Practically all microelectronic assemblies in use today utilize Pb–Sn solders for interconnection. With the advent of chip scale packaging technologies, the usage of solder connections has increased. The most widely used Pb–Sn solder has the eutectic composition. Emerging environmental regulations worldwide, most notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies, due to the inherent toxicity of Pb. This has made the search for suitable “Pb-free” solders an important issue for microelectronics assembly. Approximately 70 Pb-free solder alloy compositions have been proposed thus far. There is a general lack of engineering information, and there is also significant disparity in the information available on these alloys. The issues involved can be divided into two broad categories: manufacturing and reliability/performance. A major factor affecting alloy selection is the melting point of the alloy, since this will have a major impact on the other polymeric materials used in microelectronic assembly and encapsulation. Other important manufacturing issues are cost, availability, and wetting characteristics. Reliability related properties include mechanical strength, fatigue resistance, coefficient of thermal expansion and intermetallic compound formation. The data available in the open literature have been reviewed and are summarized in this paper. Where data were not available, such as for corrosion and oxidation resistance, chemical thermodynamics was used to develop this information. While a formal alloy selection decision analysis methodology has not been developed, less formal approaches indicate that Sn-rich alloys will be the Pb-free solder alloys of choice, with three to four alloys being identified for each of the different applications. Research on this topic continues at the present time at a vigorous pace, in view of the imminence of the issue.

Lead-free Solders in Microelectronics

Next-generation high-performance structural materials are required for lightweight design strategies and advanced energy applications. Maraging steels, combining a martensite matrix with nanoprecipitates, are a class of high-strength materials with the potential for matching these demands. Their outstanding strength originates from semi-coherent precipitates, which unavoidably exhibit a heterogeneous distribution that creates large coherency strains, which in turn may promote crack initiation under load. Here we report a counterintuitive strategy for the design of ultrastrong steel alloys by high-density nanoprecipitation with minimal lattice misfit. We found that these highly dispersed, fully coherent precipitates (that is, the crystal lattice of the precipitates is almost the same as that of the surrounding matrix), showing very low lattice misfit with the matrix and high anti-phase boundary energy, strengthen alloys without sacrificing ductility. Such low lattice misfit (0.03 ± 0.04 per cent) decreases the nucleation barrier for precipitation, thus enabling and stabilizing nanoprecipitates with an extremely high number density (more than 1024 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.

Ultrastrong steel via minimal lattice misfit and high-density nanoprecipitation

Lead-free soldering has emerged as one of the key technologies for assembling in environmental-conscious electronics. Among several candidate alloys, the Sn–Ag–Cu alloy family is believed to be the first choice with the combination of other alloys such as Sn–Zn–Bi, Sn–Cu and Sn–Bi–Ag. Phase diagrams of lead-free alloy systems have been intensively examined by using careful thermal and microstructural analysis combined with the thermodynamic calculation such as the CLAPHAD method. The Cu6Sn5/Cu3Sn layers are formed at most lead-free solder alloy/Cu interfaces, while Cu–Zn compound layers are formed in the Sn–Zn/Cu system. Growth kinetics of intermetallic layers both in solid-state and in soldering are also discussed. Creep and fatigue phenomena are also reviewed. In many aspects of lead-free soldering, much more work is required to establish a sound scientific basis to promote their applications.

Advances in lead-free electronics soldering

The microstructures and tensile properties of three typical Sn–Ag–Cu alloys, Sn–30wt%Ag–05wt%Cu, Sn–35wt%Ag–07wt%Cu and Sn–39wt%Ag–06wt%Cu, prepared under three different cooling conditions were evaluated after casting The microstructures of all rapidly cooled specimens consisted of the eutectic phase of β-Sn with fine fibrous Ag3Sn dispersion surrounding primary β-Sn grains The slowly cooled Sn–35Ag–07Cu and Sn–39Ag–06Cu alloys exhibited additional large primary Ag3Sn platelets, while the Sn–30Ag–05Cu did not For all alloys, both ultimate tensile strength and 02% proof stress increased with increasing strain-rates in tensile tests Lowering cooling speed decreased tensile strength Elongation increased with an increasing strain rate from 10−5 to 10−2 s−1, and decreased slightly at 10−1 s−1 for the rapidly cooled specimens Elongation remarkably decreased for the slowly cooled Sn–35Ag–07Cu and Sn–39Ag–06Cu alloys, a degradation attributable to the formation of large primary Ag3Sn platelets

Effects of cooling speed on microstructure and tensile properties of Sn–Ag–Cu alloys

Review of non-reactive and reactive wetting of liquids on surfaces.

This paper reviews work on fabricating light-weight optics made from inorganic composite materials. The goal of the project is to make mirrors in the 5-10 kg/m 2 area density regime based on Northwestern Universitys previous technology used to make X-ray optics. The goal of this replication process is to end up with a smooth and also potentially accurate surface such that post figuring and polishing will not be necessary. This report covers work on fabricating witness samples up to 10 cm in diameter. Keywords: Light weight, optics, micro-balloons 1. INTRODUCTION Replication technology applied to optics fabrication has a long history. An introduction to this can be found in Refs 1 and 2. The technology in the past 10 years has been used to fabricate X-ray optics for space, e.g. XMM-Newton 3 . Northwestern University has the infrastructure in place to carry out the fabrication of X-ray optics, which includes a vacuum deposition system as well as el ectroforming tanks and surf ace characterization instru ments. Two key elements of replication technology are the ability to replicate surface finishes down to the sub-nm level and slope errors of approximately 10 microns/m. We have teamed up with Advanced Power Solutions Inc(APS), to apply our techniques to fabricating and attaching fr ont surfaces to substrates provided by APS. Th e goal was to learn how to attach the front surface to APS substrates and demonstrate the basic feasibility of this approach. We report here the results of this collaboration. These materials have many other uses besides mirrors for astronomy, but the proposed goal of our project is to demonstrate that it is possible to fabricate 1 meter diam eter mirrors in a cost effec tive manner. These mirrors would have a low enough total mass and price to appeal to the amateur astronomy market as well as the professional . At least in the US, the fabrication technologies for the two major 30-m class telescopes funded by the US government

Mirrors from light-weight sintered microspheres combined with replication techniques

Microstructural characterization of interface-engineered Si3N4 for improved fracture toughness and wear resistance

In one aspect of the invention, an alloy includes a first element comprising magnesium (Mg), titanium (Ti), zirconium (Zr), chromium (Cr), or nickelaluminum (NiAl), a second element comprising lithium (Li), calcium (Ca), manganese (Mn), aluminum (Al), or a combination thereof, and a third element comprising zinc (Zn) According to the invention, nanoscale precipitates is produced in the magnesium alloy by additions of zinc and specific heat-treatment These precipitates lower the energy for dislocation movements and increase the number of available slip systems in the magnesium alloy at room temperature and hence improve ductility and formability of the magnesium alloy

Semyon Vaynman

Papers

Mirrors from light-weight sintered microspheres combined with replication techniques

Microstructural characterization of interface-engineered Si3N4 for improved fracture toughness and wear resistance

Alloys and methods of forming same