Gaseous nitriding of binary Ni-Cr solid-solution alloys was studied at 1125ºC over the range 1 to 6000 bar of N2-pressure. At the specified temperature the nitriding response of the Ni-Cr alloys depends on the Cr-content in the initial alloy and activity (fugacity) of nitrogen at the gas/metal interface. Transition from cubic δ-CrN to hexagonal β-Cr2N precipitation occurs within the reaction zone after nitrogenization at 1125ºC under nitrogen pressure 100-6000 bar when chromium content in the initial alloy is 28 at. % or higher. It was found that a ternary phase, π (Cr12.8Ni7.2N4.0) is formed inside the Ni32Cr alloy upon cooling in nitrogen after nitriding at 1125ºC and 1 bar of N2. Experimental evidence is presented that π-phase is involved in peritectoid relations with β-Cr2N and γ-(Ni-Cr) solid solution. It was also demonstrated that nitriding behaviour of the Ni-Cr alloy can be rationalized using pertinent phase diagram information, but, in some cases, effect of mechanical stresses induced upon the internal precipitation can vitiate this prediction.

Conventional and effective interdiffusion coefficients have been determined in a Cu-Fe-Ni system for 1000°C based on our experimental data [13] on the distribution of concentrations of components in six diffusion couples of this system. The possibilities of applying these coefficients to interpret peculiar features of interdiffusion in ternary systems are analyzed. It has been demonstrated that effective interdiffusion coefficients are in a certain correlation with thermodynamic characteristics of the system.

A stereological analysis was carried out in order to obtain the kinetics parameters of the (Cu1−xNix)6Sn5 growth in the diffusion soldered (Cu–5 at.%Ni)/Sn/(Cu–5 at.%Ni) interconnections where previously anomalous fast growth of this phase was described. The n-parameter in the equation x = ktn was found to be 0.27–0.15 in the temperature range 240–260 °C, respectively. This is far away from the volume control process (n = 1 if total surface of forming phase is the reference). The TEM/EDX microanalysis made across the (Cu1−xNix)6Sn5/Sn – solder showed sudden change of Sn and Cu content typical for the grain boundary diffusion as the rate controlling mechanism.

Interdiffusion in a Cu-Ni-Fe system was studied at a temperature of 1000°C for three different diffusion couples. It has been shown that, in the studied diffusion couples, the distributions of some components of the system have a distinctly nonmonotonous character and, apart from the local extremes of the concentration curves, there are also zero flux planes in five diffusion zones. Special features of the behavior of the concentration curves and diffusion paths are discussed using thermodynamic data for the system. It has been found that diffusion fluxes of the components are unambiguously associated with a derivative of thermodynamic activity by the concentration taken along the diffusion path. There are experimental data in the appendix concerning the concentration profiles for all of the studied components of the diffusion couples.

