Microstructure and chemistry of the SAC/ENIG interconnections

The microstructure and chemistry of the interface between SAC305 solder and electroless Ni–P coating (ENIG) was examined using scanning and transmission electron microscope equipped with an energy dispersive X-ray spectrometer. Four layers have been distinguished after 5 min of interaction at 503 K. There were: NiP-amorphous layer corresponding to Ni–P coating, Ni₁₂P₅ and Ni₂SnP. The discontinuous Ni₂SnP layer at SAC/ENIG interface was composed of fine crystalline particles. The last layer was formed mainly of (Ni,Cu)₃Sn₄ with some amount of (Cu,Ni)₆Sn₅ and Ag₄Sn phases. The voids observed in the NiP-amorphous layer after processing can be considered as the potential source of the “black pad” failure.     

Microstructural and Phase Composition Differences Across the Interfaces in Al/Ti/Al Explosively Welded Clads

The microstructure and phase composition of Al/Ti/Al interfaces with respect to their localization were investigated. An aluminum-flyer plate exhibited finer grains located close to the upper interface than those present within the aluminum-base plate. The same tendency, but with a higher number of twins, was observed for titanium. Good quality bonding with a wavy shape and four intermetallic phases, namely, TiAl₃, TiAl, TiAl₂, and Ti₃Al, was only obtained at the interface closer to the explosive material. The other interface was planar with three intermetallic compounds, excluding the metastable TiAl₂ phase. As a result of a 100-hour annealing at 903 K (630 °C), an Al/TiAl₃/Ti/TiAl₃/Al sandwich was manufactured, formed with single crystalline Al layers. A substantial difference between the intermetallic layer thicknesses was measured, with 235.3 and 167.4 µm obtained for the layers corresponding to the upper and lower interfaces, respectively. An examination by transmission electron microscopy of a thin foil taken from the interface area after a 1-hour annealing at 825 K (552 °C) showed a mixture of randomly located TiAl₃ grains within the aluminum. Finally, the hardness results were correlated with the microstructural changes across the samples.

     

Wetting and premelting of triple junctions and grain boundaries in the Al–Zn alloys

Phase transitions in grain boundaries (GBs) and GB triple junctions (TJs) can change drastically the properties of polycrystals. The GB and TJ wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid and solid phases are in equilibrium. The GB and TJ wetting tie-lines can continue in one-phase area of the bulk phase diagram as a GB or TJ solidus line. This line represents the GB or TJ premelting phase transition. The structure and composition of grain boundaries and GB triple junctions were studied by high-resolution electron microscopy and analytical transmission electron microscopy in the Al–5 at.% Zn polycrystals and by differential scanning calorimetry (DSC) in the Al–7.5 at.% Zn polycrystals. Between bulk solidus and GB or TJ solidus the metastable Zn-rich β_m-phase was observed in the GB triple junctions of quenched samples. This phase appears neither in the samples annealed above the bulk solidus nor in those annealed below the GB solidus. Zn-content in this β_m-phase corresponds to that of bulk liquidus. This is a structural indication that if the melt wets the GBs or TJs, the GB (or TJ) solidus line appears in the bulk phase diagram, and the liquid-like phase exists in GBs and TJs between bulk solidus and GB (or TJ) solidus lines. The structural observation of this phase is also supported by our data obtained by means of DSC.     

Formation of a quasicrystalline phase in Al–Mn base alloys cast at intermediate cooling rates

Al-rich 94Al–6Mn and 94Al–4Mn–2Fe alloys were suction-cast to evaluate the feasibility of obtaining bulk quasicrystal-strengthened Al-alloys at intermediate cooling rates alloyed with non-toxic, easily accessible and affordable additions. The influence of different cooling rates on the potential formation of a quasicrystalline phase was examined by means of scanning and transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Increased cooling rates in the thinnest castings entailed a change in sample phase composition. The highest cooling rates turned out to be insufficient to form an icosahedral quasicrystalline phase (I-phase) in the binary alloy. Instead, an orthorhombic approximant phase occurred (L-phase). The addition of Fe to the 94Al–6Mn binary alloy enhanced the formation of a quasicrystalline phase. At intermediate cooling rates of 10²–10³ K/s, various metastable phases were formed, including decagonal and icosahedral quasicrystals and their approximants. Rods (1 mm in diameter) composed of I-phase particles embedded in Al matrix exhibited a hardness of 1.5 GPa, much higher than the 1.1 GPa of 94Al–6Mn.     

Effects of PCB Substrate Surface Finish and Flux on Solderability of Lead-Free SAC305 Alloy

The solderability of the SAC305 alloy in contact with printed circuit boards (PCB) having different surface finishes was examined using the wetting balance method. The study was performed at a temperature of 260 °C on three types of PCBs covered with (1) hot air solder leveling (HASL LF), (2) electroless nickel immersion gold (ENIG), and (3) organic surface protectant (OSP), organic finish, all on Cu substrates and two types of fluxes (EF2202 and RF800). The results showed that the PCB substrate surface finish has a strong effect on the value of both the wetting time t ₀ and the contact angle θ. The shortest wetting time was noted for the OSP finish (t ₀ = 0.6 s with EF2202 flux and t ₀ = 0.98 s with RF800 flux), while the ENIG finish showed the longest wetting time (t ₀ = 1.36 s with EF2202 flux and t ₀ = 1.55 s with RF800 flux). The θ values calculated from the wetting balance tests were as follows: the lowest θ of 45° was formed on HASL LF (EF2202 flux), the highest θ of 63° was noted on the OSP finish, while on the ENIG finish, it was 58° (EF2202 flux). After the solderability tests, the interface characterization of cross-sectional samples was performed by means of scanning electron microscopy coupled with energy dispersive spectroscopy.     

On Constituents of the Periodic Layered Microstructure Developed in the Solid-State Reaction Between Zinc and Co2Si

Metallurgical and Materials Transactions A - The composition and crystal structure of the constituents of the periodic layered microstructure developed during the solid-state reaction between Co2Si...     

Interaction between liquid aluminum and yttria substrate: microstructure characterization and thermodynamic considerations

The systematic microstructure and thermodynamic studies of the reactions between liquid aluminum and dense polycrystalline yttria (Y₂O₃) substrates at 1,273 K are reported in this article. The microstructure observations showed the presence of three reaction product zones extending ~1 mm into the oxide substrate of typical C4 (Co-Continuous-Ceramic-Composites) structure. The first zone starting from the drop side was composed of fine crystalline precipitates of the Al₅Y₃O₁₂ (YAG) phase dispersed in the Al₃Y matrix. The second zone was built of larger AlYO₃ (YAP) crystals. The third zone formed elongated oxide precipitates (YAP) surrounded by the Al₂Y intermetallic channels. The thermodynamic calculation indicated that, depending on the amount of the yttrium dissolved in aluminum, the YAG (up to 5 at.% Y), YAP (5–13 at.% Y), or Al₂Y for higher content of yttrium might form at 1,273 K, while the Al₃Y phase might appear during cooling.