Substrate Surface Finish Research Articles

Effects of surface finish of single crystal superalloy substrate on cyclic thermal oxidation of its nanocrystalline coating

Cycling oxidation behavior of the nanocrystalline coating on the N5 superalloy substrate of different surface finishing is investigated at 1050°C. Results indicate that the surface finishing of the alloy substrate affects oxidation behavior of the sputtered nanocrystalline coating. After cyclic oxidation, oxide scale on the nanocrystalline coating rumples heavily and spalls off following a way of layer-by-layer in case that the alloy substrate is pretreated by sand blasting. However, it just roughens rather than rumples when the alloy substrate is pre-polished or ground by 2000# SiC paper and its spallation follows a way of bulk fracture.

Scale deposit removal by means of ultrasonic cavitation

This paper presents a study of the mechanism of acoustic cavitation-generated erosion on metallic specimens coated with calcium carbonate (scale). Specimens were produced by scale deposition on steel substrates with different surface roughness, and then exposed to sonotrode-generated acoustic cavitation. The rate of scale removal was determined to be proportional to the pressure amplitude generated by the sonotrode. Descaling can also be intensified by reducing the substrate surface roughness. However, the scale destruction mechanism is largely dependent on the microstructure of the scale deposit, as local inhomogenities may intensify the erosion rate. Scale deposits can, depending on the hardness of its surface and substrate surface finish, either be eroded layer by layer, or by breakage of larger areas. Regardless of the exact descaling mechanism, the edge length-to-area ratio of descaled surfaces decreases with time, leading to a gradual reduction of descaling efficiency.

Investigation of Morphological and Mechanical Features of Polyurea

Polyurea coatings are very reactive and fast curing even at very low temperatures with exceptional mechanical properties, chemical resistance and durability. Polyurea spray coating technology is used to overcome the initial problems in surface coating such as substrate wetting,mixing and surface finish. The study deals with the analysis of morphology and tensile properties of polyurea coating. The polyurea sample is characterized by using SEM, FTIR and XRD in addition to EDAX to determine the microstructure and chemical composition. Finally Tensile Test was carried out to examine the ultimate tensile strength and young’s modulus of Polyurea using UTM.

Effect of Electromigration on the Type of Drop Failure of Sn–3.0Ag–0.5Cu Solder Joints in PBGA Packages

Board-level drop tests of plastic ball grid array (PBGA) packages were performed in accordance with the Joint Electron Devices Engineering Council standard to investigate the effect of electromigration (EM) on the drop reliability of Sn–3.0Ag–0.5Cu solder joints with two substrate surface finishes, organic solderability preservative (OSP) and electroless nickel electroless palladium immersion gold (ENEPIG). In the as-soldered state, drop failures occurred at the substrate sides only, with cracks propagating within the interfacial intermetallic compound (IMC) layer for OSP solder joints and along the IMC/Ni–P interface for ENEPIG solder joints. The drop lifetime of OSP solder joints was approximately twice that of ENEPIG joints. EM had an important effect on crack formation and drop lifetime of the PBGA solder joints. ENEPIG solder joints performed better in drop reliability tests after EM, that is, the drop lifetime of ENEPIG joints decreased by 43% whereas that of OSP solder joints decreased by 91%, compared with the as-soldered cases. The more serious polarity effect, i.e., excessive growth of the interfacial IMC at the anode, was responsible for the sharper decrease in drop lifetime. The different types of drop failure of PBGA solder joints before and after EM, including the position of initiation and the propagation path of cracks, are discussed on the basis of the growth behavior of interfacial IMC.

Contact damage resistance of TiN-coated hardmetals: Beneficial effects associated with substrate grinding

Contact loading is a common service condition for coated hardmetal tools and components. Substrate grinding represents a key step within the manufacturing chain of these coated systems. Within this context, the influence of surface integrity changes caused by abrasive grinding of the hardmetal substrate, prior to coating, is evaluated with respect to contact damage resistance. Three different substrate surface finish conditions are studied: ground (G), mirror-like polished (P) and ground plus heat-treated (GTT). Tests are conducted by means of spherical indentation under increasing monotonic load and the contact damage resistance is assessed. Substrate grinding enhances resistance against both crack nucleation at the coating surface and subsequent propagation into the hardmetal substrate. Hence, crack emergence and damage evolution is effectively delayed for the coated G condition, as compared to the reference P one. The observed system response is discussed on the basis of the beneficial effects associated with compressive residual stresses remnant at the subsurface level after grinding, ion-etching and coating. The influence of the stress state is further corroborated by the lower contact damage resistance exhibited by the coated GTT specimens. Finally, differences observed on the interaction between indentation-induced damage and failure mode under flexural testing points in the direction that substrate grinding also enhances damage tolerance of the coated system when exposed to contact loads.

