We analysed the mechanical response of samples, representing Flip-Chip bonded type structures. Two sort of distributions of the PbSn solder bumps, interconecting a chip to a substrate were investigated. The mechanical response was established from macroscopic shear and tensile tests. The microstructural evolution to complete mechanical failure, was observed during similar tests, but performed in a Scanning Electron Microscope. These in-situ tests revealed a preferential crack nucleation site in the samples, specific to one of the interfaces. A complementary numerical analysis was developed, for trying to explain the preferential crack opening site.

Eutectic Sn-Ag and Sn-Zn solders are currently being investigated as the basis for replacement of Sn-Pb solders. Some mechanical properties of these solder systems - fatigue, tensile, stress relaxation and creep are presented here along with some damage feature observations for eutectic Sn-Ag solder. Semi-empirical modeling, constitutive modeling and numerical simulation based on damage models have been carried out for lifetime prediction. Preliminary agreement of this numerical simulation with experimental results appears encouraging, but the models need further refinement.

Reliability of solder interconnects is normally characterized by extrapolation of bulk solder reliability data, such as crack growth measurements of solder materials in the bulk form. In the present study, experimental evidence will be presented to show that crack growth, which is the most commonly observed failure mode in small solder joints strongly depends on the size and shape of the joint cross-section. Furthermore, the crack growth rate can be appropriately correlated to the size of the intense plastic yielding zone at the crack tip using a plastic decohesion argument.

We have studied the surface morphology and wetting reaction of eutectic SnBi, eutectic SnPb, and pure Sn on Cu and Pd surfaces. In the case of Pd, the reactions were so fast that no quasi-equilibrium wetting angle could be measured. The compound formation has changed not only the interfaces but also the surfaces. Due to the formation of a reaction band outside the solder cap, the SnPb solder is no longer wetting the Cu surface but rather the Cu-Sn compound surface. In the wetting interface between eutectic SnPb and Cu, the morphology of the scallop-like Cu-Sn compound grains shows that we may not apply the classical model of layered compound growth to analyze the rate of soldering reactions.

The spreading of molten 60Sn40Pb drops on higher melting point Pb-Sn alloy substrates (3 to 10 wt.% Sn) was investigated for reflow temperatures of 205° to 300°C. Following melting the drop assumed the form of a slightly flared, spherical cap with some penetration into the substrate beneath the contact area. The effects of time and temperature on the contact angle θ and the depth of penetration h were of the form

where the apparent activation energy Q was 4.2 kcal/mole for θ and 16 kcal/mole for h. The time exponent m (negative for θ and positive for h) decreased with temperature from ∼ 0.2–0.3 at 205°C to ∼0.05 at 260° and then increased again at higher temperatures. The magnitude of Q for θ is in accord with that for the viscosity of molten Pb-Sn alloys and that for h with a combined liquid-solid diffusion involved in the dissolution. Further work is however needed to identify unequivocally the mechanisms which govern the wetting in these duplex Pb-Sn alloy systems.

This paper discusses the use of Ink Jet printing techniques to dispense small (50 to 75 micrometer diameter) particles of molten eutectic solder individually at programmable dispense rates from drop on demand to several thousand per second. Alternative jet dispensing techniques are discussed. The technology could allow the selective application of programmable amounts of solder on precision circuit boards and wafer substrates, while avoiding the high cost and flexibility limits associated with hard tooling. Large solder features can be constructed by dispensing individual droplets and relying on surface tension to draw them together to form a large single feature. Alternatively, columnar features can be created by successively dispensing solder droplets at the same site, allowing time between successive droplets to avoid forming a single large spherical feature.

Several potential application areas in industry are discussed along with some of the issues associated with the projected performance of the method in the accuracy and speed domains.

Stencil apertures of various interior angle, i.e. angle between two sides of a polygon, are analyzed for print performance. Aperture size and sidewall taper are also examined for their individual and combined effects on printing. The results show no effect of interior angle within the bounds of the experiment. Sidewall taper becomes relevant when aperture geometry is nearly square.

The continuing shrinkage of feature size and environmental concerns regarding the use of lead containing solder are major driving forces in the search for alternative technologies for metal interconnects in electronic packaging. Electrically conductive adhesives are considered a potential substitute for the lead-tin solder. This study concentrated on interfacial properties of eutectic solder and silver-filled epoxy adhesives. A freeze fracture technique was employed in order to preserve interfacial information. The chemical composition of the fractured faces varied; the adhesive side was rich in tin, whereas the eutectic solder side showed lead-rich characteristics.

A 200°C fluxless process is developed to produce In-Cu joints. The fluxless feature is achieved by the prevention of indium oxidation during the solder fabrication and the bonding process. Indium and copper are deposited on an object in high vacuum to inhibit indium oxidation. Copper interacts with indium to form CuIn compound that further protects the inner indium from oxidation. For a specific design, the resulting joints consist of mainly CuIn intermetallic grains surrounded by a small amount of pure indium as revealed by SEM with EDX. Scanning acoustic microscope examination indicates that the joints are nearly void-free. This technology enables versatile control of the alloy composition, thus leading to several remelting temperatures and various physical properties.

