IMC Layer Research Articles

The Effect of Thermal Annealing on the Microstructure and Mechanical Properties of Sn-0.7Cu-xZn Solder Joint

The microstructural properties of a Pb-free solder joint significantly affect its mechanical behaviours. This paper details a systematic study of the effect of the annealing process on the microstructure and shear strength of a Zn-added Sn-0.7Cu solder joint. The results indicated that the IMC layer’s thickness at the solder/Cu interface increases with annealing time. The interfacial IMC layer of the Sn-0.7Cu solder joint gradually thickened with increasing annealing time and annealing temperature, while the interfacial IMC layer’s morphology changed from scallop-type to layer-type after the annealing process. However, the addition of 1.0 wt.% and 1.5 wt.% Zn in the Sn-0.7Cu effectively altered the interfacial IMC phase to Cu-Zn and suppressed the growth of Cu3Sn during the annealing process. The single-lap shear tests results confirmed that the addition of Zn decreased the shear strength of Sn-0.7Cu. The interfacial IMC of the Cu6Sn5 phase in Sn-0.7Cu changed to Cu-Zn due to the addition of Zn. The shear fractures in the annealed solder joint were ductile within the bulk solder instead of the interfacial IMC layer. Increased annealing time resulted in the increased presence of the Cu-Zn phase, which decreased the hardness and shear strength of the Sn-0.7Cu solder joint.

Influence of doping Ti particles on intermetallic compounds growth at Sn58Bi/Cu interface during solid–liquid diffusion

In this paper, 0.1 wt% Ti particles (2–5 μm) was incorporated into the Sn58Bi solder to improve the properties of Sn58Bi/Cu solder joint. The interface reaction and growth kinetics of the Sn–Cu IMC at the solder/Cu interface during solid–liquid diffusion at temperature of 180, 190 and 200 °C for three different time (10, 20 and 30 min) was systematically investigated. The solder/Cu interface presented a duplex structure of Cu6Sn5 and Cu3Sn IMC. The IMC thickness was developed with the higher soldering temperature and prolonged soldering duration. The diffusion coefficient (D) of different IMC layer increased with the increasing of soldering temperature. The activation energies (Q) of the overall IMC layer were calculated as 48.22 kJ/mol for Sn58Bi solder joint and 58.20 kJ/mol for Sn58Bi–0.1Ti solder joint. Especially, adding the 0.1 wt% Ti particles can inhibit the interfacial IMC growth during solid–liquid diffusion, which may be conducive to the improvement of the solder joint reliability.

Investigation of FeCoNiCu properties: Thermal stability, corrosion behavior, wettability with Sn-3.0Ag-0.5Cu and interlayer formation of multi-element intermetallic compound

High-entropy alloy (HEA) FeCoNiCu (FCNC) can be potentially employed in electronic packaging because it contains commonly used substrate metals. However, its microstructure, thermal stability, corrosion behavior, and wettability toward Sn-3.0Ag-0.5Cu (SAC305, wt%) solder have never been examined in detail, especially in terms of the microstructure and reactivity of the SAC305/FCNC interface. In this study, FCNC consisted of a dominant phase and a Cu-rich phase characterized by excellent thermal stability during thermal aging at 150–450 °C for 150 h and high corrosion resistance against 3.5% NaCl solution. Additionally, its good wettability toward SAC305 solder corresponding to a contact angle of 20.1° after reflow at 250 °C. Furthermore, a multi-element intermetallic compound layer composed of 17.73% Fe, 11.68% Co, 6.54% Ni, 3.81% Cu, and 60.23% Sn (at%) with a thickness of 0.68 μm was formed at the SAC305/FCNC interface. Finally, (Cu-rich, Fe, Co, Ni)6Sn5 IMC was formed from the Cu atoms sourced by the solder on the (Fe, Co, Ni, Cu)Sn2 IMC layer during thermal aging at 150 °C for 150 h, while the growth of the latter compound was limited. The findings of this work suggest possible applications of FCNC in HEA-related fields and electronic packaging.