Due to the obsolescence of SnPb BGA components, electronics manufacturers that use SnPb solder paste either have to use lead-free BGAs and adjust the reflow process or re-ball these components with SnPb balls. The reliability of Lead-Free and Lead-Containing solder joints for BGA’s has been investigated after re-balling using optimal microscopy. The goal was to compare the quality of the connections for both options. For the lead-free BGA, voids produced by the release of volatile species in flux during soldering were present. Large voids have been observed at the interface component/solder. Using components that were re-balled did not show the amount of voiding observed for the lead-free BGA. Kirkendall voiding has been observed for the lead-free component at the component/solder interface. It has been therefore concluded that the use of the reballed components is to be preferred to adjusting the reflow profile and using lead-free components. Introduction The use of lead containing solders for low temperature soldering in electronics has been commonplace for many decades. Due to the toxicity of lead the use of lead in electronic manufacturing has been regulated in Europe by RoHS. ii Eutectic SnPb solders have primarily been replaced by Sn, Ag and Cu (SAC) based solders at or close to eutectic composition, mainly because of its wetting ability and low melting point. The reliability of these SAC solder connections has not been proven to such an extent to satisfy the requirements in place for critical application, such as aviation, aerospace, military and medical industries. These industries therefore are still exempt from the RoHS regulations and continue to use SnPb solders for the production of electronics. The use of SAC solders for the consumer electronics on one hand and SnPb solders for exempted industries on the other brings on its own issues. One issue is the obsolescence of components compatible with a SnPb soldering process, due to the bulk of the electronic producing industry requiring lead-free compatible components. This causes component manufacturers to quit the production of SnPb compatible products because of economical reasons. In particular Ball Grid Arrays (BGA) can be affected as this could lead to the forced mixing of SAC and SnPb as the balls of the BGA are lead-free while the solder paste used is a SnPb variant. It has been shown that this leads to products that are not able to withstand the necessary life cycling test as stresses arise from different thermal expansion coefficients of the SAC balls and SnPb solder. This leads to cracking of the connection. The stresses might be avoided by either of two processes. First process is to re-ball BGA’s with SnPb balls if the only version of a BGA component is lead free. Second process was to ensure complete mixture of the SAC BGA and SnPb solder by adjusting peak temperature and dwelling time. Both processes have their advantages and drawbacks. The re-balling process leads to additional process steps, which could have a detrimental effect on reliability. It requires two additional heat processes and could lead to the extra growth of the brittle intermetallic layer between component and solder ball. Next to that because the original finish layer applied for better wettability such Au or Ag was dissolved in the old ball and therefore removed during de-balling. It will no longer protect the surface to oxidation and can decrease wetting properties for the new ball. The third issue comes from the application of flux used for re-balling, which could give rise to more and larger voids. The fourth aspect lies in the extra costs occurring due to the extra process steps. The main advantage is that if reballing is done properly and the quality of the BGA is maintained, the remainder of the soldering process would stay the same and the quality of the connection could be assumed to be as good with original SnPb BGA components. Adjusting peak temperature and dwelling time to ensure complete mixture of the SAC BGA and SnPb solder paste during reflowing leads to a solder connection which has different mechanical behavior compared to a eutectic SnPb solder connection. The different reflow profile could also lead to more oxidation and cause the problems commonly associated with oxidation during soldering. The adjusted profile could also lead to a thicker intermetallic layer. The main advantage is the relative ease and low cost of this process. It only involves a minimal adjustment of the reflow process. The goal of the research reported in this work was to investigate the quality of re-balled BGA components compared to the connections produced with an adjusted reflow process. This was done by means of metallographic research. Experimental Two similar BGA components have been investigated. The first sample consisted of a BGA with a SAC solder ball and was connected to a PCB using SnPb solder paste. The second sample consisted of a SAC BGA which was re-balled with SnPb balls and was connected to a similar PCB using SnPb solder paste. The original SAC solder balls were removed using flux and a desoldering device. The new SnPb balls were attached by a reflow process in air using a reballing mask and BGA holder. The reflow process used to connect the first sample was adjusted to ensure complete mixture of the SAC BGA and SnPb solder paste. Both samples have been subjected to metallographic analyses by means of optical microscopy. The samples were cut along the line shown in Figure 1 using a low speed diamond saw and embedded in Technovit 5000. The embedded components were coarsely grinded with sandpaper and fine polished with a diamond suspension (0.25 m particles). After polishing the microstructure has been investigated by optical microscopy. The microstructural analysis has been done with an Olympus BX-41 light microscope. For both samples eight solder connections have been investigated. For each connection the thickness of both intermetallic layers at boundaries was measured at two different points. Figure 1 The BGA component with the red dotted line indicating the cross section area. Results and Discussion

The isothermal cross section through the ternary phase diagram Nb–Ni–Cr at 1,100 °C was constructed by means of diffusion couples and equilibrated alloys. It was found that nearly 28 at.% of Cr can be dissolved in the μ phase (Nb7Ni6) at this temperature, and the solubility of chromium in NbNi3 is approximately 5 at.%. Under these circumstances the low-temperature (cubic) modification of the NbCr2 Laves phase can dissolve up to 6 at.% of nickel, but further increase of the Ni content (up to approximately 10 at.%) stabilizes the hexagonal (high-temperature) modification of the Laves phase. The presence of this pseudo-ternary compound which is in equilibrium with all binary intermetallics and body-centred cubic (BCC) Nb- and Cr-based solid solutions largely determines the topology of the isotherm at 1,100 °C. The formation of this phase was also observed in the reaction zone between Nb and Ni–Cr solid solution when chromium concentration exceeded 15 at.%.

