Papers by Author: Stuart D. McDonald

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: 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: 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: 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.

Abstract: This research examines the relationship between Cu and Ni concentration and the formation of primary Cu₃Sn in high Cu containing Sn-Cu solder alloys. Through thermal analysis and optical microscopy, it was determined that Ni additions still have a significant effect in minimising or eliminating Cu₃Sn for Cu concentrations as high as 30wt%. In addition, it is clear that a relationship exists between Cu concentration and the effect of Ni addition and the volume fraction of Cu₃Sn increases as the Cu content increases. It is likely that the Ni addition has a significant effect on the interdiffusional coefficients of the diffusing species of Cu₃Sn and Cu₆Sn₅, slowing the growth of Cu₃Sn and encouraging primary Cu₆Sn₅ nucleation.

Abstract: Phosphorus (P) is often added to wave-solder baths as an anti-oxidation agent for older generation eutectic Sn-37wt%Pb solders. For Pb-free solder alloys trace amounts of germanium (Ge) have been added successfully in Sn-0.7wt%Cu-0.05wt%Ni for anti-oxidation purposes during the wave soldering process. Despite this practice, there is little information on how P and Ge distribute in solder alloys and dross in this alloy system. In this paper, the effects of combinations of trace levels of Ge (< 100ppm) and P (< 100ppm) in Sn-Cu-Ni solder alloys and their dross has been investigated by XRD and SEM/EDS. It was found that the weight fraction of tin oxides in the dross is increased with an addition of less than 100ppm P. The dross consists on SnO oxide with Sn in samples containing Ge and no P, while SnO and SnO₂ oxide are both present when P and Ge additions are made. In samples containing P, (Cu,Ni)₆Sn₅ and Ni-P and/or Ni-Sn-P intermetallics particles were found.

Abstract: This paper demonstrates the development of an experimental technique of in-situ observation for soldering of Sn-0.7wt%Cu lead-free solder on a Cu substrate which was achieved for the first time by synchrotron X-ray imaging. Reactions between liquid solder and Cu substrate during a soldering process were able to be recorded in real-time. Individual stages of the soldering process consisted of flux activation in removal of Cu oxide, solder melting and contact with the Cu substrate (wetting) and intermetallic compound (IMC) and void formation between the solder and Cu substrate. The technique development which includes experimental setup with calculated optimum beam energy in the range of 20 – 30 keV appears to result in a clear observation of real-time X-ray imaging of the soldering process. This technique provides a key method to understand the mechanism of formation of micro-electronic inter-connects for future electronic packaging applications.

Abstract: Machining titanium is challenging due to its low thermal conductivity which results in very high temperatures at the tool/workpiece interface and in addition there is a tendency for titanium to react with most cutting materials, resulting in surface and subsurface deformation in the workpiece. This paper investigates the relationship between vibration and surface deformation that occurs while machining commercially pure titanium and Ti6Al4V alloy materials under both wet and dry machining conditions. The results have demonstrated that vibration monitoring (normalised peak frequency amplitude) can be used as a predictive tool for optimising the surface quality of the machined workpiece. Twinning plays a prominent role in the subsurface of the machined Grade 2 material.