A new Au-Ge die attachment system for next-generation SiC power electronics is proposed that is capable of withstanding both higher junction temperatures Tj and greater ΔTj cycle stress. In this system, a Au-Cu fine anchor structure is skillfully formed at the surface of the Cu conductor on a SiN ceramic substrate. In addition, the usual Cu conductor is replaced with a Cu/Invar/Cu (thickness ratio of 1/8/1) clad plate, comparable in coefficient of thermal expansion (CTE) to an SiC die. The anchors effectively prevent power chip delamination caused by the progress of thermochemical reactions among the Au-Ge layer, Cu conductor, plated Ni and air. The application of the CIC clad plate significantly reduces Au-Ge die attachment fatigue caused by repetitive thermal stress during thermal cycling. A storage test and a thermal cycle test were conducted for 3000 hours at Tj > 250°C and 3000 cycles at ΔTj > 290°C, respectively. It was found that the new attachment system maintained ample joint strength even after rigorous reliability testing.
This paper reports the results of a study on the reliability of aluminum wire for use with SiC devices operating at temperatures above 200°C. In recent years, SiC-based power modules operating at such temperatures have been proposed for the purposes of miniaturization. However, this requires the complete reassessment of all packaging components under high-temperature conditions. We evaluated the reliability of aluminum wire during high-temperature exposure at 250°C, thermal-cycle testing between -40 and 250°C, and power-cycle testing with ΔTj=165°C. The results indicated that the wire meets the IEC607439-22 guidelines for pull strength and the industrial reliability requirements for power-cycle tests.
In this study, time-dependent mechanical properties of the sintered nano-sized Ag particles were investigated. Stress relaxation was observed from the ambient temperature (0.24Tm), lower than 0.4Tm (493K), at which, as a rule of thumb, creep occurs in metal materials. The stress exponent and the activation energy of the compact portion of Ag are similar to those of dislocation creep. The results in this study, however, do not adapt to the high temperature creep theory from the viewpoint of diffusion. In addition, the elastic modulus of the compact portion of Ag is one fourth of the elastic modulus of bulk Ag. The crystallinity in the vicinity of grain boundaries is considered to be low in quality compared to bulk Ag. Thus, it can also be considered that the grain boundary of such poor quality is closely related to the low modulus of elasticity and the occurrence of low temperature creep.
The silver paste containing glass frit was studied as metal interlayer for Aluminum. On this research, the adhesion tests between glass frit and Al interfaces were demonstrated. From these results, it was revealed that the glass composition to ensure the enough adhesion was determined. In addition, Low temperature die bonding test were demonstrated on the silver layer using Ag2O paste. As the results, good adhesion between die / Ag bonding layer / Ag sintered layer / Al were obtained on the evaluation of die-shear tests.
The conductive Cu nano ink can be printed by ink-jet printing on polyimide substrate, and then photo-sintered in the air atmosphere within 1 second. In the present work, the spin coated Cu nano ink on polyimide substrate was photo-sintered under various conditions. The microstructures of the sintered Cu films were observed by scanning electron microscopy. Neck formation and grain growth were promoted with increasing of irradiation energy. The mechanism of the photo-sintering process is studied in comparison with a firing process in nitrogen atmosphere. The obtained Cu film by photo-sintering process has low resistivity of 3μΩ·cm and good adhesion vs. polyimide. In contrast, the sintered Cu film by firing process in nitrogen atmosphere has high resistivity of 15μΩ·cm and poor adhesion. It was suggested that good adhesion between Cu film and polyimide is contributed by the bite into the substrate.
This paper discusses electro-migration behavior of eutectic Sn-Bi solder with Cu-pillar bumps. Two types of under bump metal (UBM) of organic substrate were studied, that is, Cu pad and electroless Ni/Au plated on Cu pad. The current density was 4x104A/cm2 at 125 and 150 degree C. Bi quickly migrated to accumulate on the anode side(Cu-pillar) and Sn migrated to the cathode side (Substrate pad). Both the case of surface finishes, although the resistivity was increased to approximately 80 % during approximately 80 hours, it was stabilized more than 2800 hours and there were no electrically break failure. From the cross-sectional analyses of eutectic Sn- Bi solder joints after the test, it was found that Bi layer and intermetallic compound (IMC) behaved as the barriers of the Cu atom migration into Sn solder.
The mechanical integrity of wirebonds is sensitive to structures under the bond pads of ultra low-k dielectric devices. The authors calculated homogenized material properties of two basic wiring structures with three different wiring pitches in the device. Stress in the ULK layer under the bond pad was analyzed using 3D finite element method (FEM) models of ball bond, Al pad, dielectric film stacks and Si susbtrate with twelve different variations. The analyses results indicate that higher density of Cu wiring and via reduces stress in the ULK layer. The results also indicate that thicker FTEOS layer, thicker SiO2 layer or thicker Al pad reduces stress in the ULK layer.
Laser Induced Forward Transfer (LIFT) of metal was performed to directly write the micro deposited pattern using KrF Excimer laser in this study. Effects of conditions of laser irradiation on the deposited pattern were investigated. Time resolved imaging of laser-induced plume was also performed. Electric resistance of deposited micro wires was measured to investigate the possibility for the application of the LIFT. We found smaller film-substrate distance or smaller laser spot size enables better spatial resolution of deposited pattern. Electric resistance of the micro-wire patterned was almost equivalent to that fabricated using other direct writing methods such as laser CVD.