Seung Boo Jung

Dissimilar friction stir spot welding of low carbon steel and Al–Mg alloy by formation of IMCs

Abstract Dissimilar lap joints of low carbon steel and Al–Mg alloy were obtained by friction stir spot welding. Mechanically mixed layer between top and bottom plates was not formed at the weld nugget due to the limited tool penetration and the pin height of welding tool lower than the thickness of Al plate laid in top side. These welding conditions made it possible to weld steel plate using welding tools made out of a general tool steel. With increasing tool penetration depth (TPD), tensile shear force of joint increased and maximum value of 3·0 kN was obtained at the TPD of 0·5 mm, but excessive tool penetration beyond 0·5 mm was caused in a deformation of Al plate of top side. In the result of interface observation, interaction layer was formed between Fe and Al alloy, which was constituted by various intermetallic compounds (IMCs). Consequently, the size of strongly bonded area containing Fe3Al and Fe4Al13 IMCs increased with TPD, which resulted in the increase in joint strength under the limited TPD.

Reaction diffusions of Cu6Sn5 and Cu3Sn intermetallic compound in the couple of Sn-3.5Ag eutectic solder and copper substrate

The growth kinetics of intermetallic compound layers formed between Sn-3.5Ag solder and Cu substrate were investigated as a consequence of solid-state isothermal aging. Isothermal aging was carried out in a temperature range between 70°C and 200°C for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. The diffusion couples showed a composite intermetallic layer comprised of Cu6Sn5 and Cu3Sn. The growth of intermetallic compounds followed diffusion-controlled kinetics and the layer thickness reached only 9 μm after 60 day of aging at 150°C. The apparent activation energies were calculated for the growth of the total intermetallic compound (Cu6Sn5+Cu3Sn); Cu6Sn5 and Cu3Sn intermetallic are 65.4, 55.4 and 75.7 kJ/mol, respectively.

Mechanical and Electrical Properties of Sn-3.5Ag Solder/Cu BGA Packages during Multiple Reflows

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.

Effect of fixed location variation in friction stir welding of steels with different carbon contents

Abstract Dissimilar friction stir welding between steels with different carbon contents was carried out by placing the parent materials at two different fixed locations: the advancing side (AS) and the retreating side (RS). The phase transformation and the depth and width of the weld were affected by the fixed location. A smaller weld nugget was formed, and more martensite transformation occurred when the stronger SK5 steel was fixed at AS, compared to that at RS. This variation was attributed to the difference of temperature and materials flow between AS and RS.

Influence of Shear Speed on the Shear Force of Eutectic Sn-Pb and Pb-Free BGA Solder Joints

The experimental investigation and non-linear finite element analysis using elastic-viscoplastic constitutive model were conducted to study the effect of ball shear speed on shear force of BGA solder joints. The Ag3Sn intermetallic compound (IMC) particles were found inside the Sn-3.5Ag solder, while the fine intermixed structure of lead-rich phases and tin-rich phases was investigated inside the eutectic Sn-37Pb solder. The shear force linearly increased with shear speed and reached to the maximum value at the highest shear speed in both experimental and computational results. All of the test specimens were fractured like ductile mode, and the plastic strain energy density distributions were deeply related to the failure mechanism.

Evaluation of Joint Resistance Between Bi-2223/Ag Superconducting Tapes

We fabricated Bi-2223/Ag jointed tape and evaluated its current carrying capacity and joint resistance at 77.3 K as a function of the contact length. The tapes were jointed by the conventional resistive joint method with 40Pb-60Sn solder as an insert material and their joint resistance was measured by two different techniques, i.e., the standard four-probe and field decay methods. It was observed that the current carrying capacity and joint resistance were significantly dependent on the contact length. As the contact length increased, the current carrying capacity improved and increased to the level of critical current in the unjoined tape at the contact length of 5 cm. On the other hand, the joint resistance linearly decreased as the contact length increased. In addition, the joint resistances measured by the two methods were similar to each other (7.3times10-9 Omega~8.0times10-9 Omega), indicating that the standard four-probe method maintained a moderate degree of accuracy at the level of resistance of 10-9 Omega

A Comparative Evaluation of Friction-Welded and Brazed Ti and AISI 321 Stainless Steel Joints

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.

Mechanical Properties of Copper Hollow Sphere Manufactured by Sintering Process

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.

The Joint Characteristics of Friction Stir Welded Mg-Zn-Y Alloy

Friction stir welding of similar Mg-Zn-Y alloy was successfully carried out under various welding conditions. The external shape of the joints showed good quality and the weld zone perpendicular to the welding direction didn’t also include any defects. In the weld zone, the strengthening particles were homogeneously dispersed in the Mg matrix. The morphology of these particles was slightly varied with the welding speed and with the location such as SZ, TMAZ, HAZ and BM. The shear band caused by the severe deformation was observed in the TMAZ. SZ had a higher hardness than that of the base metal. Though UTS of the joints slightly varied with welding speed, it reached the equal value with that of BM.

Analysis on thermal resistance of LED module with various thermal vias

Light Emitting Diode (LED) has been already familiar that is used as lighting sources of general electronic devices and various displays. LED has many advantages such as long life, low power consumption and high reliability. In the future, as alternative to fluorescent lighting, it is sure that LED in lighting products is expected to receive much attention. However, the components related with advanced LED packages or modules have been issued on the heat from LED chip. And the LED chip is still being developed to the high power devices which are generating more heat. In this study, we investigated the variation of thermal resistance in LED modules embedded with thermal via. Through the analysis of thermal resistance with various test vehicles, we could obtain the concrete relationship between thermal resistance and structure of thermal via.