Sn Intermetallic Compound Layer Research Articles

The interfacial Cu–Sn intermetallic compounds (IMCs) growth behavior of Cu/Sn/Cu sandwich structure via induction heating method

Forming full IMC solder joint technology has become one of the leading candidates to replace conventional high temperature Pb-based solder for wide bandgap (WBG) power modules (SiC and GaN) due to its excellent thermo-stability and high electrical/thermal conductivity. Few studies were investigated the formation and growth mechanism of interfacial Cu–Sn IMCs layer of Cu/Sn(10 μm)/Cu system during induction heating method. The results showed that the growth behavior of Cu–Sn IMCs of the Cu/Sn interface by induction heating method was different from Cu–Sn IMCs growth behavior of the Cu/Sn interface by conventional soldering method. The Cu6Sn5 thickness increased by Cu/Sn solid–liquid interfacial reaction before the bonding time of 45 s and then decreased by Cu6Sn5/Cu solid–solid interfacial reaction within the bonding time of 60 s. The Cu3Sn thickness increased by solid Cu/liquid Sn interfacial reaction and solid Cu6Sn5/solid Cu interfacial reaction within the bonding time of 60 s. A formation mechanism model of Cu–Sn compounds was also presented. The mathematic model of Cu layer consumption at different temperature was also illustrated, which indicated that higher temperature is in favor of the consumption of Cu layer. By knowing the relationship between the thickness of Cu layer on SiC power device at different temperature could be designed when evaluating the reliability of device by induction heating process.

Influences of Mono-Ni(P) and Dual-Cu/Ni(P) Plating on the Interfacial Microstructure Evolution of Solder Joints

The interfacial microstructures of Sn-3.0Ag-0.5Cu (SAC305) solder systems with thin Ni(P) mono-coatings and Cu-Ni(P) dual-coatings were investigated after reflowing and isothermal aging. The ultrathin mono-Ni(P) plating of the SAC305/Ni(P) solder joint was found to rapidly decompose and then transform into a Ni2SnP phase. An intermetallic compound (IMC) formed at the plating/substrate interface, indicating that the ultrathin mono-Ni(P) plating was not an effective diffusion barrier. However, only a single IMC layer ((Cu,Ni)6Sn5) formed at the solder/plating interface in the SAC305/Cu/Ni(P)/Cu system. The (Cu,Ni)6Sn5 IMC effectively suppressed atomic diffusion, protecting the Ni(P) plating and Cu substrate. Although P-Sn-O pores formed in the root of the (Cu,Ni)6Sn5 IMC layer, the dual-Cu/Ni(P) plating protected the solder system for an extended period. The IMC growth rate constants of the SAC305/Cu, SAC305/Ni(P), and SAC305/Cu/Ni(P)/Cu solder joint systems were 0.180, 0.342, and 0.068 μm/h1/2, respectively. These results indicate that the application of dual-Cu/Ni(P) plating can effectively hinder the growth of IMC.

Highly thermostable joint of Cu/Ni plating/composite Sn–0.7Cu solder with added Cu balls for die attachment in power modules

Improving the reliability of solder joints for die attachment in power modules is one of the most important issues in creating environmentally friendly vehicles such as hybrid electric vehicles. Power modules must have highly reliable solder joints that must be thermostable at temperatures over 175 °C in the future. In die attachment, soldering surfaces are often finished with Ni plating, so for Cu/Ni plating/Sn–Cu solder joints it is necessary to suppress both Ni diffusion into the solder as well as growth of the (Ni,Cu)3Sn4 intermetallic compound (IMC). Ni diffusion in Ni plating can be suppressed by the presence of a continuous (Cu,Ni)6Sn5 IMC layer at the Ni plating/solder interface. To form this IMC, we investigated the interfacial reactions and growth behavior of IMC layers in the presence of composite Sn–0.7Cu solder with added Cu balls. Adding 2.5 mass% of Cu balls prompted the formation of a continuous (Cu,Ni)6Sn5 IMC layer at both the electroless Ni–P and the electrolytic Ni plating, and this IMC layer worked well as a Ni diffusion barrier during a high-temperature storage test at 200 °C for 1000 h.

