The shear strength of copper and brass soldered with Sn-40% Pb containing 0 to 10% Sb and/or 0 to 15% Zn

The single lap shear strength of copper and brass soldered with Sn-40% Pb containing 0 to 10% Sb and/or 0 to 15% Zn has been determined and the microstructure examined using metallographic techniques. For any solder composition, brass joints were stronger than copper joints. The strength of copper joints decreased monotonically with the increase of antimony in Sn-Pb solder, and the strength of brass joints increased to a peak with about 3% Sb in the solder and thereafter decreased on further additions of antimony. With less than a few per cent antimony in the solder, 1 % Zn in the solder decreased the strength of both copper and brass joints; with more than 1 % Zn in the solder the strengths of both copper and brass joints were increased substantially. Fracture occurs mainly in the Cu₆Sn₅. The microstructure and the presence of zinc in the intermetallic compounds were determined, and the results are discussed.     

Creep behavior in Cu and Ag particle-reinforced composite and eutectic Sn-3.5Ag and Sn-4.0Ag-0.5Cu non-composite solder joints

Creep properties were determined for small, geometrically realistic Pb-free solder joints. Solder joints were prepared with eutectic Sn-3.5Ag and Sn-4.0Ag-0.5Cu solder alloys. Composite solder joints were made using the eutectic Sn-3.5Ag alloy as the matrix with 15 vol % of mechanically added 6 m size Cu and 4 m size Ag reinforcing particles. Creep tests were conducted on these joints at 25 °C, 65 °C and 105 °C representing homologous temperatures ranging from 0.61 to 0.78. Qualitative and quantitative evaluations of creep behavior were obtained from the distortion of excimer laser-induced surface ablation markings on the solder joint. Various creep parameters, such as global and localized creep strain, variation of creep strain and strain-rate, activation energy for creep, and the onset of tertiary creep were determined. General findings in this study revealed that the creep resistance in composite solder joints is significantly improved with Cu particle reinforcements. In contrast, the improvement in the creep properties of Ag particle-reinforced composite solder joints was far less even though highly uniform deformation in the joint was observed. The strain noted at the onset of tertiary creep for Cu and Ag reinforced composite solder joints was typically lower compared to non-composite solder joints. The activation energies for creep were similar for all the solder materials investigated in this study. © 2001 Kluwer Academic Publishers     

The effect of substrate surface roughness on the wettability of Sn-Bi solders

The effect of substrate surface roughness on the wettability of Sn-Bi solders is investigated by the eutectic Sn-Bi alloy on Cu/Al₂O₃ substrates at 190 °C. To engineer the surface with different roughnesses, the Cu-side of the substrates is polished with sandpaper with abrasive number 100, 240, 400, 600, 800, 1200, and 1 m alumina powder, respectively. Both dynamic and static contact angles of the solder drops are studied by the real-time image in a dynamic contact angle analyzer system (FTA200). During dynamic wetting, the wetting velocity of the solder drop decreases for the rougher surface. However, the time to reach the static contact angle seems to be identical with different substrate surface roughness. The wetting tip of the solder cap exhibits a waveform on the rough surface, indicating that the liquid drop tends to flow along the valley. As the solder drops reach a static state, the static contact angle increases with the substrate surface roughness. This demonstrates that the wettability of solders degrades as the substrates become rough.     

Intermetallic compound layer formation between Sn–3.5 mass %Ag BGA solder ball and (Cu, immersion Au/electroless Ni–P/Cu) substrate

The growth kinetics of intermetallic compound layers formed between eutectic Sn–3.5Ag BGA (ball grid array) solder and (Cu, immersion Au/electroless Ni–P/Cu) substrate by solid-state isothermal aging were examined at temperatures between 343 and 443 K for 0–100 days. In the solder joints between the Sn–Ag eutectic solder ball and Cu pads, the intermetallic compound layer was composed of two phases: Cu₆Sn₅ (-phase) adjacent to the solder and Cu₃Sn (-phase) adjacent to the copper. The layer of intermetallic on the immersion Au/electroless Ni–P/Cu substrate was composed of Ni₃Sn₄. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the intermetallic compound was mainly controlled by a diffusion-controlled mechanism over the temperature range studied. The growth rate of Ni₃Sn₄ intermetallic compound was slower than that of the total Cu–Sn(Cu₆Sn₅+Cn₃Sn). The apparent activation energy for growth of total Cu–Sn(Cu₆Sn₅+Cu₃Sn) and Ni₃Sn₄ intermetallic compound were 64.82 and 72.54 kJ mol^–1, respectively.     

