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In this paper, we report a comparison of interfacial reactions of Sn-3.8Ag-0.7Cu (SAC 387) and SAC (0-1.5 Zn) solder pastes on Cu (organic solderability preservative finish) and Au/Ni-P/Cu [electroless Ni immersion gold (ENIG)] substrate metallizations with Ni/Sn and Cu/Sn plated component leads. Zn added to the paste in the form of surface-coated micrometer-sized particles dissolves into the solder during reflow. High-temperature aging (150 °C and 185 °C), thermal cycling experiments ( -20°C to 175 °C for FR4 substrate, -40°C to 185 °C for ENIG polyimide substrate), and shear testing of the solder joints were carried out. At a Cu interface, adding Zn to the solder joint improves the shear strength and suppresses Cu3Sn and overall interfacial intermetallic compound (IMC) and Kirkendall void formation . However, above this temperature, the presence of Zn accelerates IMC growth. At a Ni interface, IMC suppression with Zn was noted at all temperatures. The amount of IMC suppression depends on the Zn concentration in the IMCs, which in turn depends on the geometry of joint as well as the original concentration of Zn in the solder.

Metalized micron scale polymer particles have been utilised in anisotropically conductive adhesives for a number of years and more recent work has investigated their selective deposition to provide interconnects. In this work such particles have successfully been deposited onto substrates that included both unpatterned Cr/Au coated chips and test chips with “daisy-chain” interconnected bond pads, using an electrophoretic deposition technique. These were then bonded to matching substrates using a flip-chip thermocompression bonder. The interconnections formed using this method were then characterised in terms of their mechanical and electrical properties. Silicon wafers coated with gold were used as substrates for shear strength studies, which showed that the failure point of the bond produced was at the polymer-metal interface, suggesting that strong bonds had been formed between the particle surface metal layer and the substrate. The effect of the duration of the bonding process has been investigated via shear strength measurement. The particles were also deposited onto the bondpads of a daisy chain structure to allow the electrical properties of the bonds to be studied. This investigation showed that, for the same range of bonding conditions, the electrical resistance of the interconnection decreases as the mechanical shear strength increases. This occurs due to an increase in the bonded interfacial region between the particle and the substrate.

PurposeThe purpose of this paper is to present an outline of the development of a new process for the formation of flip‐chip interconnections using metal coated polymer microparticles.Design/methodology/approachMetal coated polymer microparticles were selectively deposited onto the bondpads of integrated circuits using electrophoresis. Thermocompression bonding was then used to bond the devices to substrates.FindingsParticles obtained a positive surface charge following immersion in an acidic solution and this surface charge allowed the particles to be deposited electrophoretically directly onto the bondpads of an integrated circuit without the need for patterning. Thermocompression bonding of nickel/gold coated particles to gold coated substrates was achieved at temperatures as low as 160°C.Research limitations/implicationsFurther work is needed to test this approach using integrated circuits with finer pitch, and to use patterned substrates for assembly and reliability measurements.Originality/valueThis paper presents a new method for the deposition of metal coated polymer microparticles without the need for any masking or patterning processes for the formation of interconnections on integrated circuits.

We deposit dense, ordered, thin films of Au–dodecanethiol core/shell nanoparticles by the Langmuir–Schafer (LS) printing method, and find that their resistance at ambient temperature responds selectively and sensitively to alkane odours. Response is a rapid resistance increase due to swelling, and is strongest for alkane odours where the alkane chain is similar in length to the dodecane shell. For decane odours, we find a response to concentrations as low as 15 ppm, about 600 times below the lower explosive limit. Response is weaker, but still significant, to aromatic odours (e.g. Toluene, Xylene), while potential interferants such as polar and/or hydrogen-bonding odours (e.g. alcohols, ketones, water vapour) are somewhat rejected. Resistance is weakly dependent on temperature, and recovers rapidly and completely to its original value within the error margin of measurement.

The optical and electrical properties of dodecanethiol-stabilized nanoparticles (6 nm diameter gold core) have been investigated over a range of film thicknesses and temperatures. The surface plasmon resonance absorbance is found to be dependent on temperature. Heating of the nanoparticle film causes desorption of the thiol from the surface of the gold nanoparticle, resulting in irreversible changes to the absorption spectra of the nanoparticle film. Atomic force microscopy images of the samples before and after heating for different film thicknesses reveal structural changes and increased domain connectivity for thicker films leading to sub-10 ohm resistances measured for the 15-layer film.

Flash pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS) was applied to freeze dried cells of eleven bacterial species. Six species known to synthesise bacteriohopanepolyols (BHPs) were found to produce easily identifiable C27–C35 hopenes and hopanes upon flash pyrolysis. These compounds could not be observed in the pyrolysis products of five species, two of which are known not to produce BHPs. In the search for BHP-containing bacteria this rapid screening approach prevents unnecessary time-consuming laboratory work to characterise species that would produce negative results.

The distributions of hopanoic acids, ranging from C 30 to C 34 , in the Messel oil shale were characterized in both the free and bound states. The bound acids were released by thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH). These were compared with the distributions of the hopanoic acids and hopanes released or generated from Messel oil shale kerogen following closed system microscale pyrolysis. This comparison revealed that epimerization had occurred at C-17, C-21 and C-22 during heating. It was also clear that the residual bound hopanoic acids had undergone configurational isomerization. During the pyrolysis there is a large loss of hopanoic acids following their rapid release from the kerogen into the free fraction even at 250 °C. In these particular experiments this loss does not appear to result in exclusive formation of hopanes, by way of decarboxylation or reduction reactions, unless the resulting hopanes are either themselves rapidly transformed into other compounds or the reaction rates are a function of the total number of carbon atoms in each hopanoic acid precursor.