High Cycle Fatigue Behaviour of Cu/Sn Intermetallic Compounds Prepared by Transient Liquid Phase Bonding Process

Transient liquid phase (TLP) bonds using Cu-Sn system have been suggested as high strength and temperature resistant joints for power electronics applications. While the physical and mechanical properties of these joints has been investigated to some extent, studies on fatigue properties and long term reliability of TLP joints are scarce. In this work TLP bonding was performed to produce thin Cu-Sn intermetallic joints by using Cu and 97Sn3Cu solder alloy as interlayer. Different processing conditions resulted in three types of thin joints consisting of three phases (Cu3Sn/Cu6Sn5/solder remnants), two phases (Cu3Sn/Cu6Sn5) and a single phase (Cu3Sn) with an overall thickness of ≤ 20 μm. The shear strength of the TLP joint containing one or two high melting point IMC layers showed a significant temperature resistance up to 200°C. Fatigue studies of TLP joints were conducted by using a 3-point-cyclic bending test system operating at 20 kHz. The highest fatigue resistance was obtained for the single-phase Cu3Sn joints with superior shear and flexural resistance. The two phase joints (Cu3Sn/Cu6Sn5) showed a slightly lower lifetime than the three phase system containing IMCs and residual solder. Fracture surfaces analysis in correlation with static and cyclic mechanical properties, provided insight into the failure mechanism of the Cu-Sn TLP joints.

Joining of Aluminum and Steel using AlSi12 Brazing Filler in a Protective Atmosphere Furnace: Microstructure and Mechanical Properties

Brazing of pure aluminum (Al) to steel using an Al-12 wt. %Si (AlSi12) filler metal and an Al brazing flux was performed in furnace filled with protective atmosphere. Microstructure characterizations of the full/fractured joints, tensile shear strength and micro-hardness tests were performed on the samples with holding time from 5-30 min at brazing temperature of 600 °C and additional thermal exposure of 30 min at temperature of 480 °C. It is found that the joint seam for all samples features roughly into four layers, among them, the layer adjacent to steel is an IMC layer and dominantly distributed with h phase. The tensile shear strength of joints is inversely proportional to the thickness of h phase layer and particularly governed by a specific zone which is located in the h phase layer, directly adjacent to the interface between h phase layer and steel and scattered with a lot of visible pores and cracks. Micro-hardness tests show the hardness of the h phase layer remains the highest for each holding time and increases with the increase of holding time. The higher hardness leads to the limited plasticity of the η phase and more fragile of this layer. Furthermore, great differences of hardness exist between the η phase layer and steel may also generate great stresses that induce the crack initiation in the specific zone and finally result in the failure of brazed joints.

Interface evolution and mechanical properties of Sn–36Pb–2Ag solder joints under different aging conditions

Electronic packaging products are often subjected to reliability evaluation, Sn–Pb solder is widely used in high-reliability military products due to its excellent soldering performance. Sn–36Pb–2Ag eutectic solder joints were subjected to aging experiments at 75 °C, 100 °C, 125 °C, and 150 °C for 100 h, 250 h, 500 h, and 1000 h, respectively. Continuous Cu3Sn and (Au, Cu)6Sn5 compounds were formed on the printed circuit board (PCB) substrate side after aging experiments, and continuous (Au, Ni)Sn4 and (Au, Ni)3Sn4 compounds were formed on the electronic component side. In addition, the Pb–rich phase on the IMC layer side increased with the increase in aging time and aging temperature. Thus, the mechanical properties of the solder joints seriously reduced, and the push force of the fractured solder joint measured in the push–pull force experiment was reduced from approximately 85 N (125 °C–500 h) to approximately 42 N (150 °C–1000 h). The fracture mode of solder joint under 150 °C–1000 h aging conditions was brittle. Furthermore, the diffusion coefficient and activation energy of the Cu3Sn phase and (Au, Cu)6Sn5 phase on the PCB substrate side were calculated.

Ultrasound-assisted friction stir transient liquid phase spot welded dissimilar copper-aluminum joint

Ultrasound assistance was applied to friction stir transient liquid phase spot welding (FSTLPSW) to attain a high-quality dissimilar copper-aluminum joint in this study. Different ultrasound-assisted times were selected for comparing with the conventional condition without ultrasound. The interfacial microstructure evolution of conventional joint was analyzed combined with the measured temperature cycle. The ultrasound assistance improved the joint quality by regulating the interfacial microstructure. Cavitation erosion, broken IMC particles and thicker eutectic layer appeared in ultrasound-assisted joint. The highest tensile shear strength of 7.38 kN was obtained under the ultrasound-assisted time of 60 s which was 43.1 % higher than that of the conventional joint. The fracture location transferred from IMC layers to eutectic layer under the ultrasound assistance.