Highly thermostable joint of a Cu/Ni–P plating/Sn–0.7Cu solder added with Cu balls

Solder joint reliability in power modules is one of the most important issues for hybrid, electric, and fuel cell vehicles; these modules must have highly reliable solder joints, i.e., they must be highly thermostable at temperatures over 175 °C in the future. The soldering surfaces in power modules are often finished with electroless Ni–P plating. Thus, for Cu/Ni–P plating/Sn–0.7Cu joints, it is necessary to suppress Ni diffusion into the solder. Ni diffusion can be suppressed in the presence of a continuous Cu6Sn5 intermetallic compound (IMC) layer at a Ni–P plating/solder interface. To form this IMC, we investigated the composite Sn–0.7Cu solder added with Cu balls. It was confirmed that the addition of 2.5 wt% Cu balls formed a continuous (Cu, Ni)6Sn5 IMC layer between the solder and the Ni–P plating. It is concluded that the IMC layer works well as a Ni diffusion barrier in multiple reflow tests, of which the peak temperature was 330 °C, and in a high-temperature storage test at 200 °C for 1000 h.

Harsh service environment effects on the microstructure and mechanical properties of Sn–Ag–Cu-1 wt% nano-Al solder alloy

This paper investigates the electrical and mechanical performances of eutectic Sn-3Ag-0.5Cu (wt%) solder with the addition of Al nanoparticles. The study revealed that the elastic moduli, electrical resistivity and damping properties of such solder alloy were improved. Further, interfacial reaction phenomena on Au/Ni-plated Cu pad ball grid array substrate during isothermal aging and thermal cycle was evaluated in terms of the formation and growth kinetics of intermetallic compound (IMC) layer. A structural analysis confirmed that at their interfaces a ternary (Cu, Ni)-Sn IMC layer was adhered at the substrate surface. The thickness of this IMC layer was increased with increasing the duration of the isothermal aging and thermal cycle without any defects. In addition, the formation of Ag3Sn, Cu6Sn5, Sn–Al–Ag and AuSn4 IMC phases were evenly distributed in the solder matrix which acts as the second phase reinforcement. The measured shear strength and microhardness indicated that the exposure of the solder joints to the thermal cycles make the joints degraded faster than the situation in isothermal aging.

Growth mechanism of intermetallic compound and mechanical properties of nickel (Ni) nanoparticle doped low melting temperature tin–bismuth (Sn–Bi) solder

This paper investigates the effects of Ni nanoparticles on the formation of intermetallic compound (IMC) layers and mechanical properties of low melting temperature Sn–58Bi (wt%) based solders on copper (Cu) substrate. At the initial reaction for the plain Sn–Bi solder/Cu substrate system, an island-shaped Cu6Sn5 IMC layer was found to adhere at the substrate surface. A very thin Cu3Sn IMC layer was also observed between the Cu6Sn5 IMC layer and Cu substrate as the reaction time increased. However, in the composite solders doped with Ni nanoparticles, a scallop-shaped ternary (Cu, Ni)–Sn IMC layer appeared at the interface without Cu3Sn IMC layer. In the solder ball region, the Bi phase with bright contrast was homogeneously distributed in the β-Sn matrix. After adding the Ni nanoparticles, an additional very fine Sn–Ni IMC particle was found to have been distributed in the β-Sn matrix. The IMC layer thicknesses were increased with the reaction time and temperature. However, the IMC growth behavior of composite solder was slower than that of the plain solder system. Furthermore, the mechanical properties of the composite solder exhibited higher values than that of the plain Sn–Bi solder due to the strengthening effect of fine Sn–Ni IMC particles.