Substrate surface finish effects on scratch resistance and failure mechanisms of TiN-coated hardmetals

In this study, the influence of substrate surface finish on scratch resistance and associated failure mechanisms is investigated in the case of a TiN-coated hardmetal. Three different surface finish conditions are studied: as-sintered (AS), ground (G), and mirror-like polished (P). For G conditioned samples, scratch tests are conducted both parallel and perpendicular to the direction of the grinding grooves. It is found that coated AS, G and P samples exhibit similar critical load for initial substrate exposure and the same brittle adhesive failure mode. However, the damage scenarios are different, i.e. the substrate exposure is discrete and localized to the scratch tracks for G samples while a more pronounced and continuous exposure is seen for AS and P ones. Aiming to understand the role played by the grinding-induced compressive residual stresses, the study is extended to coated systems where ground substrates are thermal annealed (for relieving stresses) before being ion etched and coated. It yielded lower critical loads and changes in the mechanisms for the scratch-related failure; the latter depending on the relative orientation between scratching and grinding directions.

Investigation of Wettability of Lead Free-Solder on Bare Copper and Organic Solder Preservatives Surface Finishes

Wettability plays an important role in the integrity of solder joints especially in critical safety operations. It helps in attaining successful soldering and reliable solder joints. The angle of wettability determines the degree of solder contact and strength of the solder joint. Moreover, the contact angle between the solid substrate and the molten solder determines the interconnection reliability. This paper focuses on the wettability of lead free solder on OSP and bare copper substrate surface finishes. The wetting behaviour of lead free solder pastes on bare copper board and Organic Solderability Preservatives (OSPs) substrates were investigated. The test vehicles consist of FR4 PCB single sided 100% copper clad with a thick film metallization and FR4 PCB single sided OSP. The substrate dimension was 100 mm×160 mm×1.6 mm. A total of 480 bumps of Lead-free solder pastes (96Sn-3.8Ag-0.5Cu and 96.5Sn-3.0Ag-0.7Cu alloy composition) were deposited for the investigation. The printing parameters and their values for the experiments include print gap of 0 mm, squeegee speed of 20 mm/s, pressure of 8kg and separation speed of 3 mm/sec. The pastes are screen printed on the substrates using 0.125mm thick stencil mounted on DEK 260 series printing machine. After the stencil printing process, the solder bumps deposited on the substrates were reflowed using a Novostar 2000HT horizontal convention reflow oven with 4-stage (preheat-soak-reflow-cooling). The FTA188 Analyser was employed for measuring the angle of contact between the substrates and the bumps. The results showed that the wetting contact angle of copper surface finish is lower as compared to the organic solderability preservatives surface finish and the wetting angle decreases as the contact surface area increases.

Effects of PCB Substrate Surface Finish, Flux, and Phosphorus Content on Ionic Contamination

The ionic contamination on printed circuit boards (PCB) having different surface finishes was examined using ionograph. The study was performed at the RT on three types of PCBs covered with: (i) hot air solder leveling (HASL LF), (ii) electroless nickel immersion gold (ENIG), and (iii) organic surface protectant (OSP), all on Cu substrates, as well as two types of fluxes, namely EF2202 and RF800. In the group of boards without soldered components, the lowest average value of contamination was for the ENIG 18 µm surface (0.01 μg NaCl/cm2). Boards with soldered components were more contaminated (from 0.29 μg NaCl/cm2 for the HASL LF 18 µm surface). After spraying boards with fluxing agents, the values of contaminants were the highest. The influence of phosphorus content in Ni-P layer of ENIG finish on ionic contamination was examined. In the group of PCBs with Au coating, the smallest amount of surface contaminants (0.32 μg NaCl/cm2) was for Ni-2-5%P layer. PCBs with Ni-11%P layer were higher contaminated (0.47 μg NaCl/cm2), and another with Ni-8%P layer had 0.81 μg NaCl/cm2. PCBs without Au coating, had the lowest contamination (0.48 μg NaCl/cm2) at phosphorous content equal 11%P. Higher contamination (0.67 μg NaCl/cm2) was at 2-5%P, up to 1.98 μg NaCl/cm2 for 8% of P. Boards with Au finish have lower value of contamination than identical boards without Au layer thus contributing to better reliability of electronic assemblies, since its failures due to current leakage and corrosion can be caused by contaminants.

