Metallographic Cross Sections Research Articles

Influence of heat flow direction on solder ball interfacial layer

Abstract This paper deals with the research of an intermetallic layer of SAC305 solder balls soldered from three directions of the heat flow in the ball-attach process for BGA package. From the point of view of the heat flow direction, the samples were soldered by infrared heating. The heat sources were placed on the top, bottom and both lateral sides of the BGA package. After the solder balls-attach process, a metallographic cross-section was performed, followed by selective etching to visualize the relief of the intermetallic layer. Images of the interfacial between the solder and solder pad were taken from the created samples, followed by measurement of the average thickness and root mean square roughness of the intermetallic layer. The results showed changes in the intermetallic layer. The largest thickness of the intermetallic layer was observed on samples soldered from the top and both sides. Soldering with the bottom infrared heater resulted to the smallest thickness of the intermetallic layer. The same trend was in the roughness of the IMC layer. The greatest roughness was found for samples soldered by the top and both side heaters. The top soldered samples exhibit the smallest roughness.

Reliability studies of InnoLot and SnBi joints soldered on DBC substrate

PurposeThis paper aims to investigate the quality and reliability of solder joints prepared from Pb-free alloys on direct bounded Cu (DBC) substrates. Two types of solder alloys were studied: Sn90.95Ag3.8Cu0.7Sb1.4Ni0.15Bi3.0, with a high melting point of 225°C, and Sn42Bi58, with low a melting point of 138°C.Design/methodology/approachCapacitor components of size 1806 were soldered on DBC substrates by using convection reflow soldering and vacuum vapor-phase soldering technologies. A part of the samples was subjected to the thermal shock test. The structure of the solder joints and the content of the voids were investigated using three-dimensional X-ray tomography. The mechanical strength of the joints was evaluated using the shear force test, and the microstructure of the joints was studied on metallographic cross sections by using scanning electron microscopy.FindingsIt was found that the number of voids is not related directly to the mechanical strength of the solder joints. The mechanical strength of the solder joints depends more on the amount of Ag3Sn precipitation, Au precipitation and the intermetallic layer in the solder joints. In some cases, the thermal shock test caused micro-cracks around the Au precipitation because of a mismatch of Au, AuSn4 and Sn in terms of coefficients of thermal expansion.Originality/valueDBC substrates are usually used for power electronics, where the quality of the solder joints is even more important than in the case of commercial electronics.

Microstructure, Wear Behavior and Corrosion Resistance of WC-FeCrAl and WC-WB-Co Coatings

The paper is focused on investigating the quality of two grades of thermally sprayed coatings deposited by high-velocity oxygen fuel (HVOF) technology. One grade contains WC hard particles in an environmentally progressive Ni- and Co-free FeCrAl matrix, while the second coating contains WC and WB hard particles in a cobalt matrix. The aim of the experimental work was to determine the effect of thermal cyclic loading on the coatings’ resistance to adhesive, abrasive and erosive wear. Abrasive wear was evaluated using abrasive cloth of two grit sizes, and erosive wear was evaluated by a dry-pot wear test in a pin mill at two sample angles. Adhesion wear resistance of the coatings was determined by a sliding wear test under dry friction conditions and in a 1 mol water solution of NaCl. Corrosion resistance of the coatings was evaluated using potentiodynamic polarization tests. Metallographic cross-sections were used for measurement of the microhardness and thickness and for line energy-dispersive X-ray (EDX) analysis. The tests proved the excellent resistance of both coatings against adhesive, abrasive, and erosive wear, as well as the ability of the WC-WB-Co coating to withstand alternating temperatures of up to 600 °C. The “green carbide” coating (WC-FeCrAl) can be recommended as an environmentally friendly replacement for Ni- and Co-containing coatings, but its operating temperature is strictly limited to 500 °C in air.

A physical modeling and predictive simulation of the laser cladding process

A 3D finite element simulation model of the laser cladding process has been developed taking into account heat transfer, fluid flow, surface tension and free surface movement. All input parameters and data, which are independent of the process parameters but depend only on the material and machine properties, have been obtained from measurements. Thereby the melt pool and the resulting surface contour can be simulated without compromising assumptions or calibration, because the machine parameters are the only variable input parameters of the model. Thus, the model can easily be transferred to other material combinations or other machines. For the surface contour calculation a modified height function method is applied. The model surface follows this contour as an arbitrary Lagrangian Eulerian (ALE) method is used allowing for mesh deformations. The model was implemented using the commercial finite element software COMSOL Multiphysics and validated by comparing the simulation results with caloric measurements of the effective heat input and metallographic cross sections from experiments, where the nickel-base alloy MetcoClad® 625 in powder form was deposited on structural steel S235JRC + C and the process parameters of laser power, feed speed, laser beam spot size and powder mass flow were varied within a range of at least 50% of their mean value each. The maximum deviation of the simulation results compared to the experimental data regarding track geometry is 14% for the parameter sets without weld defects so that these parameter sets could be industrially applied, whereas the average deviation of track width and height is below 5.1%. Moreover, the maximum difference of the simulated absorptivity compared to the measurement results is 6.9%, while the average difference is only 1.7%, meaning the model shows a high predictive capability especially regarding the heat input.