Influence of the aggregated Ag3Sn on the improvement of electromigration phenomenon in the doped Sn58Bi solder joints

Electromigration (EM) phenomenon has become more troublesome in the Sn–Bi solder joints as the increase of current density. Doping method is reported as one of the solution to improve solder joints. However, the doped particles are easily aggregated and formed large sized intermetallic compounds (IMCs) in the solder matrix during reflow soldering. In this research, EM phenomenon was studied in the Sn57.6Bi0.4Ag, Sn58Bi with 0.4 wt% Ag doping (Sn58Bi + 0.4Ag) and Sn58Bi solder joints to fully understand the influence of the aggregated Ag3Sn IMCs. The grain refinement effect of the Ag element was found to be the main reason of the improvement of EM phenomenon. Also, it was proved that Sn57.6Bi0.4Ag solder joints with uniformly distributed Ag3Sn IMCs performed well in preventing the segregation of Bi phase. However, Sn58Bi + 0.4Ag solder joints with the aggregated Ag3Sn IMCs did not perform well, forming thicker Bi rich layer at the anode. On the other hand, the addition of Ag element had minimal effect on the formation and consumption of Cu–Sn IMCs in the Sn–Bi solder joints. The thickness of Cu–Sn IMCs layer at the cathode in the Sn57.6Bi0.4Ag and Sn58Bi solder joints were found to be similar. However, the aggregated Ag3Sn IMCs in the Sn58Bi + 0.4Ag solder joints could promote the migration of Cu–Sn IMCs to the cathode.

Influence of cerium oxide (CeO2) nanoparticles on the microstructure and hardness of tin–silver–copper (Sn–Ag–Cu) solders on silver (Ag) surface-finished copper (Cu) substrates

Nano-sized, non-reacting, non-coarsening CeO2 particles with a density close to that of solder alloy were incorporated into Sn–3.0 wt%Ag–0.5 wt%Cu solder paste. The interfacial microstructure and hardness of Ag surface-finished Cu substrates were investigated, as a function of reaction time, at various temperatures. After the initial reaction, an island-shaped Cu6Sn5 intermetallic compound (IMC) layer was clearly observed at the interfaces of the Sn–Ag–Cu based solders/immersion Ag plated Cu substrates. However, after a prolonged reaction, a very thin, firmly adhering Cu3Sn IMC layer was observed between the Cu6Sn5 IMC layer and the substrates. Rod-like Ag3Sn IMC particles were also clearly observed at the interfaces. At the interfaces of the Sn–Ag–Cu based solder-Ag/Ni metallized Cu substrates, a (Cu, Ni)–Sn IMC layer was found. Rod-like Ag3Sn and needle-shaped Cu6Sn5 IMC particles were also observed on the top surface of the (Cu, Ni)–Sn IMC layer. As the temperature and reaction time increased, so did the thickness of the IMC layers. In the solder ball region of both systems, a fine microstructure of Ag3Sn, Cu6Sn5 IMC particles appeared in the β-Sn matrix. However, the growth behavior of the IMC layers of composite solder doped with CeO2 nanoparticles was inhibited, due to an accumulation of surface-active CeO2 nanoparticles at the grain boundary or in the IMC layers. In addition, the composite solder joint doped with CeO2 nanoparticles had a higher hardness value than the plain Sn–Ag–Cu solder joints, due to a well-controlled fine microstructure and uniformly distributed CeO2 nanoparticles. After 5 min of reaction on immersion Ag-plated Cu substrates at 250 °C, the micro-hardness values of the plain Sn–Ag–Cu solder joint and the composite solder joints containing 1 wt% of CeO2 nanoparticles were approximately 16.6 and 18.6 Hv, respectively. However after 30 min of reaction, the hardness values were approximately 14.4 and 16.6 Hv, while the micro-hardness values of the plain Sn–Ag–Cu solder joints and the composite solder joints on Ag/Ni metallized Cu substrates after 5 min of reaction at 250 °C were approximately 15.9 and 17.4 Hv, respectively. After 30 min of reaction, values of approximately 14.4 and 15.5 Hv were recorded.

