Conductive polymer matrix composites have been a focus of research for the past 30 years, due to applications such as electromagnetic shielding. The electrical resistance of these materials can be broken into three major categories: the intrinsic resistance of the filler and matrix, the particle‐particle contact resistance, and the tunneling resistance. Both the particle‐particle contact and tunneling resistance in nickel filled low density polyethylene (LDPE) composites can be reduced by the addition of an ultrathin polypyrrole (PPy) coating on the nickel surface. Coating the nickel particles with PPy leads to increases in conductivity as much as three order of magnitude at concentrations well above the percolation threshold without significantly changing the mechanical properties. We believe at concentrations above the critical region PPy forms “molecular wires” reducing the loss of conductivity associated with the uniaxial orientation of the polymer matrix.

Chains of conductive particles within a polymer matrix can be used in place of solder joints in electronics manufacturing. Such materials can be made such that electrical conduction occurs in only one direction. Fabricating such an electrical contact requires that the conducting particles be aligned and touching, thus providing a continuous path. This can be done by applying an electric field to a mixture containing conducting particles in a polymer. Preliminary work will be presented assessing the effect of electric field strength (applied voltage), viscosity (temperature), frequency (DC and AC) on the kinetics of chain formation. A designed experiment showed only viscosity or temperature to be statistically significant. However, it is believed that voltage and frequency are factors which will effect the chain kinetics. Specifically it is proposed that there is an optimal combination of frequency and voltage for chain formation. Based on these results a more systematic study has been proposed.

An experimental technique of environmental moiré interferometry has been developed for in‐situ monitoring and analysis of thermomechanical deformation of microelectronics packages subjected to thermal loading under a controlled atmosphere. Coupled with fractional fringe analysis and digital image processing, the environmental moiré interferometry technique achieves accurate and realistic deformation monitoring with high sensitivity and excellent spatial resolution. It has been applied to investigate the thermomechanical deformations induced by thermal loading in an underfilled flip‐chip‐on‐board packaging. The effects of temperature change in the range of 102 °C to 22 °C are analyzed for underfill and solder bumps. In addition, shear deformation and shear strains across the solder bumps are determined as a function of temperature. The experimental results are compared with the results of a finite element analysis for modeling verification. Good agreement between the modeling results and experimental measurements has been found in the overall displacement fields. Through this study, the role of underfill in the thermomechanical deformation of the underfilled flip‐chip package is determined.

We show that the thermal diffusivity of a dielectric film and the heat‐transfer coefficient between the film and the substrate to which it is adhesively bonded can be determined from the electret‐like response stimulated by laser pulses applied to the voltage‐biased dielectric. The temperature profile is calculated as a function of time from the heat‐conduction equation with boundary conditions appropriate for the model structure representing the metallic electrode/ dielectric film/ glue layer/ substrate structure. The electrode, acting as a thermal mass, and the substrate, acting as a thermal sink, affect the temperature profile in different time ranges.

The response is a convolution of the temperature and the electric‐field profiles across the dielectric. For nonideal dielectrics that acquire bulk charge under bias, deconvolution is necesssary to determine the field profile and the thermal properties. Deconvolution is accomplished using the Tikhonov‐regularization technique with a self‐consistent regularization parameter. This procedure yields information about the thermal properties of both the dielectric and the glue joint, as shown by actual measurements.

Stresses and deformation in microelectronic packaging are affected by the viscoelastic behavior of polymer materials during manufacture or operation. Predicting and measuring these thermo‐mechanical effects is important for new devices, components, and materials. The viscoelastic response of Nycoa 851 polyimide thin‐films during thermal loading is investigated. The time‐dependent relaxation of polyimide films was measured in‐situ, focusing on the change in thermo‐mechanical properties based on the thickness of the polyimide layer. The curvature change of the multilayer structure (silver‐polyimide‐quartz heterostructure) was obtained for different temperatures and polymer film thicknesses. The polyimide relaxation time constant and activation energy were determined. Results indicate that the thermo‐mechanical properties of polyimide thin films are dependent on the thickness of the polymer layer.