Irreversible degradation of Nb3Sn Rutherford cables due to transverse compressive stress at room temperature

In the framework of the Future Circular Collider design study for a 100 TeV circular collider, 16 T superconducting bending magnets based on Nb3Sn technology are being developed. A pre-stress on the conductor during magnet assembly at room temperature (RT) is needed to counteract the Lorentz forces during operation. The superconducting properties of the brittle Nb3Sn superconductor are strain sensitive and excessive pre-stress leads to an irreversible degradation of the superconductor. In order to determine the level of acceptable pre-stress during the magnet assembly process, reacted and impregnated Nb3Sn cables were exposed to increasing transverse compressive stress up to a maximum stress level of 200 MPa at RT. After each stress cycle, the critical current of the cable specimens were characterized at 4.3 K in the FRESCA cable test station. No significant critical current degradation was observed up to 150 MPa, followed by degradation less than 4% after a nominal stress of 175 MPa. A dramatic permanent critical current degradation occurred after applying a nominal stress of 200 MPa. A comprehensive post analysis consisting of non-destructive micro-tomography followed by microscopic characterization of metallographic cable cross sections was carried out after the critical current test to reveal cracks in the Nb3Sn sub-elements of the loaded specimen.

Behaviour of S31400 and S32205 steels in HCl‐ and H2S‐containing gas atmospheres under a low oxygen partial pressure between 480 and 680 °C

The effects of gaseous hydrogen chloride and hydrogen sulphide on the high‐temperature corrosion of the austenitic SS. S31400 and the duplex steel S32205 were investigated in an atmosphere without molecular O2. The testing gas atmosphere consisted of 3.8 vol% HCl, 1.9 vol% CO2, 0.3 vol% CO, 2.8 vol% H2, 0.02 vol% H2S, bal. N2. It showed a similar composition, which is present in the thermal cracking of anthropogenic resources. Tests were performed between 480 and 680 °C for 1, 3 and 10 days. Mass loss measurements were done for all experiments. After the reaction times the samples showed non‐adherent and porous layers of corrosion products, which were investigated by SEM/EDX and XRD. Metallographic cross sections of the corroded samples were prepared by water‐free polishing to investigate the microstructure of the corrosion products. Both materials formed large chromium sulphide crystals on top of a chromium oxide layer and a chlorine‐containing layer beneath the oxide. The ferrite phase of the duplex steel is selectively attacked, while the austenite phase remained without severe corrosion. Steel S31400 showed a uniform corrosion.

X-ray Microanalysis of Precious Metal Thin Films: Thickness and Composition Determination

Measuring the thickness and the composition of precious metal thin films is a challenging task. Currently, the available techniques for thickness measurements are either destructive or need heavy assumptions on the nature of the sample, relying on information that are not always available with sufficient accuracy. In this paper we propose a new methodology based on X-ray microanalysis that can complement, with better lateral resolution, the use of X-ray Fluorescence, the most widely employed technique for measuring the thickness of electrodeposited coatings. The proposed method employs a combination of energy dispersive microanalysis spectra acquisition and Monte Carlo simulation. The effectiveness of the technique has been demonstrated by the measure of the thickness and the composition of a thin 24 kt gold electroplated film that contained small amount of nickel. Results have been validated by comparing data with those obtained by X-ray fluorescence and the scanning electron microscopy of metallographic cross-sections.

Effect of Width of Kerf on Machining Accuracy and Subsurface Layer After WEDM

Wire electrical discharge machining is an unconventional machining technology that applies physical principles to material removal. The material is removed by a series of recurring current discharges between the workpiece and the tool electrode, and a ‘kerf’ is created between the wire and the material being machined. The width of the kerf is directly dependent not only on the diameter of the wire used, but also on the machine parameter settings and, in particular, on the set of mechanical and physical properties of the material being machined. To ensure precise machining, it is important to have the width of the kerf as small as possible. The present study deals with the evaluation of the width of the kerf for four different metallic materials (some of which were subsequently heat treated using several methods) with different machine parameter settings. The kerf is investigated on metallographic cross sections using light and electron microscopy.