Effects of SiC nanowires on reliability of Sn58Bi-0.05GNSs/Cu solder joints

In this work, SiC nanowires (SiC NWs) reinforced SBG (Sn-58Bi-0.05GNSs) composite solder was prepared using powder metallurgy route. The effect of SiC NWs on melting temperature, wetting behavior, shear properties, microstructure of the prepared solder joints and interfacial reaction were studied in detail. Results reveal that incorporating SiC NWs can develop the wetting behavior and shear properties of solder joint but has a little effect on melting temperature. The microstructure of solder is refined markedly with the addition of SiC NWs, which is one of the reasons for the increase in the shear strength of the solder joints. Additionally, the dimension of Cu6Sn5IMC grains diminishes with the doping of SiC NWs, which resulted in the thinning of Cu6Sn5IMC layer. Thence, the addition of SiC NWs may be an effective way to improve the reliability of solder joints.

Microstructural characterisation and corrosion behaviour of aluminium alloy/steel hybrid structure produced by friction welding

Abstract An aluminium (Al) alloy to stainless steel friction welded joint, being of potential as an alternative for the existing structural component, was obtained under specifically designed heat inputs at three different levels (low, medium and high). Heat input significantly affected the mechanical properties of the joint where the joint welded at the medium level posed the highest tensile strength (325 ± 14 MPa). When the heat input was at a low level, an un-welded zone was observed with no obvious signs of IMCs. Upon increasing heat input to a higher level, weld defects disappeared with sub-micron sized IMCs layer formed. The IMCs layer demonstrated rather distinct thickness characteristics at different interfacial regions, but a higher heat input resulted in a thicker layer at similar regions. Although it was identified that a thicker IMCs layer might pose an adverse effect on the corrosion resistance, the friction welded Al alloy/steel joints had only a marginal change in corrosion potential and corrosion current intensity under various heat inputs, presumably due to fine (sub-micron) sized IMCs formed at various regions along weld interface. Therefore, friction welding could realise the sound joining of Al alloy and steel with both satisfying mechanical properties and corrosion resistance by restricting the IMCs at sub-micron level.

Characterization of dissimilar friction stir welded lap joints of AA5083 and GL D36 steel

Dissimilar AA5083 to GL D36 steel welds produced by Friction Stir Welding in lap joint configuration, with the aluminum plate placed on the advancing side, are studied regarding their mechanical, microstructural and interfacial properties for varying process parameters, i.e. welding and tool rotational speed. An increase of welding speed or decrease of the rotational speed causes the formation of tunnel defects, a decrease of the steel hook height and reduction of grain size in the aluminum stir zone. The maximum hardness is observed at the weld interface, due to the presence of intermetallic compound layers, identified as the Fe-rich phases as FeAl and Fe3Al. As the rotational speed increases, an increase of the IMCs thickness in the weld interface is found, which contributes to the degradation of the lap shear strength, due to the brittleness and high hardness of these phases. Overall, the maximum lap shear strength is obtained for welds showing macro (steel hook) and micro interlocks, as well as the formation of thin IMC layers at the weld interface.

Properties and microstructure evolution of Sn–Cu–Ni/Cu joints bearing carbon nanotubes and graphene nanosheets for solar cell

In this paper, 0.5 wt% carbon nanotubes (CNTs) and 0.05 wt% graphene nanosheets (GNSs) were selected as additives to modify the properties of Sn–Cu–Ni solder joints for solar cell. The mechanical properties of Sn–Cu–Ni solder joints can be improved by 20–25%. The interface growth behavior of Cu–Sn intermetallic compounds at Sn–Cu–Ni/Cu and Sn–Cu–Ni–0.5CNTs–0.05GNSs/Cu couples after 250 °C and 260 °C soldering with 10 min and 20 min. The experimental results indicate that the growth rate of Cu6Sn5 IMC of Sn–Cu–Ni/Cu solder joints is higher than that of Sn–Cu–Ni–0.5CNTs–0.05GNSs/Cu solder joints. That is, adding 0.5 wt% CNTs and 0.05 wt% GNSs can reduce the growth rate of the Cu6Sn5 IMC layer, which can be attributed to the reduction of the element diffusion coefficient. In addition, the stress–strain of the two solder joints were calculated using finite element method, it is found that the Sn–Cu–Ni–0.5CNTs–0.05GNSs/Cu solder joints have significantly less stress–strain than Sn–Cu–Ni/Cu solder joints, due to the reduction of Cu6Sn5 layer thickness, fatigue life of solder joints can be enhanced by three times.