Dependences of the parameters of interdiffusion (effective coefficients, diffusion paths, Kirkendall shift) on the variation of the initial compositions of a diffusion couple have been studied on the concrete example of the Co-Fe-Ni system. The experimental results obtained show that the diffusion paths in the three-component system depend substantially on the selection of initial conditions, even when the initial alloys of the “narrower” diffusion couple lie on the diffusion path of the “wider” pair. At the same time, when represented in reduced concentrations, the diffusion paths of all these diffusion couples practically coincide. The latter makes it possible to predict the type of diffusion paths for any diffusion couple whose initial com- position lies in the same concentration region. In one of the diffusion couples studied, the instability of the Kirkendall plane was revealed, which thus far was observed only in binary systems.

In a diffusion-controlled interaction, the Kirkendall plane, identified by inert particles placed at the initial interface between the reactants, need not be unique. The Kirkendall plane can microstructurally (spatially) be stable as well as unstable, and can, under predictable circumstances, bifurcate and even trifurcate. The movement of the Kirkendall markers during the interaction can be rationalized using the classical diffusion theory in terms of the Kirkendall velocity construction. The position of a Kirkendall plane is revealed in the reaction zone not only by the presence of inert markers, but also by a different crystal morphology developed on either side of the plane. The role of the Kirkendall plane in the morphogenesis of multiphase interdiffusion systems can be elucidated using equations of the interfacial reactions occurring in the diffusion zone. The appearance of one or more Kirkendall planes, characterized by morphology changes in the reaction layers is related to different nucleation sites of the product grains. The presence or absence of a Kirkendall plane in certain product phases provides insight into the initial stages of the reactive diffusion. Besides, the sometimes observed spatial (and temporal) patterns in a diffusion zone can be interpreted (and globally predicted) as a Kirkendall-effect mediated phenomenon. These conclusions will alter some previous notions about the diffusional growth of reaction layers and will influence the educational treatment in textbooks. It also will have strong technological implications, e.g., in the field of composite materials, thin-film electronic devices, etc.

Formation of diffusion zone morphologies periodic in time and space during metalceramic reactions is considered as a manifestation of the Kirkendall effect. In a diffusion-controlled interaction, the Kirkendall marker plane can bifurcate, which is attributed to diverging vacancies fluxes in the reaction zone. When the Kirkendall plane is present in a phase layer, it attracts in situproduced inclusions of “secondary-formed phase”, which, in turn, can result in a highly patterned microstructure.

The forthcoming European legislation restriction of hazardous substances (RoHS), in effect at July 1st 2006, has prohibited the use of certain substances for electronic and or electrical equipment. One of these prohibited materials is lead, which has been used for the past decades in the solder used to connect the components to the printed circuit boards (PCBs). Therefore, lead-free solders have been developed to replace the SnPb solders. The military, medical and aerospace industries work exclusively with proven technologies and are consequently exempted from these directives. Consumer electronics already has experience with lead-free technology, although the products have a shorter lifespan, whereby the maintaining of the product is of lower importance. Industrial electronics has almost no experience with lead-free technology yet remains confronted with longevity problems. The main alternatives solder alloys are Sn, Ag and Cu (SAC) near eutectic composition. As these alloys have higher melting points and different physical characteristics in comparison to SnPb, research is needed to discover the eventual problems with the replacement of SnPb. In this work PCBs with Ni/Au finish have been soldered with SAC and SnPb alloys. Reflow and wave soldering have been performed on three type of industrial. The components soldered to the board were: ceramic resistor, ceramic capacitor, coil, diode, jumper, plastic spacer, hybride module, flexible PCB, QFP, LED, electrolytic capacitor, capacitor, oscillator, cable or connector. The component finishes encountered were: pure Sn, SnPb, SnPb40 and AgPd. Environmental and bump tests have been conducted and analyzed by means of metallurgical research. Visual and light microscopy as well as in-circuit tests have been preformed

