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.

Stress management and reliability assessment in electronic packaging

This project compared the formability of modified ASTM A710 Grade B50 ksi yield strength steel, jointly developed by Northwestern University and the Illinois Department of Transportation, with ASTM A606 Type 4 weathering steel used in Illinois and many other states for sign and signal structures, light poles, and other highway structures. ASTM E290 Type 1 free-bend 180° guided-bend test was used to evaluate formability, with side clearances as per ASTM E190. Bend and tensile specimens were cut in longitudinal and transverse directions from plates and sheets of numerous thicknesses (from 0.38" down to 0.10" thick). Mandrels of three diameters (0.25", 0.313", and 0.375") were used, each having a rounded nose with radius of bend equal to ½ of the mandrel diameter. All specimens of A710 and A606 steels of all thicknesses passed the guided-bend tests. No cracks, side tears, or fractures were observed. Tensile tests showed excellent ductility in both longitudinal and transverse rolling directions, making the modified ASTM A710 steel very suitable for use in sign and signal structures, light poles, and highway structures, along with structural tubing and other applications requiring weathering steel sheets and plates. Comparable A606 Type 4 steel had a yield strength range of 65 to 73 ksi and an ultimate tensile strength range of 79 to 89 ksi. The elongation to failure for the A606 steel sheets was in the range of 41% to 49%, averaging 43% ± 2.5% based on a 1" gage length. The microstructure of A606 steel is primarily ferrite with a limited amount of pearlite. The grain size of A606 in thinner plates was much smaller; grains were more rounded in thicker plates. Two heats of modified A710 Grade B steel were tested. Variations in mechanical properties were a function of steel composition, hot-rolling procedures, and thickness of the plates or sheets. In the first heat, yield strength varied in a range of 48 to 119 ksi, and tensile strength ranged from 64 to 119 ksi. The variation in the strength in the first heat was most likely caused by excessive section reduction during hot-rolling. The microstructure of the A710 steel was fine-grained ferrite and was significantly smaller than the A606 steel grains of similar thickness, which accounted for the higher strengths of the A710 steel in the first heat. Some bands of pearlite and fine-grained ferrite were observed in the modified A710 sheets, but did not affect the formability of the steel. Copper, nickel and manganese contents were decreased and better rolling procedures were used in the second heat, which had a uniform 49 to 56 ksi yield strength range. The second heat also had a uniform yield to tensile ratio of 0.74, high ductility and formability, making this alloy a very attractive weathering steel for many transportation and structural applications. An optimized 50 ksi minimum yield strength composition with an ASTM G101 index of 6.0 or more was developed for A710 sheet steel, and is proposed for general use in transportation and other construction applications.

https://www.ideals.illinois.edu/bitstream/handle/2142/47234/FHWA-ICT-14-002.pdf?sequence=2

Formability of New High Performance A710 Grade 50 Structural Steel

This project addressed the goal of National Strategy for Surface Transportation Research to improve highway structures by enhanced materials, in particular by design and implementation of new, drastically improved steels with respect to strength, low-temperature-fracture toughness, weldability, and weatherability. The goal of the project was to develop, standardize and commercialize a family of low-cost, high-performance steels for infrastructure applications. As a result we developed steels with yield strength of 50 to 80 ksi, with Charpy absorbed fracture energy significantly exceeding requirements of bridge construction codes at temperatures down to -100°F, steels that are easy to weld without pre-heat or post-heat, steels that exhibit best weathering performance among other commercially available steels. In addition these steels are easy to produce; production does not involve any heat treatment or thermo-mechanically controlled processing which is available only to several United States Steel companies. The steels developed in this project were commercially produced in the form of plate and wide-flange I-beams. The steels were used for seismic retrofitting of one bridge (in 2000) and for construction of two new bridges (in 2006 and 2010).

Development and Marketing of Low-Cost, High-Performance Steels for Infrastructure Applications

Higher toughness in steels permits the presence of longer fatigue cracks prior to rupture making it less likely that they would escape detection. Since steels become brittle at cryogenic temperatures, a steel with higher fracture toughness at low temperatures than existing steels is of interest for infrastructure applications such as bridges and tank cars that transport cryogenic liquids such as liquid chlorine or compressed natural gas. Such a steel (ASTM A710 Grade B) was previously developed at Northwestern University and was successfully used in construction of a bridge at Lake Villa, Illinois. This steel is produced by hot-rolling and air-cooling, is easily weldable without pre- and post-heating, is very corrosion-resistant and possesses very good fracture toughness at low temperatures. During development of this steel, the effects of different elements on the mechanical properties of the steel were investigated. In one steel composition, 0.1% titanium was added to further improve its welding properties. That steel had phenomenal fracture toughness. This project investigated the effect of the addition of various levels of titanium on the mechanical and fracture properties and microstructure of A710 Grade B steel. This study resulted in an invited paper for American Society for Metals Materials and Technology Conference, MT a full copy of this paper is attached and highlights of it are summarized in this report.

/pdf/super-tough-steel-for-bridges-and-other-applications-4jwioxfv9k.pdf

Semyon Vaynman

Papers

Stress management and reliability assessment in electronic packaging

Formability of New High Performance A710 Grade 50 Structural Steel

Development and Marketing of Low-Cost, High-Performance Steels for Infrastructure Applications

Super-Tough Steel for Bridges and Other Applications

Experimental and micromechanics analysis on fatigue crack propagation behavior in Sn-Ag eutectic solder