Baolei Liu

Influence of electric current on the grain orientation of Cu-Sn intermetallic compounds in Cu/molten Sn/Cu interconnection system

In this paper, the interfacial reaction and the grain orientation of Cu₆Sn₅ intermetallic compound was investigated in Cu/molten Sn/Cu interconnection system under the current density of 1.0 × 10²A/cm² at 260 °C. The imposed electric current significantly accelerated the Cu₆Sn₅ growth rate at anode side under the effect of solid-liquid electromigration, while it has no obvious effect on the Cu₃Sn growth rate. The growth kinetics calculation results showed that with the passage of electric current, the growth of Cu₆Sn₅ compound at the cathode was determined by reaction process, while the Cu₃Sn growth was diffusion-controlled. In addition, the current can strongly influence the orientation of Cu₆Sn₅ phase in Cu-molten Sn-Cu system. There was a strong texture of [0001] direction in Cu₆Sn₅ phase, which was paralleled with the direction of electron flow. This result indicated that the electrons traveled along some particular directions and were scattered least by the lattices. The newly formed Cu-Sn compounds orientated themselves in those particular growth directions to facilitate electron flow.

Relationship between crack propagation trends and grains in SnAgCu interconnects

Thermal shock test was performed on the SnAg3.8Cu0.7 (SAC387) solder bump interconnecting AlN and Cu substrate. Finite element modeling (FEM), cross-polarized light microscopy, scanning electronic microscopy (SEM) and EBSD were used to analyze the failure mechanism of them. It was realized that during thermal shock, the areas with higher stress would suffer from more serious IMC coarsening and then lead to easier propagation for the cracks. The recrystallization phenomena and crack could also be observed after 1500 cycles. The coefficient of thermal expansion (CTE) mismatch in thermal shock also caused serious cracks and was transgranular fracture along the diagonal the same direction with the stress. The energy bring by the crack tip was the necessary condition of the recrystallization and sliding and rolling of refined grains. The radius of the influenced area was only about 10 μm. The recrystallization could consume some of the crack energy but then allowing the crack propagates in a more plastic mode.

Growth kinetics of Cu 6 Sn 5 intermetallic compound in Cu-liquid Sn interfacial reaction enhanced by electric current

In this paper, electric currents with the densities of 1.0 × 102 A/cm2 and 2.0 × 102 A/cm2 were imposed to the Cu-liquid Sn interfacial reaction at 260 °C and 300 °C with the bonding times from 15 min to 960 min. Unlike the symmetrical growth following a cubic root dependence on time during reflowing, the Cu6Sn5 growth enhanced by solid-liquid electromigration followed a linear relationship with time. The elevated electric current density and reaction temperature could greatly accelerate the growth of Cu6Sn5, and could induce the formation of cellular structures on the surfaces because of the constitutional supercooling effect. A growth kinetics model of Cu6Sn5 based on Cu concentration gradient was presented, in which the dissolution of cathode was proved to be the controlling step. This model indicates that higher current density, higher temperature and larger joint width were in favor of the dissolution of Cu. Finally, the shear strengths of joints consisted of different intermetallic compound microstructures were evaluated. The results showed that the Cu6Sn5-based joint could achieve comparable shear strength with Sn-based joint.

Interconnection of Cu wire/Au plating pads using parallel gap resistance microwelding process

Because of the complicated configuration in MEMS devices, significant technical challenges are presented for the interconnection of dissimilar materials. Since tin soldering is an effective method for joining dissimilar materials, there are still some reliability problems in later service process, such as remelting of Sn solder, excessive microstructure evolution. In the present work, a parallel gap resistance microwelding method was developed for forming reliable Cu wire/Au plating interconnection. In an attempt to avoid fragile electronic components, an ohmic contact tip onto the electrodes was utilized. Experimental results revealed that solid state welding was obtained with a relatively electric current intensity within 20 ms. No weld nugget was generated at the Cu/Au interface. The joule heat-induced temperature affected microstructures as well as mechanical properties of the joints, mainly depending on three parameters (welding voltage, welding time and electrode force). The maximum joint breaking force was 24 g with the welding voltage of 0.5 V for 16 ms and under the pressure of 0.56 N. The ANSYS simulation results indicated that the weld geometry changed with welding time.

Rapid formation of Cu-Sn intermetallic compounds by strong electric current

A new approach of rapid fabrication of intermetallic compounds (IMCs) solder joints at ambient temperature was investigated in this work. The high share strength Cu/Sn/Cu solder joints were obtained by using different strong electric currents 0.8KA, 0.9KA, 1.0KA, and 1.2KA for only about 0.2s. The Sn foils with the thickness of 10μm were used as solders. The results showed that with the current intensities increasing from 0.8KA to 1.2KA, the thickness of IMCs, Cu6Sn5, increased from less than 1μm to 4.2μm for 0.2s, which was induced by thermoelectric coupling interfacial interaction at the liquid solder/solid metallization interface. In addition, amount of dendritic Cu6Sn5 distributed in the Sn matrix at the bottom right corner of solder joint under the current intensity of 1.2KA, which was caused by the current crowding effects. Lastly, the share strengths of Cu/Sn/Cu solder joints under the rapid bonding process were improved with the bonding current increasing. The share strength was up to 51.9MPa at 1.2KA.

Electromigration failure of SnAgCu lead-free BGA package assembled with SnPb solder paste

Electromigration damage in Sn-3.0Ag-0.5Cu (SAC305) lead-free solder joints of Ball Grid Array (BGA) package assembled with eutectic SnPb paste was investigated after current stressing at 100°C with a density of 1×104 A·cm-2 for up to 180 h. The under bump metallization (UBM) for the mixed solder joints on the substrate side was Cu/electroplated Ni, while the pad on the printed circuit board (PCB) side was Cu plate. The results show that open failure occurred in the Cu trace at the upper-right corner of the solder joint with a downward current flow in the thermo-eletromigration test. As the current crowding occurred at that point and induced the Ni and Cu atoms excessive migration. The Pb atoms were found to move in the same direction as with the electron current flow, and accumulated at the anode. The two new factors, the initial concentration gradient of Pb atoms and current density distribution in the solder bump, for the redistribution of Pb-rich phases to be considered to clarify that phenomenon. Meanwhile, the polarity effect of electro-migration on the interfacial reaction of mixed solder bump was obvious. Electric current enhance the growth IMC layers at anode, and the interfacial microstructure of triple layers ((Cu, Ni)6Sn5, Cu6Sn5, and Cu3Sn) was observed. While the growth of IMC at anode, Cu6Sn5, was retarded.