Hyo-Sok Ahn

Tribochemical Interactions of Si-doped DLC Film Against Steel in Sliding Contact

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a darkgray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction (∼0.06), the Si-DLC film showed a significant lower coefficient of friction (∼0.022) with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.

Effect of relative vapor pressure on separation of nanoscale contact in atomic force microscope

The separation of nanoscale contact junction is investigated in an atomic force microscope at various relative vapor pressure conditions. Gradual increase in adhesion force is observed as the relative vapor pressure increases. However, the force-deformation behaviors of the water-mediated nanoscale contacts vary extensively with the relative vapor pressure conditions. At low relative vapor pressure (p/ps<0.06), water molecules play a role as a weak glue contributing solid extension. In contrast, at high relative vapor pressure (p/ps=0.8), the highest adhesion force is observed without indication of the solid extension. The meniscus collapses and forms a water column after solids separates at an intermediate relative vapor pressure condition (p/ps=0.4). The detailed analysis revealed the transition of adhesion mechanism from the solid-dominant adhesion to liquid-dominant adhesion as the relative vapor pressure increases.

Reliability of Various Types of Wafer Level Package (WLP) for Mobile Application

According to expansion of the market for portable products such as mobile phone, digital camera and PMP, adoption of WLP for various devices was increasing. Recently, WLP was being applied to higher pin count device as well as lower pin count device. In case that WLP is applied to higher pin count device, various reliability issues are expected from large die size and fine solder ball pitch. The more die size increase and contact area decrease, the more solder joint is fragile, especially at the outermost. To apply the WLP to mobile application, the reliability of various types of WLP in terms of die size, UBM (Under Bump Metallurgy) type, solder material and solder ball pitch have been evaluated at package and board level. First, the reliability for WLP of 3 mm ? 3 mm size and 0.5 mm solder ball pitch using solder ball with Sn3.0Ag0.5Cu solder composition on electroplated Cu/Ni/Au UBM had been evaluated by employing TC (Thermal Cycling) and HTS (High Temperature Storage) at package level and passed TC 2000cycles and HTS 1000hrs. Second, the reliability for WLP of 3.7 mm ? 3.9 mm size and 0.5 mm solder ball pitch using solder ball with Sn3.0Ag0.5Cu solder composition on electroplated Cu/Ni/Au UBM had been evaluated at package level and passed TC 500cycles, HTS 1000hrs and PCT 168hrs. Third, the reliability for WLP of 4 mm ? 4 mm size and 0.4 mm solder ball pitch using Sn3.0Ag0.5Cu on electroplated Ni UBM and ENIG (Electroless Nickel Immersion Gold) finished substrate has been evaluated by employing TC, THT (Temperature Humidity Test), 4-point bending and Drop test at board level. All items electrically passed except the one TC sample without underfill. As a result of cross-section analysis of TC samples without underfill, some solder crack on PCB side were found. For the 4-point bending samples, separation between Cu pad and TiW were found at die side of outermost row and crack between IMC and Ni Pad was found at PCB side of outermost. In case of drop test, there was no issue except the PCB resin crack under the outermost bonding pad. Finally, WLP of 6 mm ? 6 mm size and 0.4 mm solder ball pitch has fabricated and the reliability of TC, HTS, PCT, drop and electro-migration are being evaluated.