Tadashi Takemoto

Laser ablation at solid–liquid interfaces: An approach from optical emission spectra

The emission spectra from the solid–liquid interface irradiated by a pulsed laser were studied. The solid target used in this study was graphite and boron nitride, and the liquid in which the target was immersed was water, benzene, n-hexane, and carbon tetrachloride. The results showed strong continuous spectrum immediately after a pulse shot, whereas after ≈100 ns later from the irradiation it was greatly reduced, and instead, the emission from small molecules dominated the spectra. The line spectra of small molecules observed in the later time range indicate the chemical reaction between the ablated species and the species originated from the liquid molecules. The intensity of the continuous spectrum was very prominent compared to what has been observed for solid–gas interfaces. This is due to rapid electron ion recombination or bremsstrahlung due to highly confined interface plasma.

Interfacial reaction between Sn-0.7Cu (-Ni) solder and Cu substrate

The interfacial reaction between Sn-0.7mass%Cu-(Ni) solders and a Cu substrate was investigated to reveal the effect of the addition of Ni to Sn-Cu solder on the formation of intermetallic compounds (IMCs). Sn-0.7Cu-xNi solders (x=0, 0.05, 0.1, 0.2 mass%) were prepared. For the reflow process, specimens were heated in a radiation furnace at 523 K for 60 sec, 300 sec, and 720 sec to estimate the interfacial reaction between the molten solder and Cu substrate. Then, for the aging process, some specimens were heat-treated in an oil bath at 423 K for 168 h and 504 h. The cross sections of soldered specimens were observed to measure the dissolution thickness of the Cu substrate and the thickness of the IMC and to investigate the microstructures of IMC. The results showed that, just after the reflow process, the dissolution thickness of the Cu substrate increased with the increase of Ni content in the Sn-0.7Cu-xNi solder and the thickness of the IMC between the solder and Cu substrate was the minimum in the Sn-0.7Cu-0.05Ni solder. After the aging process, the IMC grew with the increase of aging time. In the case of 0.05% Ni, the IMC thickness was the thinnest regardless of aging time. It is clear that 0.05% Ni addition to Sn-0.7Cu solder very effectively inhibits the formation and growth of the IMC between solder and Cu substrate. Electron probe microanalysis of the IMC showed that the IMC layer in the Sn-0.7Cu-Ni solder contained Ni, and the IMC was expressed as (Cu1−y,Niy)6Sn5.

Intermetallic compounds formed during brazing of titanium with aluminium filler metals

The effect of aluminium filler metal composition on the formation of AI-Ti intermetallic compounds was investigated in brazed aluminium-to-titanium (Al/Ti) joints and titanium-totitanium (Ti/Ti) joints. The clearance filling ability was also studied. In Ti/Ti joints, the thickness of the intermetallic compound layer was strongly dependent on the aluminium filler metal composition, whereas the clearance filling ability was independent of the composition. The maximum intermetallic compound layer thickness was observed in 99.99% highly pure aluminium filler metal; therefore all additional elements reduced the layer thickness. Above all, the addition of 0.8% Si greatly reduced the thickness. After brazing at 680° C for 3 min, the intermetallic compound formed by Al-0 to 0.8% Si filler metal was found to be of type Al3Ti. Other compounds, of types Ti9Al23 and Ti7Al5Si12, were also found in joints brazed by Al-3 to 10% Si filler metals. AI-0.8% Si filler metals maintained a higher joint strength than pure aluminium filler metal under brazing conditions of high temperature and long heating time. In Al/Ti joints, AI-Cu-Sn and AI-Cu-Ag filler metal mainly formed Al3Ti, and Al-10Si-Mg filler metal mainly formed Ti7Al5Si12 at the brazed interface of the titanium side after brazing at 600 to 620° C.

Influence of Joining Conditions on Bonding Strength of Joints: Efficacy of Low-Temperature Bonding Using Cu Nanoparticle Paste

We investigated a new low-temperature bonding process utilizing Cu nanoparticle paste without addition of sintering promoter. Joint bonding strengths above 30 MPa were achieved even at a low bonding temperature of 250ºC. We attribute the higher bonding strengths of joints fabricated using the vacuum preheating process to the rapid progression of Cu nanoparticle sintering due to the activated nanoparticle surface at lower temperatures. The increase in bonding strength depended on the applied pressure, in addition to the bonding temperature. The formation of a dimple-like morphology was confirmed in the ductile fracture area. This indicated that the joint bonded strongly with the bonding layer, in agreement with the results of bonding tests carried out on strongly bonded joints. The bonding ability of the joints obtained using Cu nanoparticle paste could be improved by controlling the joint fabrication conditions.

