Haoran Ma

Abnormal Intermetallic Compound Evolution in Ni/Sn/Ni and Ni/Sn-9Zn/Ni Micro Solder Joints Under Thermomigration

Interfacial reactions in Ni/Sn/Ni and Ni/Sn-9Zn/Ni micro solder joints during thermomigration (TM) have been studied by reflowing solder joints on a hot plate. Asymmetrical growth and transformation of interfacial intermetallic compounds (IMCs) were clearly observed. The growth of the Ni3Sn4 IMC in the Ni/Sn/Ni solder joints was always fast at the cold end and relatively slow at the hot end. Only asymmetrical growth of the Ni5Zn21 IMC in the Ni/Sn-9Zn/Ni solder joints occurred at the beginning because Zn was the dominant TM species; however, asymmetrical transformation of the Ni5Zn21 IMC also occurred under the combined effect of Zn depletion and Ni dissolution and migration, resulting in formation of a thin τ-phase layer at the hot end and a thick τ-phase/Ni5Zn21/τ-phase sandwich structure at the cold end. TM of Ni and Zn atoms was identified towards the cold end, being responsible for the abnormal IMC evolution. Addition of Zn was found to slow the TM-induced IMC growth and Ni dissolution.

Effects of rare earth Ce addition on the microstructure,wettability and interfacial reactions of eutectic Sn–0.7Cu solder

The effects of trace RE Ce addition on the microstructure, melting point and wettability of eutectic Sn–0.7Cu solder as well as on the interfacial reactions of Sn–0.7Cu–xCe/Cu(Ni) during soldering were investigated. The Ce bearing solders owned slightly higher melting temperatures than eutectic Sn–0.7Cu solder, with small melting intervals. The addition of trace RE Ce had a diverse effect on the wettability of the solders, and better wettability was obtained on Cu substrate than on Ni substrate. Due to the metamorphism of RE elements, the β-Sn dentrites were refined and the eutectic regions became more uniform in the Sn–0.7Cu–xCe solders with the increase of Ce addition. The intermetallic compound (IMC) formed at Sn–0.7Cu–xCe/Cu interfaces during soldering was scallop-like Cu6Sn5, and those formed at Sn–0.7Cu–xCe/Ni interfaces were (Ni,Cu)3Sn4 and (Cu,Ni)6Sn5. With the increase of Ce addition, the average size, the aspect ratio and the thickness of the Cu6Sn5 grains on the Cu substrate decreased, while those of the IMC grains on the Ni substrate increased. The different growth behavior of the interfacial IMCs on Cu and Ni substrates were discussed.

Characters of multicomponent lead-free solders

In the process of electronic packaging, such as flip chip technology, under bump metallization (UBM) can be consumed gradually by solder during soldering. Then dissolution of Ni, Au and Cu from UBM into the solder may change the original solder to a multicomponent one especially under the trend of miniaturization. It is quite necessary to evaluate the properties of the multicomponent solders that have new composition after soldering. In this study, the microstructure, thermal and mechanical properties of five types of multicomponent lead-free solders, i.e. Sn–2Cu–0.5Ni, Sn–2Cu–0.5Ni–0.5Au, Sn–3.5Ag–0.5Ni, Sn–3.5Ag–1Cu–0.5Ni and Sn–3.5Ag–2Cu–0.5Ni (all in wt% unless specified otherwise) were investigated. Comparison with eutectic Sn–0.7Cu, Sn–3.5Ag and Sn–3.5Ag–0.7Cu solders was made. There was no obvious difference of the melting point between the multicomponent lead-free solders and the eutectic ones. For Sn–2Cu–0.5Ni solder, Cu6Sn5 and (Cu,Ni)6Sn5 intermetallic compounds (IMCs) formed. In the case of Sn–2Cu–0.5Ni–0.5Au, besides (Cu,Ni)6Sn5, (Cu,Au)6Sn5 and (Cu,Ni,Au)6Sn5 were also observed. The IMCs formed in Sn–3.5Ag–0.5Ni solder were Ag3Sn and Ni3Sn4. In both Sn–3.5Ag–1Cu–0.5Ni and Sn–3.5Ag–2Cu–0.5Ni solders, Ag3Sn and (Cu,Ni)6Sn5 were detected. The mechanism for the formation of the IMCs was discussed. Tensile test was also conducted. The fractography indicated that all of the multicomponent lead-free solders exhibited a ductile rupture.