Interfacial microstructure and hardness of nickel (Ni) nanoparticle-doped tin–silver–copper (Sn–Ag–Cu) solders on immersion silver (Ag)-plated copper (Cu) substrates

Sn–Ag–Cu composite solder has been prepared by adding Ni nanoparticles. Interfacial reactions, the morphology of the intermetallic compounds (IMC) that were formed, the hardness between the solder joints and the plain Cu/immersion Ag-plated Cu pads depending on the number of the reflow cycles and the aging time have all been investigated. A scallop-shaped Cu6Sn5 IMC layer that adhered to the substrate surface was formed at the interfaces of the plain Sn–Ag–Cu solder joints during the early reflow cycles. A very thin Cu3Sn IMC layer was found between the Cu6Sn5 IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nanoparticles, a scallop-shaped (Cu, Ni)–Sn IMC layer was clearly observed at both of the substrate surfaces, without any Cu3Sn IMC layer formation. Needle-shaped Ag3Sn and sphere-shaped Cu6Sn5 IMC particles were clearly observed in the β-Sn matrix in the solder-ball region of the plain Sn–Ag–Cu solder joints. Additional fine (Cu, Ni)-Sn IMC particles were found to be homogeneously distributed in the β-Sn matrix of the solder joints containing the Ni nanoparticles. The Sn–Ag–Cu–0.5Ni composite solder joints consistently displayed higher hardness values than the plain Sn–Ag–Cu solder joints for any specific number of reflow cycles–on both substrates–due to their well-controlled, fine network-type microstructures and the homogeneous distribution of fine (Cu, Ni)–Sn IMC particles, which acted as second-phase strengthening mechanisms. The hardness values of Sn–Ag–Cu and Sn–Ag–Cu–0.5Ni on the Cu substrates after one reflow cycle were about 15.1 and 16.6 Hv, respectively–and about 12.2 and 14.4 Hv after sixteen reflow cycles, respectively. However, the hardness values of the plain Sn–Ag–Cu solder joint and solder joint containing 0.5 wt% Ni nanoparticles after one reflow cycle on the immersion Ag plated Cu substrates were about 17.7 and 18.7 Hv, respectively, and about 13.2 and 15.3 Hv after sixteen reflow cycles, respectively.

Shear strengths and fracture behaviors of Cu/Sn37Pb/Cu soldered joints subjected to different displacement rates

The effects of displacement rate and reflow duration on the shear strengths of the Cu/Sn37Pb/Cu soldered joints as well as the interfacial microstructure were investigated after reflowing. The samples were reflowed at 300°C for different durations (5min, 15min and 30min) and then shear tested with different displacement rates ranging from 2.5×10−3 to 5×10−2mms−1. The intermetallic compounds (IMCs), including Cu6Sn5 and Cu3Sn phases were observed between Sn37Pb solder and Cu substrate, and their thicknesses increased with increasing reflow duration up to 30min. The single lap shear test results indicated that the shear forces of the joints increased with increasing displacement rate, but decreased with increasing reflow duration. Failure mechanisms of soldered joints in different displacement rate regimes were investigated based on the fractography analysis. After reflowing for 5 and 15min, the fractures of the Sn37Pb soldered joints mainly occurred inside the bulk solder irrespective of the displacement rate. While some broken Cu6Sn5 particles could be observed at the bottom of dimples in solder bulk as the high displacement rates were adopted (such as 1×10−2 and 5×10−2mms−1). In case of reflow duration of 30min, as the low displacement rates (such as 2.5×10−3 and 5×10−3mms−1) were adopted, the fracture patterns of soldered joints were similar to that of soldered joints reflowed for 5 and 15min. In contrast, little solder and many IMCs were detected on the fracture surface under high displacement rates condition (such as 1×10−2 and 5×10−2mms−1), which indicated that the fracture mainly occurred in the interior of Cu–Sn IMC layer. The displacement rate sensitivities of the soldered joints reflowed for different durations were also investigated, and it is found that the displacement rate sensitivity decreased with increasing reflow duration due to the increased interfacial IMC layer thickness.

Interfacial reactions between SnAg1.0Ti and Ni metallization

The effect of Ti on the solid state reactions between Sn and Ni has been investigated in this work. Based on the experimental results the following statements can be made: Firstly, the presence of Ti does not have measurable effects on the thickness evolution of Ni3Sn4 during solid state annealing. Secondly, the results from long term heat treatments show that there is no marked solubility of Ti to Ni3Sn4. Rather Ti reacts with Sn to form large Ti2Sn3 platelets inside the solder matrix. The Sn-rich part of the Ni–Sn–Ti phase diagram was assessed in order to rationalize the experimental results. By utilizing this information, the absence of any marked effects of Ti on the growth of Ni–Sn intermetallic compounds (IMC) was analysed. As there is no solubility of Ti to SnAg solder or to Ni–Sn IMC’s, Ti cannot change activities of components in the solder nor influence the stability of the IMC layers. Hence, these results throw significant doubts over the concept of trying to influence the Ni–Sn IMC layer thickness or quality by Ti alloying.