A series of rosin mildly activated, halide free, low residue fluxes have been evaluated for use with hot bar reflow of Tape Carrier Packages (TCP) to printed circuit boards (PCB). Materials were studied with respect to their wettability, SIR performance, water reflux extractable ionics and residue levels. Ionic levels were measured by FTIR and Ion chromatography. A flux was selected which has demonstrated acceptable performance in 156C/85%RH 10V biased stress testing.

With ongoing demand for high density wiring and high I/O on VLSI chips, the requirement of high wire bond yield is a challenge to achieve low cost, high performance and reliable products. Secondary Ion Mass Spectrometry (SIMS) was used to investigate the metallurgical contaminants on the gold wire bond pads and their impact on wire bond yields. SIMS depth profile studies showed that copper and nickel in concentrations greater than 1 wt% caused poor wire bondability, while copper concentration at less than 0.1 wt% resulted in good bondability of Al ultrasonic wire bonded to the gold pads.

The reliability of high power Insulated Gate Bipolar Transistor (IGBT) modules is mainly limited by the aluminum wire bonds. Bond failure occurs due to fatigue fracture close to the welded interface inside the large bonding wire caused by thermal cycling at extreme operating conditions. This paper presents the results of the metallographic and electron microscopic examinations of the welded interface as well as fracture morphology investigations of failed parts after power cycling. Infrared thermal imaging and investigations of the stress/strain relationship analyze the differential expansion caused by temperature distribution and thermal expansion mismatch between large bonding wire and semiconductor.

Fabrication of metal lines on aluminum nitride (AIN) ceramic is important for application of AIN as a microcircuit board. An Al line can be made directly from AIN by excimer laser exposure. All lines in this paper were written by pulsed KrF excimer laser (248 nm) in air and vacuum at different scan speeds with a constant laser energy. The morphology of the Al lines was observed by optical and scanning electron microscopies. The lines become more metallic when the scan speed decreases. Consequently the resistance, measured by the four-point probe technique, decreased to a minimum value in the scan speed range of 400 μm/s to 900 μm/s for the samples made in air. The resistance of Al lines written in air is greater than that in vacuum because of oxidation. The samples made in air with scan speed lower than 400 μm/s were heavily oxidized. SAXPS spectra showed a strong oxidation state and that the excimer laser exposure depletes nitrogen from AMN. The Al lines produced by laser-induced decomposition of AMN board are being evaluated as catalysts for laser assisted CVD and conventional electroless plating. The oxidation layer on the surface of Al lines may effect the adhesion between the Al lines and the deposited or plated metal.

Aluminum Nitride (AlN) has not attained the type of success alumina has had with thick film materials. This project is aimed at increasing the number of thick film inks compatible with AIN. Excimer laser reduced AIN is metallic, but not sufficiently conductive to act as an interconnection for microelectronic circuits. The reduced surface acts as a bonding layer to promote adhesion of screen printed metal films.

Design of Experiments (DOE) plays a critical role in determining the success of a research project. Completely Randomized Nested Classification design strategy is used in these experiments. The Ag-Pd ink formulated for alumina is screen printed on AIN and alumina substrates in a random order and fired. Wires are soldered to the pads and a pull test is done. Analysis of Variance (ANOVA) is used to compare the adhesion strength of Ag-Pd ink formulated for alumina screen printed on Al2O3 and laser reduced AIN substrates.

A via metallization technique involving excimer laser ablation of aluminum nitride substrate with a metal backing, attached with cyanoacrylate adhesive was investigated. The objective was to develop a procedure to ensure good electrical interconnection between the thick film conductor pads and the walls of the via. An aluminum nitride compatible Ag-Pd thick film paste was used in this experiment. Two different sequences were tested: In the first sequence both sides of the aluminum nitride substrate were printed and fired and then the vias were laser drilled. In the second sequence, one side of the substrate was printed and fired first, then the vias were laser drilled and the second side was printed and fired. In both of the above sequences, laser ablation of the printed conductor and redeposition of the ablated metal backing are necessary for making a proper contact with the thick film conductor pads.

In electronic packages consisting of glass-alumina composite dielectric layers and thick film conductors, Au and Ag conductors on separate layers may alloy. Alloys are formed by Ag+ ion diffusion through the intervening dielectric. The extent of alloy formation is dependent upon whether the Au and Ag conductors are electrically isolated or connected. When the Au and Ag conductors are electrically isolated, alloying is self-limiting and occurs by reduction of Ag+ ions and dissolution of the metallic Ag into the Au. In this case, the reduction of Ag+ is accompanied by oxidation of the lattice oxygen anions forming O2 gas which accumulates to form blisters. If the Au and Ag conductors are electrically connected, then the alloying is not self-limiting and occurs without the formation of O2. In this study, we quantitatively characterize the alloying reactions which occur during firing and propose mechanisms for specific dielectric and conductor packages. The presence of these alloying reactions can affect the reliability and the performance of the electronic ceramic package.