Microstructuring of joining areas in aluminum alloy sheets by Jet Electrochemical Machining

In this work the increase of the tensile shear strength by means of microstructuring of the metallic part for ultrasonic vibration assisted joining of hybrid compounds is presented. The aluminum alloy EN AW-5083 and a carbon fibre-reinforced plastic (CFRP) from Bond Laminates are used as a material combination. A suitable method is electrochemical processing (ECM). The microstructuring is carried out with continuous electrolyte free jet machining (Jet-ECM): Characteristic of this technology is the restriction of the electric current to a limited area of the electrolyte jet. After describing the materials and sample geometry used, the Jet-ECM technology and the ultrasonic vibration assisted joining process are explained. The strength of the joint is assessed by means of a tensile shear test. The determined results of the tensile shear strength for hybrid connections between microstructured aluminum sheets and CFRP are compared with those of unstructured aluminum sheets. Furthermore, the influence of the microstructure on the tensile shear strength achieved is discussed using metallographic cross-sections of the joining area.

In Situ Acoustic Monitoring of Thermal Spray Process Using High-Frequency Impulse Measurements

In order to guarantee their protective function, thermal spray coatings must be free from cracks, which expose the substrate surface to, e.g., corrosive media. Cracks in thermal spray coatings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology and coating defects were inspected using light microscopy on metallographic cross sections. Additionally, the resulting crack patterns were imaged in 3D by means of x-ray microtomography.

Influence of Build Orientation, Heat Treatment, and Laser Power on the Hardness of Ti6Al4V Manufactured Using the DMLS Process

This contribution is focused on the influence of build orientation on hardness of materials sintered using direct metal laser sintering (DMLS) technology. It builds on the current research works that has monitored the influence of build orientation on a fatigue life, mechanical properties, roughness after machining, etc. In the mentioned work, a slight influence of build orientation on the above properties was shown. The hardness was measured on a Ti6Al4V alloy which was made of powder by DMLS technology. The individual materials were sintered at different laser powers, then annealed to remove internal stresses. Part of the experiment examined the metallographic analysis of materials in the direction perpendicular to the sintered layers and parallel with the sintered layers. Microhardness was measured on metallographic cross-sections and the results were statistically processed. The influence of laser power on a respective material hardness was assessed by one-way analysis of variance (ANOVA), a comparison of the hardness between sintered and sintered-annealed samples, as well as the comparison of hardness in the two considered directions was performed by t-test and F-test. A statistically significant difference in the hardness of the materials prepared at different laser powers was found. The influence of heat treatment, as well as the direction of material building also showed a statistically significant difference.

Fiber laser welding of dissimilar titanium (Ti-6Al-4V/cp-Ti) T-joints and their laser forming process for aircraft application

The weldability of dissimilar T-joints between commercially pure titanium (cp-Ti) Grade 2 skin and Ti-6Al-4V Grade 5 stringer using a continuous wave 8kW ytterbium fiber laser as well as the possibility of subsequent laser straightening process of these joints were investigated. Based on the industrial standards ISO 4578:2011 and AWS D17.1:200, process development to compensate inherent angular distortion after welding by laser heating with the same equipment as for welding was carried out. The obtained results were effectively transferred to a 6-stringer-demonstrator with a length up to 500mm. To investigate the shape and morphology of the welding seam as well as to verify its freedom from defects using the defined process parameters, metallographic transverse cross-sections and X-ray analyses were realized. In addition, the behavior of the welding seam geometry and the bending behavior of the specimens for varied process parameters were elucidated. For the welding process special attention to the shielding conditions and to the local and angular laser beam positioning was payed. To straighten the welded joints, laser straightening parameters inducing no microstructural changes were identified.

Characterization of the melt flow direction and cut front geometry in oxygen cutting with a solid state laser

Laser oxygen cutting is one of the most established laser based manufacturing processes and mainly used for cutting thick metal sheets of mild steel up to a thickness of 25 mm. Besides local absorption of the laser beam, the exothermal FeO oxidation reaction of iron and oxygen provides additional heat input to the cutting process. A lack of energy input can lead to dross formation and decreased cut quality. A detailed, phenomenological description of the melt flow direction and cut front geometry can help understanding dross formation and improving the stability of the cutting process at the cutting limit. Until now, the correlation of the melt flow direction and cut front geometry in case of dross formation is not completely understood. Therefore, high speed videos and metallographic cross and longitudinal sections have been prepared and correlated to dross formation. In order to describe the transition from a quality cut with optimized energy input to a fail cut with dross formation caused by a lack of energy input, three characteristics have been identified: The melt flow direction, characterized by the angle of melt expulsion, the local cut front angle in the lower part of the cut front and the width of the cut kerf at the bottom side of the sheet.

