A simplified low-temperature wafer-level hybrid bonding process using Cu pillar bumps and photosensitive adhesive was reported, wherein Cu/SnAg/Ni–P micro interconnects were formed to achieve electrical interconnect and the sealing around adhesive played the role of mechanical reinforcement. The proposed hybrid bonding method has been applied to 8 inch wafer to wafer bonding. Two kinds of photosensitive adhesives, i.e., polyimide and dry film, were selected for adhesive bonding. Excess adhesive on the Cu/SnAg micro bumps was properly removed using simple and low cost lithograph process. In order to prevent the adhesive trapping in the metal bonding interface, the height of the Cu/SnAg micro bumps was 2 μm higher than that of the adhesives. Although hybrid bonding using polyimide and dry film can achieve seam free bonding interface, shear test results indicate that bonding strength using dry film is more robust, and dry film is more suitable for hybrid bonding in three-dimensional integration applications.

Fluorescence X-ray Absorption Fine Structure (XAFS) was used to in situ study the local structure around Cu atoms in liquid Sn solder during Sn/Cu liquid–solid interfacial reaction. The extended edge data were analyzed by cumulant expansion method. The results showed that both the atomic distance and the coordination number in the first shell decreased with the increase of temperature. It is an intrinsic trend that high-coordinated polyhedrons could transform into low-coordinated ones with relatively high stability in the liquid Sn solder with increasing temperature, and the high-coordinated polyhedrons always had larger atomic distance. The atomic distance in the first shell decreased with decreasing coordination number. Based on the fitting results, the proportion of Cu–Sn atomic coordination increased with rising soldering temperature from 250 to 310 °C. That is, Cu atoms preferred to form Cu–Sn interaction with Sn atoms, which promoted the nucleation and growth of Cu6Sn5 intermetallic compound at the Sn/Cu interface during soldering process.

•The outline evolution of interfacial bubbles and grains has been in situ observed by Synchrotron Radiation Facility.•The growth behavior of solder bubbles is analyzed for each reflow cycle.•The diffusion velocity of Cu in interface area including a bubble is calculated.•The size-inhibiting interaction of interfacial solder bubbles and IMCs is studied.By utilizing synchrotron radiation X-ray imaging technique for in situ observation, the geometrical outline evolution and size-inhibiting interaction of solder bubbles and IMCs during multiple reflows were investigated in this work. It was found that the interfacial bubbles can inhibit the growth of IMC grains caused by the hindering effect of bubbles on contact and reaction between solder and substrate; meanwhile, following the growth of IMC grains, the bubble size in return got smaller and its aspect ratio became larger with reflow cycle, which is the result of inhibiting effect of solid IMC grains on solder voids; Because of boundary segregation and thermomigration, Cu atoms tend to gather on the bubble boundary and form a Cu-rich phase, consequently resulting in the formation of IMCs at the gas/solder interface area.

•The distribution of Cu concentration in liquid bulk for solder bumps in different volumes has been simulated.•The pinning effect of Ag3Sn nano particles for different reflow cycles is studied.•The IMC growth kinetics for Sn3.5Ag/Cu joints in different sizes are calculated.•The competition between promoting of Cu and pinning of Ag3Sn for IMC growth in specimens is analyzed.Size effect on growth kinetics of interfacial intermetallic compound (IMC), induced by Cu concentration gradient and pinning effect of Ag3Sn particles during multiple reflows, was investigated in this article. The simulation results, for Cu distributions in solder bulks of different volumes after a single reflow for 60 s at 250 °C, show that Cu concentration gradient in liquid increases with the growing size of solder bump. On the contrary, resistive pressure of nano particles decreases gradually with the increasing bump size. In conclusion, the pinning effect of Ag3Sn particles on IMC grains plays a dominant role in small samples, whereas the inhibiting effect of Cu concentration gradient is mainly functional in big samples. Combining the two factors, solder bump in an intermediate diameter of 800 μm benefits most and has the largest IMC thickness during multiple reflows.Download high-res image (206KB)Download full-size image

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.