Wirebonding is the weakest area of device packaging of power IGBT modules. Accelerated thermal fatigue testing causes cracks to form and propagate in the aluminum wirebond at the foot area. This study examined the relationship of the wirebond reliability and the aluminum wire grain structure, which can be affected by post‐wirebond heat treatment. A series of wirebonded IGBTs were annealed at a temperature range from 280°C to 400°C for up to 60 minutes. Wirebond shear strengths versus temperature cycles were examined. Cross‐sectional SEM was used to examine both aluminum grain size development by annealing and crack initiation and propagation in the wirebonds after temperature cycling. It was found that aluminum grain size was increased by post‐wirebond annealing. With temperature cycling, the wirebond shear strengths of the as‐wirebonded samples decrease rapidly, and lifted wirebonds were present after 1500 temperature cycles. The lifted wirebonds typically break within the aluminum wire near the wire/metallization interface. The shear strength of the wirebonds with post‐wirebond annealing showed no significant change even after 5000 temperature cycles, and there were no signs of significant deterioration of the wirebonds either. The wirebond crack initiation and growth rates were depressed substantially by larger aluminum grains. Annealing of the aluminum wire after wirebonding provided increased aluminum grain size resulting in improved reliability of the wirebonds of high power modules.

An electrical contact surface is described which combines the flexibility of a low resistance elastic interface with bonded conductive particles for penetration during compressive mating to metallic oxide layers. The processes for applying both technologies is described including the types of interfacial bonds provided. Both technologies permit high conductivity across the layers. Such technologies offer advantages for connectors requiring low normal forces, superior oxidative resistance to the environment, elastic conductors with the same resistance as bare copper, and high volume processing capabilities.

The first layer of conductive coating includes grafting an elastomeric bridge from the primary surface to the outer layer of metal particles and flakes. The secondary outer coating involves deposition of sharp, hard, conductive particles onto the grafted surface. Each coating bonds to the other forming several layers outward from the primary elastomeric substrate.

Product applications include a conductive elastic interface for interconnecting a BGA to a PCB via a socketed solution. The elastomeric interface overcomes co‐planarity issues associated with mating the BGA chip to a PCB. Compressive forces are reduced as compared to metal connectors providing lower total force for high I/O counts. Environmental constraints are reduced due to the durability of the elastomeric system.

A surface vibrational study of a stearic acid/silver (Ag) flake system is reported. The chemical nature of this interfacial system was examined with surface enhanced Raman scattering (SERS) as a function of sample temperature. The SERS spectrum from the room temperature flake exhibited vibrational peaks that were attributed to a carboxylate species bound to the Ag substrate through the oxygen atoms in the terminal end group. The surface species remained stable to temperatures up to 150 °C, above which a partial decomposition to an amorphous carbon layer was observed. In addition to the Raman studies, an IR‐visible sum‐frequency (SFG) measurement from a stearic acid/smooth Ag substrate proved a useful probe of the C‐H stretches in the terminal methyl group of the molecule. The SFG signal from the asymmetric C‐H stretch in the methyl group exhibited a temperature dependence that was similar to the SERS response from the surface carboxylate group, namely, a marked and irreversible decrease in signal levels for temperatures above 150 °C.

We have used conversionelectron Mössbauer spectroscopy (CEMS) to examine the surfaces of solder foils treated in a plasma assisted dry soldering (PADS) system which has been developed1 to convert surface oxides to more desirable fluorides in order to protect the solder surface prior to assembly and to eliminate the need for flux during the soldering process. CEMS samples only the outer 1 μm of a solder foil and this, coupled with the fortuitous fact2 that Mössbauer spectroscopy is 10–20 times more sensitive to oxides and fluorides than to the metallic form of solder, means that surface layers of oxides and fluorides as thin as 10 nm can be detected and chemically identified. We have followed the evolution of surface layers produced in PADS treatments ranging from 7 minutes to several hours. Our results show that fully treated foils contain surface layers of SnF2 and Sn2F6. Partially treated foils also contain some SnO2 and perhaps a small amount of SnO. No Sn2OF2 or SnF4 was produced during any portion of the PADS treatment cycle.

