Experimental characterization and mechanical behavior analysis of intermetallic compounds of Sn–3.5Ag lead-free solder bump with Ti/Cu/Ni UBM on copper chip

Due to today’s trend towards ‘green’ products, the environmentally conscious manufacturers are moving toward lead-free schemes for electronic devices and components. Nowadays the bumping process has become a branch of the infrastructure of flip chip bonding technology. However, the formation of excessively brittle intermetallic compound (IMC) between under bump metallurgy (UBM)/solder bump interface influences the strength of solder bumps within flip chips, and may create a package reliability issue. Based on the above reason, this study investigated the mechanical behavior of lead-free solder bumps affected by the solder/UBM IMC formation in the duration of isothermal aging. To attain the objective, the test vehicles of Sn–Ag (lead-free) and Sn–Pb solder bump systems designed in different solder volumes as well as UBM diameters were used to experimentally characterize their mechanical behavior. It is worth to mention that, to study the IMC growth mechanism and the mechanical behavior of a electroplated solder bump on a Ti/Cu/Ni UBM layer fabricated on a copper chip, the test vehicles are composed of, from bottom to top, a copper metal pad on silicon substrate, a Ti/Cu/Ni UBM layer and electroplated solder bumps. By way of metallurgical microscope and scanning-electron-microscope (SEM) observation, the interfacial microstructure of test vehicles was measured and analyzed. In addition, a bump shear test was utilized to determine the strength of solder bumps. Different shear displacement rates were selected to study the time-dependent failure mechanism of the solder bumps. The results indicated that after isothermal aging treatment at 150 °C for over 1000 h, the Sn–Ag solder revealed a better maintenance of bump strength than that of the Sn–Pb solder, and the Sn–Pb solder showed a higher IMC growth rate than that of Sn–Ag solder. In addition, it was concluded that the test vehicles of copper chip with the selected Ti/Cu/Ni UBMs showed good bump strength in both the Sn–Ag and Sn–Pb systems as the IMC grows. Furthermore, the study of shear displacement rate effect on the solder bump strength indicates that the analysis of bump strength versus thermal aging time should be identified as a qualitative analysis for solder bump strength determination rather than a quantitative one. In terms of the solder bump volume and the UBM size effects, neither the Sn–Ag nor the Sn–Pb solders showed any significant effect on the IMC growth rate.     

The influence of solder volume and pad area on Sn–3.8Ag–0.7Cu and Ni UBM reaction in reflow soldering and isothermal aging

This paper examines various aspects of SAC (Sn–3.8Ag–0.7Cu wt.%) solder and UBM interactions which may impact interconnection reliability as it scales down. With different solder joint sizes, the dissolution rate of UBM and IMC growth kinetics will be different. Solder bumps on 250, 80 and 40 μm diameter UBM pads were investigated. The effect of solder volume/pad metallization area (V/A) ratio on IMC growth and Ni dissolution was investigated during reflow soldering and solid state isothermal aging. Higher V/A ratio produced thinner and more fragmented IMC morphology in SAC solder/Ni UBM reflow soldering interfacial reaction. Lower V/A ratio produced better defined IMC layer at the Ni UBM interface. When the ratio of V/A is constant, the IMC morphology and growth trend was found to be similar. After 250 h of isothermal aging, the IMC growth rate of the different bump sizes leveled off. No degradation in shear strength was observed in these solder bump after 500 h of isothermal aging.     

Interfacial Reaction of Sn and Cu-<Emphasis Type="Italic">x</Emphasis>Zn Substrates After Reflow and Thermal Aging

Zn additions to Cu under bump metallurgy (UBM) in solder joints were the subject of this study. An alternative design was implemented to fabricate pure Sn as the solder and Cu-xZn (x = 15 wt.% and 30 wt.%) as the UBM to form the reaction couple. As the Zn content increased from 15 wt.% to 30 wt.% in the Sn/Cu-Zn system, growth of both Cu₃Sn and Cu₆Sn₅ was suppressed. In addition, no Kirkendall voids were observed at the interface in either Sn/Cu-Zn couple during heat treatment. After 40-day aging, different multilayered phases of [Cu₆Sn₅/Cu₃Sn/Cu(Zn)] and [Cu₆Sn₅/Cu(Zn,Sn)/CuZn] formed at the interface of [Sn/Cu-15Zn] and [Sn/Cu-30Zn] couples, respectively. The growth mechanism of intermetallic compounds (IMCs) during aging is discussed on the basis of the composition variation in the joint assembly with the aid of electron-microscopic characterization and the Sn-Cu-Zn ternary phase diagram. According to these analyses of interfacial morphology and IMC formation in the Sn/Cu-Zn system, Cu-Zn is a potential UBM for retarding Cu pad consumption in solder joints.     