Backscattered electron imaging of dental amalgam

A study of the microstructure of set dental amalgams has been undertaken using techniques of back-scattered electron imaging, transmission electron microscopy and X-ray analysis. It has been demonstrated that small ( 0.2, m) particles of a copper-tin phase are present in the matrix of the conventional low-copper amalgams and that the reaction zone around the silver copper eutectic phase in the high copper dispersion type amalgams is of a duplex nature consisting of the Cu₆Sn₅, phase and an intervening layer of silver-mercury phase.     

Surface damage accumulation in Sn–Ag solder joints under large reversed strains

During service, solder joints may encounter repeated reversed stress states. Since realistic solder joint specimen geometry employed for creep/stress relaxation studies does not facilitate stress reversal, shear test specimens consisting of 3/8×3/8×1 copper blocks joined with eutectic Sn–Ag solder of realistic thickness of about 400 m were used for this study. The mechanical behavior and microstructural features of the solder joint subjected to repeated reversed straining were investigated by imposing large shear strains. Damage accumulation on the surface of the solder joints due to a few such large shear strain reversals is very similar to that observed on the surface of thermomechanically fatigued joints made with the same solder.     

Micromechanical characterization of thermomechanically fatigued lead-free solder joints

Nanoindentation testing (NIT) was used to investigate micromechanical properties of (i) as-fabricated, (ii) thermomechanically fatigued (TMF), and (iii) TMF and crept lead-free solder joints. NIT also served to generate information for a database on lead-free solder joints. Sn–Ag-based solder materials used in this study included a binary eutectic alloy, one ternary alloy, and two quaternary alloys. TMF solder joints were thermally cycled for 0, 250, 500, 1000 cycles between –15 and 150 °C. Using NIT, mechanical properties such as hardness, elastic modulus, strength trends, creep behavior, and stress exponent for power-law creep were obtained on small (nominally, 100 m thick) solder joints. Because the volume of material probed by the indenter during NIT is small and highly localized, the properties observed depended strongly on the particular joint microstructure of the indent location. Scanning electron microscopy (SEM) was used to image the nanoindents and monitor deformation and fracture events that resulted from the indenting.     

Effect of isothermal aging on intermetallic compound layer growth at the interface between Sn-3.5Ag-0.75Cu solder and Cu substrate

Intermetallic compound (IMC) growth during solid-state isothermal aging at temperatures between 100 and 200°C up to 60 days for Sn-3.5Ag-0.75Cu solder on Cu substrate was investigated. A quantitative analysis of the IMC layer thickness as a function of aging time and temperature was performed. Diffusion couples showed a composite IMC layer comprised of Cu₆Sn₅ and Cu₃Sn. After isothermal aging at temperature over 120°C, the solder/Cu interface exhibited a duplex structure of Cu₆Sn₅ and Cu₃Sn intermetallics. The growth of IMCs followed diffusion-controlled kinetics and the layer thickness reached 13 m after 60 day of aging at 170°C. The apparent activation energies calculated for the growth of the total IMC (Cu₆Sn₅ + Cu₃Sn), Cu₆Sn₅ and Cu₃Sn intermetallic are 62.6, 49.1 and 80.1 kJ/mol, respectively.     

Structural observations of the interface of explosion-bonded Mo/Cu system

The structure of the interface of explosion-bonded Mo/Cu system has been examined by optical and electron (TEM, AEM) microscopy. Molybdenum and copper can be directly bonded along the flat interface without a diffusion zone, but only on the copper side of the interface is a thin (about 10 m thick) bond layer, which consists of very fine subgrains with an ill-defined boundary. The microstructure in the bond layer is generated by severe dynamic deformation due to jetting and subsequent recovery with frictional heating. On the molybdenum side, originally-existing and elongated subgrains are observed just adjacent to the interface, even after bonding. These results indicate that jetting can occur only on the copper side, with a strength much lower than molybdenum because bonding must be carried out at an impact pressure as low as possible to avoid cracking of molybdenum as well as melting of copper during welding.     