Corrosion properties of steel protected by nanometre-thick oxide coatings

A comprehensive study of the corrosion properties of low alloy steel protected by 40–50nm aluminium and tantalum mixed oxide coatings grown by atomic layer deposition is reported. Electrochemical and surface analysis was performed to address the effect of substrate surface finish and whether an oxide mixture or nanolaminate was used. There was no dissolution or breakdown for nanolaminate alumina/tantala stacks in acidic NaCl solution. Localised corrosion (pitting) took place when defects exposing the substrate pre-existed in the coating. Substrate pre-treatment by brushing and H2–Ar plasma was instrumental to block or slow down pit initiation by reducing the defect dimensions.

WC/C Coating Protection Effects on 7075-T6 Fatigue Strength in an Aggressive Environment

Thin hard coatings are typically employed to improve surface material characteristics such as hardness, wear and contact fatigue resistance, and friction behavior. Their effect on the component fatigue strength may vary, depending on the substrate surface finishing and to the microstructural modification of the base material due to the Physical Vapor Deposition (PVD) process thermal loads, particularly critical for a 7075-T6 low ageing temperature (121°C) alloy. The present work investigates the effects of a low temperature (180°C) Tungsten Carbide/Carbon (WC/C) PVD coating, in terms of corrosion protection under fatigue loads. The effects of an aggressive environment over uncoated and coated 7075-T6 hourglass specimens were analyzed, by rotating bending (R = -1) step loading fatigue tests at 2·105 cycles. A chemically aggressive external condition has been investigated, considering a methanol environment. Results show a certain protective capability of the WC/C coating, which is however offset by the fatigue strength drop found for coated samples at 2·105 cycles.

Influence of Surface Finish on the Cyclic Oxidation Lifetime of an EB-PVD TBC, Deposited on PtAl and Pt-diffused Bondcoats

Thermal barrier coatings (TBCs) are well established as protective systems for gas turbine hot path components, due to their ability, with substrate cooling, to reduce the maximum surface temperature experienced by the metal component. However, when subject to high temperature oxidation, cyclic heating and cooling during service, TBCs degrade in both thermal protection capability and mechanical stability as a result of a combined thickening of the alumina-thermally grown oxide and sintering of the ceramic top coat. Eventually the ceramic top coat spalls from the metallic substrates. The detailed failure mechanisms for the TBC often are complicated, reflecting a balance between defects introduced into the TBC during manufacture and service and the stored energy generated in the TBC as a result of cyclic thermal exposure. It has been shown that the surface finish influences the residual stress in the thermally grown oxide and thus the stored energy. In this study, the influence of substrate surface finish, prior to bondcoat manufacture, on the cyclic oxidation lifetime is examined. Two EB-PVD TBC systems, a zirconia 8 wt% yttria topcoat on a platinum aluminide bondcoat and a zirconia 8 wt% yttria topcoat on a platinum diffused γ+γ′ bondcoat have been studied. For these two systems, various substrate surface finishes have been investigated, including ground, grit blasted and polished and grit blasted surfaces. The lifetime data for these cyclic oxidation tests of EB-PVD TBCs on these two diffusion bondcoats, platinum aluminide and platinum diffused, on CMSX4, have been analysed statistically for the various surface finishes. It is shown that the variability in measured lifetime can be modelled using Weibull statistics. The role of surface finish on the Weibull model parameters, characteristic life (η) and Weibull modulus (β), are discussed in this paper and hence the role surface finish plays on the likelihood of early, short life, TBC failure. Based on this analysis a more optimised surface finish is recommended to extend TBC lifetimes with diffusion based bondcoats. Further, the platinum diffusion bondcoat is shown to outperform the platinum aluminide system once the substrate surface finish has been optimised.

Effects of PCB Substrate Surface Finish and Flux on Solderability of Lead-Free SAC305 Alloy

The solderability of the SAC305 alloy in contact with printed circuit boards (PCB) having different surface finishes was examined using the wetting balance method. The study was performed at a temperature of 260 °C on three types of PCBs covered with (1) hot air solder leveling (HASL LF), (2) electroless nickel immersion gold (ENIG), and (3) organic surface protectant (OSP), organic finish, all on Cu substrates and two types of fluxes (EF2202 and RF800). The results showed that the PCB substrate surface finish has a strong effect on the value of both the wetting time t 0 and the contact angle θ. The shortest wetting time was noted for the OSP finish (t 0 = 0.6 s with EF2202 flux and t 0 = 0.98 s with RF800 flux), while the ENIG finish showed the longest wetting time (t 0 = 1.36 s with EF2202 flux and t 0 = 1.55 s with RF800 flux). The θ values calculated from the wetting balance tests were as follows: the lowest θ of 45° was formed on HASL LF (EF2202 flux), the highest θ of 63° was noted on the OSP finish, while on the ENIG finish, it was 58° (EF2202 flux). After the solderability tests, the interface characterization of cross-sectional samples was performed by means of scanning electron microscopy coupled with energy dispersive spectroscopy.

