Papers by Author: Seung Boo Jung

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 electrochemical migration (ECM) with flexible printed circuit board (PCB) was affected by factors such as distance between the electrodes and bias voltage using water drop (WD) test. The rate of migration was increased by decreasing the distance between electrodes of the opposite polarity and increasing the bias voltage.

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.