Reflow Time Research Articles

Justification of strengthening of working bodies of forestry machines with self-fluxing alloys during gas-flame spraying

The article deals with the possibility of strengthening the working bodies of tillage machines with self-fluxing alloys by the method of gas-flame sputtering with subsequent melting of the coating. As a result of the research, the analysis of the physical and mechanical properties of the coating made of a self-fluxing alloy and the changes in these properties depending on the technological modes of spraying was carried out. It is established that the structure of self-fluxing alloys depends mainly on the modes of heat treatment(time and temperature of reflow). Metallographic analysis shows, that the self-fluxing alloy PR-N70H17C4R4 forms a pore-free multiphase structure after melting. The presence of components in the structure (borides and carbides) with a hardness of about 11 GPa allows the coating to have significant abrasive wear resistance.

Shear strength and fracture analysis of Sn-9Zn-2.5Bi-1.5In and Sn-3.0Ag-0.5Cu pastes with Cu-substrate joints under different reflow times

Sn-Zn solders have been considered as promising lead-free solders. This study investigated the shear strength and fracture mechanism of Sn-9Zn-2.5Bi-1.5In and Sn-3.0Ag-0.5Cu pastes with Cu-substrate joints. It is found that the intermetallic compounds (IMC) in Sn-Ag-Cu joints are Cu6Sn5 and Cu3Sn, while the IMC of Sn-Zn joints are particular Cu5Zn8, different from other Sn-based solders, which grow more slowly. Both solder joints fracture by a mixed mechanism. The cracks tend to form and propagate at the interface of the solder and IMC in the cases of too short or too long reflow times, resulting in more interfacial fracture and lower shear strength. An optimum soldering zone of reflow time will bring about more reliable joints, which fracture mainly in the solder. Sn-Zn joints have more solder fracture and less interface fracture, therefore have higher shear strength and reliability, compared with Sn-Ag-Cu joints. For Sn-Zn joints, the optimum reflow times are 20 s–120 s with the shear strength of 62 MPa–70 MPa, while for Sn-Ag-Cu joints, the optimum reflow times are 50 s–180 s, with the strength of 50 MPa–62 MPa.

Effect of microwave operating power and reflow time on the microstructure and tensile properties of Sn–3.0Ag–0.5Cu/Cu solder joints

PurposeThis paper aims to investigate the morphology and tensile properties of SAC305 solder alloy under the influence of microwave hybrid heating (MHH) for soldering at different microwave parameters.Design/methodology/approachSi wafer was used as susceptor in MHH for solder reflow. Microwave operating power for medium and high ranging from 40 to 140 s reflow time was used to investigate their effect on the microstructure and strength of SAC305/Cu solder joints. The morphology and elemental composition of the intermetallic compound (IMC) joint were evaluated on the top surface and cross-sectional view.FindingsIMC formation transformed from scallop-like to elongated scallop-like structure for medium operating power and scallop-like to planar-like structure for high operating power when exposed to longer reflow time. Compositional and phase analysis confirmed that the observed IMCs consist of Cu6Sn5, Cu3Sn and Ag3Sn. A thinner IMC layer was formed at medium operating power, 80 s (2.4 µm), and high operating power, 40 s (2.5 µm). The ultimate tensile strength at high operating power, 40 s (45.5 MPa), was 44.9% greater than that at medium operating power, 80 s (31.4 MPa).Originality/valueMicrowave parameters with the influence of Si wafer in MHH in soldering have been developed and optimized. A microwave temperature profile was established to select the appropriate parameter for solder reflow. For this MHH soldering method, the higher operating power and shorter reflow time are preferable.