Impact strength of Sn–3.0Ag–0.5Cu solder bumps during isothermal aging

In order to investigate the fracture behavior of Sn–3.0Ag–0.5Cu solder bump, solder balls with the diameter of 0.76 mm were soldered on Cu pad in this study, then high speed impact test and static shear test of solder bumps were carried out to measure the joint strength of the soldering interface. The effect of isothermal aging on joint strength as well as fracture behavior of solder bumps was investigated, and the composition of the fracture surface was identified by means of EPMA. The results indicate that the fracture is inside the bulk solder in low speed shear test regardless of the aging effect, thus the maximum load reflects the solder strength rather than the interfacial strength. It is also found that under 1 m/s impact loading, the crack initiation position is changed from solder/Cu₆Sn₅ interface to Cu₃Sn/Cu interface after long time isothermal aging, and the fracture occurs inside the bulk solder accompanying with intermetallic compound in both of the as-soldered and aged joints. The thickened multiple IMC layers during isothermal aging account for the degraded impact resistance, and the change of the solder matrix is another factor for reduced impact resistance owing to Sn residue on the fracture surface.     

Isothermal aging effects on the microstructure and solder bump shear strength of eutectic Sn37Pb and Sn3.5Ag solders

The reliability of the eutectic Sn37Pb (63%Sn37%Pb) and Sn3.5Ag (96.5%Sn3.5%Ag) solder bumps with an under bump metallization (UBM) consisting of an electroless Ni(P) plus a thin layer of Au was evaluated following isothermal aging at 150 °C. All the solder bumps remained intact after 1500 h aging at 150 °C. Solder bump microstructure evolution and interface structure change during isothermal aging were observed and correlated with the solder bump shear strength and failure modes. Cohesive solder failure was the only failure mode for the eutectic Sn37Pb solder bump, while partial cohesive solder failure and partial Ni(P) UBM/Al metallization interfacial delamination was the main failure mode for eutectic Sn3.5Ag solder bump.     

Effect of displacement rate on ball shear properties for Sn–37Pb and Sn–3.5Ag BGA solder joints during isothermal aging

The interfacial reactions and ball shear properties of ball grid array (BGA) solder joints aged at 170 °C for up to 21 days were investigated with different displacement rates. Two different kinds of solders, Sn–37Pb and Sn–3.5Ag (all wt.%), and an electroplated Ni/Au BGA substrate were employed in this work. A continuous Ni₃Sn₄ intermetallic compound (IMC) layer was formed at the interfaces between both the Sn–37Pb and Sn–3.5Ag solders and the substrate during reflow. After aging, two different reaction layers, consisting of (Au_xNi_1−x)Sn₄ IMC and Pb-rich phase, were additionally observed between the Sn–37Pb solder and the Ni₃Sn₄ IMC layer. The thicknesses of these interfacial reaction layers increased with increasing aging time. After reflow, all the fractures occurred inside the bulk solder. The fracture location of the Sn–37Pb solder joints was shifted toward the solder/Ni interface with increasing aging time and displacement rate, whereas the fracture of the Sn–3.5Ag solder joints mainly occurred inside the bulk solder, irrespective of the aging time and displacement rate. Consequently, the shear properties of the Sn–37Pb solder joints significantly decreased with increasing aging time, whereas those of the Sn–3.5Ag solder joints slightly decreased. The tendency toward brittle fracture of the Sn–37Pb solder joints was intensified with increasing displacement rate. The shear properties of the ductile solder joints increased with increasing displacement rate, while the displacement until fracture, deformation energy and displacement rate sensitivity of the brittle solder joints significantly decreased with increasing displacement rate.     