Characteristics of solderable electrically conductive adhesives (ECAs) for electronic packaging

This study investigated the effect of the viscosity of the ECAs using a low-melting-point alloy (LMPA) filler on its bonding characteristics. The curing behaviors of the ECAs were determined using Differential Scanning Calorimetry (DSC), and ECA temperature-dependant viscosity characteristics were observed using a torsional parallel rheometer. The wetting test was conducted to investigate the reduction capability of ECAs and the flow-coalescence-wetting behavior of the LMPAs in ECAs. Electrical and mechanical properties were determined and compared to those with commercial ECAs and eutectic tin/lead (Sn/Pb) solder. In the metallurgically interconnected Quad Flat Package (QFP) joint, a typical scallop-type Cu–Sn intermetallic compound (IMC) layer formed at the upper SnBi/Cu interface after curing process. On the other hand, a (Cu, Ni)6Sn5 IMC layer formed on the SnBi/ENIG interface. In addition, the fracture surface exhibited by cleavage fracture mode and the fracture was propagated along the Cu–Sn IMC/SnBi interface. The extremely low-level viscosity of ECAs had a significant influence on the flow-coalescence-wetting behavior of the LMPAs in ECAs and also on the interconnection properties. Stable interconnected assemblies showed good electrical and mechanical properties.

In-situ observations on fracture behaviors of Cu–Sn IMC layers induced by deformation of Cu substrates

Abstract The fracture behaviors of the interfacial Cu–Sn intermetallic compound (IMC) layers induced by deformation of the Cu substrates were investigated in this study. The IMC layers at the Sn–Ag/Cu single crystal and the Sn–Ag/cold-drawn Cu interfaces were driven to deform synchronously with the Cu substrates, and their fracture morphologies were in situ observed. The results show that fractures in the IMC layers at both of the two interfaces occur shortly after the Cu yields, and the cracks correlate well with the slip bands in the Cu substrates. In micro-scale, the fractures are predicated to be induced by dislocation pile-up. After the Cu substrates yield, the dislocations in them slip toward their surfaces. Since the dislocations cannot get across the IMC/Cu interface, they pile-up at the interface and generate a high cumulative stress ahead the pile-up group. When the cumulative stress reaches the fracture strength of the IMC, fractures inside the IMC layer occur. Before yielding of the Cu substrates, the IMC layer can withstand a high strain without fracture, because there are thin gaps between the IMC grains that can be opened to accommodate deformation. The yield behavior of the Cu substrate is the major influencing factor on the fracture behaviors of the interfacial IMC layers. Thermal aging makes the IMC layers thicker, more compact and easier to fracture.

Relation between Kirkendall voids and intermetallic compound layers in the SnAg/Cu solder joints

Several different kinds of intermetallic compound (IMC) layers were fabricated through adding trace elements into the Sn-3.5Ag solder, to investigate the relationship between the Kirkendall void (KV) and the IMC layers. The results show that a considerable amount of KVs were observed at the Cu3Sn/Cu interface in the Sn-3.5Ag/Cu joint after isothermal aging. A proper amount of Zn (0.5 wt%) and Ge (0.1 and 0.3 wt%) were found to effectively suppress the formation of the Cu3Sn layer, and no obvious KVs were observed at the Cu6Sn5/Cu interface, while more Zn induced the formation of the Cu6Sn5 plus Cu–Zn mixed IMC layer, and voids (not KVs) were observed at the Cu6Sn5/Cu–Zn interface. (Cu,Ni)6Sn5 IMC layer replaced the initial Cu6Sn5 at the SnAg-Ni/Cu joints, likewise, the Cu3Sn was suppressed at the thermal aging stage. Moreover, voids were found at the IMC/solder interface, while not at the IMC/Cu. Therefore, the formation of KVs is greatly determined by the characteristic of the IMC layer, this is consistent with the previous reports. On other hand, the KV can be suppressed by controlling the interfacial phase through adding trace elements into the solder.

