Reactive wetting of Sn–2.5Ag–0.5Cu solder on copper and silver coated copper substrates

In the present work, wetting characteristics and morphology of intermetallic compounds (IMCs) formed between Sn–2.5Ag–0.5Cu lead-free solder on copper (Cu) and silver (Ag) coated copper substrates were compared. It was found that, Ag coated Cu substrate improved the wettability of solder alloy. The average values of contact angles of solder alloy solidified on Ag coated Cu substrate were reduced to about 50 % as compared to contact angles obtained on Cu substrates. Flow restrictivity for spreading of solder on Ag coated Cu was found to be lower as compared to Cu substrate. The spreading of solder alloy on Ag coated Cu exhibited halo zone. Coarse needle shaped Cu₆Sn₅ IMCs were observed at the solder/Cu substrate interface whereas at the solder/Ag coated Cu interface Cu₆Sn₅ IMCs showed scallop morphology. The formation of Cu₃Sn IMC was observed for the spreading of solder alloy on both substrates. The solder/Ag coated Cu substrate interface exhibited more particulates of Ag₃Sn precipitates as compared to solder/Cu substrate interface. The improved wettability of solder alloy on Ag coated Cu substrate is due to the formation of scallop IMCs at the interface.     

Effects of trace amount addition of rare earth on properties and microstructure of Sn–Ag–Cu alloys

Ternary lead free solder alloys Sn–Ag–Cu were considered as the promising alternatives to conventional SnPb alloys comparing with other solders. In the present work, effects of trace amounts of rare earth Ce on the wettability, mechanical properties and microstructure of Sn–Ag–Cu solder have been investigated by means of scanning electron microscopy and energy dispersive X-ray analysis systematically. The results indicate that adding trace amount of rare earth Ce can remarkably improve the wettability, mechanical strength of Sn–Ag–Cu solder joint at different temperature, especially when the content of rare earth Ce is at about 0.03%, the tensile strength will be 110% times or more than that of the lead free solder joint without rare earth Ce addition. Moreover, it was observed that the trace amount of rare earth Ce in Sn–Ag–Cu solder may refine the joint matrix microstructure, modify the Cu₆Sn₅ intermetallic phase at the copper substrate/solder interface, and the intermetallic compound layer thickness was reduced significantly. In addition, since rare earth Ce possesses a higher affinity to Sn in the alloy, adding of rare earth Ce can also lead to the delayed formation and growth of the intermetallic compounds of Ag₃Sn and Cu₆Sn₅ in the alloy.     

Solder joint reliability of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloys solidified on copper substrates with different surface roughnesses

Abstract

The bond shear test was used to assess the integrity of Sn–0·7Cu and Sn–0·3Ag–0·7Cu lead-free solder alloy drops solidified on copper substrates with smooth and rough surface finishes. Solder alloys solidified on smooth substrates required higher shear force compared to that on rough substrates. Sn–0·3Ag–0·7Cu alloy required higher shear energy than Sn–0·7Cu alloy. Solder alloys solidified on smooth substrate surfaces exhibited complete ductile failure. On rough copper surfaces, solder alloys showed a transition ridge characterized by sheared intermetallic compounds (IMCs) and the presence of dimples. The peak shear strength decreased with increase in contact area of the solder bond on the substrate. Smooth surface and the presence of minor amount of Ag in the solder alloy enhance the integrity of the solder joint.     

Effect of temperature and substrate surface texture on wettability and morphology of IMCs between Sn–0.7Cu solder alloy and copper substrate

In the present work, the effect of soldering temperature (270 and 298?°C) and substrate surface texture (0.02 and 1.12?μm) on wetting characteristics and morphology of intermetallic compounds (IMCs) between Sn–0.7Cu lead-free solder on copper substrates was investigated. It was found that increase in temperature and substrate surface roughness improved the wettability of solder alloy. However, the effect of surface roughness on wettability was significant as compared to that of temperature. The spreading of solder alloy was uniform on smooth substrate, whereas spreading of the alloy on rough substrate resulted in an oval shape. The morphology of IMCs transformed from long needle shaped to short and thick protrusions of IMCs with increase in surface roughness of the substrate. Needle shaped and thick protruded intermetallics formed at the solder/Cu interface were identified as Cu₆Sn₅ compounds. The formation of Cu₃Sn IMC was observed only for the spreading of solder alloy at 298?°C which contributed to improvement in the wettability of solder alloy on both smooth and rough substrate surfaces.     

