Masamoto Tanaka

Processing and properties of QMG materials

Abstract Recent progress of Quench and Melt Growth (QMG) process is reviewed. QMG process is one of the melt processes for Y based superconductor. A pseudo single crystal (QMG crystal) which includes finely dispersed 211 phase in the matrix of 123 phase was produced by the original QMG process. The critical current density of the QMG crystal that has no high angle grain boundaries has been exceeding 104A/cm2 at 77K and 1T. The original QMG process was improved to make larger QMG crystals of which crystallographic orientation was controlled. Modified QMG process can be characterized by Pt addition in precursor preparation and seeding technique in growth process. Crystallinity of the QMG crystal made by this modified process was investigated. It appeared that there were two kinds of domain structures that depend on the growth direction. QMG crystals enlarged by this modified process are applicable to various fields. For instance, QMG crystal can operate as a bulk magnet. The maximum value of flux density was 4.5T at 40K on the surface. It was found that QMG crystal is thermally stable above 40K at least.

High Magnetic Flux Trapping by Melt-Grown YBaCuO Superconductors

Trapped magnetic flux density of melt-grown YBaCuO bulk superconductors was evaluated. A single-grained disc-shaped bulk sample with a size of 45 mm×15 mm trapped very high magnetic flux density after field cooling. One disc trapped a maximum surface flux density of 0.72 T, while the maximum of double-piled discs was 1.35 T after 100 seconds. A polygrained sample showed a much lower maximum. The pinning potential of this single-grained disc was 0.17 eV, and the overall critical current density estimated from the trapped flux density was about 7×103 A/cm2. Since these values are lower than those of a small-sized sample, weak links are still expected to exist even in the single-grained sample. Domain structure within a grain is a strong candidate for this weak link.

Effect of Pt Addition on Melt-Processed YBaCuO Superconductors

Platinum addition in precursors of melt processing could produce finely dispersed Y2BaCuO5 (211) inclusions of about 1 µm in YBa2Cu3Ox matrix. This fine dispersion of 211 resembles that of quench-and-melt-grown materials. The platinum-added sample exhibited a critical current density exceeding 2×104A/cm2 at 77 K and 1 T. Platinum is considered to enhance the nucleation of 211 which forms by the pertitectic reaction between Y2O3 and liquid phase.

Critical Current Characteristics in Superconducting Y-Ba-Cu-O Prepared by the Melt Process

Critical current densities in superconducting Y-Ba-Cu-O specimens with different sizes of 211 particles prepared by the melt process were measured under various magnetic fields and temperatures. Discussion is given on the flux pinning properties by 211 particles and background pinning centers. At temperatures above 60 K, 211 particles are expected to be dominant pinning centers. A rapid degradation of the critical current density with elevating temperature at low temperatures below 25 K seems to be attributed to weak links in specimens.

Effect of eddy current dampers on the vibrational properties in superconducting levitation using melt-processed YBaCuO bulk superconductors

Abstract The effect of eddy current dampers on the vibrational properties in superconducting levitation has been investigated using melt-processed YBaCuO bulk superconductors. In vertical vibration, the damping was about 100 times improved by inserting eddy current dampers into the gap between a superconductor and a magnet. On the other hand, in horizontal vibration, eddy current dampers did not greatly affect the vibrationaal properties of superconducting levitation up to a given thickness of eddy current dampers.

Solder joint reliability of a polymer reinforced wafer level package

Wafer level packages (WLPs) have demonstrated a very clear cost-advantage vs. traditional wire-bond technologies, especially for small components that have a high number of dice and I/O per wafer. Solder joint reliability of traditional WLPs is the weakest point of the technology. The ability of the solder joint to survive the required thermal cycle testing has limited WLP use to products having relatively small die sizes and a small number of I/O. To increase solder joint reliability without underfill new technologies needed to be developed. To address this need, the K&S Flip Chip Division has developed a polymer reinforcement technology called Polymer Collar WLP/sup TM/. Polymer Collar WLP utilizes a polymer reinforcement structure surrounding the solder joint and it has demonstrated a full 50-60% increase in solder joint life in various sets of TC (thermal cycling) tests. The strengthening mechanism of the polymer collar is well understood and was documented after the second TC test.