Effect of Ti on the interfacial reaction between Sn and Cu

The effect of Ti on the solid state reactions between Sn and Cu has been investigated in this work. Based on the experimental results the following statements about the effect of Ti can be made: Firstly, the presence of Ti does not have measurable effect on the thickness of either Cu6Sn5 or Cu3Sn during solid state annealing. However, the unevenness of both Cu6Sn5 and Cu3Sn layers is increased by the addition of Ti. Secondly, there is no marked solubility of Ti to either Cu6Sn5 or Cu3Sn. Rather Ti reacts with Sn to form large Ti2Sn3 platelets inside the solder matrix. These findings were subsequently rationalized with the help of the assessed Cu–Sn–Ti phase diagram. By utilizing this phase diagram information, the absence of any marked effects of Ti on the growth of Cu–Sn intermetallic compound (IMC) formation was rationalized. As there is a very low solubility of Ti to SnAg solder and to Cu–Sn IMC’s, Ti cannot change activities of components in the solder nor influence the stability of the IMC layers. Hence, these results throw significant doubts over the concept of trying to influence the Cu–Sn IMC layer thickness or quality by Ti alloying.

Influence of Dopant on Growth of Intermetallic Layers in Sn-Ag-Cu Solder Joints

The interfacial interaction between Cu substrates and Sn-3.5Ag-0.7Cu-xSb (x = 0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0) solder alloys has been investigated under different isothermal aging temperatures of 100°C, 150°C, and 190°C. Scanning electron microscopy (SEM) was used to measure the thickness of the intermetallic compound (IMC) layer and observe the microstructural evolution of the solder joints. The IMC phases were identified by energy-dispersive x-ray spectroscopy (EDX) and x-ray diffractometry (XRD). The growth of both the Cu 6 Sn 5 and Cu 3 Sn IMC layers at the interface between the Cu substrate and the solder fits a power-law relationship with the exponent ranging from 0.42 to 0.83, which suggests that the IMC growth is primarily controlled by diffusion but may also be influenced by interface reactions. The activation energies and interdiffusion coefficients of the IMC formation of seven solder alloys were determined. The addition of Sb has a strong influence on the growth of the Cu 6 Sn 5 layer, but very little influence on the formation of the Cu 3 Sn IMC phase. The thickness of the Cu 3 Sn layer rapidly increases with aging time and temperature, whereas the thickness of the Cu 6 Sn 5 layer increases slowly. This is probably due to the formation of Cu 3 Sn at the interface between two IMC phases, which occurs with consumption of Cu 6 Sn 5 . Adding antimony to Sn-3.5Ag-0.7Cu solder can evidently increase the activation energy of Cu 6 Sn 5 IMC formation, reduce the atomic diffusion rate, and thus inhibit excessive growth of Cu 6 Sn 5 IMCs. This study suggests that grain boundary pinning is one of the most important mechanisms for inhibiting the growth of Cu 6 Sn 5 IMCs in such solder joints when Sb is added.

Interfacial reaction and mechanical reliability of eutectic Sn–0·7Cu/immersion Ag-plated Cu solder joint

In this study, the interfacial reaction and joint reliability of immersion Ag-plated Cu substrate with the Sn–0·7Cu (wt-%) ball–grid array (BGA) solder was investigated. During reflow, the Ag plating layer was dissolved completely into the molten Sn–Cu solder and some of the Cu layer was also dissolved into the molten solder. The dissolved Ag and Cu were precipitated as Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs) in the solder matrix. Upon reflow, the Sn–Cu solder exhibits an off-eutectic reaction to produce the eutectic phase and precipitate (Cu6Sn5 and Ag3Sn). The Cu–Sn IMC layer was formed at the solder/Cu interface after reflow, and the IMC layer grew during aging treatment. During the shear tests, the failure mode switched from a bulk-related failure to an interface-related failure. After aging for 250 h, the joint failed partially at the solder/Cu6Sn5 interface. The brittle fracture was linked to the formation of thick Cu–Sn IMC layer.

