John A. Emerson

Indentation responses of time-dependent films on stiff substrates

A viscous-elastic-plastic indentation model was extended to a thin-film system,including the effect of stiffening due to a substrate of greater modulus. The systemmodel includes a total of five material parameters: three for the film response(modulus, hardness, and time constant), one for the substrate response (modulus), andone representing the length-scale associated with the film-substrate interface. Thesubstrate influence is incorporated into the elastic response of the film through adepth-weighted elastic modulus (based on a series sum of film and substratecontributions). Constant loading- and unloading-rate depth-sensing indentation testswere performed on polymer films on glass or metal substrates. Evidence of substrateinfluence was examined by normalization of the load-displacement traces. Comparisonswere made between the model and experiments for indentation tests at different peakload levels and with varying degrees of substrate influence. A single set of fiveparameters was sufficient to characterize and predict the experimentalload-displacement data over a large range of peak load levels and correspondingdegrees of substrate influence.

Liquid encapsulant and uniaxial calibration mechanical stress measurement with the ATC04 assembly test chip

A new assembly test chip, ATC04, designed to measure mechanical stresses at the die surface has been built and tested. This CMOS chip, 0.25 in. on a side, has an array of 25 piezoresistive stress sensing cells, four resistive heaters and two ring oscillators. The ATC04 chip facilitates making stress measurements with relatively simple test equipment and data analysis. The design, use, and accuracy of the chip are discussed and initial results presented from three types of stress measurement experiments: four-point bending calibration, single point bending of a substrate with an ATC04 attached by epoxy, and stress produced by a liquid epoxy encapsulant.<<ETX>>

SELF-ADHESION HYSTERESIS IN POLYDIMETHYLSILOXANE ELASTOMERS

Self-adhesion hysteresis has been investigated in crosslinked poly(dimethylsiloxane) (PDMS) lenses using the Johnson, Kendall, and Roberts technique. The experimental conditions involved relatively short contact times for which interchain penetration effects across the interface are minimal. Only lenses that had been extracted in toluene displayed self-adhesion hysteresis. The same lenses demonstrated no adhesion hysteresis when pressed against tethered polystyrene substrates, indicating that hysteresis was caused by surface interactions and not bulk viscoelastic effects. Extraction produces hysteresis by removing the free chains, which normally lubricate the interface, inhibiting the adhesion mechanism. Self-adhesion hysteresis was only observed for networks with a high molecular weight between crosslinks. More tightly crosslinked networks did not display self-adhesion hysteresis, even at extended contact times under load. By inhibiting the hydrosilylation reaction between residual vinyl and silane groups in the PDMS lenses, self-adhesion hysteresis was prevented, suggesting that the formation of chemical crosslinks across the interface caused the observed hysteresis. The molecular weight dependence of the hysteresis can be interpreted in terms of the Lake-Thomas model [1] for fracture in elastomers.

HAST evaluation of organic liquid IC encapsulants using Sandia's assembly test chips

Accelerated aging (HAST) experiments were conducted with special-purpose corrosion test chips (ATCs) in both bare die form and with various liquid encapsulants: epoxy, silicone and silicone elastomer, or silicone gel. The purpose of the experiment was to show what incremental improvement in die corrosion resistance was provided by the encapsulants and to determine the failure modes for the two types of samples. The test conditions were 140 degrees C/85% RH (relative humidity) and +40-V bias on outer tracks with respect to the center track. In the case of nonencapsulated parts, median lifetimes of about 1000 h were observed for the best passivations, with the predominant failure mode being triple track corrosion on the die. In the case of the encapsulated parts, the failure mode depended on the encapsulant type. Several of the silicone gel materials showed excellent HAST (high accelerated stress testing) performance, with only a few percent failures at the 1 100-h point.<<ETX>>

JKR contact mechanics for evaluating surface contamination on inorganic substrates

Abstract The JKR contact mechanics approach is employed to analyze the effects of surface contaminants on adhesive bonding, as well as quantify the level of contamination at which adhesive strength decreases. The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to join the two surfaces, and by subsequently applying a tensile load to assess the energy dissipation mechanisms involved in the debonding process. In the present work, we monitor the interactions between a diglycidyl epoxy elastomer and an aluminum oxide substrate in the presence of an organic contaminant, as a means to evaluate the level of contamination at a surface. Moreover, we present a method by which surface contamination can be quantified using a single number, referred to as the adhesion hysteresis parameter, H.