Investigations on the Additive Manufacturing and Heavy Wire Bonding Capability of Selective Laser-Melted Circuit Carriers

Abstract Additive manufacturing (AM) provides tool-free and direct manufacturing of complex geometries simultaneously integrating various functions into components. Though AM techniques are widely used in various sectors, the application into electronics production has not yet been explored. In electronics production, substrate development has high relevance due to their multifunctionality through electrically connecting components and providing mechanical support. This contribution introduces an innovative approach based on selective laser melting (SLM) in the development of high-temperature substrates through additive layered manufacturing. Furthermore, possibilities and challenges that come along with the process combination of SLM and heavy wire bonding are investigated. Wire bonding capability is analyzed on untreated as well as on postprocessed surfaces. The influence and effectiveness of various steps of postprocessing such as cleaning, sand blasting, and grinding are analyzed. Thus, interdependencies between both manufacturing process as well as postprocessing can be revealed. The effect of surface roughness and hardness of the assembly partner are investigated as well. The primary characteristics besides the bond parameters that influence the wire bonding capability are focused in this article. The process stability and the interconnection quality are evaluated by optical nondestructive laser microscopic analysis. Destructive pull and shear tests and metallographic cross sections are performed to evaluate the adhesion characteristics. By a profound understanding of all interdependencies between the two processes, a flexible manufacturing technology for power devices can be established.

A robust method to characterize rumpling in high-strength bond coats

A new method has been developed to nondestructively characterize bond coats that rumple during thermal cycling. This method employs Fourier transforms of 2D optical profilometer data and is applied to coatings that have differing tendencies to rumple such as (Pt,Ni)Al coatings and high-strength γ′ or γ+γ′ bond coats. The method isolates the 2D periodic undulations characteristic of rumpling from other inhomogeneous time-dependent phenomena that occur at the surface, including HfO2 and NiAl2O4 spinel oxide growth, bond coat swelling, and noisy data. The method is benchmarked against the traditional rumpling indicators seen in the literature, surface roughness and tortuosity, using synthetic datasets to explore the strengths and limitations of the algorithm. The versatility of this method to quantify rumpling is demonstrated on experimental datasets from a β-phase bond coat with high levels of noisy or missing data. Application of the method provides a quantification of the rumpling amplitude and wavelength that are confirmed with metallographic cross sections of the samples and predicted by the Balint and Hutchinson rumpling model. The algorithm correctly measures (in contrast to the RMS surface roughness) an increasing surface undulation amplitude during initial furnace cycling, followed by a decrease in undulation amplitude near the end of the coating life as TGO spallation becomes prevalent.

The Effect of Laser Beam Welding Parameters onto the Evolving Joints Geometry

In our research the effect of a new type of laser beam parameters during the laser welding have been investigated with 80 different welding parameters. The laser welding parameters such as the laser power, laser beam spot size on the surface and feed rate greatly affect the resulting weld geometry. S235 grade steel has been used. The operating equipment was a Trumpf 4001 4 kW disk laser with a diameter of 100 microns optical fiber. The effect of different welding parameters were evaluated from the metallographic cross-section of the welded joints. This article describes the effect of the different laser beam focusing and the welding feed rate.

Fracture analysis and embrittlement phenomena of machined brass components

Fracture analysis and metallographic characterization is performed in selected machinable brass components, for use in hydraulic installations. Evidence of brittle fracture was mainly indicated by the presence of intergranular failure and limited plasticity areas. Multiple cracking was observed at the most highly stressed areas, such as the crests of the threads, the cross section transitions and thinnest wall locations. Fractographic analysis performed using Field Emission SEM, suggested the occurrence of embrittling phenomena, probably attributed to overheating and subsequent Pb-phase grain boundary segregation. Metallographic cross section investigation rather supported the failure hypothesis, implying that hot shortness is the plausible cause of ductility trough and brittle failure.