Effect of reflow temperature and substrate roughness on wettability,IMC growth and shear strength of SAC387/Cu bonds

The effect of reflow temperature and substrate surface roughness on wettability, intermetallics and shear strength of Sn–3.8Ag–0.7Cu solder alloy on copper (Cu) substrate was studied. It was found that increase in reflow temperature and substrate surface roughness improved the wettability of solder alloy. The size of needle shaped Cu₆Sn₅ IMCs (intermetallic compounds) increased with increase in temperature. The morphology of IMCs transformed from long to short needles with increase in substrate roughness. Shear strength and shear energy of the solder bond on rough Cu surfaces were found to be higher than that on smooth Cu surfaces. However, the sheared surfaces of the solder bond on rough Cu surface exhibited a transition ridge characterised by sheared IMCs whereas solder bond on smooth Cu surfaces exhibited completely ductile failure. Although, rough surface exhibited higher shear strength and shear energy, smoother surface is preferable due to its predominant bond failure in the solder matrix.     

Solderability of SnBi-nano Cu solder pastes and microstructure of the solder joints

The solderability of the Sn58Bi (SnBi)-nano Cu solder pastes and the microstructure of the solder joints were investigated. Experimental results indicated that the addition of the nano Cu particles in the SnBi solder paste shows limited effect on the solidus. The liquidus of the SnBi-3nano Cu solder paste was 1 °C higher than the SnBi solder paste. Solid Cu₆Sn₅ intermetallic particles formed in the SnBi-3nano Cu solder paste during the heating process. The Cu₆Sn₅ intermetallic particles decreased the mobility and wettability of the molten solder. Meanwhile, the Cu₆Sn₅ nano particles worked as nucleation sites for the formation of Bi grains and Sn–Bi eutectic phase during the cooling process and led to the grain refinement of the solder bulk. The SnBi-1nano Cu solder paste showed the smallest grain size in this research. Additionally, the SnBi-3nano Cu/Cu solder joint showed a eutectic microstructure of Sn–Bi system at the center of the solder bulk but a hypereutectic microstructure with polygon Bi grains near the margin in the solder bulk.     

The effect of adding Zn into the Sn–Ag–Cu solder on the intermetallic growth rate

Due to toxicity of lead in the commercial solder, lead-free solders were proposed. Among the potential lead-free solders, the Sn–Ag–Cu solders were considered as a potential replacement. To further improve the solder properties, a fourth element was added into the Sn–Ag–Cu solder. The present study investigates the effect of different weight percentage of Zn (up to 0.7 wt%) into the Sn-3.5Ag-1.0Cu solder on intermetallic and growth rate (k) after long time thermal aging. The solders were prepared using powder metallurgy method and X-ray diffraction analysis shows that there were Cu₆Sn₅, Cu₃Sn, CuZn and Ag₃Sn phases present after solder preparation. The solders were reacted with Cu substrate at 250 °C for 1 min and aged at 150 °C until 1,000 h. The morphology of the intermetallic was observed under scanning electron microscope and the elemental distribution was confirmed by energy dispersive X-ray. Intermetallic thickness and growth kinetic result show that the additions of 0.4 % zinc is sufficient in retarding the Cu₆Sn₅ and Cu₃Sn intermetallic growth.     

Effect of Bismuth on Intermetallic Compound Growth in Lead Free Solder/Cu Microelectronic Interconnect

The intermetallic compound (IMC) growth kinetics in pure Sn/Cu and Sn10 wt%Bi/Cu solder joints was studied, respectively, after they were aged at 160-210°C for different time. It was found that the total IMC in Sn10 wt%Bi/Cu joint developed faster than it did in pure Sn/Cu solder joint, when they were aged at the same temperature. And the activation energy Qa for total IMC in Sn10 wt%Bi/Cu joint was lower than that for pure Sn/Cu interconnect. The IMC growth process was discussed. The IMC Cu6Sn5 was enhanced in compensation of reduced IMC Cu3Sn growth. The work reveals that Bi element containing in lead free solder alloys with 10 wt% can enhance IMC growth in lead free solder/Cu joint during service.     

Microstructure and kinetic analysis of the properties and behavior of nickel (Ni) nano-particle doped tin–zinc–bismuth (Sn–8Zn–3Bi) solders on immersion silver (Ag)-plated copper (Cu) substrates