Interfacial reaction of gas-atomized Sn–Zn solder containing Ni and Cu additives

Sn–Zn-based alloy is expected as an alternative low-temperature lead-free solder because of its melting temperature being close to that of Sn–Pb eutectic solder. The Sn–Zn solder is, however, inferior in terms of interfacial reaction with Cu substrate; that is, a hard and brittle Cu–Zn intermetallic compound (IMC) layer is formed at the joint interface, which leads to degradation of adhesion strength. In order to prevent the formation of Cu–Zn IMCs, we have investigated the effects of atomizing treatment on the Sn–Zn solders with additives Ni and Cu. In the solder containing Cu and Ni additives, the Cu–Zn IMC layer commonly observed is formed for the non-atomized solder, whereas the Cu–Sn IMC layer is formed for the atomized solder as well as Sn–Pb solder alloy. This is significant improvement for practical use of Sn–Zn based solder alloy.

IMC Growth Mechanisms of SAC305 Lead-Free Solder on Different Surface Finishes and Their Effects on Solder Joint Reliability of FCBGA Packages

Intermetallic compound (IMC) growth behavior of lead-free solder plays an important role in ball grid array (BGA) solder joint reliability for flip chip BGA (FCBGA) packaging applications. The growth mechanism of IMC is reported based on a diffusion model. Thermal treatment such as accelerated thermal cycling (ATC) and isothermal aging exposure also contribute to the growth rate and morphology of lead-free solder IMC. Among the lead-free solder alloys, Sn-3.0wt.%Ag-0.5wt.%Cu (SAC305) solder is a promising substitute for Sn-Pb because of its good mechanical properties and wettability with current surface finishes. After thermal exposure, BGA solder joint reliability is degraded due to IMC formation and growth. In this study, two different thermal treatments, ATC and isothermal aging, and two different pad surface finishes, solder on pad (SOP) and electroless Ni immersion gold (ENIG), are considered in terms of IMC growth rate and mechanical solder joint reliability. An SOP finished interface forms a thin ε-phase Cu3Sn layer and a scallop-like η-phase Cu6Sn5 layer. In contrast, the ENIG finished interface forms a thick (Cu,Ni)6Sn5 IMC layer and prevents overall IMC growth. Different surface finished test vehicles are evaluated in an ATC test in a 0°C to 100°C temperature range and the Ni diffusion layer shows a longer solder joint fatigue lifetime than the nondiffusion barrier interface based on the micro cross-section and dye penetration analysis results. In an isothermal aging test at 100°C and 150°C, the aging temperature and time are valid factors to decide mechanical shock reliability. Interfacial fractures are found in the 100°C aged test vehicle due to easier crack propagation at the interface between the thin Cu3Sn layer and the scallop-like Cu6Sn5 layer based on SEM microstructure analysis results. Finally, this investigation proposes how to improve solder joint reliability and prevent interfacial fracture for SAC305 lead-free application.

Influence of minor Bi additions on the interfacial morphology between Sn–Zn–xBi solders and a Cu layer

The influence of minor Bi additions on the interfacial morphology between Sn–Zn–xBi (x = 0, 1, 3) solders and a Cu layer after reflowing were investigated by microstructural observations. The addition of minor amount of Bi into Sn–Zn solder reduced the tendency to form cracks at the solder/Cu interface. This is because alloying with Bi reduced the mismatch of the coefficient of thermal expansion (CTE) between the solder alloys and the Cu plate. Moreover, the Sn–Zn solder with Bi reduced the melting temperature of the solder alloy, and, this resulted in the coarsening of the gains and thickening of the intermetallic compound (IMC) layers because solder alloys with a lower melting temperature experienced a longer molten period during reflow. Because the Bi atoms accumulated at the surface of the IMC layers in homogeneously, partially impeding the IMC dissolving into the molten solder, a serrated-like Cu–Zn–Sn IMC layer was formed at the Sn–8Zn–3Bi/Cu interface.