Microstructure Of Solder Research Articles

Effect of Nanoparticles Addition on the Microstructure and Properties of Lead-Free Solders: A Review

With the development of microelectronic packaging and increasingly specific service environment of solder joints, much stricter requirements have been placed on the properties of lead-free solders. On account of small size effect and high surface energy, nanoparticles have been widely used to improve the microstructure and properties of lead-free solders. Therefore, the composite solders bearing nanoparticles have recently attracted wide attention. This article reviewed the recent research on SnAgCu, SnBi, and SnZn composite solder alloys and introduced the effect of nanoparticles on their microstructure, mechanical properties, wettability, and reliability. The mechanism of nanoparticles strengthening was analyzed and summarized. In addition, the shortcomings and future development trends of nanoparticle-reinforced lead-free solders were discussed, which is expected to provide some theoretical reference for the application of these composite solder in 3D IC package.

Fine Microstructured In-Sn-Bi Solder for Adhesion on a Flexible PET Substrate: Its Effect on Superplasticity and Toughness.

A novel In-Sn-Bi solder with a low electrical resistivity of 14.3 × 10-6 Ω cm and a melting temperature of 99.3 °C was produced for use in adhesive joining on a flexible poly(ethylene terephthalate) substrate. We determined that the fine microstructure of the In-based solder (which had an average phase size of 62.2 nm) strongly influenced its superplasticity and toughness at diffusive temperatures of 55-85 °C because the late-forming BiIn intermetallic compound (IMC) suppressed the growth of two other IMCs, In3Sn and In0.2Sn0.8, which formed earlier in the soldering process. Thus, an elongation of 858.3% and toughness of 36.0 MPa were obtained at a temperature of 85 °C and a strain rate of 0.0020 s-1. However, due to phase boundary fracturing, the phase-refined solder exhibited a slightly more brittle nature (with an elongation of 74.3%) at room temperature compared with a standard In-Sn solder consisting only of the In3Sn and In0.2Sn0.8 IMCs, which had a slightly larger phase size of 84.9 nm and higher ductility (with an elongation of 80.7%). In terms of superplastic deformation, the conventional fracture system based on the Hall-Petch effect transformed into phase boundary sliding at the solder operating temperature, significantly enhancing ductility.

Improved Reliability and Mechanical Performance of Ag Microalloyed Sn58Bi Solder Alloys

Ag microalloyed Sn58Bi has been investigated in this study as a Pb-free solder candidate to be used in modern electronics industry in order to cope with the increasing demands for low temperature soldering. Microstructural and mechanical properties of the eutectic Sn58Bi and microalloyed Sn57.6Bi0.4Ag solder alloys were compared. With the addition of Ag microalloy, the tensile strength was improved, and this was attributed to a combination of microstructure refinement and an Ag3Sn precipitation hardening mechanism. However, ductility was slightly deteriorated due to the brittle nature of the Ag3Sn intermetallic compounds (IMCs). Additionally, a board level reliability study of Ag microalloyed Sn58Bi solder joints produced utilizing a surface-mount technology (SMT) process, were assessed under accelerated temperature cycling (ATC) conditions. Results revealed that microalloyed Sn57.6Bi0.4Ag had a higher characteristic lifetime with a narrower failure distribution. This enhanced reliability corresponds with improved bulk mechanical properties. It is postulated that Ag3Sn IMCs are located at the Sn–Bi phase boundaries and suppress the solder microstructure from coarsening during the temperature cycling, hereby extending the time to failure.

Indentation creep behavior of thermally aged Sn-5wt%Sb-1.5wt%Ag solder integrated with ZnO nanoparticles

In the present study the microstructure changes and their impacts on creep parameters from the indentation creep testing are investigated. For this propose the alloys Sn-5Sb-1.5Ag (SSA) plain solder and SSA reinforced with ZnO nanoparticles composite solder have been fabricated. The specimens were isothermally aged at 125, 175 °C for 24 h, and then quenched in air at 27 °C. The microstructure of the present solders is identified by X-ray diffraction (XRD). A crystalline β-Sn and intermetallic compounds (IMCs) SnSb and Ag3Sn are detected. Microstructural evolutions revealed refinement of SnSb and Ag3Sn IMCs as a result of ZnO addition. The reinforcing ZnO nanoparticeles are found to decrease the creep strain. The calculated values of the stress exponent n for the indentation creep process are found to decrease with increasing aging temperature. The value of n of the composite solder was found less than that of the plain solder at the same testing conditions.