Intermetallics-induced directional growth of Sn whiskers in Sn-3.5Ag coating on Al substrate

In this paper, the directional growth behavior of Sn whiskers was found to be associated with the interfacial γ-Ag2Al IMCs at Sn-3.5Ag/Al interface. A 100 μm thick columnar structured Sn-3.5Ag coating consisted of β-Sn phases and γ-Ag2Al phases was fabricated on Al substrate. The as-formed interfacial IMCs between Sn-3.5Ag and Al were confirmed as γ-Ag2Al and μ-Ag3Al. After aging treatment, the growth direction of Sn whiskers was found to be consistent. μ-Ag3Al phases transformed into γ-Ag2Al. The newly formed hexagonal γ-Ag2Al grew out of the IMCs layer and provided the out-of-plane stress for whisker growth. Ag atoms diffused into Sn sub-grain boundaries and formed γ-Ag2Al IMCs leading to the formation of in-plane stress. The c-axis of the β-Sn and γ-Ag2Al is parallel, leading to the similar growth direction of the whiskers. The misorientation between the whisker and the underneath grain was lower than 2°. A transitional area formed between the whisker and underneath grain, which consists of extra half-planes of atoms. A mathematical model was proposed to calculate the heterogeneous distribution of Ag atoms. By comparing the calculated growth rate with the measured growth rate of whisker, the Sn atomic transport mode was pipe diffusion.

Effects of Extreme Thermal Shock on Microstructure and Mechanical Properties of Au-12Ge/Au/Ni/Cu Solder Joint

Extreme temperature change has generally been the great challenge to spacecraft electronic components, particularly in long, periodic, deep-space exploration missions. Hence, researchers have paid more attention to the reliability of component packaging materials. In this study, the microstructure evolution on the interface of Cu/Ni/Au/Au-12Ge/Au/Ni/Cu joints, as well as the effects of extreme thermal shock on mechanical properties and the fracture mode in the course of extreme thermal changes between −196 and 150 °C, have been investigated. Results revealed that the interface layers comprised of two thin layers of NiGe and Ni5Ge3 compounds after Au-12Ge solder alloy was soldered on the Au/Ni/Cu substrate. After extreme thermal shock tests, the microstructure morphology converted from scallop type to planar one due to the translation from NiGe to Ni5Ge3. Meanwhile, the thickness of interface layer hardly changed. The shear strength of the joints after 300 cycles of extreme thermal shock was 35.1 MPa, which decreased by 19.61%. The fracture location changed from the solder to solder/NiGe interface, and then to the interface of NiGe/Ni5Ge3 IMC layer. Moreover, the fracture type of the joints gradually transformed from ductile fracture mode to brittle mode during thermal shock test. Simultaneously, the formation and extension of defects, such as micro-voids and micro-cracks, were found during the process of thermal shock due to the different thermal expansion coefficient among the solder, interface layer and substrate.

The Microstructure, Thermal, and Mechanical Properties of Sn-3.0Ag-0.5Cu-xSb High-Temperature Lead-Free Solder