Influence of rare earth Ce addition on the microstructure, properties and soldering reaction of pure Sn

The influence of trace rare earth (RE) Ce addition on the microstructure, melting point and wettability of pure Sn as well as on the soldering reactions in Sn-xCe/Cu(Ni) solder joints was investigated. In bulk Sn-xCe solders, large β-Sn grains were observed with the Ce addition less than 0.2 wt%; while the β-Sn grain size decreased markedly when the Ce addition was 0.2 wt%, resulting in a refined microstructure. The addition of trace RE Ce had little effect on the melting temperature of the solders. Smaller wetting angles of Sn-xCe solders on both Cu and Ni substrates were measured when the samples were reflowed at a higher temperature. The Sn-0.2Ce solder owned the best wettability on Cu substrate. Scallop-like Cu6Sn5 intermetallic compound (IMC) grains formed at the Sn-xCe/Cu interfaces, while a continuous Ni3Sn4 IMC layer formed at each Sn-xCe/Ni interface. With the increase of Ce addition, the interfacial IMC grain size and the interfacial IMC layer thickness on both Cu and Ni substrates decreased gradually. The activity of Sn was lowered with the Ce addition, which depressed the growth of the interfacial IMC. In the current study, the Ce addition of 0.2 wt% exhibits the optimized performance.

Interfacial reactions of sequentially electroplated Au/Sn/Au films on Si chips

Abstract Au−30 at-%Sn eutectic alloy was fabricated by sequentially pulse electroplating Au and Sn films on Si chips. Three kinds of Au/Sn/Au triple layer films were prepared in the present work: Au/Sn/Au (6/6/1 μm) films, Au/Sn/Au (6/6/6 μm) films and Au/Sn/Au (8/6/1 μm) films. The microstructure and phase transformation in Au/Sn/Au films during aging and reflow soldering were investigated. For Au/Sn/Au (6/6/1 μm) films during aging at 100 and 150°C, the layered AuSn/AuSn2/AuSn4 structure formed in the reaction region. Furthermore, the Sn film was completely consumed, and AuSn4 finally transformed into AuSn and AuSn2 after aging at 150°C for 15 h. For Au/Sn/Au (6/6/6 μm) films during aging at 150°C, the electroplating sequence had an important effect on the formation of Au−Sn phases. An Au5Sn layer was present at the Au II/Sn interface but not at the Au I/Sn interface. For Au/Sn/Au (8/6/1 μm) films, the micropores that formed preferentially along the Au5Sn/AuSn interface remarkably decreased with increasing reflow temperature from 280 to 310°C. After reflowing for 10 s, the microstructure was not an Au−Sn eutectic; however, after reflowing for 60 s, coarsened primary Au5Sn phase and typical Au−30 at-%Sn eutectic microstructure of fine eutectic phases (AuSn+Au5Sn) formed.

Modelling the melting of Sn0.7Cu solder using the enthalpy method

The temperature and velocity distribution during the transient melting phase change in Sn-0.7Cu solder alloy has been modelled using finite element method. The simulation of melting front dynamics has been performed by an Eulerian method, commonly known as enthalpy method. CALPHAD technique is utilized for the computation of enthalpy of the solder material. The flow in the liquid regime of the solder is assumed incompressible and natural convection effects are incorporated through the Boussinesq approximation. The asymptotic viscosity is employed for the mushy zone. For a horizontal finite temperature gradient imposed on the square domain, the mushy zone is thinner at the upper half whereas thicker at the lower of the initially liquid region. Consequently, the melting front within the temperature range 500 K - 503 K moves faster towards the solid in the upper portion of the geometry. The velocity at the peripherial zones of the liquid undergoing convection flow reaches its maximum magnitude of 13 mm/s.

