Materials Science Forum Vols. 580-582

Abstract: An optical subassembly which is suitable for high speed and parallel optical modules was introduced. The optical subassembly has features of low optical coupling loss less than 1.5 dB and wide 3-dB bandwidth more than 20 GHz in the view of the transmission line. And it was fabricated with a vertically stacked structure cost-effectively and precisely.

Abstract: In an optical module with high density and small size, it is important to maintain the operating temperature by adequate heat dissipation. The thermal simulation of an optical module for communication has been performed to reduce the operating temperature. To improve the behavior of heat dissipation, we have simulated heat path and heat resistance using commercial code, COMSOL. We have chosen simulation parameters, which include thickness, thermal conductivity, interface area and surface roughness of heat sink and submount as an element of heat resistance. To reduce heat resistance, it was desirable to use the material of higher thermal conductivity, single crystalline silver, and to control the surface roughness of the interfaces. We could lower the operating temperature of an optical module about 18 degrees by controlling the above parameters.

Abstract: The focus of this study was on discussing the novel reflow method ISHR (Induction spontaneous heat reflow) used for high-density area array packaging and assembly. Multi-layer under bump metallization (UBM) and Sn3.5Ag lead-free solder ball were laid in the high frequency electromagnetic field. Because of the induction heating, solder balls melted and spread onto the UBM to form solder bumps. The solder bumps could be formed within 2 seconds through this method; meanwhile, the infrared temperature measurement results showed that the rosin substrate temperature was lower. The shear test indicated that the solder bumps made by ISHR can satisfy the mechanical requirement. Finally, the feasibility experiment was performed to demonstrate the application feasibility of this ISHR technology. Through all these experiments, conclusion can be made that the ISHR as a novel reflow method can be applied in microelectronics packaging.

Abstract: In this paper, the characteristics of bond interface and bonding mechanism were investigated with peeling-off method. The fracture was observed and interfacial composition was certified by map scanning of EDX (Energy dispersive X-ray analysis). Based on the features of interfacial characters, the actual joining area mainly distributed at bond periphery; non-bonded at bond center. When the bonding time was lower, the ratio of the bond length to its width was larger and elemental aluminum distributed discontinuously on the bond fracture, primarily at the periphery. After aging, the fractures were also analyzed and Cu2Al3 intermetallic compound (IMC) was identified. The phenomena of bond interfacial tracings were analyzed, and the bonding mechanism was ascribed to plastic flow analyzed by finite element method based on the contact issues.

Abstract: Bending fatigue behavior of eutectic Sn-3.5Ag solder bump bonded on FR4-PCB was characterized by experimental and finite element method (FEM). To investigate an effect of stress state on bump failure, which had not been weighed in conventional Coffin-Manson model of Nf=K ⋅εp -1~-2, ‘fatigue frequency variable’ and ‘bump viscoplasticity’ were included in analysis procedure. As experimental results, with increasing fatigue cycles from 3,000 to 10,000, bond strength decreased from 98.9% to 76.5%, and from 97.5% to 67.1% at the fatigue frequencies of 2.5Hz and 5.0Hz, respectively. Stress state could be critical components to determine fatigue life, which should be combined in Coffin-Manson criteria. FEM calculation showed that higher bending frequency led to higher normal stress development at the solder and IMC interface, but smaller plastic strain in bump. However, bending fatigue experiment revealed discrepant results from that of Coffin-Manson criteria. Higher bending frequency, which was predicted to give rise to smaller εp at solder, showed dramatic bond deterioration of solder bump on UBM (under bump metallurgy). This was confirmed experimentally through SEM (scanning electron microscopy) observation as cracks were found at the solder bump and UBM interfacial IMC, Ni3Sn4, in case of the higher bending frequency.