In this paper, we discuss a newly characterized compound, ZrW2Og, that has been introduced into the composite materials with an adjustable and low thermal expansion for electronic applications. Offering a negative coefficient of thermal expansion (CTE) of approximate ‐9xlO–6/°C in a large temperature range, ZrW2Og was used as a particle filler in polymer‐matrix composites. The paper presents two kinds of composites, that is, polyester and epoxy with various volume fractions of ZrW20g. The CTEs of the polyester/ZrW2Og and epoxy/ZrW2Og composites have been proven adjustable in the ranges of 94 to 56x10–6 /°C and 54 to 18х 10–6 /°C, respectively, with ZrW2Og filler from 0 to 30 vol%. In addition, the analysis about the interfaces between the matrices and filler indicated that the interfaces may be beneficial to reduce the overall thermal expansion of the composites. The methods to further decrease composite CTEs are also discussed.

Electromagnetic interference (EMI) shielding effectiveness reaching 130 dB at 1 GHz was achieved by using flexible graphite (Grafoil). Due to the resilience of flexible graphite, it is particularly attractive as an EMI gasket. The high shielding effectiveness is due to the large specific surface area and large skin depth. In contrast, the shielding effectiveness was only 90 dB for polycrystalline graphite of low specific surface area.

The reliability of plastic packaged integrated circuits was assessed from the point of view of interfacial mechanical integrity. It is shown that the effect of structural weaknesses caused by poor bonding, voids, microcracks or delamination may not be evident in the electrical performance characteristics, but may cause premature failure. Acoustic microscopy (C-SAM) was selected for nondestructive failure analysis of the plastic integrated circuit (IC) packages. Integrated circuits in plastic dual in line packages were initially subjected to temperature (25 °C to 85 °C) and humidity cycling (50 to 85 %) where each cycle was of one hour duration and for over 100 cycles and then analyzed. Delamination at the interfaces between the different materials within the package, which is a major cause of moisture ingress and subsequent premature package failure, was measured. The principal areas of delamination were found along the leads extending from the chip to the edge of the molded body and along the die surface itself. Images of the 3-D internal structure were produced that were used to determine the mechanism for a package failure. The evidence of corrosion and stress corrosion cracks in the regions of delamination was identified.

Organic adhesives are commonly used in the microelectronics industry to bond silicon chips to a wide variety of substrates. The substrates include copper, silver, nickel / palladium, polyimide, and glass‐filled epoxies. The surfaces of these substrates are affected by the processing steps used in the manufacturing process used to produce the final package. The most common adhesives used in the assembly of microelectronic packages are epoxy‐based and there are numerous grades and types to chose from. Therefore, the packaging engineer is faced with the dilemma of what criterion to use to select the best adhesive for a particular package design.

At the crux of the problem is how does one predict adhesion and how does one measure it? It is proposed that the surfaces play an important role in forming the interfacial bond and that strength of the mating surfaces can be characterized in terms of the thermodynamic work of adhesion. Roughness is also a factor but will not be dealt with here. The thermodynamic work of adhesion describes the energy to reversibly separate two surfaces (elastic). Interfacial fracture mechanics may be used to quantify the strain energy release rate for separating two surfaces and contains both elastic and inelastic contributions. This talk will contain a discussion of our studies on the use of a three liquid probe method to determine the thermodynamic work of adhesion and a mixed‐mode bending method to measure interfacial fracture toughness. Moreover, we will comment on the perils of relating the thermodynamic work of adhesion to the interfacial fracture toughness.