Effect of crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag–0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects

Abstract An effect of the crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag– 0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects were discussed. From an orientation imaging microscopic observation, three types of microstructures (single crystal-like, fine grain type and large grain type) were observed in both solders. The single crystal-like microstructure disappeared and the large grain type occurred by further fatigue due to recrystallisation. Because single crystal-like microstructure had the {100} plane approximately parallel to strain concentrated areas, recrystallisation could be retarded if the slip systems of {100}<011> or {100}<010> operate and an amount of thermal strain decreases because these slip systems have the larger critical resolved shear stress due to an anisotropic nature of β-tin. One of the reasons Sn–3Ag–0˙5Cu had longer thermal fatigue life than Sn–1Ag–0˙5Cu can be the number of the single crystal-like or the fine grain type microstructures in Sn–3Ag–0˙5Cu were larger.

Improvement in drop shock reliability of Sn-1.2Ag-0.5Cu BGA interconnects by Ni addition

The drop shock reliability of a Sn-Ag-Cu solder system in ball grid array (BGA) interconnects were improved by selecting a lower Ag chemical content and the addition of a small amount of Ni. The drop shock reliability of Sn-Ag-Cu solder in BGA interconnects was enhanced by the addition of small amount of Ni, and LF35(Sn-1.2Ag-0.5Cu-Ni) had twice better drop shock reliability than LF45(Sn-3.0Ag-0.5Cu). One of the main reasons is the solder with lower Ag contents in the Sn-Ag-Cu solder system demonstrated softer properties in terms of hardness. Different intermetallic compound (IMC) layer morphologies were found on the Cu electrodes after reflow between Sn-1.2Ag-0.5Cu-Ni and Sn-3.0Ag-0.5Cu, in which the former is smooth IMC and the latter is like a peninsula IMC. Different cracks modes were also detected. Cracks in Sn-1.2Ag-0.5Cu-Ni were mainly inside solder and cracks in Sn-3.0Ag-0.5Cu were near the solder/electrode interface for all drop shocks tested. From SEM, EPMA mapping and TEM analysis, the small amount of doped Ni was mostly segregated at the interface. The doped Ni mainly existed in the Cu6Sn5 IMC layer region as formed (Cu,Ni)6Sn5, and the Cu3Sn IMC region contains less Ni. Both different IMC morphologies and crack locations were discussed on the basis of the lattice distortion relaxation. The Cu 6Sn5 peninsula IMC growth was discussed that was caused by the compression stress of itself, and the Sn-3.0Ag-0.5Cu crack inside the IMC was also discussed that was caused by the difference stress between Cu6Sn5 (compression stress) and Cu 3Sn (tensile stress). Namely, the doped Ni was substituted for the Cu site of Cu6Sn5 and their lattice distortions were relaxed due to the smaller atomic radius of Ni compared with Cu. The stress difference between (Cu,Ni)6Sn5 and Cu3Sn was relieved by the Ni substitutions, which improved the drop shock reliability in BGA of Sn-1.2Ag-0.5Cu-Ni

Thermal fatigue properties and grain boundary character distribution in Sn–x Ag–0˙5Cu (x = 1, 1˙2 and 3) lead free solder interconnects

Abstract Thermal fatigue properties of Sn–x Ag–0˙5Cu (x = 1, 1˙2 and 3, mass%) lead free solder interconnects were discussed from the viewpoints of both morphology and grain boundary character distribution. 3Ag showed the longer thermal fatigue life than 1Ag and 1˙2Ag. Both cracks, which were initiated by thermal strain, and grain boundary damage due to grain boundary sliding degraded the thermal fatigue lives. From a microstructural observation using orientation imaging microscopy, recrystallisation of tin grains was observed, and 3Ag suppressed coarsening of tin grains after further thermal fatigue as compared with 1Ag and 1˙2Ag. Moreover, 3Ag showed a larger amount of coincidence site lattice boundaries than 1Ag and 1˙2Ag. It is suggested that in 3Ag not only smaller tin grains but also larger amount of coincidence site lattice boundaries suppressed crack propagation and grain boundary sliding.

Magnetic Shielding by Superconducting Y-Ba-Cu-O Prepared by the Modified Quench and Melt Growth (QMG) Process

Magnetic shielding capability has been investigated on Y-Ba-Cu-O crystals prepared by the modified quench and melt growth (QMG) process at 77 K and 4.2 K. Overall critical current densities in large single-grained specimens were estimated to be about 104 A/cm2 based on the Bean model in order to predict shield effect. At 77 K, a single grain cylinder shields the inside up to about 0.2 T of the external field, and this value is the highest among all the values reported so far for tubes fabricated of oxide superconductors. A comparison between shielding characteristics of a single grain and those of a polygrain demonstrated that weaklinking high-angle grain boundaries should be eliminated for high field shielding. Lorentz force, flux creep, and flux jump are discussed.