Abstract: Granulation is a precious metal craft process method that decorates a metal surface using tiny metal granules. It was imported into Korea during the Shilla Dynasty around 1500 years ago, and many granulation ornaments have been found with the process’s unique bonding features. The granules show a direct bonded interface with a neck. The key technology of making granules and bonding the granules is not well known. Thus, it is a technology of the Lost World. Although the exact bonding method is unidentifiable, it is known that the traditional method of preparing gold granules was time consuming and costly. Therefore, we proposed a process to reproduce the Shilla’s granulation ornament using a modern method. First, we employed atomization to produce 22K gold granules. Direct bonding was accomplished using a spot welder and vacuum jig instead of using the traditional method of graphite bed melting and direct annealing. 0.8 mm granules were successfully fabricated and bonded directly to the substrate with a necking and 35% bonding ratio, which is very similar to Shilla’s granule bonding. Moreover, to estimate the bond strength, K factors (fracture toughness index) at different bonding ratios were evaluated using a finite element method simulation. Our proposed direct bonded granule process and design were expected to have enough bond strength to be used as a key element for fine modern jewelry.

Abstract: Pulsed Nd;YAG laser was applied to investigate the bond characteristics of Sn- 3.5wt%Ag (hereafter, Sn-3.5Ag) solder ball. The ball diameter is 500µm, and the UBM (Under Bump Metallurgy) on a FR4-PCB consisted of Cu/Ni/Au from bottom to top with thickness of 50µm/25µm/0.05µm, respectively. A RMA (rosin mildly activated) flux for BGA (Ball Grid Array) was applied before laser soldering to set up the solder ball on the UBM. Laser power was 250W, and its beam diameter at the focal point was around 250µm. Experimental results reveal that the solder ball showed a good bonded state when the laser energy was in the range between 0.8 and 1.0 J. Laser energy lower than 0.8J caused non-wetting of the solder ball, and energy higher than 1.2J showed crack along the UBM. Microstructure of the laser soldered Sn-3.5Ag was examined with the number of pulse, when the frequency, pulse width and laser energy were fixed at 5 Hz, 20 ms and 0.8 J, respectively. The IMC (Intermetallic Compound) of Ag3Sn in the solder changed from acicular to rod type with increasing laser pulses. Ni3Sn4 was produced between the solder and the UBM, and its thickness increased from 0.65 to 1.80µm with increasing number of pulse from 2 to 4. These are thinner than those of general air reflow. Shear strength of the laser soldered ball with output energy of 1 J showed 652.3 gf, and it was similar value compared to the result from hot plate soldering, 654.5 gf.

Abstract: To alleviate concerns of manufacturing cost and efficiency of solar cell fabrication, a belt type-high temperature furnace with three temperature zones is employed in order to increase the solar cell manufacturing throughput. In addition, characterization of the firing process for the contact formation is investigated to achieve the most efficient solar cell device fabricated. Statistical design of experiment (DOE) is used in order to perform a set of firing experiments in a systematic way. Response surface models for the efficiency are established for the characterization of multivariate problem. Furthermore, the optimized process recipe that can provide 15.9% of efficiency is proposed through the process optimization.

Abstract: Tail breaking forces (TBFs) are measured for various process conditions to understand phenomena such as short tail formation. TBFs obtained with several Cu wires are compared to find the most suitable Cu wire type that improves consistent tail formation. In situ online TBF measurement method is developed. The highest TBF obtained is 61.59 + 9.10mN. The highest Cpk value obtained is 2.97 + 0.33 when lower specification limit of 10 mN is assumed.

Abstract: Sn-based lead-free solders have some characteristics that are different from the conventionally used Sn-Pb eutectic. One is the high dissolution rate of metals and alloys into molten lead-free solder. Several problems due to this high dissolution rate were reviewed. The prominent materials that suffered from this high dissolution rate are copper and iron based. High dissolution rate of Cu resulted in thick Cu-Sn intermetallic compound formation during soldering and Cu enrichment in wave soldering bath. The iron-based materials used for soldering equipments are damaged by erosion. Suppression of the dissolution rate is important to secure quality of micro-soldered joints, to prevent contamination of solder bath and to prolong the life of soldering equipments. The method of suppressing dissolution in practical soldering operation and some examples of erosion damage are introduced.