Microstructure and mechanical properties evolution of intermetallics between Cu and Sn-3.5Ag solder doped by Ni-Co additives

The evolution of intermetallic compounds (IMCs) generated between Sn-3.5Ag solder doped by additive couples (namely, 0.2mass%Co and 0.1mass%Ni) and Cu substrate was characterized. After soldering, the additive couples, Co-Ni, were all detected at the intermetallic region. The microstructure of intermetallic was identified as (Cu, Ni, Co)6Sn5 by electron probe microanalysis (EPMA) and x-ray diffraction (XRD). However, the morphology of (Cu, Ni, Co)6Sn5 was converted to columnar like and was not as dense as the typical scallop-like Cu6Sn5. A duplex structure of (Cu, Ni, Co)6Sn5, namely, two distinct regions bearing different concentrations of Ni and Co, was observed. Much higher Ni and Co concentrations were probed in the outer intermetallic region adjacent to the solder matrix, while lower concentration at the inner region was verified. After aging, the intermetallic (Cu, Ni, Co)6Sn5 tended to be dense, while the growth rate was depressed at the early stage. In addition, the Cu3Sn phase was not detected after aging at 110°C, while it appeared at 130°C and 150°C for 504 h. Using the nanoindentation technique, some mechanical properties of (Cu, Ni, Co)6Sn5 were investigated. The lower hardness and Young’s modulus of the outer intermetallic region was revealed. After aging treatment, both the hardness and Young’s modulus values were elevated.

Dissolution rates of iron plating on soldering iron tips in molten lead‐free solders

The dissolution rates of iron and alloyed steels in molten lead‐free solders were investigated in order to clarify the effect of erosion of iron plating on soldering iron tips. The dissolution rates of iron‐based alloys in lead‐free solders were found to be about three times greater than in conventional Sn‐Pb eutectics, indicating that the iron plating of a soldering iron tip is subjected to heavier damage when used with lead‐free rather than eutectic Sn‐Pb. Several steel alloys showed dissolution rates similar to that of pure iron, suggesting that compositional changes in the iron plating may have little influence on the erosion depth. Decreases in the reaction temperature and time, and a small addition of iron into the solder was found to be effective in suppressing both dissolution of iron wire and erosion of iron plating.

Effects of Joining Conditions on Joint Strength of Cu/Cu Joint Using CuNanoparticle Paste

High-temperature bonding, or joining, is a key technology for electronic component assembly and other high- temperature applications. Recently, focusing on the sintering behavior of nanoparticles, the joining process using a nanoparticle paste has been proposed as an alternative to soldering for high-temperature applications. In this study, Cu nanoparticle paste was used to join two Cu discs, and the effect of joining conditions on the joint strength of the Cu-to-Cu joint was investigated. Joining using Cu nanoparticle paste was successfully achieved, but the effect of joining conditions such as heating temperature and joining atmosphere on joint strength of the Cu-to-Cu joint was significant.

Transformation of Sn–Cu alloy from white tin to gray tin

Abstract Sn–0.8 wt.% Cu alloy is prepared to investigate the transformation of white tin to gray tin. The relationship between transformation percentage and time is determined by measuring the areas of transformed gray tin, and the effect of cold rolling on the transformation is investigated. It is concluded that both the additions of copper to pure tin and the severe deformation are effective on the enhancement of transformation. The incubation period before initiation of transformation is also dependent on the cold reduction.

Tensile test for estimation of thermal fatigue properties of solder alloys

A tensile test is proposed to evaluate the thermal fatigue resistance of solder alloys. The test is based on the strain rate change method to obtain the strain rate sensitivity index, m. The m value is obtained at various strains during the tensile test. The plots of m and strain where m is measured showed a linear relation; therefore, the m value at zero strain, m0, and the gradient, k, are obtained by extrapolation. The m value at zero strain related to the coarseness or fineness of solder alloy microstructure; m0 becomes lower with coarsening of the microstructure. The solder alloys with low m0 and low k have excellent thermal fatigue resistance when compared with alloys with high m0 and high k. m0 and k would be good measures to estimate the thermal fatigue properties of solder alloys; highly resistant alloys have low m0 and low k.

Dissolution of stainless steels in molten lead‐free solders

Purpose – To determine the endurance of stainless steels used for wave soldering container materials in molten lead‐free solders.Design/methodology/approach – A dissolution test on stainless steel wires in molten lead‐free solders was performed. The effects of the composition of the stainless steel, test period, and composition of the lead‐free solder on the dissolution rate were investigated. Dissolution was measured by cross‐sectioning the wires and measuring the reduction in radius.Findings – The Sn‐Ag lead‐free solder showed faster dissolution than did the conventional Sn‐Pb eutectic. A severe dissolution rate was also observed for the Sn‐Zn system.Practical implications – Quantitative data for the reaction rate between stainless steels and molten lead‐free solder is useful for the design of soldering machines and in planning their maintainance.Originality/value – This paper shows the effect of basic factors on the dissolution rate of stainless steels in molten solder. It can give a basic understandin...