Papers by Author: Kazuhiro Nogita

Abstract: Sn-0.7Cu-0.05Ni is a widely used Pb-free solder that solidifies into a near-eutectic microstructure and a small fraction of primary Cu₆Sn₅. This paper overviews in-situ time-resolved imaging experiments on the solidification of Sn-0.7Cu-0.05Ni solder under three conditions: (i) directional solidification, (ii) continuous cooling in a near-uniform thermal field, and (iii) solder joint solidification on a Cu substrate. Primary Cu₆Sn₅ grow as rods along [0001] in each case but can also grow as X-shaped crystals in (iii). There are significant differences in eutectic growth due to nucleation difficulties for tin in conditions (ii) and (iii).

Abstract: Additions of trace elements such as Phosphorus (P) and Germanium (Ge) are common practice to improve the oxidation resistance in Tin-Copper (Sn-Cu) wave solder systems, however, little insights are available regarding their combined role. In this article, the effect of trace P (<100ppm), in the presence of Ge (<100ppm), on the phase composition and microstructure of Sn-Cu-Ni wave solder dross is studied using various techniques including Synchrotron XRPD, SEM, FIB and TEM. We find that P additions, in the presence of Ge, result in the formations of SnO, SnO₂ and Ni₂SnP intermetallic in the dross whereas only SnO is present in the P-free equivalent. The crystal structure of Ni₂SnP is identified as orthorhombic with the space group Pnma. Based on the findings, it is evident that P not only influences the oxidation state of tin oxides but also reduces the concentration of effective Ni in the alloys via the formation of Ni₂SnP intermetallic.

Abstract: Cu₆Sn₅ is an important intermetallic compound in soldering and electronic packaging. It is formed at the interface between molten solder and substrate during the soldering process, and the evolution of microstructure and properties also occurs in service. Previous studies revealed that Au and Ni are stabilization alloying elements for hexagonal η-Cu₆Sn₅ intermetallic. For better understanding of stabilization mechanisms at atomic resolution level, in this work, we made an attempt atomic structure analysis on a stoichiometric (Cu, Au, Ni)₆Sn₅ intermetallic prepared by direct alloying. High-angle annular dark-field (HAADF) imaging and atomic-resolution chemical mapping were taken by the aberration-corrected (Cs-corrected) scanning transmission electron microscopy (STEM). It is found that Au and Ni doped Cu₆Sn₅ has hexagonal structure. The atom sites of Cu1 and Sn can be distinguished in atomic-resolution images after being observed from orientation [2110], which is also confirmed by atomic-resolution chemical mapping analysis. Importantly, atomic-resolution about distribution of alloying Au atom was directly observed, and Au atoms occupy the Cu1 sites in η-Cu₆Sn₅.

Abstract: Ga and Ga-based alloys appear to be promising materials for low temperature soldering in microelectronics. This research involved an analysis of the joint interfaces that resulted from reactions between a eutectic Ga-Sn alloy and Au coated Cu substrates at both room temperature and 100°C. At both temperatures the intermetallic CuGa₂ accounted for the majority of the interfacial microstructure. This study has shown the possibility of using eutectic Ga-Sn alloys in low temperature soldering applications, as well as the advantages of Synchrotron XFM techniques in characterising trace element distributions in solder joints.

Abstract: Liquid metal structure of Sn-Cu alloys studied by synchrotron high energy X-ray diffraction have been performed to investigate the effect of the trace elements Ni and Al. It has been demonstrated that trace Ni and Al additions are able to alter the liquid Sn-4wt%Cu structure. The refinement of the primary Cu₆Sn₅ phase after trace Al additions is proposed to be caused by the consumption of Cu atoms involved in a AlCu chemical ordering and an associated change in the undercooling for Cu₆Sn₅ nucleation.