Effect of Intermetallic Compound Bridging on the Cracking Resistance of Sn2.3Ag Microbumps with Different UBM Structures under Thermal Cycling

In this study, the effect of intermetallic compound (IMC) bridging on the cracking resistance of microbumps with two different under bump metallization (UBM) systems, Cu/solder/Cu and Cu/solder/Ni, under a thermal cycling test (TCT) is investigated. The height of the Sn2.3Ag solders was ~10 µm, which resembles that of the most commonly used microbumps. We adjusted the reflow time to control the IMC bridging level. The samples with different bridging levels were tested under a TCT (−55–125 °C). After 1000 and 2000 TCT cycles (30 min/cycle), the samples were then polished and characterized using a scanning electron microscope (SEM). Before IMC bridging, various cracks in both systems were observed at the IMC/solder interfaces after the 1000-cycle tests. The cracks propagated as cyclic shapes from the sides to the center and became more severe as the thermal cycle was increased. With IMC bridging, we could not observe any further failure in all the samples even when the thermal cycle was up to 2000. We discovered that IMC bridging effectively suppressed crack formation in microbumps under TCTs.

Facile preparation of Cu foam/Sn composite preforms for low-temperature interconnection of high-power devices

A simple and low-cost Cu foam/Sn composite preform was proposed for the low-temperature interconnection of high-power devices. The composite preform was prepared by pressing a Cu foam as the skeleton between two Sn foils as low-melting point metals with different pressures. The composite preform retains the high re-melting temperature superiority of transient liquid phase (TLP) bonding, as well as shortens the reflow time of intermetallic compounds (IMCs). The microstructures of the bondlines and the electrical and mechanical properties of the composite preform pressed at different pressures were studied. After sintering at 260 °C, the interconnection layer becomes denser with the increasing pressures of composite preform, and the bondline is composed of mainly Cu6Sn5, Cu3Sn and Ag3Sn phase. When the pressure of the composite preform increases from 200 to 400 MPa, the electrical resistivity decreases and the shear strength of bonded joints increases. At the pressure of 400 MPa, the electrical resistivity and the shear strength are 8.83 μΩ cm and 36.4 MPa, respectively, which are far better than the traditional Sn performs. Furthermore, two fracture failure models were obtained to analyze the breaking mechanism of the bonding joints under different sintering temperatures.

Strategies for inducing heredity of β-Sn texture in micro solder joints during multi-reflow process

The strategies for inducing heredity of β-Sn texture and thus controlling β-Sn orientation in micro solder joints during multi-reflow process were investigated. The temperature gradient (TG) induced gene clusters that carried the information similar to the solid-state crystals and conical cavities that provided preferred nucleation sites at one side of the joints were identified as the keys for generating heredity effect. There was non-heredity effect in Cu/Sn/Cu joints whether the joints were reflowed under TG or not for the first time due to the absence of gene clusters and in Cu/Sn-3.0Ag-0.5Cu(SAC305)/Cu joints during isothermal multi-reflow due to the absence of deep and narrow conical cavities. However, the Cu/SAC305/Cu joints that had orientated β-Sn grains after the first reflow under TG exhibited remarkable heredity effect during the subsequent second and third reflow. The critical peak reflow temperature was confirmed to be 255 °C over which the heredity effect disappeared. A general guidance of the strategies for inducing heredity effect in Cu/SAC305/Cu joints during multi-reflow process was demonstrated focusing on reflow temperature, times, duration, cooling rate and TG. The novel reflow method is believed to be beneficial to improve the reliability and anisotropy of micro solder joints containing few β-Sn grains.

Temperature-induced effect and control of Cu3Sn on the rotation of orientation-preferred Cu6Sn5 crystals at Sn-3.0Ag/(001)Cu interface

Crystal rotation of the orientation-preferred Cu6Sn5 formed on (001)Cu usually happens in soldering, and its rotation rate depends on the temperature closely. In this work, the evolutions of intermetallic orientation, phase and thickness at Sn-3.0Ag/(001)Cu interface were systematically investigated under temperatures of 230 °C, 250 °C and 300 °C. Results illustrate that the nucleation of orientation-preferred Cu6Sn5 on (001)Cu can be realized at any temperature above the solder melting point, but subsequently crystal rotation from <112¯0> to 〈0001〉 in ND happens as well. High temperature was found to greatly maintain the initial orientation of Cu6Sn5 crystals as well as facilitate the formation and growth of Cu3Sn, which can effectively reduce the driving force of Cu6Sn5 rotation and consequently inhibiting the rotation of Cu6Sn5 crystals. Finally, a theoretical model of Cu6Sn5 crystal rotation based on interfacial thermodynamic calculation was established and a two time reflow profile of 300 °C–230 °C was designed to furthest inhibit the rotation of Cu6Sn5 crystals and maintain the preferred orientation of Cu6Sn5 texture according to our proposed theory. The results are of great significance for the intermetallic orientation control in soldering.

