Fig. 1 Typical microstructure in Pb-free solder joints: a primary faceted Cu6Sn5, b non-faceted β-Sn dendrite [1], c Sn-0.7Cu-0.05Ni (wt.%) solder joint on Cu substrate, with interfacial Cu6Sn5 layer. d and e Top views of interfacial Cu6Sn5, under conditions of as-soldered and 1500 h ageing at 150 °C, respectively [3]

Fig. 2 Experimental set up for a monochromatic high flux synchrotron X-ray radiography imaging apparatus at BL20XU beamline, SPring-8 synchrotron, Japan [46], b micro-XRF mapping for trace element analysis at BL37XU beamline, SPring-8 synchrotron, Japan, and c glancing angle incidence X-ray Diffraction at Powder Diffraction Beamline, Australian Synchrotron [47]

Fig. 3 Synchrotron radiograph of a dynamic β-Sn dendrite growth in Sn-0.7Cu-0.15Zn at the rate of 0.2 K/min, and this image was captured with?~?0.6 K undercooling [9]. b Primary Cu6Sn5 IMC growth in Sn-4Cu solidifying at 10 K/min [66]

Fig. 4 In situ synchrotron radiography of solidification of Sn-0.7Cu-0.05Ni at 2 K/min [2]

Fig. 5 a Solidification path predicted by TCSLD3.2 database in ThermoCalc 2018a, b thermal analysis of Sn-0.7Cu-0.5Ag, c DSC curve of Sn-0.7Cu-0.5Ag alloy at the rate of 10 K/min, and d-f solidified microstructure of the alloy at cooling rate of ～?0.3 K/s. Synchrotron radiography of solidifying Sn-0.7Cu-0.5Ag alloy for (g1) fully liquid, (g2) developing β-Sn dendrite, (g3) competitive growth of secondary dendrite arms, (g4) tertiary dendrite arm growth, (g5) Cu6Sn5 eutectic growth and (g6) Cu6Sn5 eutectic coarsening and Ag3Sn growth [8]

Fig. 6 Synchrotron radiography in Ref. [6] of (a-e) nucleation and growth of primary Cu6Sn5 in a Sn-0.7Cu/Cu joint. Cu6Sn5 crystals are dark. (f) A processed image with each Cu6Sn5 was coloured by its nucleation time in s. t?=?0 is the onset of cooling from the peak temperature of 250 °C

Fig. 7 Synchrotron XRF mapping (100 nm spatial resolution) of interfacial (Cu, Ni)6 (Sn, Zn)5 IMC layers of as-reflowed Sn-0.7Cu-0.06Zn-0.05Ni/Cu BGA solder joint [47]

Fig. 8 Cu-Sn binary phase diagrams in the area around the Cu6Sn5 phases: a Saunders [139]. b Kattner [140], Cu-Sn system with the η′ phase as a line-compound and a non-stoichiometric η phase. c Wieser et al. proposed a revised version of the phase diagram Cu-Sn in the range of the η′-η order-disorder transition. This transition actually is composed of a peritectoid reaction ε-Cu3Sn?+? η?↔? η′ and a eutectoid reaction β-Sn?+? η′?↔? η [135, 136]. Redrawn from Refs. [135-137]

Fig. 9 Kinetics studies on polymorphic phase transformation in Cu6Sn5 using in situ synchrotron XRD: a temperature conditions for XRD measurements, b schematic TTT diagrams of hexagonal?→?monoclinic transformations [138]

Fig. 10 In situ synchrotron XRD patterns of interfacial IMCs on Cu substrates (X-ray energy 15 keV, wavelength 0.8266 Å): a example of phase identification and refinement by the whole-pattern fitting method; b Sn-0.7Cu; c Sn-0.7Cu-0.05Ni; d Sn-0.7Cu-0.15Zn; e Sn-0.7Cu-0.4Zn; f Sn-0.7Cu-0.06Zn-0.05Ni; and g Sn-0.7Cu-0.4 Zn-0.03Ni [2]

Fig. 11 Refinement of temperature variable synchrotron XRD patterns to calculate a CTE ellipsoids of η Cu6Sn5 at 180 °C, b η′ Cu6Sn5 at 100 °C in the Cartesian coordinate system (red axes), relative to crystal axes[152], and c volumetric thermal expansion behaviour of hexagonal Cu6Sn5 and Cu [2]

Fig. 12 Shear-impact test of 600 μm BGA joints at 2000 mm/s rates using DAGE4000S microtester. The hammer height above the substrate was fixed at 60 μm