In order to identify the effect on the properties and behavior of tin–zinc–bismuth (Sn-8 wt% Zn-3 wt% Bi or Sn-13.6 at.% Zn-1.6 at.% Bi) based solders produced by adding nickel (Ni) nano-particles, the interfacial microstructure between plain and composite solders with newly developed immersion silver (Ag) plated copper (Cu) substrates has been investigated as a function of reaction time, at various temperatures. For plain Sn–8Zn–3Bi solder joints, a scallop-shaped Cu–Zn–Ag intermetallic compound layer was found to adhere to the surface of the immersion Ag-plated Cu substrate. However, after addition of Ni nano-particles into the Sn–8Zn–3Bi solder, Cu–Zn–Ag (at the bottom) and (Cu, Ni)–Zn (at the top) intermetallic compound layers were observed at the interfaces. In addition, these intermetallic compound layer thicknesses increased substantially with increases in the temperature and reaction time. In the solder ball region, needle-shaped α-Zn rich phase and spherically-shaped Bi-particles appeared to be homogeneously distributed throughout a beta-tin (β-Sn) matrix. However, after the addition of Ni nano-particles, needle-shaped α-Zn rich phase appeared that exhibited a fine microstructure, due to the heterogeneous nucleation of the Ni nano-particles. The calculated activation energy for the Cu–Zn–Ag intermetallic compound layer for the plain Sn–8Zn–3Bi solder/immersion Ag-plated Cu system was 29.95 kJ/mol—while the activation energy for the total [Cu–Zn–Ag + (Cu, Ni)–Zn] intermetallic compound layers formed in the Sn–8Zn–3Bi–0.5Ni (Sn-13.6 at.% Zn-1.6 at.% Bi ~1 at.% Ni) composite solder/immersion Ag-plated Cu system was 27.95 kJ/mol. Addition of Ni nano-particles reduces the activation energy which enhanced the reaction rate as we know that lower the activation energy indicates faster the reaction rate.     

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

Abstract

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.     

Solder Size Effect on Early Stage Interfacial Intermetallic Compound Evolution in Wetting Reaction of Sn3.0Ag0.5Cu/ENEPIG Joints

Solder size effect on early stage interfacial intermetallic compound(IMC) evolution in wetting reaction between Sne3.0Age0.5Cu solder balls and electroless nickel electroless palladium immersion gold(ENEPIG) pads at 250 C was investigated. The interfacial IMCs transformed from initial needle- and rodtype(Cu,Ni)6Sn5to dodecahedron-type(Cu,Ni)6Sn5and then to needle-type(Ni,Cu)3Sn4at the early interfacial reaction stage. Moreover, these IMC transformations occurred earlier in the smaller solder joints, where the decreasing rate of Cu concentration was faster due to the Cu consumption by the formation of interfacial(Cu,Ni)6Sn5. On thermodynamics, the decrease of Cu concentration in liquid solder changed the phase equilibrium at the interface and thus resulted in the evolution of interfacial IMCs; on kinetics, larger solder joints had sufficient Cu flux toward the interface to feed the(Cu,Ni)6Sn5growth in contrast to smaller solder joints, thus resulted in the delayed IMC transformation and the formation of larger dodecahedron-type(Cu,Ni)6Sn5grains. In smaller solders, no spalling but the consumption of(Cu,Ni)6Sn5grains by the formation of(Ni,Cu)3Sn4grains occurred where smaller discrete(Cu,Ni)6Sn5grains formed at the interface.     

Activation energies of intermetallic compound growth at interface between Sn–5Bi–3.5Ag solder and Cu substrate

Abstract

The growth kinetics of intermetallic compound layers formed between Sn–5Bi–3.5Ag solder and Cu substrate were investigated at temperatures between 70°C and 200°C for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. Diffusion couples showed a composite intermetallic layer comprised of Cu₆Sn₅ and Cu₃Sn. The growth of intermetallic compounds followed diffusion controlled kinetics and the layer thickness reached only 10 μm after 60 days of aging at 150°C. The apparent activation energies calculated for the growth of the total intermetallic compound (Cu₆Sn₅ + Cu₃Sn), Cu₆Sn₅ and Cu₃Sn intermetallic are 88.6, 84.3 and 70.28 kJ mol^-1, respectively.     

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.     

Intermetallic compounds formed at the interface between Cu substrate and an Sn-9Zn-0.5Ag lead-free solder

The intermetallic compounds (IMCs) formed at the interface between Cu substrate and an Sn-9Zn-0.5Ag lead-free solder alloy have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron diffraction (ED). The XRD patterns show that the main IMCs formed at the interface of Sn-9Zn-0.5Ag/Cu are γ-Cu₅Zn₈ and η′-Cu₆Sn₅. The Ag₃Sn IMC with orthorhombic structure was also observed at the Sn-9Zn-0.5Ag/Cu interface by TEM and ED analyses. The interfacial adhesion strength between the Cu substrate and Sn-9Zn-0.5Ag lead-free solder alloy is higher than that of the Sn-9Zn alloy due to the formation of Ag₃Sn IMC at the interface.     