Analysis of Defects Generated by the Reflow Soldering in SMT (Surface Mount Technology) Assembly Applying the Six Sigma Method

The Six Sigma method using the DMAIC methodology is being applied for analyzing the reflow soldering process in an SMT assembly line. The Define phase (D) and Measure phase (M) were concluded, the Analysis (A) phase is being implemented, and the Improve (I) and Control (C) phases will be the next ones. Defects generated during the reflow process were classified and measured both on assembled memory modules and on virgin laminates that were passed through the oven during the reflow of these modules. Spots of solder and flux were found on the edge connector of the modules and also on the surface of the virgin laminates. It was found that these defects are generated inside the reflow oven, indicating that the oven is contaminated. Two solder pastes were analyzed and consequently, two temperature profiles were used. The amount of defects generated by the oven was found to be independent on the temperature profile. On the other hand the amount of defects depends on the solder paste that is used. The FMEA (Failure Mode and Effect Analysis) was also accomplished. As a result, the main failure modes of the reflow process were determined, namely, the heating rate, the soak temperature, the conveyor velocity, the reflow temperature, the reflow time, and the cooling rate.

Failure Mechanism of the SnAgCu/SnPb Mixed Soldering Process in a Ball Grid Array Structure

In this study, the influence of different reflow profiles and dosages of SnPb solder paste on the reliability of the SnAgCu/SnPb mixed assembly process was investigated. The interfacial microstructures were carefully examined through scanning electron microscopy, and two failures modes were found. Firstly, when reflowed at 245°C for 95 s, Pb-rich phases were buried within an intermetallic compound (IMC) layer and led to micro-voids and cracks under thermal stress or mechanical stress. However, upon increasing the reflow temperature or time further led to melting the solder ball totally, and the Pb element dissolved into a liquid phase and precipitated into uniform and small granules during the cooling process. Further, among the samples with different dosages of SnPb solder paste, severe shrinkage occurred at the solder joints with the addition of 12 wt.% eutectic SnPb component. Electron back-scattered diffraction analysis was conducted in order to determine the mechanism of different shrinkages. The solder ball consisted of several primary Sn grains, and the shrinkage and Pb-rich phase are preferred to appear at the grain boundary. When the solder ball consisted of a single grain, no preferred orientation of the shrinkage and Pb-rich phase were detected. The shrinkage appeared at the final solidification region, which is near the IMC layer. Moreover, the shrinkages connect to each other, and a continuous crack formed on the chip side, leading to the failure.

Impact of multiple reflow on intermetallic compound of nickel‐doped tin‐silver‐copper on ENImAg substrate

AbstractThe growth of intermetallic compounds (IMCs) of the tin‐silver‐copper‐nickel/electroless nickel immersion silver (SACN/ENImAg) at the 230 °C during multiple reflows was investigated. The characterization analysis of the samples was conducted using scanning electron microscopy and assisted by a dispersive x‐ray analysis. A layer of continuous (Cu, Ni)6Sn5 and discontinuous (Ni, Cu)3Sn4 intermetallic were formed at the interface after up to the third reflow, while Ag3Sn nano‐sized was formed on the intermetallic surface. The total thickness of the intermetallic layer increased as the reflow increased. The findings also revealed that the intermetallic thickness became thicker with the mass percentage of nickel increased. Hence, the intermetallic growth was controlled mainly by reflow times and nickel content in solders.