Liquid encapsulant stress variations as measured with the ATCO4 assembly test chip

We have examined the use of the ATCO4 piezoresistive Assembly Test Chip to measure the mechanical surface stresses produced by liquid encapsulation of die mounted directly on ceramic substrates. In our experiment, two groups of parts, each with about 70 samples, were encapsulated with two different materials, one a standard and one a low stress formulation. Results are presented for the measured stress components; /spl sigma//sub xx/-/spl sigma//sub zz/, /spl sigma//sub xy/, and /spl sigma//sub xx/-/spl sigma//sub yy/. We observe an /spl ap/20% reduction in the compressive stress between the normal stress and low stress materials, and a small but measurable difference in the average in-plane shearing stress. This experiment demonstrates the ability to resolve stress differences produced by different encapsulants and provides guidelines for selecting appropriate sample sizes.

Process-Based Quality Tools to Verify Cleaning and Surface Preparation

A test method, the Tensile Brazil Nut Sandwich (TBNS) specimen, was developed to measure mixed-mode interfacial toughness of bonded materials. Interfacial toughness measured by this technique is compared to the interfacial toughness of thin film adhesive coatings using a nanoindentation technique. The interfacial toughness of solvent-cast and melt-spun adhesive thin films is compared and found to be similar. Finally, the Johnson-Kendall-Roberts (JKR) technique is used to evaluate the cleanliness of aluminum substrates.

Simultaneous Measurements of Electrical Resistivity and Raman Scattering from Conductive Die Attach Adhesives

The development of electrical conductivity in silver (Ag)-filled conductive polymer adhesives is dependent on the thermal profile of the curing process. Previous studies of polymer adhesive systems have shown that chemical reactions at the interface of the micronsized Ag filler are a key factor in determining the subsequent performance of the conductive system. In an attempt to correlate the behavior of electrical conductivity with the chemical nature of the Ag particle interface, we have simultaneously performed electrical resistivity and surface enhanced Raman scattering (SERS) measurements on a commercial conductive adhesive. At room temperature in the low conductance state (∼10 −9 ohms −1 ), the SERS spectrum from the uncured adhesive exhibited peaks that were identified with a molecular species bound to Ag surface via the carboxylate functionality of the adsorbate. During the thermal cure processing, the SERS data showed a partial decomposition of the carboxylate species and the formation of an amorphous carbon layer at the Ag surface. A comparison of the simultaneously recorded electrical resistance and SERS data showed a strong correlation between the development of high conductance (∼ 1 ohm −1 ) in the adhesive and the formation of the amorphous carbon layer.

Coupling agent studies : reaction of thiol-containing esters with aluminum

Abstract Double cantilever beam experiments performed by Joseph et al. showed increases in strain energy release rate and cohesive failure for mercaptoester-treated aluminum substrates as compared to untreated controls. This paper presents evaluation of the mercaptoester–aluminum reactivity as a first step toward assessing the effectiveness of incorporating the mercaptoester functionality into a polymeric coupling agent for an epoxy-aluminum bond system. Grazing-angle infrared spectroscopy and XPS of mercaptoester-treated solid aluminum substrates did not show evidence of a mercaptoester/aluminum reaction. The reaction was also attempted in solution where potential spatial limitations associated with the solid surface were eliminated. IR and NMR showed no evidence of reactivity between the mercaptoester and aluminum ions in solution. Theoretically, the Hard–Soft Acid–Base Theory, in conjunction with the Mison equation, does not support a mercaptoester–aluminum reaction. Thus incorporating a mercaptoester into a polymeric coupling agent for an epoxy–aluminum bond system would not result in chemical bonding. In addition, no significant reaction between thiols and aluminum in general, in the presence of water and moist air, is expected.

Evaluation of β-diketone-containing Polymeric Coupling Agents for Enhancing the Adhesion of Epoxy to Aluminum

Abstract β-diketone-containing polymeric coupling agents (PCA) were evaluated as potential adhesion enhancers for an epoxy/aluminum bond system. Torsional shear joint measurements revealed that the β-diketone-containing PCA did not influence the joint strength and durability, positively or negatively, as compared with untreated controls. Grazing angle infrared spectroscopy revealed that despite reactivity of the β-diketone-containing monomer with aluminum substrates, no reactivity of the β-diketone was observed once the monomer was incorporated into the polymer. Deposition studies showed that the resulting PCA coating thickness following treatment and solvent rinsing was not a function of solvent solubility parameter, solution concentration, or immersion time. It was hypothesized that preferential physisorption of the phenyl and/or epoxy functionalities in the PCA inhibited reactivity of the β-diketone functionality.