Wettability of molten Sn-Bi-Cu solder on Cu substrate

The wetting behavior of a new Sn-Bi-Cu Pb-free solder on Cu substrate was investigated by sessile drop method under an Ar-H₂ flow in the temperature range from 493 K to 623 K. The contact angle curves tested at 548 K and 623 K are found to fit exponential rule very well. However, the contact angle curve tested under 493 K is not well consistent with exponential rule, for which the spreading course may be classified into three stages. Equilibrium contact angles between Sn-Bi-Cu solder and Cu substrate decrease monotonously with the increase in temperature, which are 28°, 24° and 18° at 493 K, 548 K and 623 K, respectively. The results show that 69.5Sn-30Bi-0.5Cu exhibits good wettability on Cu substrate. Intermetallics formed at the 69.5Sn-30Bi-0.5Cu/Cu interface are identified as Cu₆Sn₅ adjacent to the solder and Cu₃Sn adjacent to the Cu substrate, respectively. Formation of intermetallic seems to improve strong wetting of the substrate by the solder.     

Interface analysis of embedded chip resistor device package and its effect on drop shock reliability

In this study, the drop reliability of an embedded passive package is investigated under JESD22-B111 condition. Chip resistors were buried in a PCB board, and it was electrically interconnected by electroless and electrolytic copper plating on a tin pad of a chip resistor without intermetallic phase. However tin, nickel, and copper formed a complex intermetallic phase, such as (Cu, Ni)6Sn5, (Cu, Ni)3Sn, and (Ni, Cu)3Sn2, at the via interface and via wall after reflow and aging. Since the amount of the tin layer was small compared with the solder joint, excessive intermetallic layer growth was not observed during thermal aging. Drop failures are always initiated at the IMC interface, and as aging time increases Cu-Sn-Ni IMC phases are transformed continuously due to Cu diffusion. We studied the intermetallic formation of the Cu via interface and simulated the stress distribution of drop shock by using material properties and board structure of embedded passive boards. The drop simulation was conducted according to the JEDEC standard. It was revealed that the crack starting point related to failure fracture changed due to intermetallic phase transformation along the via interface, and the position where failure occurs experimentally agrees well with our simulation results.     

Role of Wetting Front in Dewetting of Liquid Solder Drop on Cu Thin Films

The thermodynamic conditions for dewetting of a liquid solder drop on copper thin films were examined under a hot-stage optical microscope in a flowing protective atmosphere.Dewetting of liquid solder was found to depend strongly on the copper film thickness and preceded by spalling of Cu 6 Sn 5 intermetallic compounds.However,the loss of interfacial bonding by spalling was not sufficient to cause immediate dewetting of solder drops if the wetting tip was still strongly bonded to the copper film.By introducing a pinning force on the wetting front,a sufficient condition was found from a force balance analysis for dewetting of the liquid solder drop,in general agreement with the experimental results.     

The effect of reflow time on reactive wetting,evolution of interfacial IMCs and shear strength of eutectic Sn–Cu solder alloy

In the present work, the effect of reflow time on wetting behaviour, microstructure and shear strength of the eutectic Sn–0.7Cu lead-free solder on Cu substrate were studied. The reflow time was varied from 10 to 10,000 s. The contact angle decreased with the increase in reflow time. The growth of intermetallic compounds (IMCs) increased with the reflow time. The thickness of the Cu₆Sn₅ IMC layer formed during a reflow time of 10 s was about 3.76 μm and its thickness increased to 3.89, 6.2, 6.68, 7.6 and 19.83 μm during 100, 300, 500, 1,000 and 10,000 s reflow time respectively. The joint shear test was performed to assess the integrity of Sn–0.7Cu solder solidified on copper substrate surfaces. The shear strength decreased with increase in reflow time after an optimum value.     

Interfacial reaction and growth behavior of IMCs layer between Sn–58Bi solders and a Cu substrate

This paper reports on the interfacial reaction and growth behavior of intermetallic compounds (IMCs) layer (η-Cu₆Sn₅ + ε-Cu₃Sn) between molten Sn–58Bi solder and Cu substrate for various liquid–solid soldering temperatures and times. In addition, the Bi segregation at the Cu₃Sn/Cu interface was also discussed, too. It was found that the Cu₆Sn₅ IMC could be observed as long as the molten solder contacted with the Cu substrate, while the Cu₃Sn IMC was formed at the interface between Cu₆Sn₅ and Cu substrate as the higher soldering temperature and/or longer soldering time were applied. Both thickness of total IMCs layer and Cu₆Sn₅ grains size increased with increased soldering temperature or time. The growth of the Cu-Sn IMCs layer during soldering exhibited a linear function of the soldering temperature and 0.27 power of soldering time. With soldering temperature increasing (above 280 °C in this present study), Bi was accumulated at the Cu₃Sn/Cu interface and resulted in some isolated Bi particles were formed.