The formation and growth of intermetallic compounds and shear strength at Sn–Zn solder/Au–Ni–Cu interfaces

The microstructures and shear strength of the interface between Sn–Zn lead-free solders and Au/Ni/Cu interface under thermal aging conditions was investigated. The intermetallic compounds (IMCs) at the interface between Sn–Zn solders and Au/Ni/Cu interface were analyzed by field emission scanning electron microscopy and transmission electron microscopy. The results showed the decrease in the shear strength of the interface with aging time and temperature. The solder ball with highly activated flux had about 8.2% increased shear strength than that with BGA/CSP flux. Imperfect wetting and many voids were observed in the fracture surface of the latter flux. The decreased shear strength was influenced by IMC growth and Zn grain coarsening. In the solder layer, Zn reacted with Au and then was transformed to the β-AuZn compound. Although AuZn grew first, three diffusion layers of γ-Ni₅Zn₂₁ compounds were formed after aging for 600 h at 150 °C. The layers divided by Ni₅Zn₂₁ (1), (2), and (3) were formed with the thickness of 0.7 μm, 4 μm, and 2 μm, respectively.     

Cu Substrates with Different Grain Sizes

Cu₆Sn₅ and Cu₃Sn are easily formed at the interface between Sn and Cu during reflow and aging processes. Thick Cu-Sn compounds at the interface become brittle, reducing the mechanical strength of solder joints and increasing the consumption of under bump metallization (UBM). It is noted that intermetallic compound (IMC) growth and substrate consumption are affected by factors such as substrate fabrication, substrate orientation, and substrate microstructure. In this study, to determine the effects of substrate grain size on IMC growth and substrate consumption, pure Sn solder was reflowed on annealed Cu substrates with different grain sizes at 250°C for 30 s to 600 s. It was revealed that Cu substrates with smaller grain sizes exhibited reduced IMC growth. In addition, the interdiffusion coefficients of Cu₆Sn₅ and Cu₃Sn were decreased for the Cu substrate with the smaller grain size. The influence of the Cu substrate grain size on IMC growth and substrate consumption is discussed.     

The influence of Sn–Cu–Ni(Au) and Sn–Au intermetallic compounds on the solder joint reliability of flip chips on low temperature co-fired ceramic substrates

The formation of intermetallic compounds in the solder joint of a flip chip or chip scale package depends on the under bump metallurgy (UBM), the substrate top surface metallisation, the solder alloy and the application conditions. To evaluate the influence of intermetallic compounds on the solder joint reliability, a detailed study on the influence of the UBM, the gold finish thickness of the substrate top surface metallisation, the solder alloy and the aging conditions has been conducted. Flip chips bumped with different solder alloys were reflow-mounted on low temperature co-fired ceramic substrates. The flip chip package was then aged at high temperature and a bump shear test followed to examine the shear strength of the solder joint at certain aging intervals. It was found that the type of UBM has a great impact on the solder joint reliability. With Ni(P)/Au as the UBM, well-documented gold embrittlement was observed when the gold concentration in the eutectic SnPb solder was about 3 wt%. When Al/Ni(V)/Cu was used as the UBM, the solder joint reliability was substantially improved. Copper dissolution from the UBM into the solder gives different intermetallic formations compared to Ni(P)/Au as UBM. The addition of a small amount of copper in the solder alloy changed the mechanical property of the intermetallic compound, which is attributed to the formation of Sn–Cu–Ni(Au) intermetallic compounds. This could be used in solving the problem of the AuSn₄ embrittlement. The formation and the influence of this Sn–Cu–Ni(Au) intermetallic phase are discussed. The gold concentration in the solder joint plays a role in the formation of intermetallic compounds and consequently the solder joint reliability, especially for the Sn–Ag–Cu soldered flip chip package.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Effects of fourth alloying additive on microstructures and tensile properties of Sn–Ag–Cu alloy and joints with Cu