•Both Zn and Cu atoms were driven to migrate towards cold end by temperature gradient.•Temperature gradient significantly enhanced interfacial IMC growth at the cold end.•Temperature gradient accelerated the Cu dissolution at hot end.•Cu5Zn8 layer hindered the dissolution of hot end Cu until it spalled from interface.•Crystallization of β-Sn grains followed a highly preferred orientation.Synchrotron radiation real-time imaging technology was used to in situ study the interfacial reactions of Cu/Sn–9Zn/Cu solder joints during reflow under a temperature gradient. It was clarified that both Zn and Cu atoms were driven to migrate towards the cold end by the temperature gradient, which enhanced the intermetallic compounds (IMCs) growth locally and the Cu consumption at the hot end. A Cu5Zn8 layer adhered to the hot end interface could drastically restrict the consumption of the neighboring Cu substrate till it spalled from the interface. Additionally, the crystallization of β-Sn grains was observed to follow a highly preferred orientation with the c-axis of β-Sn grains departing from the direction of temperature gradient by about 30°–40°. These peculiar phenomena may inspire future exploration of interfacial reaction and grain orientation control by temperature gradient in 3D IC packaging technology.

•Asymmetrical evolution of pre-formed Cu6Sn5 under temperature gradient was in situ characterized by synchrotron radiation.•The pre-formed Cu6Sn5 grew linearly at cold end whereas dissolved linearly at hot end till a critical layer.•The critical Cu6Sn5 layer developed from dynamic equilibrium between chemical potential gradient and temperature gradient.•A small temperature gradient can dramatically decrease the equilibrium IMC thickness at hot end.•The proposed model well explains the dissolution and precipitation kinetics of interfacial Cu6Sn5.Multiple reflows are often required in 3D packaging. To elucidate the effect of temperature gradient during subsequent reflow on existing intermetallic compounds (IMCs), Cu6Sn5 IMC layers were initially formed in Cu/Sn/Cu micro interconnects. Upon subsequent reflow, synchrotron radiation real-time imaging technology was used to in situ study the dissolution and precipitation behavior of the pre-formed Cu6Sn5 under different temperature gradients. The pre-formed Cu6Sn5 IMC at the cold end continued to grow linearly with increasing aspect ratio, whereas that at the hot end dissolved linearly and then maintained a critical thin layer. The thick pre-formed Cu6Sn5 IMC at the hot end significantly hindered the dissolution of the neighboring Cu substrate until a dynamic equilibrium between chemical potential gradient and temperature gradient was satisfied. The thermomigration of Cu atoms from the hot end towards the cold end was responsible for the asymmetrical evolution of the interfacial Cu6Sn5 between the cold and hot ends. A theoretical model was proposed based on Cu diffusion flux to calculate the IMC thickness at the both ends as a function of reflow time and the equilibrium IMC thickness at the hot end under temperature gradient.Figure optionsDownload full-size imageDownload as PowerPoint slide

In the process of electronic packaging, the dissolution of under bump metallizations, such as Cu and Ni, into liquid solder occurs during soldering, which can change the original solder to a multicomponent one. Under the trend of miniaturization, it is quite necessary to evaluate the properties of multicomponent solder with excessive Cu and Ni compositions. In this study, the tensile creep behavior of Sn–3.5Ag–2.0Cu–0.5Ni multicomponent lead-free solder alloy is investigated at three temperatures, i.e., 303, 348 and 393 K. The steady-rate creep rates are obtained in the range of 10−4–10−8 s−1, when the normalized stress, σ/E, is in the range of 10−4–10−3. Based on the Dorn equation, the apparent stress exponent (na), threshold stress (σth), and activation energy of creep (QC) are calculated at the three temperatures. It is found that the Sn–3.5Ag–2.0Cu–0.5Ni solder alloy shows a better creep performance than pure tin and eutectic Sn–3.5Ag solder due to the strengthening effect of Ag3Sn and (Cu,Ni)6Sn5 IMC precipitations. The true stress exponent for creep is identified to be 7, indicating that the creep behave is controlled by the dislocation-pipe diffusion in the tin matrix.

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.