six drops with a plastic leaded chip carrier (PLCC) 44 package from a height of 1.52 m (60 in). Promising candidate adhesives identified in the screening study were evaluated on a circuit board test vehicle with a variety of components. Assembly revealed a narrow processing window when compared to solder. Impact testing demonstrated that current materials have inadequate impact strength for many components and accelerated T/H aging reveals unstable electrical resistance with Sn/Pb finished components. Preliminary data is included on a new adhesive formulation that has been developed to address the shortcomings of currently available ICAs.