Po Chun Liu

Electromigration in Pb-free SnAg3.8Cu0.7 solder stripes

Electromigration behavior in the eutectic SnAg3.8Cu0.7 solder stripes was investigated in the vicinity of the device operation temperature of 100°C by using the edge displacement technique. Measurements were made for relevant parameters for electromigration of the solder, such as drift velocity, threshold current density, activation energy, as well as the product of diffusivity and effective charge number (DZ*). The threshold current densities were estimated to be 4.3×104A∕cm2 at 80°C, 3.2×104A∕cm2 at 100°C, and 1.4×104A∕cm2 at 120°C. These values represent the maximum current densities that the SnAg3.8Cu0.7 solder can carry without electromigration damage at the three stressing temperatures. The electromigration activation energy was determined to be 0.45eV in the temperature range of 80–120°C. The measured products of diffusivity and the effective charge number, DZ*, were −1.8×10−10cm2∕s at 80°C, −5.0×10−10cm2∕s at 100°C, and −7.2×10−10cm2∕s at 120°C.

Electromigration in Sn-Cu intermetallic compounds

As the shrinking in bump size continues, the effect of intermetallic compounds (IMCs) on electromigration becomes more pronounced. Electromigration in Sn–Cu intermetallic compounds was examined using edge displacement method. It was found that Cu6Sn5 compounds are more susceptible to electromigration than Cu3Sn compounds. The lower solidus temperature and higher resistivity of the Cu6Sn5 IMCs are responsible for its higher electromigration rate. Length-dependent electromigration behavior was found in the stripes of various lengths and the critical length was determined to be between 5 and 10 μm at 225 °C, which corresponded to a critical product between 2.5 and 5 A/cm. Furthermore, the Sn–Cu compounds were proven to have better electromigration resistance than eutectic SnAgCu solder.

Relieving Sn whisker growth driven by oxidation on Cu leadframe by annealing and reflowing treatments

Owing to environmental concern, Pb-free solders are replacing eutectic tin lead in electronic packaging industry. Thus, whisker growth becomes a serious reliability issue for Sn finishes. In this study, the mechanism of whisker growth from Sn finish on Cu leadframe was investigated under the temperature/humidity storage test. It is found that oxidation of the Sn finish was the driving force behind the whisker growth. Thermal treatments including annealing at 220 °C and reflowing at 260 °C were employed to mitigate the whisker growth. It is found that both heat treatments can significantly reduce the whisker growth rate. It is speculated that the heat treatments can relieve the residual stress in the Sn finishes and can modify their grain structure, resulting in a slower oxidation rate. Thus, they can slow down the grow rate of Sn whiskers. In addition, reflowing treatment can change the columnar grain structure of the Sn film to the equiaxed grain structure in some of its regions, resulting in a lower gra...

Microstructural Evolution During Electromigration in Eutectic SnAg Solder Bumps

Microstructural changes induced by electromigration were studied in eutectic SnAg solder bumps jointed to under-bump metallization (UBM) of Ti/Cr–Cu/Cu and pad metallization of Cu/Ni/Au. Intermetallic compounds (IMCs) and phase transformations were observed during a current stress of 1 × 10 A/cm at 150 °C. On the cathode/ substrate side, some of the (Cuy,Ni1−y)6Sn5 transformed into (Nix,Cu1−x)3Sn4 due to depletion of Cu atoms caused by the electron flow. It is found that both the cathode/ chip and anode/chip ends could be failure sites. On the cathode/chip side, the UBM dissolved after current stressing for 22 h, and failure may occur due to depletion of solder. On the anode/chip side, a large amount of (Cuy,Ni1−y)6Sn5 or (Nix,Cu1−x)3Sn4 IMCs grew at the low-current-density area due to the migration of Ni and Cu atoms from the substrate side, which may be responsible for the electromigration failure at this end.

Measurement of electromigration parameters of lead-free SnAg3.5 solder using U-groove lines

Measurement of electromigration parameters in the lead-free solder SnAg3.5 was carried out by utilizing U-groove solder lines and atomic force microscopy in the temperature range of 100–150 °C. The drift velocity was measured, and the threshold current densities of the SnAg3.5 solder were estimated to be 4.4 × 10 A/cm at 100 °C, 3.3 × 10 A/cm at 125 °C, and 5.7 × 10 A/cm at 150 °C. These values represent the maximum current densities that the SnAg3.5 solder can carry without electromigration damage at the three stressing temperatures. The critical products for the SnAg3.5 solder were estimated to be 462 A/cm at 100 °C, 346 A/cm at 125 °C, and 60 A/cm at 150 °C. In addition, the electromigration activation energy was determined to be 0.55 eV in the temperature range of 100–150 °C. These values are very fundamental for current carrying capability and mean-time-to-failure measurement for solder bumps. This technique enables the direct measurement of electromigration parameters of solder materials.

High-temperature healing of interfacial voids in GaAs wafer bonding

Artificial voids were introduced at bonding interfaces to study how processing parameters affected the healing mechanism of interfacial voids in GaAs wafer bonding. These voids were created by placing unpatterned wafers in contact with topographically patterned wafers. During the bonding process, crystallites formed within these voids and corresponded to bonded regions within the voids. Their formation depended strongly on the height of the surface irregularities at the wafer interfaces. When the void depth (h) was ⩾200 nm, most of the crystallites were diamond shaped. The edges of the diamond features were elongated in the 〈100〉 direction. On the other hand, when the void depth was small (h⩽70 nm), dendrites grew quickly in the 〈110〉 direction.

Effects of annealing temperature on electrical resistance of bonded n-GaAs wafers

The electrical characteristics and microstructures of n-type (100) GaAs bonded interfaces were systematically investigated. Experimental results indicated that GaAs did not bond directly to itself, but via an amorphous oxide layer at 400°C. When temperatures increased above 400°C, the oxide bonded area declined and finally disappeared. Electrical resistance decreased with bonding temperature. However, the resistance increased with temperatures exceeding 850°C.

Bonding line-patterned In0.5Ga0.5P layer on GaP substrate for the successive growth of high-brightness LED structures

The heteroeptixial integration of the III-V semiconductor compound, which was limited by lattice mismatch, can now be accomplished by using the wafer bonding process. In this study, a line-patterned In 0 . 5 Ga 0 . 5 P layer was successfully transferred to the GaP wafer through the wafer bonding process for the successive growth of high-brightness light-emitting diode (LED) structures.