Improved microstructure and mechanical properties for SnBi solder alloy by addition of Cr powders

In this work, Cr powders with several weight fractions (0, 0.1, 0.2 and 0.3 wt%) were doped into SnBi solder. The effects of Cr on the microstructure and mechanical properties of composite solders were investigated. The test results show that the microstructure of SnBi-0.2Cr solder is distinctly refined compared to the pure SnBi solder. The elongation of solder slab and the UTS of solder joint are respectively enhanced by 56% and 10% with 0.2 wt% Cr in tensile tests. Moreover, the creep depth decreases by 34% when the content of Cr is 0.2 wt%. The morphologies of Cu6Sn5 IMC layers transforms from the elongated scallop-like to the continuous rough scallop-like and the thicknesses of Cu6Sn5 IMC layers are decreased in composite solder joints. It is obvious that the fracture mechanism changes from brittle fracture to ductile fracture in both solder joint and slab.

Electroplating of 3D Sn-rich solder for MEMS packaging applications

Electroplating is a popular method to produce solders in microelectromechanical systems (MEMS) encapsulation and interconnection applications. In this work, a fabrication process is introduced for the generation of three-dimensional (3D) Sn solder microstructures. An aluminum membrane was employed as the conducting layer while electroplating. Based on the proposed method, Sn solders with different heights can be easily obtained after reflow. Taking advantage of these 3D structures, MEMS packaging, electrical interconnections, hermetic seals and metal-armoring can be formed simultaneously in one step. Metal-armoring provided by the solder–solder contact can be used at the extremes of the suspension travel, greatly increasing the capability of shock resistance. The helium leak rates of the packaged samples are lower than the reject limit (5 × 10−8 atm cc s−1 based on MIL-STD-883E standard). The bonding strength of samples is measured by a die shear strength tests with an average value of 7.4 MPa. In addition, the ohmic behavior of the Sn solder bonding is verified as well. These excellent results show an outstanding packaging quality in MEMS encapsulation applications.

Copper foam enhanced Sn58Bi solder joint with high performance for low temperature packaging

The brittle Sn58Bi solder was modified with the ductile copper foam (Cu-F) with high plasticity to improve its mechanical properties. It is observed that the coarse microstructure of Sn58Bi solder is notably refined by the uniform distribution of copper foam. Moreover, it is found that the shear force of Sn58Bi soldered sample increases sharply with increasing shear distance and brittle fracture occurs once the shear force reaches the maximum value. However, thanks to the existence of large amount of copper frameworks in Sn58Bi + Cu-F soldered sample, significant plastic deformation is generated after reaching the maximum shear force and no fracture is observed. Also, the Sn58Bi + Cu-F soldered sample demonstrates a higher resistance to elastoplastic deformation as the average maximum indentation depth decreases from 2.48 μm to 1.80 μm compared to the Sn58Bi solder. In addition, the creep of soldered samples becomes stable after 200 s, and a lower creep rate is gained in Sn58Bi + Cu-F soldered sample.

Effect of In Addition on Microstructure, Wettability and Strength of SnCu Solder

Abstract Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength.

Abstracts

Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength.

Microstructures and Mechanical Properties of the Sn58wt.%Bi Composite Solders with Sn Decorated MWCNT Particles

The mechanical properties and microstructures of Sn58Bi (in wt.%) composite solders with Sn decorated multiwalled carbon nanotube (Sn MWCNT) particles were investigated. The contents of Sn MWCNT particles were 0, 0.05, 0.1, and 0.2 wt.%. Sn58Bi composite solder pastes were printed on printed circuit board (PCB) substrates and bonded to the substrates using reflow processes (1, 2, 3, 5, and 7 times). This study describes the effect of Sn MWCNTs content and the number of reflow processes on the mechanical properties and microstructures of Sn58Bi composite solders. Mechanical properties were investigated using a low speed ball shear test. In the shear test, the shear strength increased by 7.07%, and the fracture energy increased by 14.4% with 0.1 wt.% of Sn MWCNT particles after 1one reflow. The number of reflows did not significantly affect the shear strength, but the fracture energy increased with increasing content of Sn MWCNT particles. Cross-sectional microstructures and fracture surfaces were observed by scanning electron microscopy (SEM). Sn MWCNT particles in the solder matrix were observed on polished surfaces of SEM images and were also identified by a Raman spectrometer. After the shear test, brittle failure occurred in all joints, indicating no influence of MWCNT particles.

