•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

•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.

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.

The morphology evolution mechanism and dynamics of Cu6Sn5 intermetallic compound (IMC) in cooling stage were studied by using pure Sn solder ball with a diameter of 1 mm to react with polycrystalline Cu substrate and form Cu6Sn5. A nearly uniform height of the scallop-like IMC grains is attained by using high pressurized air to remove excess liquid solder (that is, for acquisition of the IMC morphology identical to heat preservation stage). But the morphology evolution of IMC in the solder joint was greatly affected by the cooling phase of interfacial reaction, and IMC morphology partition phenomenon was found in the spherical joints. The IMC at the edge portion characterized scallop morphology whereas the central portion possessed prismatic shape morphology. Finite element simulation for the temperature field distribution in the solder ball-substrate domain showed temperature gradient in solder’s internal core was significantly greater than that at the edge. Using this simulation results with kinetic equations, it could be understood that the central core region is conducive to the formation of small plane structure morphology. These small planes would eventually form the prismatic morphology IMC in the central region. As the soldering temperature increases, the area ratio of IMC with scallop-like morphology to the prismatic shaped IMC reduced from 0.2178 down to 0.1680. Moreover, for an elevated temperature of 300 °C, it is observed that the central region IMC (with prismatic structure) grows upto an average thickness of 7.7245 μm, whereas the thickness value for scalloped IMC at the edge portion is around 4.556 μm.

Highlights•Synchrotron radiation imaging technique is applied for thermomigration study.•The thermotransport of Cu in Sn has been modelled using FEM.•Higher temperatures enhances the rate of increase of cold end intermetallic compound size.•The smaller cold end Cu6Sn5 in Sn–3.5Ag than Sn may be attributed to the role of Ag3Sn.The thermal gradient induced intermetallic compound growth at the cold side of the Cu/Sn/Cu and Cu/Sn–3.5Ag/Cu solders heated at 350° has been in situ studied using synchrotron radiation imaging technique. Finite element model for thermotransport of Cu from hot end to cold end in Cu/Sn/Cu solder is implemented for the description of the associated growth of the compound. Higher temperature and bigger solder volume are observed as favorable parameters for the rate of thickness increase in both Sn and SnAg solders. In case of SnAg system, the formation of Ag3Sn particle, causing retardation effects in the growth of Cu6Sn5 compound, subsequently induces a lowered resulting thickness.

The effects of compressive stresses on the oxide-scale morphologies formed on an Fe–20Cr alloy were investigated by comparison of the oxidation behavior in air under classical conditions, i.e., without any applied mechanical stresses and under static compressive stresses, at 900 °C. The study was carried out mainly by comparisons of oxidation kinetics gained by thermogravimetric analysis (TGA), surface morphologies of oxidized specimens observed by scanning electron microscopy (SEM), oxidized products examined by X-ray diffraction (XRD). It was found that the application of compressive stresses induced an increase in oxidation rate, but a decrease of oxide grain size. When the stresses are in the range of 5–8 MPa, both chromium- and iron-oxides formed but, at other stresses, only chromia was present. In particular, there was a maximum in oxidation rate when the applied stress was 5 MPa. The paper places emphasis on analyzing the cause of this phenomenon.

The oxidation kinetics and mechanism of oxide-scale failure of pure Ni oxidized under external static compressive and tensile loads were studied. The results showed that both types of mechanical loads accelerated the oxidation rate, but the effect was different for the two types. Compressive loading (CL) affected it by improving the plasticity of oxide scales, and tensile loading (TL) affected it by amplifying the compaction of the oxide–metal interface. As for the oxide-scale failure, CL can delayed cracking, TL accelerated brittle failure. The study analyzed the effect of external load on the oxidation kinetics and the failure mechanism of oxide scales.