A Comprehensive Study of Intermetallic Compounds in Solar Cell Interconnections and their Growth Kinetics

Intermetallic compounds (IMC) in soldered interconnections influence the reliability of PV modules. Thus, the microstructure of solar cell interconnections and the growth of IMCs are to be investigated in this paper. Sn60Pb40 and Sn41Bi57Ag2 are chosen as alloy coatings of copper interconnectors and semi-automatically soldered to screen-printed front Ag-busbars of industrial mono-crystalline solar cells. The microstructure of the solder bonds is characterized with metallographic cross sections and confocal laser microscopy, as well as scanning electron microscopy and electron dispersive x-ray spectroscopy. The cross section samples are isothermally aged between 85°C to 150°C and for 15 hour to 155 hour to obtain the kinetic parameters of a diffusion-based growth model of the IMCs. The model is used to estimate the IMC thickness after 3000h at 85°C, and after 600 thermal cycles as well as after 25 years in the outdoor location Freiburg, Germany. It is found that extensive microstructural changes take place within the solder bonds during thermal aging. Grain coarsening within the solder matrix, in particular for Sn41Bi57Ag2 solders, is observed, which can lead to an entire Sn depletion of the solder matrix. Moreover, non-uniform Sn penetration and IMC growth at cavities and lead-glass particles of the busbar are observed for both solders, which is discussed in terms of its effect on metallization adhesion. Eventually, simulating the IMC growth for 3000h at 85°C forecasts a 3.7μm thick Ag3Sn IMC at the busbar for the Sn41Bi57Ag2 solder compared to 2.6μm for Sn60Pb40. The prognosis of the IMC thickness after 25 years in Freiburg yields an Ag3Sn thickness of 1.3μm for Sn41Bi57Ag2.

Investigations on the Wire Bonding Capability on Selective Laser Melted Structures

Abstract The paper displays the influencing factors, as well as the possibilities and challenges that come along with the process combination of selective laser melting (SLM) and heavy wire bonding. For the investigations, test samples were created from bronze powder on a SLM-machine. Then, 300 μm aluminum and copper wires were bonded on the SLM generated structures. Wire bonding capability is analyzed on untreated as well as on post-processed surfaces. The influence and effectiveness of various steps of post-processing such as cleaning, sandblasting and grinding are analyzed. Thus, interdependencies between both manufacturing process as well as the post-processing can be revealed. The effect of surface roughness and hardness of the assembly partner are investigated as well. To draw statistically backed conclusions, all tests are performed using DoE (Design of Experiment) studies. The primary characteristics besides the bond parameters that influence the wire bonding capability are focused in this paper. The process stability as well as the interconnection quality are evaluated by optical non-destructive laser microscopic analysis. Destructive pull and shear tests and metallographic cross sections are performed to evaluate the adhesion characteristics. The process stability and the yield obtained are important factors to describe the process and to evaluate the industrialization potential. By a profound understanding of all interdependencies between the two processes, a flexible manufacturing technology for power devices can be established.

Dealloying Mechanism of Bulk-AlCu Intermetallic Phases (Al2Cu, Al7Cu2Fe, Al2CuMg) : Effect of Crystallographic Structure and Electrolyte

The aluminum alloys with high mechanical properties, the 2xxx and 7xxx series, are very sensible to corrosion because of the presence of numerous coarse intermetallic phases in the aluminum matrix. These phases are often responsible of the formation of pits in corrosion conditions, or the formation of defects in the anticorrosion coatings such as anodized layers. The objective of this work is to study the behavior of different AlCu intermetallic phases (Al2Cu, Al7Cu2Fe, Al2CuMg) in the 7XXX and 2XXX alloys in NaCl electrolyte and H2SO4 electrolyte.Firstly, bulk-samples of phases (centimeter size) were synthesized by quick melting in an induction furnace followed by several annealing steps under argon. The electrochemical behavior of the three phases was characterized by stationary electrochemical measurements and electrochemical impedance spectroscopy in function of immersion time at open-circuit potential. The surface layers on each phase were observed on surface and metallographic cross-section by FEG-SEM, EDX spectroscopy and X-ray Photoelectron Spectroscopy.In NaCl electrolyte, all the studied intermetallic phases undergo a dealloying phenomenon which leads to the formation of a pure copper nanometer sponge. On the surface, the copper sponge can be partially embedded and covered in an aluminum hydroxide gel according to the hydrodynamic conditions. In sulfuric acid, the dealloying phenomenon depends on the copper content in the intermetallic phase and on its crystallographic structure. According to the density of the Cu plan, some phases can form a compact layer of copper that passivate the phase, some others form a nanometer sponge of copper. In conclusion, after the dissolution of aluminum, magnesium or iron atoms, copper atoms diffuse on the surface of phases to form either crystalline copper nanometer particles, or compact copper layer according to the electrolyte properties and their crystallographic structure.