Abstract: The effect of bismuth (Bi) micro-alloying additions on wettability and mechanical properties of Sn-0.7Cu lead-free solder were explored. This paper also investigates the influences of various Bi percentages on the suppression of intermetallic compound formation. Scanning electron microscope (SEM) was used to observe the microstructure evolution of solder joint including the thickness of interfacial intermetallic layers. Overall, with the addition of Bi to Sn-0.7Cu solder, the size of primary Cu₆Sn₅ become smaller and suppresses the thickness of interfacial intermetallic compound between solder and the Cu substrate. Microhardness value and wetting properties also increased with Bi addition which resulted in smaller size of β-Sn and Cu₆Sn₅.

Abstract: This paper investigated the effect of trace addition of Al and Mg on the grain refinements of Cu₆Sn₅ in Sn-3wt%Ag-5wt%Cu high temperature solder alloys. Furthermore, the effect of Al and Mg addition on the Sn/Ag₃Sn eutectic were also investigated. It was found that the addition of both Al and Mg successfully refined the Cu₆Sn₅ in Sn-3wt%Ag-5wt%Cu solder alloy. In addition, Al suppresses the formation of Ag₃Sn in the Sn/Ag₃Sn eutectic; while Mg promotes the formation of fine Sn/Ag₃Sn eutectic microstructure. The refinement of Cu₆Sn₅ is believed to be due to heterogeneous nucleation by Al and Mg rich intermetallic particles respectively. Effect of Al and Mg addition on the undercooling of the Sn/Ag₃Sn eutectic was found to be similar, both reducing undercooling effectively at a low addition rate of 0.025wt%. The addition of Al and Mg have mixed effect on the nucleation temperature of Cu₆Sn₅. It is found that the nucleation temperature of Cu₆Sn₅ is increased with 0.025wt% Al and 0.1wt% Mg addition to the unmodified alloy, while the nucleation temperature slowly decreases again as the trace element addition rate increases.

Abstract: This paper presents the use of the transient liquid phase concept to grow the high temperature Cu₆Sn₅ intermetallic compound between Cu₃Sn-rich powdered alloys and molten Sn. In this study, high temperature powdered alloys containing high fractions of Cu₃Sn were fabricated from a chill-cast Sn-60 wt.%Cu alloy. A ternary alloy with composition of Sn-59 wt.%Cu-1 wt.%Ni was also prepared to investigate the effect of Ni. The reaction products were obtained at 250°C over a period of 30 minutes. The results provide new insight into the mechanism of the interfacial reaction between liquid Sn and solid Cu₃Sn-rich alloy with and without Ni additions.

Abstract: A sessile drop experiment involving slow heating and cooling of lead-free solder alloys under inert gas revealed segregation of trace elements to the sample surface. Addition of germanium or gallium to Sn-0.7Cu-0.05Ni alloys promoted a metallic lustre in samples, in contrast with the blue/purple colour of the parent alloy. Alloys with Ge or Ga additions showed oxidation resistance. Depth profiling of surfaces of sample alloys with Ge or Ga showed a significant concentration of these elements within the oxide film, which may be responsible for oxidation resistance of these alloys.

Abstract: Extending the use of the Sn-Cu system to high-temperature solders poses additional challenges as the necessary high Cu content is in a region of the binary phase diagram which is dominated by the peritectic reaction and has the intermetallic compound (IMC) Cu₃Sn as the primary phase, which is known to have negative effects on soldering properties. Minor additions of nickel (Ni) have been reported to suppress the formation of Cu₃Sn in low Cu content Sn-Cu solder alloys though higher Cu content alloys have not been investigated. As such, the objective of this paper was to investigate the effect of more significant concentrations of Ni on the microstructure of a Sn-30wt%Cu alloy. An initial addition of 2wt%Ni greatly reduced the volume fractions of Cu₃Sn and the amount of eutectic present whilst significantly increasing the volume fraction of Cu₆Sn₅; however, further additions of Ni had a less pronounced affect. The Sn-30wt%Cu morphology was changed from a plate-like structure to a dendritic structure by adding Ni, which would improve solder performance by decreasing the possible crack plane length.