TiN coatings on copper and phosphor-bronze plates by the CVD process,and their oxidation and corrosion stabilities

Copper and phosphor-bronze plates were coated with TiN layers by the chemical vapour deposition (CVD) process, and their oxidation and corrosion stabilities were examined in relation to the thickness of the TiN layer. TiN-coated copper plates were very stable to oxidation up to 700° C, and the weight increase of a specimen with a TiN layer thickness of 0.8 m when exposed in air at 800° C was 1/8 of that of a bare specimen. The corrosion stabilities of copper and phosphor-bronze plates to concentrated HCl and 3.2 N HNO₃ solutions were improved outstandingly by TiN coatings with thicknesses of only 0.5 to 1.0 m.     

A crack in a viscoelastic functionally graded material layer embedded between two dissimilar homogeneous viscoelastic layers – antiplane shear analysis

A crack in a viscoelastic functionally graded material (FGM) layer sandwiched between two dissimilar homogeneous viscoelastic layers is studied under antiplane shear conditions. The shear relaxation modulus of the FGM layer follows the power law of viscoelasticity, i.e., = ₀ exp (y/h) [t₀ exp (y/h) /t]^q, where h is a scale length, and ₀,t ₀,, and q are material constants. Note that the FGM layer has position-dependent modulus and relaxation time. The shear relaxation functions of the two homogeneous viscoelastic layers are =₁(t ₁/t)^q for the bottom layer and =₂(t ₂/t)^q for the top layer, where ₁ and ₂ are material constants, and t ₁ and t ₂ are relaxation times. An elastic crack problem of the composite structure is first solved and the `correspondence principle' is used to obtain stress intensity factors (SIFs) for the viscoelastic system. Formulae for SIFs and crack displacement profiles are derived. Several examples are given which include interface cracking between a viscoelastic functionally graded interlayer and a viscoelastic homogeneous material coating. Moreover, a parametric study is conducted considering various material and geometric parameters and loading conditions.     

Question of determining the composition of composites by a tomographic method

One possible method of determining the volume content of composite components by using x-ray calculational tomography is examined.Notation , _T, _fi, P_b, _b, _w, _im material densities, theoretical, fiber, binder, pore, water, impregnating fluid - _fi, _b, _p fiber, binder, pore volume contents - f_fi, f_b, f_p, f_di, f_di ^*, f_cij, f_im linear coefficients of attenuation of the fiber, binder, air of the i-th layer of the dry specimen - n quantity of cells in the i-th layer, m_fi, m₁ m₂, weight of the dry and impregnated specimens in air and in water - C_cl closed porosity - S_f rms deviation of the linear attenuation coefficients - volume closed pore content Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 5, pp. 738–745, May, 1989.     

Characteristics of porosity in solder pastes during infrared reflow soldering

The effects of solder pastes and infrared reflow temperature profiles on the characteristics of porosity in solder joints are described. Porosity in solder joints can be detected by X-ray radiography. It was found that the composition and structure of solder pastes had the most significant effect on pore formation. However, a lower metal content and/or a higher heating rate did not necessarily cause a higher percentage of pores in solder joints. Results of experiments on pore formation processes during the whole infrared reflow soldering cycle show that high area fraction of pores in solder joints correspond to the peak temperatures in infrared reflow temperature profiles. To evaluate the thermal effect on the performance and structure of solder pastes, tests were also conducted using differential scanning calorimetry, thermogravimetric analysis (TGA) and weight loss in infrared reflow. It was found that the weight loss rate in the TGA curve and infrared reflow and floating speed of porosity are useful to predict the pore formation behaviour in the solder joint. It is recommended that an IR reflow process is chosen to fit with a suitable solder paste in order to decrease porosity in surface-mount solder joints.Nomenclature A Area sum of pores - A _i Area of one pore - C Constants - d, d _i Diameter of a pore - d _i,max Maximum axial dimension of a pore - d _i,min Minimum axial dimension of a pore - g Gravitation constant - L _j Length of a copper land - M Atomic weight of alloy - M ₁, M ₂ Atomic weight of pure metals - n Number of molecules of gas - P _a Atmospheric pressure - P _g Internal gas pressure in a gas bubble - P _h Hydrostatic pressure - P _s Shrinkage pressure - R Gas constant - r Radius of a pore - S _j Area of a copper land - S Area sum of copper lands - T, T _i Temperature - T _m Melting point of alloy - V Volume of a pore - V _m Atomic volume at the melting point - W _j Width of a copper land - w, w _i Weight loss of solder paste - x ₁, x ₂ Mole fraction in alloy - Area fraction of pores - _v Volume fraction of pores - Slope of TGA curve - Density of liquid alloy - _m Density of liquid alloy at the melting point - _p Tap density of solder paste - _s True density of solder metal - Viscosity of liquid solder metal - Rising speed of a gas bubble - Surface tension at the gas-metal interface     