Characterization of SAC – x NiO nano-reinforced lead-free solder joint in an ultra-fine package assembly

PurposeThis study aims to investigate the NiO nano-reinforced solder joint characteristics of ultra-fine electronic package.Design/methodology/approachLead-free Sn-Ag-Cu (SAC) solder paste was mixed with various percentages of NiO nanoparticles to prepare the new form of nano-reinforced solder paste. The solder paste was applied to assemble the ultra-fine capacitor using the reflow soldering process. A focussed ion beam, high resolution transmission electron microscopy system equipped with energy dispersive X-ray spectroscopy (EDS) was used in this study. In addition, X-ray inspection system, field emission scanning electron microscopy coupled with EDS, X-ray photoelectron spectroscopy (XPS) and nanoindenter were used to analyse the solder void, microstructure, hardness and fillet height of the solder joint.FindingsThe experimental results revealed that the highest fillet height was obtained with the content of 0.01 Wt.% of nano-reinforced NiO, which fulfilled the reliability requirements of the international IPC standard. However, the presence of the NiO in the lead-free solder paste only slightly influenced the changes of the intermetallic layer with the increment of weighted percentage. Moreover, the simulation method was applied to observe the distribution of NiO nanoparticles in the solder joint.Originality/valueThe findings are expected to provide a profound understanding of nano-reinforced solder joint’s characteristics of the ultra-fine package.

Enhanced microstructural, thermal and tensile characteristics of heat treated Sn-5.0Sb-0.3Cu (SSC-503) Pb-free solder alloy under high pressure

The Sn-5.0Sb-0.3Cu (SSC-503) solder alloy was pressurized by 1.0 GPa during heat treatment at 130 °C for 2 h (SSC-PHT) in order to refine its β-Sn grains and enhance the thermal and tensile properties. Microstructure of SSC-503 solder has irregular wavy coarsened grains. Refined grains with random irregular texture were observed after pressuring. This can be attributed to work hardening and shortened diffusion paths which promoted the crystal defects. XRD calculations revealed that the crystallite size of β-Sn phase of SSC-PHT sample was decreased by ~23% with respect to SSC-503 sample while the average lattice strain (η) was higher than heat treated sample (SSC-HT) by ~ 51%. Hence, pressuring during heat treatment causes an increase in dislocation density which strains the lattice and stores more strain energy. Differential scanning calorimeter (DSC) measurements indicated that the applied pressure inhibited the degree of undercooling from 3.04° to 1.47°C. This could endorse the solidification and restrain the growth rate of β-Sn dendrites due to the creation of new nucleation paths through preferential heterogeneous starting sites. The solidification activation energy of SSC-HT and SSC-PHT samples was evaluated as 60.52 and 52.31 kJ/mol respectively by utilizing modified Kissinger equation. After Appling pressure, a detectable enhancement was recorded in ultimate tensile strength (σUTS) by 63% and in yield stress (σYS) by 61% without deterioration of the ductility. These improvements related to the change in microstructure of SSC-PHT solder since high-pressure generated plenty of mixed dislocations as well as more defects. Moreover, the solid solution and dispersion hardening strengthening restricted the motion of the dislocations.

The effects of ultrasonic treatments on the microstructure and mechanical properties of SAC0307 solder

The microstructures and physical properties of SAC0307 lead-free solder formed by an ultrasound-assisted semi-solid casting process were analyzed. The damages to a thin aluminium foil placed in the alloy melt at 226 °C showed strong effects of acoustic streaming. Significant erosion due to cavitation was found for powers higher than 450 W. Numerical simulations of a collapsing cavitation bubble obtained the highest peak pressure and temperature when the ultrasonic power was around 550 W. When the ultrasonic power was between 500 W and 600 W, sub-grains were observed and the crystal grains became equiaxed with the smallest average grain size of about 140 μm. The highest tensile strength found in this range was about 36.6 MPa. The density was 7.35 g/cm3. The elongation at break was about 60%, which was 40% higher than the value obtained by traditional casting techniques.