A general treatment of a diffusion-controlled growth of a stoichiometric intermetallic in reaction between two two-phase alloys is introduced. A reaction couple, in which a layer of Co2Si is formed during interdiffusion from its adjacent saturated phases is used as a model system. On the basis of chemical reaction equations occurring at the interphase interfaces, data on relative mobilities of diffusing species and the integrated diffusion coefficient in the product phase are deduced. The analysis yields numerical results identical to those calculated from classical Wagner's theory for the case in which the terminal phases of the diffusion couple are initially saturated.

A diffusion-controlled growth of intermetallic phases and the role of the Kirkendall effect in morphological evolution of the product phase layers can be described in terms of an alternative theory considering chemical reactions at the interphase interfaces. Application of this “physicochemical” treatment to diffusional growth of intermediate phases with fairly wide homogeneity ranges is illustrated by the example of interaction in the Ag-Zn system. The model is purely phenomenological, and its use is convenient, since no explicit assumption of the underlying diffusion mechanism is required.

Since conventional production of high-temperature materials involves high investments and costly consumption of both energy and time, reaction engineering methodology combined with near-net shaping is often the answer to problems associated with the fabrication of advanced materials. Over the last decades, the number of different reaction–based processing methods for near-net-shaped ceramics has gradually increased. In this review, different reactive processing techniques and their potential for near-netshaping are treated, e.g. SHTS (self-supporting high temperature synthesis), the Lanxide method DIMEX®, reaction bonding (RB), reactive processing of Alumina-Aluminide Alloys (3A) and Al2O3-Al alloyed metal composites (3AMC). In addition to their potential for near-net shaping, other advantages to reactive processing routes are recognized to be reduced processing temperatures, reduced glassy phase formation at the grain boundaries, fine grained microstructures and improved mechanical strength. Since the exothermic reactions constitute the base for reactive processing of high quality materials in an economic way, control of these reactions is essential. The process flows are described together with characteristic features of process and materials. In addition, specific aspects of reaction-based synthesis will be illustrated with examples from own work in the area of reaction bonding of silicon nitride and alumina.

When a moderately stable phase is precipitated out during an internal reaction, the behaviour of the penetrating atoms within the diffusion zone can be interpreted based on thermodynamic considerations. Evidence for "up-hill" diffusion of the penetrating species through the matrix towards the precipitation front during the internal nitridation of Ni-Cr alloys at 1125 degrees C and 6000 bar of N-2-pressure was predicted. Such behaviour of nitrogen is opposite to the boundary conditions in Wagner's description of internal reactions. A volume change associated with die precipitation reaction resulted in a stress gradient between the alloys surface and the internal nitridation front. Stress relief occurred mainly by transport of nickel to the gas/metal interface. Pipe diffusion-controlled creep is the dominant stress accommodation mechanism during nitriding of dilute Ni-Cr alloys at 700 degrees C under a flowing NH3 + H-2 gas mixture.

Magnesium alloys show strong susceptibility to localized corrosion when immersed in aggressive solutions (e.g. chlorides). The existence of second phase particles in the microstructure might represent initiation sites for localized corrosion. This is due to the formation of galvanic couples between the particles and the matrix. Extruded AZ80 magnesium alloy has been investigated by means of scanning Kelvin probe force microscopy (SKPFM) in order to measure the Volta potential of different phases relative to the matrix. The phases present in the alloy have been identified by optical microscopy and scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDXS). Three different phases were observed: Al8Mn5, Mg2Si and Mg17Al12 (β phase). All phases exhibited positive Volta potentials relative to the matrix indicating a cathodic behaviour. The Volta potential depends on the composition of second phase particles. The Al8Mn5 intermetallics showed the strongest cathodic behaviour. Based on the SKPFM results, it is expected that the cathodic phases are effective sites for the initiation of localized corrosion in extruded AZ80 magnesium alloy.