To obtain Sn-3.0Ag-0.5Cu-xSb (x = 0, 25, 28, and 31) high-temperature lead-free solder antimony was added to Sn-3.0Ag-0.5Cu solder. The microstructure, thermal properties, and mechanical behavior of the solder alloy prepared were studied by using JSM-5610LV scanning electron microscope, Germany STA409PC differential scanning calorimeter, AG-I250KN universal tensile testing machine, and other methods. The SEM-EDS results showed that after adding Sb, SnSb phase was formed in the β-Sn matrix phase. The newly formed SnSb phase and the existing Sb in the solder alloy can inhibit the generation of IMC and refine the IMC layer. The addition of Sb significantly increased the melting temperature of the solder alloy. Among them, the thermal performance of Sn-3.0Ag-0.5Cu-25Sb is the best. The melting temperature of Sn-3.0Ag-0.5Cu-25Sb is 332.91 °C and the solid–liquid line range of Sn-3.0Ag-0.5Cu-25Sb solder alloy is 313.28–342.02 °C. Its pasty range is 28.74 °C, lower than 30 °C, which is beneficial for soldering. The test results of the mechanical behavior of Sn-3.0Ag-0.5Cu-xSb solder alloy show that with the increase of Sb addition, the ultimate tensile strength of the solder alloy also increases. However, the change of the elongation of the solder alloy is the opposite. The ultimate tensile strength of the solder alloy increased from 29.45 MPa of Sn-3.0Ag-0.5Cu solder to 70.81 MPa of Sn-3.0Ag-0.5Cu-31Sb solder. The reason for the increase in the strength of the solder alloy is the reduction of the thickness of IMC and the solid solution hardening effect of Sb.

Cryorolling of SAC305 solder : Microstructure analysis and shear strength of solder joint

Miniaturization of devices along with demand of higher performance in electronic packaging industries led to development of higher reliability solder alloys. Solder material must meet the requirements of high solder joint strength, better wettability, high fatigue and creep resistance in order to provide highly reliable solder joint. This work attempts to evaluate effectiveness of cryorolling in providing ultrafine grains which could lead to higher solder strength. The microstructure of bulk solder and IMC layer at interface between solder and Cu substrate, and lap joint shear strength of cryorolled commercial SAC305 solder alloy are presented here. Solder alloys were dipped in liquid nitrogen at different dipping times (0, 10, 30 and 60 minutes) prior to rolling. Cryorolling was observed to encourage grain refinement in solder with 30 min dipping gave lowest grain size of 5.88 µm, compared to 11 µm for as-cast solder. Longer dipping up to 60 minutes resulted in larger grain size and tend to be brittle with small cracks started to appear during rolling. Shear strength test showed an improved and stronger joint for SAC305 dipped in liquid nitrogen for 30 minutes compared to sample without dipping in liquid nitrogen. As a conclusion, cryorolled solder alloys had smaller grain size and this led to an improved solder joint shear strength.

Structural, Thermal and Wetting Characteristics of Novel Low Bi-Low Ag Containing Sn–x.Ag–0.7Cu–1.0Bi (x = 0.5 to 1.5) Alloys for Electronic Application

Low drop failure tendency and reduced cost of low silver SAC alloys has provoked researchers to exploit their potential for portable electronics. Low Bi addition in near-eutectic SAC alloys has proven confinement of IMC layer. In order to exploit potential of Ag variation (0.5–1.5 wt%) on structural, thermal, wettability and IMCs layers growth during thermal aging in low Bi-low Ag, Sn–xAg–0.7Cu–1.0Bi alloys were investigated. The resulting microstructure were characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Furthermore, structure–property correlations were established. This study signifies that 1.5 wt% Ag is responsible for lower melting point (220.18 °C) and lower contact angle (33°) in low Bi-low Ag, Sn–xAg–0.7Cu–1.0Bi alloys. Moreover, increment in Ag content restricted the interfacial IMC layer growth during thermal aging making them reliable for use in portable electronics. The microstructural investigations inferred that 0.5 wt% Ag solder joint with Cu substrate had the highest growth rate (0.34 μm/h1/2) of IMC layer thickness while 1.5 wt% Ag solder joint had lowest growth rate (0.17 μm/h1/2).

Influence of Novel Torch Type on Mechanical and Microstructural Characteristics of Cold Metal Transfer Brazed Joints

Abstract The present study aimed to braze interstitial free (IF) steel by using novel Braze+ and conventional Robacta torches with the aid of CuAl8 filler wire for a range of heat input 72–250 J/mm. Results showed that focused/denser arc and smaller arc action range, i.e., lower heat loss in the case of Braze+ torch resulted in better wettability, thicker intermetallic layer at the interface of deposited bead-steel, and higher dispersive phase fraction in the bead compared to Robacta torch which ultimately contributed to the evolution of mechanical properties of brazed joints. Furthermore, wettability, thickness of intermetallic layer and fraction of dispersive phases increased with the heat input due to increased deposition rate per unit time, availability of reacting atoms near the interface, and melting of base metal, respectively. Hardness variation of the brazed joints revealed that base metal was prone to failure due to comparatively higher hardness as well as improved resistance to failure of HAZ (due to the formation of acicular ferrite) and deposited bead (due to dispersion hardening). In addition, Fe, Al, and Cu were the main constituents of intermetallic layer and dispersive phases in the bead. IMC layer was harder in the case of Braze+ torch due to comparatively higher Fe content. Two modes of failure namely interface (brittle features) and base metal (ductile features) were observed during shear-tensile testing.

