Papers by Author: Ja Myeong Koo

Abstract: Microstructure and mechanical properties of friction welded and vacuum brazed Ti/AISI 321 stainless steel have been evaluated with various welding conditions. Maximum tensile strength of friction welded joints was approximately 420 MPa with the conditions of 400 MPa of upset pressure (P2) and friction time (t1) within 2.0 s. Maximum tensile strength of brazed joints was acquired under the condition of 900 °C brazing temperature and 5 min. brazing time and showed approximately 275MPa which was about 80% of that of the Ti base metal. Friction welded Ti/AISI 321 joints showed the superior tensile strength than that of brazed Ti/AISI 321 due to thinner intermetallic compound layer.

Abstract: The interfacial reactions and shear properties of Sn-37Pb (wt.%) solder bumps with two different under bump metallizations (UBMs), Cu and Ni, were investigated after high temperature storage (HTS) tests at 150
C for up to 65 days. Two different intermetallic compounds (IMCs), Cu6Sn5 and Cu3Sn, were formed at the bump/Cu interface, whereas only a Ni3Sn4 IMC layer was formed at the bump/Ni interface. The thicknesses of these IMCs increased linearly with the square root of duration time. The IMC growth rate at the bump/Cu UBM interface was much greater than that at the bump/Ni UBM interface. The shear properties of the bumps with the Cu UBM were greatly decreased with increasing duration time, compared with those with the Ni UBM.

Abstract: Thermo-mechanical reliability of the solder bumped flip chip packages having underfill encapsulant was evaluated with thermal shock testing. In the initial reaction, the reaction product between the solder and Cu mini bump of chip side was Cu6Sn5 IMC layer, while the two phases which were (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 were formed between the solder and electroless Ni-P layer of the package side. A crack was formed at the upper edge region of solder bump, and propagated through the solder region. The primary failure mechanism of the solder joints in this type of package was confirmed to be thermally activated solder fatigue failure. After thermal shocks of 2000 cycles, one more crack which was not observed in the case of non-underfill encapsulated flip chip was observed at the left side of interface between solder bump and substrate. The addition of this crack formation should be due to the underfill encapsulation between the Si chip and substrate.

Abstract: The polystyrene spheres coated with a slurry, consisting of copper (I) oxide powder, water and PA (poly acrilamide), were reduced to form copper (Cu) hollow spheres at sintering temperatures ranging from 650 to 950 °C under a hydrogen atmosphere. The microstructural evolutions and mechanical properties of the Cu hollow spheres were investigated with the sintering temperature. The increase in the sintering temperature led to the increase in the density and grain size of the spheres. The amount of the residual copper oxide in the samples decreased with increasing sintering temperature. The electrical conductivity, hardness, and compression strength of the hollow spheres increased with increasing sintering temperature.

Abstract: The interfacial reactions and bump shear properties of the electroplated Sn-37Pb (in wt.%) solder bumps with the Ni under bump metallization (UBM) were investigated as a function of the number of reflows. A continuous facetted Ni3Sn4 intermetallic compound (IMC) layer was formed at the interface between the solder bump and the Ni UBM during reflow. The thickness of the Ni3Sn4 IMC layer was 0.41 μm after 1 reflow, and then the thickness of the IMC layer increased with increasing the number of reflows. The shear properties of the bumps indicated the maximum values after 1 reflow, and then decreased with increasing the number of reflows. The fracture surfaces of the bumps showed ductile failure characteristics after 1 reflow, and then the fraction of the brittle fracture, induced by the IMC formed at the interface, increased with increasing the number of reflows.

Abstract: The mechanical and electrical properties of the Sn-3.5Ag solder/Cu BGA packages were investigated as a function of number of reflows. A continuous scallop-shaped Cu6Sn5 intermetallic compound (IMC) layer was formed at the solder/Cu interface upon 1 reflow process. After 3 reflows, very thin layer of Cu3Sn IMC was observed at the Cu6Sn5/Cu interface. As the number of reflows increased, the thickness of these IMCs increased with a cube root of reflow time. Shear force slightly increased up to 4 reflows and then gradually decreased with increasing the number of reflows. The maximum shear force was 11.7N when the thickness of the IMC layers was 3.7㎛. The fracture surfaces of all the specimens showed ductile failure characteristics up to 4 reflows, and then the fraction of the brittle IMC fracture increased with the number of reflows. The electrical resistance of the BGA packages increased with the number of reflows.