The Effect of Reflow Temperature on Time at the End of Gravity Zone (Tgz) of Sn-3.8Ag-0.7Cu Solder Alloy

Abstract The reflow time for solder until the end of the gravity zone (Tgz) is considered to be the optimum reflow time for obtaining high mechanical performance from lead-free solders. In the present work, the effect of reflow time and temperature on Tgz of Sn-3.8Ag-0.7Cu (SAC387) lead-free solder alloy reflowed on the copper substrate has been investigated. The evolution of interfacial microstructure and solder bond shear strength under different reflow temperatures and time was assessed. Solder balls weighing 0.08 ± 0.01 g were reflowed at 260°C, 280°C, and 300°C for reflow times of 30 s, 60 s, 120 s, and 240 s. Times at the end of the gravity zone for SAC387 solder were obtained as 110 ± 5 s, 55 ± 5 s, and 23 ± 3 s for reflow temperatures of 260°C, 280°C and 300°C, respectively. The contact angle for SAC387 solder on the copper substrate at Tgz was found to be 25.5° ± 0.2° for all reflow temperatures. Scanning electron microscopy revealed the formation of a Cu6Sn5 intermetallic compound (IMC) layer at the interface. The IMC layer thickness increased with increase in reflow temperature and time. Maximum solder joint strength was obtained at Tgz reflow times for all reflow temperatures. Microstructures of samples reflowed beyond the gravity zone showed secondary Cu6Sn5 precipitation in the solder bulk. The present investigation reveals a reduction in Tgz reflow time for SAC387 lead-free solder at higher operating reflow temperatures.

Effects of multiple reflows on microstructure and shear strength of (Au-20Sn)-2Ag/Au/Ni(P)/Kovar joints

The multiple reflow reliability of Au-20 wt%Sn solder has attracted great interests in optoelectronic and high power electronic industries. But the growing dendrite in the solder has a negative effect on the reliability of the solder joint. In this study, to control the microstructure and improve the reliability of the Au-20Sn electronic-packaging solders, we successfully designed and prepared a (Au-20 wt%Sn)-2wt.%Ag solder using micro-alloying method. It was found that the addition of 2 wt%Ag can change the heterogeneous nucleation conditions and refine the grains of primary Au5Sn phase, forming globular-shaped (Au,Ag)5Sn phase and eutectic (AuSn + (Au,Ag)5Sn) lamellar structure. The (Au-20Sn)-2Ag solder showed a strong reflow reliability even after 200 times of reflow (the shear strength of (Au-20Sn)-2Ag/Au/Ni(P)/Kovar joint maintained 88.33% of that measured after one reflow, and the shear strength of Au-20Sn/Au/Ni(P)/Kovar joint only maintained 63.37% of that measured after one reflow). The interface of (Au-20Sn)-2Ag/substrate joint after 200 times of reflow consisted of less accumulated (Ni,Au)3Sn2 grains and thick (Au,Ni)5Sn phase, so the refined microstructure can reduce the stress concentration and thus hinder the crack initiation and propagation, thereby significantly improving the reliability of the joint.

Dramatic morphological reservation of prism-type Cu6Sn5 formed on single crystal Cu substrates under temperature gradient

The morphology evolution of Cu6Sn5 grains in (001)Cu/Sn/Cu and (011)Cu/Sn/Cu micro solder joints reflowing with and without a temperature gradient was studied. In the as-soldered state, regular and perpendicular prism-type Cu6Sn5 grains formed on (001)Cu substrate, while a mixture of stubby prism-type and scallop-type Cu6Sn5 grains formed on (011)Cu substrate. The prism-type morphology gradually transferred into scallop-type after isothermally reflowed at 270 °C for 10 min. However, with a temperature gradient, the initial perpendicular small prismatic Cu6Sn5 grains on (001)Cu substrate and mixed-type Cu6Sn5 grains on (011)Cu continued to grow into large prism-type. All the prism-type Cu6Sn5 grains grew along same direction after long time reflow under temperature gradient due to the merging of adjacent Cu6Sn5 grain pair.