The effects of the fourth elements, i.e., Fe, Ni, Co, Mn and Ti, on microstructural features, undercooling characteristics, and monotonic tensile properties of Sn–3 wt.%Ag–0.5 wt.%Cu lead-free solder were investigated. All quaternary alloys basically form third intermetallic compounds in addition to fine Ag₃Sn and Cu₆Sn₅ and exhibit improved solder structure. The precipitates of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni, Ti and Mn) alloy are very fine comparing with the other alloys. The effective elements for suppressing undercooling in solidification are Ti, Mn, Co and Ni. All quaternary bulk alloys exhibit similar or slightly larger tensile strengths; especially Mn and Ni can improve elongation without degrading strength. The interfacial phases of Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Fe, Mn and Ti)/Cu joints are typical Cu₆Sn₅ scallops. Sn–3Ag–0.5Cu (–0.1 wt.%X; X=Ni and Co)/Cu joints form very fine Sn–Cu–Ni and Sn–Cu–Co scallops at interface. The Cu/Sn–3Ag–0.5Cu–0.1Ni/Cu joint exhibits improved tensile strength prior to thermal aging at 125 and 150 °C. The fracture surface of Cu/Sn–3Ag–0.5Cu/Cu joint exhibits mixture of ductile and brittle fractures, while Cu/Sn–3Ag–0.5Cu (–0.1X; X=Ni and Co)/Cu joints exhibit only brittle fracture at interface. The Sn–3Ag–0.5Cu–0.1Ni alloy is more reliable solder alloy with improved properties for all tests in the present work.     

High temperature reliability and interfacial reaction of eutectic Sn–0.7Cu/Ni solder joints during isothermal aging

The interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–0.7Cu (wt.%) solder and Au/Ni/Cu substrate were investigated at aging temperatures of 185 and 200 °C for aging times of up to 60 days. After reflow, the IMC formed at the interface was (Cu, Ni)₆Sn₅. After aging at 185 °C for 3 days and at 200 °C for 1 day, two IMCs of (Cu, Ni)₆Sn₅ and (Ni, Cu)₃Sn₄ were observed. The growth of the (Ni, Cu)₃Sn₄ IMC consumed the (Cu, Ni)₆Sn₅ IMC at an aging temperature of 200 °C due to the restriction of supply of Cu atoms from the solder to interface. After aging at 200 °C for 60 days, the Ni layer of the substrate was completely consumed in many parts of the sample, at which point a Cu₃Sn IMC was formed. In the ball shear test, the shear strength decreased with increasing aging temperature and time. Until the aging at 185 °C for 15 days and at 200 °C for 3 days, fractures occurred in the bulk solder. After prolonged aging treatment, fractures partially occurred at the (Cu, Ni)₆Sn₅ + Au/solder interface for aging at 185 °C and at the (Ni, Cu)₃Sn₄/Ni interface for aging at 200 °C, respectively. Consequently, thick IMC layer and thermal loading history significantly affected the integrity of the Sn–0.7Cu/Ni BGA joints.     

Effect of multiple reflows on interfacial reaction and shear strength of Sn-Ag electroplated solder bumps for flip chip package

The effect of displacement rate and intermetallic compound (IMC) growth on the shear strength of electroplated Sn-2.5Ag (in wt.%) flip chip solder with Cu under-bump metallization (UBM) were investigated after multiple reflows. Cu₆Sn₅ IMC was formed at the interface after one reflow. After five reflows, two different IMC layers, consisting of a scallop-shaped Cu₆Sn₅ phase and a planar Cu₃Sn phase, and their thicknesses increased with increasing reflow number up to 10. The shear strengths peaked after four reflows, and then decreased with increasing reflow number. Increasing displacement rate increased the shear force. The tendency toward brittle fracture characteristics was intensified with increasing displacement rate and reflow number.     

A study on interfacial reactions between electroless Ni-P under bump metallization and 95.5Sn-4.0Ag-0.5Cu alloy