Effects of Multiple Reworks on the Accelerated Thermal Cycling and Shock Performance of Lead-Free BGA Assemblies

In this paper, the effect of multiple rework cycles (up to 5×) on reliability of lead-free assemblies is investigated. The test vehicle is designed to capture the reliability impact of rework processes on interconnects of ball grid array (BGA) devices, the solder joints of its adjacent components, and BGA devices of clamshelled devices. The effects on high aspect ratio plated through hole (PTH) vias and microvias are also considered. A variety of BGA packages, such as flip-chip BGA, plastic BGA, and chip array BGA are selected to represent different package technologies and stress levels. The effect of multiple reworks on the behavior of lead-free solder joints during accelerated thermal cycling (ATC) and mechanical shock is extensively explored in this paper. The experimental and analytical findings showed that multiple reworks significantly degraded the ATC performance of the reworked assemblies by up to 50% in terms of characteristic life. It is also found that multiple reworks affect the PTH vias significantly. The combination of increased lead-free rework temperature excursions and high-end printed circuit board (PCB) aspect ratios may incur higher stresses in the PTH copper barrel. Early failures are observed post multiple reworks and the subsequent ATC test due to PTH barrel cracking. However, the effects of multiple reworks are less severe for devices with microvias compared to those with PTH vias. Multiple reworks also cause severe degradation of long-term performance of devices with clamshell designs. With proper control in rework process, multiple reworks have minimum effects on the adjacent BGA solder joints. In addition, failure analysis is performed in order to study the impact of multiple rework on the microstructure of SnAgCu solder joints. It is found that the intermetallic compounds (IMC) layer grows thicker as the number of reworks increases; however, the IMC growth rate and morphology are dependent on the surface finish of the package substrate. The failure mode and failure mechanism after ATC are also compared for different number of rework cycles. The implications of these results for the reliability of lead-free solder joints are discussed in this paper.

Electromigration Failure Mechanism in Sn-Cu Solder Alloys with OSP Cu Surface Finish

Organic solderable preservative (OSP) has been adopted as the Cu substrate surface finish in flip-chip solder joints for many years. In this study, the electromigration behavior of lead-free Sn-Cu solder alloys with thin-film under bump metallization and OSP surface finish was investigated. The results showed that severe damage occurred on the substrate side (cathode side), whereas the damage on the chip side (cathode side) was not severe. The damage on the substrate side included void formation, copper dissolution, and formation of intermetallic compounds (IMCs). The OSP Cu interface on the substrate side became the weakest point in the solder joint even when thin-film metallization was used on the chip side. Three-dimensional simulations were employed to investigate the current density distribution in the area between the OSP Cu surface finish and the solder. The results indicated that the current density was higher along the periphery of the bonding area between the solder and the Cu pad, consistent with the area of IMC and void formation in our experimental results.

Oxide scale formation and morphology on austenitic stainless steel at medium annealing temperatures

This paper considers the effect that surface finish has on early oxide growth. Austenitic stainless steel of grades 304 and 316L, with ground (240 grit) and polished (1 µm) surface finishes were subjected to temperatures of 750°C and 800°C for times of up to 24 h. It was found that the oxide morphology was heavily dependent on surface finish and substrate microstructure. The ground surfaces were found to initially produce a homogenous oxide scale, while the polished samples formed a scale made up of many oxide islands. The formation of the oxide islands was due to the fast diffusion of chromium through grain boundaries and prior ferrite regions. These fast diffusion paths provided localised oxidation resistance at the surface which caused breaks in the oxide scale. With the ground surface finish, these fast diffusion paths become negated as the grinding of the surfaces creates an even distribution of fast diffusion paths. Mechanisms of early oxide growth in relation to the substrate microstructure and surface finish have been summarised in a schematic diagram.