Design and solderability characterization of novel Au–30Ga solder for high-temperature packaging

To meet the needs of high-temperature packaging for high-power devices, a novel Au–30Ga solder was designed and prepared, and its solderability was characterized in this work. The results showed that it was a eutectic alloy with a melting point of 450 ℃, consisting of AuGa phase and AuGa2 phase. The novel Au–30Ga solder possessed high thermal conductivity, excellent wettability and higher working temperature than traditional Au-based solders. The shear strength of Au–30Ga/Ni joint can reach 77.1 MPa, which was much higher than that of Au–20Sn/Ni and Au–12Ge/Ni joints. The addition of a trace amount of Ni helped to enhance the shear strength of the Au–30Ga/Ni joint, but had no significant effect on the microstructure of the Au–30Ga/Ni joint. Two Ni–Ga intermetallic compound layers were formed at the Au–30Ga/Ni interface, which thickened with an increase in soldering time. In addition, a new Au7Ga3 phase would be observed in the seam when the reflow time was more than 5 min.

Interfacial reactions between different Sn-based lead- free solder alloys and CuNi substrates

The effect of the presence of Ni in Cu substrates on the interface reaction during soldering and the growth of Sn-Cu intermetallic compounds (IMCs) was studied. The study involved reflow soldering of Sn-0.7Cu, Sn- 3Ag-0.5Cu and Sn-0.7Cu-0.05Ni lead-free solders on pure Cu and Cu-6Ni substrates with holding times of 5, 10 and 15 minutes at 232 °C. The presence of 6 wt% Ni in the Cu substrate strongly affects the microstructural evolution and the growth of the IMCs. The presence of Ni in the substrate significantly increased the overall thickness of the (Cu,Ni)6Sn5 layer while suppressing the formation of the Cu3Sn layer. The thickness of the (Cu,Ni)6Sn5 layer increased with reflow time for both Cu and Cu-6Ni substrate.

A diffusion model and growth kinetics of interfacial intermetallic compounds in Sn-0.3Ag-0.7Cu and Sn-0.3Ag-0.7Cu-0.5CeO2 solder joints

The growth kinetics of the intermetallic compounds (IMCs) between the low-silver composite solders (Sn–0.3Ag–0.7Cu-xCeO2) and copper substrates during reflow soldering process has been studied in this work. The thickness and grain size of the interfacial IMCs increase with an increase of reflow time. The addition of CeO2 nanoparticles can effectively suppress the growth of the interfacial IMCs. The interfacial reaction between composite solders and copper substrates is a dynamic response to the comprehensive effects of IMC growth at the interface, diffusion rate of copper atoms, and changes in composition at the solder/copper interface. A diffusion-controlled kinetics model is put forward to investigate the comprehensive effects. The modeling results show that, the diffusion rate of copper atoms at the solder/copper interface is related to the interfacial IMC grain size, IMC thickness and reaction time, and the addition of CeO2 nanoparticles can lead to a relatively lower diffusion rate of Cu atoms at the solder/copper interface. The change characteristic of the copper atom diffusion introduced in this model is supported by a good agreement between experimental and theoretical results. The mechanism of CeO2 nano-additive on the diffusion behavior of Cu atoms at the interface and interfacial IMC growth is explained by using the theory of adsorption and heterogeneous nucleation.

Effects of crystalline and amorphous Pd layers on initial interfacial reactions at Sn-3.0Ag-0.5Cu/thin-Au/Pd/Ni(P) solder joints

Herein, we investigated the interfacial reactions of crystalline Pd (pure Pd) and amorphous Pd (Pd(P)) layers in thin electroless-Ni electroless-Pd immersion-Au (thin-ENEPIG) surface-finished printed circuit board with Sn-3.0Ag-0.5Cu (SAC305) solder joints reflowed at 260 °C for 10–180 s. After 20 s reflow, a thick (Pd,Au)Sn4 IMC was formed at the interface of the pure Pd joint, while thick AuSn4 and thin (Pd,Au)Sn4 IMCs were formed at the interface of the Pd(P) joint. After 30 s reflow, needle-type (Cu,Ni)6Sn5 IMC was mainly formed at the top-side interface of the pure Pd joint, while scallop-type (Cu,Ni)6Sn5 IMC was formed at the top-side interface of Pd(P) joints. Finally, the top-side (Cu,Ni)6Sn5 IMC of the pure Pd and the top- and bottom-side (Cu,Ni)6Sn5 IMCs of the Pd(P) joints coarsened with increasing reflow time up to 180 s. In the results of top-view SEM micrographs, the (Cu,Ni)6Sn5 IMCs on the Pd(P) joint were larger than those on the pure Pd joint for reflow times of 30–180 s. Therefore, P in the Pd layer was concluded to significantly affect the interfacial reactions and IMC morphology of the SAC 305 solder with ENEPIG joints during reflow reactions.