Even though electroless Ni-P and Sn-Ag-Cu solders are widely used materials in flip-chip bumping technologies, interfacial reactions of the ternary Cu-Ni-Sn system are not well understood. The growth of intermetallic compounds (IMCs) at the under bump metallization (UBM)/solder interface can affect solder-joint reliability, so analysis of IMC phases and understanding their growth kinetics are important. In this study, interfacial reactions between electroless Ni-P UBM and the 95.5Sn-4.0Ag-0.5Cu alloy were investigated, focusing on identification of IMC phases and IMC growth kinetics at various reflowing and aging temperatures and times. The stable ternary IMC initially formed at the interface after reflowing was the (Cu,Ni)₆Sn₅ phase. However, during aging, the (Cu,Ni)₆Sn₅ phase slowly changed into the quaternary IMC composed of Cu, Ni, Sn, and a small amount of Au. The Au atoms in the quaternary IMC originated from immersion Au plated on electroless Ni-P UBM. During further reflowing or aging, the (Ni,Cu)₃Sn₄ IMC started forming because of the limited Cu content in the solder. Morphology, composition, and crystal structure of each IMC were identified using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Small amounts of Cu in the solder affect the types of IMC phases and the amount of the IMC. The activation energies of (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were used to estimate the growth kinetics of IMCs. The growth of IMCs formed in aging was very slow and temperature-dependent compared to IMCs formed in reflow because of the higher activation energies of IMCs in aging. Comparing activation energies of each IMC, growth mechanism of IMCs at electroless Ni-P/SnAgCu solder interface will be discussed.     

Effects of Cu contents in Pb-free solder alloys on interfacial reactions and bump reliability of Pb-free solder bumps on electroless Ni-P under-bump metallurgy

Using the screen-printed solder-bumping technique on the electroless plated Ni-P under-bump metallurgy (UBM) is potentially a good method because of cost effectiveness. As SnAgCu Pb-free solders become popular, demands for understanding of interfacial reactions between electroless Ni-P UBMs and Cu-containing Pb-free solder bumps are increasing. It was found that typical Ni-Sn reactions between the electroless Ni-P UBM and Sn-based solders were substantially changed by adding small amounts of Cu in Sn-based Pb-free solder alloys. In Cu-containing solder bumps, the (Cu,Ni)₆Sn₅ phase formed during initial reflow, followed by (Ni,Cu)₃Sn₄ phase formation during further reflow and aging. The Sn3.5Ag solder bumps showed a much faster electroless Ni-P UBM consumption rate than Cu-containing solder bumps: Sn4.0Ag0.5Cu and Sn0.7Cu. The initial formation of the (Cu,Ni)₆Sn₅ phase in SnAgCu and SnCu solders significantly reduced the consumption of the Ni-P UBM. The more Cu-containing solder showed slower consumption rate of the Ni-P UBM than the less Cu-containing solder below 300°C heat treatments. The growth rate of the (Cu,Ni)₆Sn₅ intermetallic compound (IMC) should be determined by substitution of Ni atoms into the Cu sublattice in the solid (Cu,Ni)₆Sn₅ IMC. The Cu contents in solder alloys only affected the total amount of the (Cu,Ni)₆Sn₅ IMC. More Cu-containing solders were recommended to reduce consumption of the Ni-based UBM. In addition, bump shear strength and failure analysis were performed using bump shear test.     

Studies of electroless nickel under bump metallurgy—Solder interfacial reactions and their effects on flip chip solder joint reliability

The electroless-deposited Ni-P under bump metallurgy (UBM) layer was fabricated on Al pads for Sn containing solder bumps. The amount of P in the electroless Ni film was optimized by controlling complexing agents and the pH of plating solution. The interfacial reaction at the electroless Ni UBM/solder interface was investigated in this study. The intermetallic compound (IMC) formed at the interface during solder reflowing was mainly Ni₃Sn₄, and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the Ni-Sn IMC and the electroless Ni layer. One to four microns of Ni₃Sn₄ IMC and a 1800–5000 Å of P-rich Ni layer were formed in less than 10 min of solder reflowing depending on solder materials and reflow temperatures. It was found that the P-rich Ni layer contains Ni, P, and a small amount of Sn (～7 at.%). Further cross-sectional transmission electron microscopy (TEM) analysis confirmed that the composition of the P-rich Ni layer was 75 at.% Ni, 20at.%P, and 5at.%Sn by energy-dispersive x-ray spectroscopy (EDS) and the phase transformation occurred in the P-rich Ni layer by observing grain size. Kirkendall voids were also found in the Ni₃Sn₄ IMC, just above the P-rich Ni layer after extensive solder reflow. The Kirkendall voids are considered a primary cause of the brittle fracture; restriction of the growth of of the P-rich Ni layer by optimizing proper processing conditions is recommended. The growth kinetics of Ni-Sn IMC and P-rich Ni layer follows three steps: a rapid initial growth during the first 1 min of solder reflow, followed by a reduced growth step, and finally a diffusion-controlled growth. During the diffusion-controlled growth, there was a linear dependence between the layer thickness and time^1/2. Flip chip bump shear testing was performed to measure the effects of the IMC and the P-rich Ni layers on bump adhesion property. Most failures occurred in the solder and at the Ni₃Sn₄ IMC. The brittle characteristics of the Ni-Sn IMC and the Kirkendall voids at the electroless Ni UBM-Sn containing solder system cause brittle bump failure, which results in a decreased bump adhesion strength.     

