A Cu6Sn5 layer is an integral part of many electronic interconnections. Here we show that, although primary Cu6Sn5 is not a potent nucleant for Sn, the Cu6Sn5 layer plays a key role in Sn nucleation and microstructure formation in solder joints. Combining thermal analysis, FIB-tomography and EBSD, we show that conical cavities are present between the scallops of the Cu6Sn5 layer that act as geometric nucleation sites for Sn, that Sn grows from the Cu6Sn5 layer, and that reproducible nucleation orientation relationships (ORs) exist between Cu6Sn5 and Sn. With these ORs, a near-random distribution of Sn orientations is predicted from joint to joint even for Cu6Sn5 layers with a strong [0001] fibre texture. It is shown that the nucleation undercooling is strongly affected by manipulation of the Cu6Sn5 layer shape, and that it is possible to prevent nucleation on the Cu6Sn5 layer by adding more potent nucleants.

The growth mechanisms of primary Cu6Sn5 are studied in Sn-Cu alloys and solder joints by combining EBSD, FIB-tomography and synchrotron radiography. With increasing cooling rate and Cu content, Cu6Sn5 crystals developed from faceted hexagonal rods to grooved rods, in-plane branched faceted crystals and, finally, to nonfaceted dendrites. This range of growth morphologies has been rationalised into a kinetic microstructure map. Cu6Sn5 hexagonal rods grew along [0001] bounded by {101 ̅ 0} facets and Cu6Sn5 dendrites branched along <405> in the {101 ̅ 0} planes. The faceted to nonfaceted transition indicates a kinetic interface roughening transition and a gradual change in mechanism from lateral growth governed by anisotropic attachment kinetics to continuous growth governed by diffusion and curvature. Finally, it is shown that the full range of Cu6Sn5 morphologies that grew for different composition and cooling rate combinations in bulk alloys can be engineered to grow in solder joints made with a single composition (Sn-0.7wt%Cu/Cu) by altering the peak temperature and the cooling rate.

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The thermal decomposition of actinides oxalates greatly depends on the oxidation state of the cation, the gas involved and the physical characteristics of the precursor. In the actinides series, uranium(IV) oxalate U(C 2 O 4 ) 2 .6H 2 O can be viewed as a peculiar case, as its sensibility towards oxidation leads to a specific series of reactions when heating under oxygen atmosphere. In order to clarify the disagreements existing in the literature, particularly concerning potential carbonate intermediates and the possible transitory existence of UO 3 , we show here an extended characterization of the different intermediates through a combination of X-Ray diffraction, vibrational spectroscopies and X-Ray absorption near edge spectroscopy.In this frame, uranium oxidation was found to occur at low temperature (200°C) concomitantly to the onset of oxalate groups decomposition, leading to an amorphous oxo-oxalato compound. Pursuing the thermal conversion up to 350°C led to complete oxidation of U(IV) into U(VI), then to the formation of amorphous UO 3 still bearing adsorbed carbonates. The first pure oxide formed during the thermal conversion was further identified to sub-stoichiometric UO 3- after heating at 550°C. Finally, U 3 O 8 was obtained as the final stable phase after heating above 660°C. The mechanism of thermal conversion of uranium(IV) oxalate into oxide under oxygen is then driven by a complex interplay between redox reactions and decomposition of the organic fractions. Such chemical reactions were also found to significantly modify the morphology of the powder through HT-ESEM observations : decomposition led the size of the aggregates to reduce by 20% while uranium oxidation clearly promoted growth within the agglomerates.