Effect of Substrate Surface Finish and Proximity Distance on Compositional Shift of InGaAsP Metal–Organic Vapor Phase Epitaxy in a High-Speed-Rotation Multiple-Substrate Reactor

Shift of photoluminescence (PL) wavelengths toward shorter wavelengths (blue shift) in a specific area of InGaAsP quaternary epitaxial layers, grown in a high-speed rotating-disk type metal–organic vapor phase epitaxy (MOVPE) reactor, was found to be a deteriorative factor of PL wavelength uniformity in the epitaxial layers. Experiments were performed to generate a blue shift of PL wavelength, with two mirror-polished substrates placed in adjacent positions in the wafer carrier. Analysis of compositional distribution over 61 points was performed in InGaAsP/InP epitaxial layers with band gap corresponding to the 1310 nm PL wavelength. From this analysis, it was found that the blue shift of PL wavelength was dependent on the compositional shift of group V elements. In addition, the effect of substrate surface finish (mirror-polished or as-etched) was found to be a major cause of the compositional shift of group V elements. We found that, as the proximity distance between adjacent substrates was increased, there existed a critical proximity distance at which the blue shift disappeared.

Effect of Substrate Surface Finish and Proximity Distance on Compositional Shift of InGaAsP Metal–Organic Vapor Phase Epitaxy in a High-Speed-Rotation Multiple-Substrate Reactor

Shift of photoluminescence (PL) wavelengths toward shorter wavelengths (blue shift) in a specific area of InGaAsP quaternary epitaxial layers, grown in a high-speed rotating-disk type metal–organic vapor phase epitaxy (MOVPE) reactor, was found to be a deteriorative factor of PL wavelength uniformity in the epitaxial layers. Experiments were performed to generate a blue shift of PL wavelength, with two mirror-polished substrates placed in adjacent positions in the wafer carrier. Analysis of compositional distribution over 61 points was performed in InGaAsP/InP epitaxial layers with band gap corresponding to the 1310 nm PL wavelength. From this analysis, it was found that the blue shift of PL wavelength was dependent on the compositional shift of group V elements. In addition, the effect of substrate surface finish (mirror-polished or as-etched) was found to be a major cause of the compositional shift of group V elements. We found that, as the proximity distance between adjacent substrates was increased, there existed a critical proximity distance at which the blue shift disappeared.

Oxide scale formation and morphology on austenitic stainless steel at medium annealing temperatures

This paper considers the effect that surface finish has on early oxide growth. Austenitic stainless steel of grades 304 and 316L, with ground (240 grit) and polished (1 µm) surface finishes were subjected to temperatures of 750°C and 800°C for times of up to 24 h. It was found that the oxide morphology was heavily dependent on surface finish and substrate microstructure. The ground surfaces were found to initially produce a homogenous oxide scale, while the polished samples formed a scale made up of many oxide islands. The formation of the oxide islands was due to the fast diffusion of chromium through grain boundaries and prior ferrite regions. These fast diffusion paths provided localised oxidation resistance at the surface which caused breaks in the oxide scale. With the ground surface finish, these fast diffusion paths become negated as the grinding of the surfaces creates an even distribution of fast diffusion paths. Mechanisms of early oxide growth in relation to the substrate microstructure and surface finish have been summarised in a schematic diagram.

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Surface Finish Effects

The interaction between isothermal aging and the long-term reliability of fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects was investigated. In this study, 0.4-mm fine-pitch packages with 300-μm-diameter Sn-Ag-Cu solder balls were used. Two different package substrate surface finishes were selected to compare their effects on the final solder composition, especially the effect of Ni, during thermal cycling. To study the impact on thermal performance and long-term reliability, samples were isothermally aged and thermally cycled from 0°C to 100°C with 10 min dwell time. Based on Weibull plots for each aging condition, package lifetime was reduced by approximately 44% by aging at 150°C. Aging at 100°C showed a smaller impact but similar trend. The microstructure evolution was observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and organic solderability preservative (OSP) surface finishes, focusing on the microstructure evolution near the package-side interface. Different mechanisms after aging at various conditions were observed, and their impacts on the fatigue lifetime of solder joints are discussed.

Impact of isothermal aging on fine pitch BGA packages with Sn-Ag-Cu solder interconnects

The interaction between isothermal aging and long-term reliability of fine pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt%) solder ball interconnects are investigated. In this study, 0.4mm fine pitch packages with 0.3mm diameter Sn-Ag-Cu solder balls are used. Two different die sizes and two different package substrate surface finishes are selected to compare the internal strain impact and alloy effect, especially the Ni effect during thermal cycling. To see the thermal impact on the thermal performance and long-term reliability, the samples are isothermally aged and thermal cycled from 0 to 100°C with a 10minute dwell time. Based on weibull plots for each aging condition, the lifetime of the package reduced approximately 44% with 150°C aging precondition. The microstructure evolution is observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and OSP surface finishes, focusing on the microstructure evolution near the package side interface. Different mechanisms after aging at various conditions are observed, and their impacts on the fatigue life of solder joints are discussed.