Carbide surface coating of Co-Cr-Mo implant alloys by a microwave plasma-assisted reaction

A technique to grow a hard carbide surface coating on Co-Cr-Mo implant alloys used in artificial joints was developed. The carbide surface coating was applied to as-cast and forged Co-Cr-Mo alloys to improve their wear properties. The surface carbide layers were produced by reactions between the alloy surface and a methane-hydrogen mixed gas by a microwave plasma-assisted surface reaction. The new carbide layers showed brain coral-like surface morphology and appear to consist of mixed phases including Cr₃C₂, Cr₂C, Cr₇ C₃, Cr₂₃C₆, and Co₂C. The Vickers microhardness of thin carbide coatings (3 m thick) was about HV 1100 regardless of the test location. The Vickers microhardness of thick carbide coatings (10 m thick) showed a wide range of hardnesses from HV 1000 to HV 2100. Co-deposition of soot and diamond films occured on a small area of the forged alloy substrates and diamond particles were sparsely dispersed on as-cast alloy substrates. The carbide surface layer has the potential to increase the wear resistance of the Co-Cr-Mo alloy as a wear resistant coating.     

Ceramic joining IV. effects of processing conditions on the properties of alumina joined via Cu/Nb/Cu interlayers

Multilayer copper/niobium/copper interlayers consisting of 3 m thick cladding layers of copper on a 125 m thick niobium core layer were used to join aluminum oxides at 1150°C or 1400°C, or both. Three microstructurally distinct aluminum oxides were joined—a 25 m grain size 99.5% pure alumina, a submicron grain size 99.9% pure alumina, and single crystal sapphire. Two-phase interlayer microstructures containing both copper-rich and niobium-rich phases developed during bonding. In some cases, the initially continuous copper film evolved via Rayleigh instabilities into an array of discrete copper-rich particles along the interlayer/alumina interface with concurrent increases in the niobium/alumina contact area. Processing conditions (temperature and applied load) and the alumina microstructure (grain size) impacted the extent of film breakup, the morphologies of the copper-rich and niobium-rich phases, the interlayer/alumina interfacial microstructure, and thereby the strength characteristics. Joints possessing a large copper/alumina interfacial area fraction were comparatively weak. Increases in bonding pressure and especially bonding temperature yielded interfaces with higher fractional niobium/alumina contact area. For joined polycrystals, such microstructures resulted in higher and more consistent room temperature fracture strengths. Joined 99.9% alumina polycrystals retained strengths >200 MPa to 1200°C. Relationships between processing conditions, interlayer and ceramic microstructure, and joint strength are discussed.     

Magnetic field effects in thick proximity-induced superconductors

Low-field magnetic screening and breakdown fields have been measured in thick proximity-induced superconducting copper by means of dc magnetization and ac susceptibility in the temperature range 10 to 0.005 K. From the thickness and temperature dependence of the breakdown field the Cooper pair penetration depth in copper is obtained. This value isK _N ^–1 =0.6/T m, withT in kelvins. No saturation effects were observed in the screening length , the breakdown fieldH _b, or the supercooled fieldH _sc down to the lowest temperatures.     