Mechanism of interfacial reaction for the Sn-Pb solder bump with Ni/Cu under-bump metallization in flip-chip technology

Nickel-based under-bump metallization (UBM) has been widely used in flip-chip technology (FCT) because of its slow reaction rate with Sn. In this study, solder joints after reflows were employed to investigate the mechanism of interfacial reaction between the Ni/Cu UBM and eutectic Sn-Pb solder. After deliberate quantitative analysis with an electron probe microanalyzer (EPMA), the effect of Cu content in solders near the interface of the solder/intermetallic compound (IMC) on the interfacial reaction could be probed. After one reflow, only one layered (Ni_1−x,Cu_x)₃Sn₄ with homogeneous composition was found between the solder bump and UBM. However, after multiple reflows, another type of IMC, (Cu_1−y,Ni_y)₆Sn₅, formed between the solder and (Ni_1−x,Cu_x)₃Sn₄. It was observed that if the concentration of Cu in the solders near the solder/IMC interface was higher than 0.6 wt.%, the (Ni_1−x,Cu_x)₃Sn₄ IMC would transform into the (Cu_1−y,Ni_y)₆Sn₅ IMC. The Cu contents in (Ni_1−x,Cu_x)₃Sn₄ were altered and not uniformly distributed anymore. With the aid of microstructure evolution, quantitative analysis, elemental distribution by x-ray color mapping, and related phase equilibrium of Sn-Ni-Cu, the reaction mechanism of interfacial phase transformation between the Sn-Pb solder and Ni/Cu UBM was proposed.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

Comparison of electroplated eutectic Bi/Sn and Pb/Sn solder bumpson various UBM systems

The effect of a reflow process and under bump metallurgy (UBM) systems on the growth of intermetallic compounds (IMC) of the 57Bi/43Sn and 37Pb/63Sn solder bump/UBM interfaces was investigated. The selected UBM systems were sputtered Al/Ti/Cu, sputtered Al/NiV/Cu, Al/electroless Ni/immersion Au, and Al/Ti/electroless Cu. An alloy electroplating method was used for the solder bumping process. The microstructure and composition of intermetallic compound (IMC) phases and their morphologies were examined using scanning electron microscopy and X-ray diffraction. The Cu₆Sn₅ η'-phase IMC appeared on all Cu containing UBM cases with Pb/Sn and Bi/Sn solders and the Cu ₃Sn ϵ-phase was detected only with Pb/Sn solder bumps. The Ni₃Sn₄ IMC was found to be the main IMC phase between Ni and solder. The Ni₃Sn secondary IMC was also detected on the electroless Ni UBM with PbSn solder after ten times reflow. Through the bump shear test, Al/NiV/Cu, Al/elNi/Au, and Al/Ti/elCu UBMs showed good stability with Bi/Sn and Pb/Sn solder in terms of metallurgical aspects     

Mechanical and Electrical Properties of Cu/Sn-3.5Ag/Cu Ball Grid Array (BGA) Solder Joints after Multiple Reflows

The microstructural evolution, die shear strength, and electrical resistivity of Cu/Sn-3.5Ag (wt.%)/Cu ball grid array (BGA) solder joints were investigated after 1 to 10 reflows using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), bonding testing, and a four-point probe station. A Cu₆Sn₅ intermetallic compound (IMC) was formed at both the upper and lower interfaces after one reflow. The IMC thickness increased at the lower interface with increasing reflow number, whereas the IMC morphology and thickness remained virtually unchanged at the upper interface, irrespective of the reflow number. The amount of Cu₆Sn₅ IMC contained in the solder ball increased with increasing reflow number. These microstructural evolutions with increasing reflow number strongly affected the mechanical and electrical properties of the solder joint.