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The paper compares theoretical calculations with experimental data, to identify the metallurgical mechanisms with respect to the rework or repair that may be encountered in the transition period from Sn–Pb to Pb-free soldering. Thermodynamic calculations have been carried out to study material behaviour and possible formation of intermetallic precipitates during the reaction between Sn–Pb and Sn–Ag–Cu Pb-free alloys. Two Sn–Ag–Cu alloys that are relevant to current industrial interests, namely Sn–3.9Ag–0.6Cu* (known as ‘405 alloy’ in Europe and North America), and Sn–3.0Ag–0.5Cu (known as ‘305’ alloy in Asia), were reacted with different contamination levels of eutectic Sn–37Pb solder. The variables examined included those related to both the materials and processes, such as composition, temperature and cooling rate. Together these are the primary drivers with respect to the resultant solder microstructures, which were studied using scanning electron microscopy (SEM). Nanoindentation, which is suitable for the ultra-fine and complex microstructures, was also used to investigate the micromechanical properties, including hardness and elastic modulus, at both ambient and elevated temperatures. The results from this work provide guidance as to the consequence for microstructural evolution and hence mechanical integrity when small amounts of Pb exist in Pb-free alloys. [ABSTRACT FROM AUTHOR]

Market demands and legislation are driving the electronics-manufacturing sector to move rapidly toward a lead-free future, with Pb-containing electronics products to be banned in Europe after 2006. Although the related scientific research has been undertaken for a decade, a number of technical complications still exist, which are further exacerbated due to the concurrent developments in miniaturization and multifunctionality of microelectronic products. As the packaging joint geometry shrinks toward a microscopic scale, the joint fabrication and reliability become extremely sensitive to the composition and resulting microstructure generated from the chosen joining process and materials. The current level of understanding of such issues is still in its infancy and therefore requires further fundamental study. Thermodynamic modeling is employed in this work as a computational tool to study the sensitivity of processing ranges (e.g., reflow temperature) and the resultant reliability of the micro joints by changing the alloying elements and their content in Sn-based lead-free systems. The work is implemented using the MTDATA program developed by the National Physical Laboratory. With a newly developed database containing critically assessed thermodynamic data appropriate for lead-free solder systems, MTDATA allows the prediction of the liquid-solid transformation and phase formation, for example, as a function of chemical composition and temperature. The paper emphasizes the formation and mass fraction of intermetallic precipitates of different phases in the bulk solder joints and the modeling is also validated through experimental work and recent literature. The results are expected to assist the optimization of processing parameters and cost-effective production using lead-free solders. [ABSTRACT FROM AUTHOR]

Continuous miniaturization of solder joints in high-density packaging makes it important to study how the joint size could affect the solder microstructure and thereby the subsequent in-service reliability. In this study, a printed circuit board with electroless nickel immersion gold (i.e., Au/Ni-P) over Cu bond pads of size approximately ∼80 µm and ∼1500 µm in diameter was dipped into a Sn-3.5Ag solder bath. The study shows that the smaller bumps, which cool more quickly, include much finer Ag3Sn particles. In addition, substantial differences in the thickness of the interfacial intermetallics and the microstructure for different dipping times are observed for different bump sizes. The results from a combined thermodynamic-kinetic model also suggest that the solder bump geometry can influence the dissolution kinetics of the pad metal into the molten solder and therefore the microstructure at the solder-pad interface and within the bulk solder. [ABSTRACT FROM AUTHOR]

A thermodynamic approach was used to investigate solder alloy systems containing Sn, Ag, Sb, and Pb, during both equilibrium and Scheil cooling conditions. The modeled microstructure was used to explore recent experimental results and to establish the microstructure-property relationships in microsolder joints. This approach is shown to be very useful in the transition from Pb-Sn to lead-free solders by enabling the consideration of contamination by a small amount of Pb. Molten solder interacts with the under bump metallization or print circuit board (PCB) metallization to form intermetallic compounds (IMCs). A truncated sphere structure was used to predict the solder joint geometry, and a two-dimensional finite-element (FE) method was adopted to investigate the kinetics of the dissolution of Au during the reflow process. The dissolution of Au into different volumes of solder material for three sizes of joints has been studied. In the modeling of the dissolution kinetics, the Nernst--Brunner equation is found to have poor validity for these calculations because of the dramatic change in the microscopic geometry and boundary conditions for joints at 100 µm in size or smaller. A combined thermodynamic and kinetic modeling approach, with a novel interface for implementation, is also briefly discussed. [ABSTRACT FROM AUTHOR]

As solder joints become increasingly miniaturized to meet the severe demands of future electronic packaging, it is vitally important to consider whether the solder joint size and geometry could become reliability issues and thereby affect implementation of the Pb-free solders. In this study, three bumping techniques, i.e., solder dipping, stencil printing followed by solder reflow, and electroplating of solders with subsequent reflow, were used to investigate the interfacial interactions of molten Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn solders on a copper pad at 240°C. The resultant interfacial microstructures, coming from a variety of Cu pads, with sizes ranging from 1 mm to 25 µm, and representing different solder bump geometries, have been investigated. In addition, a two-dimensional thermodynamic/kinetic model has been developed to assist the understanding of the kinetics of interdiffusion and the formation of interfacial intermetallic compounds. Experimental results and theoretical predictions both suggest that the solder bump size and geometry can influence the as-soldered microstructure; therefore, this factor should be taken into consideration for the design of future reliable ultrafine Pb-free solder joints. [ABSTRACT FROM AUTHOR]