Improvement of Adhesion Strength of the Sprayed Material

The effective way of enhancing resistance to deterioration of the parts is the change of surface properties due to coating with the required properties. Since the deposition process takes place at an elevated temperature, it is necessary to ensure the absence of phase and structural transformations in the metal, mixing of materials, changes in the chemical composition of the coating. The use of plasma spraying at the transport management facility showed a sufficiently high quality of the coating. However, the metallographic analysis showed the presence of discontinuities at the interface in the form of pores, oxide film and sections of decarburized steel. In the case of the process of melting–spraying, the overheating areas of the base metal are formed, which leads to disadvantages similar to the deposition process. To ensure the minimal thermal impact on the part, the thermal field calculations are required. For this purpose, the time of local melting of the part by a single-phase plasma jet was calculated. The temperature field of a part in which heat is transferred by thermal conductivity, the solution of the differential equation of thermal conductivity is described. The boundary conditions of three genera are formulated. The first kind is the distribution of temperature on the enclosing surfaces. The second kind – the task on the surface of the part of the heat flux density. The third kind – setting the temperature of the medium, which acts as a plasma jet, washing the surface of the part, and the intensity of heat exchange. The mathematical model of melting of the product is constructed. The formula for calculating the reflow time is derived. The temperature distribution expressions for the molten and solid zones are obtained by the solution of the problem with moving boundaries by the Fourier’s method. On the basis of theoretical calculations it is proposed to increase the strength of the sprayed layer with the base to apply the process of plasma spraying with preliminary local melting by periodic exposure to a single-phase plasma jet.

Fabrication of a Three-Layer PDMS Pneumatic Microfluidic Chip for Micro Liquid Sample Operation.

The emphasis of this paper lies in the fabrication of a three-layer polydimethylsiloxane chip for micro liquid sample operation. In this paper, the microchannels with a rectangular control layer cross section are fabricated based on a dry-film negative photoresist mold, while the microchannels with a rounded liquid layer cross section are fabricated by a positive photoresist reflow mold. The relationships between temperature and the time of reflow and the arc level of the liquid layer mold are discussed. Different ratios, curing temperatures, and curing times are used to fabricate the two PDMS layers to improve their toughness and plasticity separately. The PDMS slabs with microstructure networks are treated with oxygen plasma to improve their surface properties. The improved surface properties serve to reduce the temperature and time, and improve the sealing strength, which is as effective as adding PDMS in varying ratios. The micro liquid sample operation experiments show that high levels of pinching off and mixing performances on pneumatic microfluidic chips are obtained more easily.

Effect of polycrystalline Cu microstructures on IMC growth behavior at Sn/Cu soldering interface

This paper investigated the morphologies of intermetallic compounds (IMCs) forming between pure Sn solders and different polycrystalline copper substrates with microstructures of fiber structure, equiaxed crystal and columnar crystal respectively as well as the effect of Cu microstructures on IMC growth kinetics. The results showed that when multiple IMC grains could be accommodated in a single grain area of substrates, the IMC growth behavior on some grains of polycrystalline substrates was similar to that on single crystalline substrate of same orientation. The difference on polycrystalline Cu substrates was that IMCs with different morphologies appeared alternately. Furthermore, the IMC distribution at soldering interface was stripy on fiber structure and blocky on equiaxed crystal substrates. With increasing of reflow temperature or time, IMCs with different morphologies followed their own growth behavior. In addition, the grain growth rate of IMCs on columnar crystal substrate was higher than that on the other two substrates due to different Cu supply and diffusion abilities.