Characterization of sintered Cu nanopaste for micro-bumping with Injection Molded Solder technology

Abstract We have previously developed a novel plating-free bumping process using Cu nanopaste and Injection Molded Solder (IMS) technology. In the present study, we investigated the further detail about the microstructural and mechanical properties of sintered Cu nanoparticles formed into a pillar shape. By analyzing cross-sections of Cu nanoparticle pillars sintered in various conditions, we clarified how the sintering conditions affect the microstructural features, including the size and numbers of Cu grains and voids inside sintered Cu nanoparticles. In addition, we conducted the shear testing for the obtained Cu pillars to evaluate relationships between the mechanical strength and the microstructural features. We found that the results of the shear testing were consistent with the microstructural features of the sintered Cu nanoparticles. Finally, we injected molten solder onto the Cu nanoparticle pillars to evaluate the overall feasibility of the developed process. It was confirmed that the molten solder injected by IMS process has good wettability against the sintered Cu nanoparticles, which resulted in the successful bump formation without solder missing. In addition, The IMC layer between the sintered Cu nanoparticles and injected solder was formed well. These results proved the quality of microbumps fabricated by the novel bumping process using Cu nanopaste and IMS.

Effect of kaolin geopolymer ceramic addition on the properties of Sn-3.0Ag-0.5Cu solder joint

This paper investigates the effects of different weight percentages (0, 0.5, 1.0, 1.5 and 2.0 wt.%) of kaolin geopolymer ceramic (KGC) on the microstructure formation, thermal properties, spreadability and joint strength in Sn-3.0Ag-0.5Cu (SAC305) lead-free solder alloys in order to develop a new composite solder system. Advanced characterization techniques such as Electron backscatter diffraction (EBSD) and synchrotron micro-XRF were used to study the behaviors of the pure SAC305 and KGC reinforced SAC305 composite solders. Experimental results shows that the addition of KGC refines the β-Sn area and increases the eutectic area with fine intermetallics formation. In addition, the thickness of the IMC layer is reduced with a reduction in undercooling value for the KGC reinforced SAC305 composite solder. The spreadability of the KGC reinforced SAC305 composite solder is significantly increased in the spreadable area with a higher strength of solder joint. Significantly, the results obtained prove that 1.0 wt.% KGC addition gives better performance in terms of microstructure formation, thermal properties, spreadability and joint strength. Synchrotron micro-XRF interestingly indicated that some Al and Si, which are the major elements in geopolymer systems, migrate into the solder area.

Microstructure and mechanical properties of Cu joints soldered with a Sn-based composite solder, reinforced by metal foam

In this study, Ni foam, Cu coated Ni foam and Cu-Ni alloy foams were used as strengthening phases for pure Sn solder. Cu-Cu joints were fabricated by soldering with these Sn-based composite solders at 260 °C for different times. The tensile strength of pure Sn solder was improved significantly by the addition of metal foams, and the Cu-Ni alloy/Sn composite solder exhibited the highest tensile strength of 50.32 MPa. The skeleton networks of the foams were gradually dissolved into the soldering seam with increasing soldering time, accompanied by the massive formation of (Cu,Ni)6Sn5 phase in the joint. The dissolution rates of Ni foam, Cu coated Ni foam and Cu-Ni alloy foams into the Sn matrix increased successively during soldering. An increased dissolution rate of the metal foam leads to an increase in the Ni content in the soldering seam, which was found to be beneficial in refining the (Cu,Ni)6Sn5 phase and inhibiting the formation of the Cu3Sn IMC layer on the Cu substrate surface. The average shear strength of the Cu joints was improved with increasing soldering time, and a shear strength of 61.2 MPa was obtained for Cu joints soldered with Cu-Ni alloy/Sn composite solder for 60 min.