Effect of Ni Addition to Sn0.7Cu Solder Alloy on Thermal Behavior, Microstructure, and Mechanical Properties

The addition of Ni on lead-free solder alloys Sn0.7Cu was tested in this study. A small amount of Ni was added to the solder alloys to evaluate the thermal behavior, microstructure, and mechanical properties of the composite solders after sintering and isothermal aging for 3 days and then compared with the results of the binary Sn0.7Cu solder. The results indicated that the ultimate tensile strength and yield tensile strength changed when adding 0.25 wt.% of Ni. The tested results of the differential scanning calorimeter showed that the addition of Ni (such as 0.5 and 1 wt.%) could obviously increase the solidification temperature of Sn0.7Cu alloys in the cooling process. When the Ni content was increased to 0.25 wt.% in the ternary condition, which includes the Sn0.7Cu-xNi (x = 0, 0.1, 0.25, 0.5, and 1 wt.%) solder alloys for the indentation creep test, the minimum creep rate reached the maximum; however, the trend was reversed as the Ni content was higher than this level. In addition, the microstructure of Sn-0.7Cu-xNi solder alloys was obviously different with the eutectic Sn-0.7Cu solder, such that the Ni gradually accumulated in the (Cu, Ni)6Sn5 IMCs within the Ni-containing solder alloys. Additionally, this process refined the microstructure of the Sn-0.7Cu solder. The fracture surface of the eutectic Sn-0.7Cu solder revealed ductile fracture modes; however, there was no mixed ductile–brittle fracture mode occurred when the Ni content was in the range of 0-1 wt.%.

Thermal cycling aging effects on the tensile property and constitute behavior of Sn–3.0Ag–0.5Cu solder alloy

Thermal cycling aging effects on the tensile property of Sn–3.0Ag–0.5Cu solder alloy are investigated experimentally and theoretically. Solder specimens were tested at temperature ranges from 77 to 293 K and 77–398 K with different cyclic numbers. Thermal cycling optimizes the microstructure of solder and causes dislocation, which can enhance both tensile strength and plasticity of solder material. It is observed experimentally that the plasticity of Sn–3.0Ag–0.5Cu solder alloy is strongly time dependent, higher cycle number leads to an increase of plastic strain. A unified creep plasticity constitutive model is developed by modifying the drag strength and taking the effects of temperature and cycling number into account. A new material damage parameter is proposed to consider the temperature effect during the treatment, which is incorporated into the developed model to describe the mechanical behavior of Sn–3.0Ag–0.5Cu solder under thermal cycling. The numerical predictions agree well with the experimental results of Sn–3.0Ag–0.5Cu solder alloys, it shows the developed constitutive model can describe the mechanical properties of Sn–3.0Ag–0.5Cu solder under thermal cycling with reasonable accuracy.

Investigation into the Structure and Properties of Solders Based on Aluminum and Zinc in the Form of Cast Rods of a Small Cross Section

Solders of the Al–Cu–Si system (A34 brand) and Zn–Al–Cu system (Welco52 brand) are investigated. It is found that the A34 solder (Al–28% Cu–6% Si) melts and crystallizes in a narrow temperature range (~18°C). Solidus and liquidus temperatures of the A34 solder are ~508 and ~526°С, respectively. Solders of the Zn–Al–Cu system (Zn–4% Al–2.5% Cu) have an eutectic composition that provides melting and crystallization at a constant temperature of ~389°С. Densities of studied solders in liquid and solid states are investigated. Their values for the solder of the A34 brand are 3.02 and 3.32 g/cm3, respectively, and, for the zinc solder, 6.28 and 6.69 g/cm3. The influence of casting conditions on the structure of cast alloys in the form of rods with a cross-section area of 13, 10, and 5 mm2 is investigated. The main structural components of solder alloys refine with a decrease in the cross-section area. Dendrites of the aluminum-based solid solution and CuAl2 phase refine in the microstructure of the A34 solder. Dendrite sizes of the zinc-based solid solution most noticeably decrease in the zinc solder. The largest castability is characteristic of alloys fabricated from rod solders with a cross section of 5 mm2 for studied gaps in the sampling of 2.0, 1.5, and 1.0 mm. The zinc solder of the eutectic composition has the best castability when compared with the A34 solder: this characteristic for the melt formed from the zinc-based solder rod with a cross section of 5 mm2 and a gap width in the sampling of 2.0 mm is 100% (the melt of the A34 solder formed from the rod of the same cross section has a castability of 98%). The results of experiments on soldering plates made of the AK12 alloy and sheets made of the AMts alloy show the presence of a tight boundary in the “solder–base material” system, as well as the absence of discontinuities in the form of pores and unsoldered regions. An insignificant interpenetration of solder alloys into the base material is observed, especially when soldering cast plates made of the AK12 alloy.