Fracture mechanisms in oxide scale on iron during substrate deformation

Oxide scales of different thickness and structure were grown on iron. Fracture of scales was studied when the underlying iron substrate was torsionally deformed at room temperature. For thin scales (5m) with a porous interface structure, the nucleation and growth of cracks occurred by the successive joining of interface pores. Slowly cooled scales of intermediate thickness (20m) failed by crack growth along oxide grain boundaries and from sharp corners of magnetite cuboids within wustite zones. For thick scales (35m), cracks nucleated from the base of the outermost magnetite crystallites. Rapidly cooled, thick scales exhibited crack nucleation from the sharp edges of voids at the scale/metal interface. Crack spacing in the oxide scale decreased with increasing substrate strain in a parabolic form.     

Stress characterization of device layers and the underlying Si1−x Ge x virtual substrate with high-resolution micro-Raman spectroscopy

Silicon–germanium (Si–Ge) epitaxially grown mismatched heterostructures are becoming increasingly important for high-frequency microelectronics applications. One option under serious consideration is that of using Si–Ge virtual substrates, i.e., compositionally graded layers designed to accommodate the lattice mismatch between the underlying Si substrate and the overlying active epilayers(s). This assists in the prevention of misfit dislocations that can impact adversely on the active device regions. The stress in both device silicon cap layers and the underlying Si_1–x Ge _x virtual substrates is characterized with high-resolution micro-Raman spectroscopy (RS). The device layers of the samples studied composed of a 7-nm thick silicon channel, a 6-nm thick SiGe layer and were capped with a 7-nm thick silicon layer. The device layers are grown over a 1-m thick constant composition Si_0.70Ge_0.30 virtual substrate capping layer, and the Si-Ge virtual substrate is grown on a p⁺-type (0 0 1) silicon wafer with a thickness of about 500 m. RS measurement results with a 488-nm Ar⁺ visible laser source indicate that the Si_0.70Ge_0.30 capping layer at the virtual substrate is fully unstrained, while the top silicon cap layer is in extremely high tension. The use of a 325-nm HeCd UV laser for the RS measurements, which probes only a very small depth into the Si cap layer (approximately 9 nm) confirms this high tensile stress is in the top silicon cap layer. The tensile stress in the top silicon cap layer is estimated to be as large as 2.4 GPa by analyzing the shift of the Si Raman peak with respect to the standard strain-free silicon sample. The measured stress value is almost equal to the theoretically predicted tensile stress that should exist in the fully strained Si cap layer. This implies that the Si cap layer remains strained in samples with this structure.     

Microstructure and mechanical properties of reaction-formed joints in reaction-bonded silicon carbide ceramics

A reaction-bonded silicon carbide (RB-SiC) ceramic material (Carborundum's Cerastar RB-SiC) has been joined using a reaction f rming approach. Microstructure and mechanical properties of three types of reaction-formed joints (350 m, 50–55 m, and 20–25 m thick) have been evaluated. Thick (350 m) joints consist mainly of silicon with a small amount of silicon carbide. The flexural strength of thick joints is about 44±2 MPa, and fracture always occurs at the joints. The microscopic examination of fracture surfaces of specimens with thick joints tested at room temperature revealed the failure mode to be typically brittle. Thin joints (<50–55 m) consist of silicon carbide and silicon phases. The room and high temperature flexural strengths of thin (<50–55 m) reaction-formed joints have been found to be at least equal to that of the bulk Cerastar RB-SiC materials because the flexure bars fracture away from the joint regions. In this case, the fracture origins appear to be inhomogeneities inside the parent material. This was always found to be the case for thin joints tested at temperatures up to 1350°C in air. This observation suggests that the strength of Cerastar RB-SiC material containing a thin joint is not limited by the joint strength but by the strength of the bulk (parent) materials.     

Measurement of the Thermal Diffusivity of Metallic Foils and Films by the Photoacoustic Method1

The thermal diffusivities of four kinds of metallic foils from 20 to 200m in thickness were measured by a photoacoustic method on the basis of the Rosencwaig and Gersho theory. The measured data for continuous foils of uniform microscopic structure almost agreed with the literature values. Measurements were also carried out on two kinds of metallic thin films with of 10m thickness produced by sputtering. The difference in thermal diffusivity between the foils and the sputtered films depended on the uniformity of the microscopic structure.