This paper describes the ultrasound assisted dispersal of a low wt./vol.% copper nanopowder mixture and determines the optimum conditions for de-agglomeration. A commercially available powder was added to propan-2-ol and dispersed using a magnetic stirrer, a high frequency 850 kHz ultrasonic cell, a standard 40 kHz bath and a 20 kHz ultrasonic probe. The particle size of the powder was characterized using dynamic light scattering (DLS). Z-Average diameters (mean cluster size based on the intensity of scattered light) and intensity, volume and number size distributions were monitored as a function of time and energy input. Low frequency ultrasound was found to be more effective than high frequency ultrasound at de-agglomerating the powder and dispersion with a 20 kHz ultrasonic probe was found to be very effective at breaking apart large agglomerates containing weakly bound clusters of nanoparticles. In general, the breakage of nanoclusters was found to be a factor of ultrasonic intensity, the higher the intensity the greater the de-agglomeration and typically micron sized clusters were reduced to sub 100 nm particles in less than 30 min using optimum conditions. However, there came a point at which the forces generated by ultrasonic cavitation were either insufficient to overcome the cohesive bonds between smaller aggregates or at very high intensities decoupling between the tip and solution occurred. Absorption spectroscopy indicated a copper core structure with a thin oxide shell and the catalytic performance of this dispersion was demonstrated by drop coating onto substrates and subsequent electroless copper metallization. This relatively inexpensive catalytic suspension has the potential to replace precious metal based colloids used in electronics manufacturing.

Electroless copper plating of insulating substrates, such as printed circuit boards, typically requires the pre-deposition of a catalyst layer onto the surface to initiate the chemical reactions. Pd/Sn based catalysts are widely used, but carry a high cost and in many cases require specialist pre-treatment of the substrate to achieve good adhesion. In this work, functionalised copper nanoparticles have been investigated as alternative catalysts for electroless deposition. Commercially available copper nanoparticles were functionalised with different organic molecules and their functionalisation was confirmed with X-ray photoelectron spectroscopy. The ability of these particles to act as a catalyst was demonstrated, however their effectiveness was found to depend on the nature of the organic molecules that were used in the functionalisation. Furthermore, significant variability was found between batches of samples in both the particle dispersion and attachment to the substrate surface, which affected the reproducibility of the coverage and adhesion of the subsequent electroless plating, for which further work is required to understand these effects.

The increasing demand for fine pitch flip-chip interconnection has led to significant interest in alternatives to the widely used solder joints. The use of mono-sized metal coated polymer based micro-spheres is emerging as a method to achieve more robust and reliable interconnects for ball grid arrays and chip scale packages. This paper presents results from a collaborative research project investigating whether polymer cored interconnects are a viable replacement for such ultra-fine pitch solder joints. The focus of this work centres on the selective deposition of these polymer microspheres onto fine pitch bond pads. An important advantage of using such microspheres is that their mechanical properties can be tailored to suit the needs of the application, in addition to the relatively low usage of metals. Initial trials have been carried out using 9.8 μm diameter polymer particles which have been coated with electroless nickel and then an immersion gold surface layer. In order to selectively deposit these particles on to the bond pads of an ASIC device they were required to be charged. The particles were immersed in an aqueous acid solution and after rinsing in deionised water they were then transferred to a suitable solvent. This process resulted in a net positive charge on the particle surface which allowed the particles to be deposited using electrophoretic techniques i.e. an electric field was applied to the particle suspension to drive the particles to deposit on the substrate. The effect of this immersion in the aqueous acid solution on the surface metal layer of the particles has been studied using electron microscopy. The rate of electrophoretic deposition of the particles onto a homogeneous gold coated silicon substrate was studied to determine how the length of chemical treatment affects the particle deposition. Following from this the same technique was then used to deposit the particles directly on to the bond pads of an ASIC device. Using the ASIC as the cathode in the electrophoretic setup the particles were able to be selectively deposited onto bond pads with a pitch of 170 μm.

The distribution of the free and bound hopanoic acids in both unheated and heated (350 degrees C for 50 h) kerogens, isolated from the Messel oil shale, were analyzed by GC-MS. The bound acids were released by subjecting the kerogen to three different treatments, namely, thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH), as well as basic and acidic hydrolyses. All of these methods gave a series of hopanoic acids ranging from C(30) to C(34), in which the biological 17beta, 21beta(H) configuration is prominent. Both 22R and 22S epimers are present for the C(30) acid, whereas the others are dominated by the sidechain 22R-configuration. Thermochemolysis in the presence of TMAH was the most efficient in releasing kerogen-bound hopanoids. Following pyrolysis, the acids are generated and released into the free fraction with apparent epimerization occurring at C-17, C-21, and C-22. The bound hopanoic acids may be both chemically bonded as well as possibly being physically encapsulated within the macromolecular fraction of sedimentary organic matter. They are therefore either generated by breaking the bonds which bind them to the kerogen or they are released as a result of the macromolecular cage being broken apart.