STUDY INTO STRUCTURE AND PROPERTIES OF SOLDERS BASED ON ALUMINUM AND ZINC IN THE FORM OF SMALL-SECTION CAST BARS

The study covers Al–Cu–Si (A34 grade) and Zn–Al–Cu (Welco52 grade) solders. It is found that A34 solder (Al–28%Cu–6%Si) melts and crystallizes in a narrow range of temperatures (~18 °C). Solidus and liquidus temperatures of A34 solder are ~508 °С and ~526 °С, respectively. Zn–Al–Cu (Zn–4%Al–2,5% Cu) solder has a eutectic composition, so it melts and crystallizes at a constant temperature of ~389 °С. Densities of investigated solders in their liquid and solid states are studied. A34 solder has a density of 3,02 and 3,32 g/cm3 , respectively. Zinc solder density is 6,28 g/cm3 in the liquid state, and 6,69 g/cm3 in the solid state. The influence of casting conditions on the structure of cast alloys in the form of bars with cross sections of 13, 10, and 5 mm2 was investigated. Main structural components of solder alloys reduce in size as cross sections decrease. The aluminium-based solid solution dendrites and CuAl2 phase are reduced in the A34 solder microstructure. In zinc solders, the most severely reduced ones are zinc-based solid solution dendrites. The best castability is observed in melts obtained from 5 mm2 section bar solders with studied sample gaps of 2,0, 1,5 and 1,0 mm. Eutectic zinc solder features better castability in comparison with A34: castability of the melt obtained from the 5 mm2 section zinc solder rod with a sample gap width of 2,0 mm is 100 % (for A34 melt solder obtained from a rod of the same cross section it was 98 %). Experiments on soldering AK12 alloy plates and 3003 alloy sheets demonstrated that there is a tight border in the solder/base material system and no any defects such as pores or unsoldered areas. There was a slight interpenetration of solder alloys into base materials, especially when soldering AK12 cast plates.

Enhancement of the mechanical strength of Sn-3.0Ag-0.5Cu/Ni joints via doping minor Ni into solder alloy

In this study, the Ni-doped solder alloy led to joints with improved shear performance and Sn-3.0Ag-0.5Cu-0.1Ni/Ni joint showed the highest strength. The mechanical response under shear testing depended on the microstructure of the solder zone. Doping minor Ni into the solder ball modified the evolution of the solder microstructure, leading to (Cu,Ni)6Sn5 precipitation in the solder zone and consequently strengthening the Ni-doped joints. Additionally, the pre-existence of Ni in the solder alloy is necessary for triggering the precipitation of (Cu,Ni)6Sn5 particles in solder.

Enhancement on the high-temperature joint reliability and corrosion resistance of Sn–0.3Ag–0.7Cu low-Ag solder contributed by Al2O3 Nanoparticles (0.12 wt%)

In this study, the evolution of interfacial microstructures and mechanical properties of the joints soldered with Sn–0.3Ag–0.7Cu (SAC0307) and SAC0307-0.12Al2O3 nanoparticles (NPs) aged at 150 °C for different hours (72–840 h) were investigated. It was found the joint soldered with SAC0307-0.12Al2O3 displayed a significantly enhanced high-temperature joint reliability, reflected in a higher shear force than that of the original. This enhancement in shear force primarily benefited from the refinement in solder microstructure contributed by Al2O3 NPs. As aging time reached 840 h, a controlled growth of interfaical IMC layer resulted from the pinning effect of Al2O3 NPs contributed to the increase in shear force. Theoretical analysis showed 0.12 wt% Al2O3 NPs effectively lowered the growth constant of total interfacial IMCs (DT) from 3.19 × 10−10 to 1.02 × 10−10 cm2 s−1. Moreover, comparative studies on the corrosion resistances of SAC0307 and SAC0307-0.12Al2O3 were also conducted by electrochemical test and analyzed by electrochemical impedance spectroscopy (EIS). The results revealed SAC0307-0.12 Al2O3 solder displayed a stronger corrosion resistance (Rt; ~ 3.1 kΩ cm2 vs ~ 7.1 kΩ cm2). This is also related with the tailored microstructure, which provides more grain boundaries for the initial nucleation of passive film.

Microstructure Evolution of Sn-Cu Based Solder Paste with Different Cu Concentration Subjected to Multiple Reflows

The evolution in microstructure of Sn-Cu based solder paste with different copper (Cu) content subjected to multiple reflow cycles was investigated. In this study, the Sn-0.7Cu (SC) solder paste was used as based material. The Cu particles were added into SC solder paste to produce new Sn-4Cu and Sn-10Cu solder paste. After that, the solder paste was then reflowed on Cu-OSP surface finished and subjected to six times reflows. Characterization focuses on the bulk solder microstructure, morphology and intermetallic compound (IMC) thickness after multiple reflows. Results reveal that solder composition significantly affect the microstructure formation and growth of IMC.