A contact angle measurement technique has been used to determine the thermodynamic adhesive strength between nano-sized copper clusters and polyimide. The copper clusters are formed under UHV conditions by using ∼ mono-layer per minute deposition rates to assist equilibrium cluster formation. Temperatures of the polyimide substrate during the deposition process are maintained below Tg, which ensures no diffusion of the copper atoms into the polyimide matrix. Values of the strength of adhesion from these contact angle measurements are in reasonable agreement with predictions made using Fowkes’ theory of non-polar interfacial adhesion, but are at least one order of magnitude less than the value obtained by mechanical tests. The limitations of this technique due to deformation at the polyimide surface, as well as kinetic factors constraining equilibrium cluster formation will be discussed.

In this paper, mixedmode (mode I + mode II) fracture toughness data are presented for the prediction of molding compound (silica filled epoxy resin) failure. The stresses necessary to cause the package cracking under mixed mode loading are experimentally determined. Measured toughness values are presented as a function of modemixity and temperature

Partially embedded copper clusters within a polyimide substrate have been utilized to act as “nano‐nails” to anchor the bulk metal overlayer to the underlying polyimide substrate resulting in increased adhesion between the metal and substrate. Isolated nano‐sized copper clusters can be formed on a polyimide surface under UHV conditions using very slow deposition rates and temperatures that are high enough to induce mobility of copper atoms on the polyimide surface. Because copper has a high surface energy with respect to the copper/polyimide interface and the polymer surface, the copper clusters exert an embedding force on the polyimide substrate. These clusters can be made to embed in the polymer by annealing in vacuum at temperatures close to the glass transition temperature of the polymer. By regulating the time and temperature of post‐deposition anneals, the degree of embedding of the copper clusters can be controlled. Cross‐sectional TEM results will illustrate the various degrees of cluster embedding, while adhesion testing results will be presented to show the improved adhesion obtained from these interfaces.

Low temperature oxidation of a Cu-base leadframe has been investigated to understand the effect of Cu oxidation on the adhesion between Cu-base leadframes (Cu L/F) and epoxy molding compounds (EMC). From the kinetic studies on the oxidation, oxide growth was found to follow the parabolic rate law in the temperature range of 150 °C to 300 °C and the activation energy for the oxidation was 17.0 kcal/mol. X-ray photoelectron spectroscopy (XPS) studies confirmed that the oxide film consisted of Cu2O, CuO, and NiO. It was shown that the early stage of oxidation improved the adhesion strength. Furthermore the optimum copper oxide thickness required for the maximum pull strength ranged between 20 nm and 30 nm. The high pull strength was presumably due to the increase of surface wettability and mechanical interlocking effects resulting from copper oxidation.

In this study a finite strain elastic-plastic finite element analysis is performed on diamond/GaAs structures. A series of models based upon the principal of superposition are proposed to investigate the mechanical deformation and thermal stress behavior of the diamond/gas structure due to coefficients of thermal expansions (CTE) mismatches. The interfacial shear and peeling stresses in multilayered stacks subjected to uniform temperature variation are studied. Finite strain elastic – plastic analysis is performed on a crack which lies on the interface between the diamond and gas materials. The ductile fracture from the tip of the interface crack, the stress and strain fields and distribution of microvoid volume fraction are analyzed.

Structural reliability is a major concern in using anisotropic conductive films (ACF) in the electrical connection of chips or chip carriers to substrates. The relationship between electrical and structural failure is experimentally examined using a custom designed test chip. In-situ electrical measurements along with post failure microstructural examination revealed that electrical failure (open circuit) is caused by interfacial cracking of ACF/bump interface. An engineering analysis of the influence of assembly geometry revealed that tall relative bump height and large underbump adhesive thickness decrease the driving force for interfacial cracking. However, when relative bump height is increased and underbump adhesive thickness is reduced, increased compression from increased relative bump height dominate over increased shear resulting in a net decrease in crack propagation driving force. In general, tall relative bump height can reduce the potential for electrical failure in ACF interconnect when the ACF/bump interfacial fracture is the dominant failure mode.