Positive feedback on imposed thermal gradient by interfacial bubbles in Cu/liquid Sn-3.5Ag/Cu joints

Synchrotron radiation x-ray imaging technique was applied for in-situ observation of interfacial bubbles in Cu/molten Sn-3.5Ag/Cu joint undergoing thermomigration. The heating plate temperature was maintained at 350 °C and reflow time of 1 h was considered. Interfacial bubbles at hot side, favoured by wetting transition for growth, increase the effective thermal gradient in the solder medium. The interfacial voids lower the resistance of Sn-based solder joints to thermomigration. The numerical model for effective thermal conductivity and heat transfer in the inhomogeneous medium was implemented using finite element method.

The effect of reflow temperature on IMC growth in Sn/Cu and Sn0.7Cu/Cu solder bumps during multiple reflows

The interfacial intermetallic compound (IMC) is a crucial factor for solder jointing reliability assessment and a suitable IMC layer size guarantees the quality of solder joints. On one hand, the interfacial grains keep growing with reaction time during soldering; On the other hand, with the coming universal application of 3D electronic packaging in modern electron industry, multiple reflows become a common soldering technology at the same time, which result in the much longer effective reaction time for solder materials. In this case, the size of interfacial IMCs at solder/substrate interface should arouse close attention of material scientists and engineering designers. In this article, the effect of welding temperature on IMC growth kinetics in Sn/Cu and Sn0.7Cu/Cu solder bumps was investigated respectively during multiple reflow process, by high pressure blowing (HP) experimental method and scanning electron microscope observing technology. It was found that the IMC thickness as well as the grain width increases with soldering temperature and reflow cycle for both pure Sn and Sn0.7Cu alloys; furthermore, the length of prism crystal is larger in higher temperature; besides, the value of time exponent n also grows with welding temperature, especially for Sn0.7Cu solder; finally, the reason for the kinetic difference between Sn and Sn0.7Cu alloys may be attributed to the fine grains at solder/substrate interface in Sn0.7Cu/Cu joint.

The study on the rapidly-solidified Sn-0.7Cu lead-free solders and the interface reactions with Cu substrate

The rapidly-solidified lead-free solders had lower melting point, narrower melting interval, better wettability and improved mechanical performance because of the fine, uniform and small segregation microstructure compared with the as-cast lead-free solders. In this study, a rapidly-solidified Sn-0.7Cu eutectic alloy was used as a sample for investigating the microstructure, wettability and the Cu6Sn5 IMCs and their growth behavior formed between the solders and Cu substrate. In addition, an as-cast Sn-0.7Cu solder was prepared as a reference. The results of the study showed that the microstructure composition was more uniform, the wettability was much better, the melting rate was much faster of the rapidly-solidified Sn-0.7Cu solders than the as-cast solders; the morphology of Cu6Sn5 grains formed between solders and substrate was different between rapidly-solidified and as-cast Sn-0.7Cu solders in short soldering duration; the rapidly-solidified Sn-0.7Cu solders were suitable for soldering with low temperature and short duration.

Effects of soldering temperature and cooling rate on the as-soldered microstructures of intermetallic compounds in Sn-0.7Cu/Cu joint

In this study, Sn-0.7Cu solder alloy, being selected as the research object, was allowed to react with polycrystalline Cu substrate at 250°C, 275°C and 300°C. After soldering reaction of 10 minutes, it underwent cooling in the three types of cooling medium: water, air and heating furnace, along with the simultaneous application of high pressure air for blowing away the liquid solder on the top of the intermetallic compounds (IMCs) of the specimens. Scalloped, faceted, prismatic and hexagonal shape Cu6Sn5 grains were observed at the solder/Cu interface, respectively. Scalloped shape grains were found at 250°C. At higher soldering temperatures such as 275°C and 300°C, faceted grains were dominant and the other two were mainly discovered under smaller cooling rates. With these observations, in-depth analyses of the morphology and size of Cu6Sn5 IMC as well as effect of brazing temperature and cooling rate on the microstructure of the joint were made. Moreover, synchrotron radiation real-time imaging technology, was utilized in observing the dynamic growth behavior of IMC during brazing process, thereby, providing direct evidence to the hypothesis regarding the IMC evolution pattern.