Ambient Temperature Ultrasonic Bonding of Si-Dice Using Sn-3.5wt.%Ag

Ultrasonic bonding of Si-dice to type FR-4 printed circuit boards (PCB) with Sn-3.5wt.%Ag solder at ambient temperature was investigated. The under-bump metallization (UBM) on the Si-dice comprised Cu/Ni/Al from top to bottom with thicknesses of 0.4 μm, 0.4 μm, and 0.3 μm, respectively. The pads on the PCBs consisted of Au/Ni/Cu with thicknesses of 0.05/5/18 μm, sequentially from top to bottom. Solder was supplied as Sn-3.5wt.%Ag foil rolled to 100 μm thickness, and inserted in the joints. The ultrasonic bonding time was varied from 0.5 s to 3.0 s, and the ultrasonic power was 1400 W. The experimental results showed that reliable joints could be produced between the Si-dice and the PCBs with Sn-3.5wt.%Ag solder. The joint breaking force of “Si-die/solder/FR-4” increased with bonding times up to 2.5 s with a maximum value of 65 N. A bonding time of 3.0 s proved to be excessive, and resulted in cracks along the intermetallic compound between the UBM and solder, which caused a decrease in the bond strength. The intermetallic compound produced by ultrasonic bonding between the UBM and solder was confirmed to be (Cu, Ni)₆Sn₅. An erratum to this article can be found at     

Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated ∼62 μm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150°C, 10 kA/cm² at 160°C, and 15 kA/cm² at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52-μm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.     

A Study on the Thermal Reliability of Cu/SnAg Double-Bump Flip-Chip Assemblies on Organic Substrates

The Cu/SnAg double-bump structure is a promising candidate for fine-pitch flip-chip applications. In this study, the interfacial reactions of Cu (60 μm)/SnAg (20 μm) double-bump flip chip assemblies with a 100 μm pitch were investigated. Two types of thermal treatments, multiple reflows and thermal aging, were performed to evaluate the thermal reliability of Cu/SnAg flip-chip assemblies on organic printed circuit boards (PCBs). After these thermal treatments, the resulting intermetallic compounds (IMCs) were identified with scanning electron microscopy (SEM), and the contact resistance was measured using a daisy-chain and a four-point Kelvin structure. Several types of intermetallic compounds form at the Cu column/SnAg solder interface and the SnAg solder/Ni pad interface. In the case of flip-chip samples reflowed at 250°C and 280°C, Cu₆Sn₅ and (Cu, Ni)₆Sn₅ IMCs were found at the Cu/SnAg and SnAg/Ni interfaces, respectively. In addition, an abnormal Ag₃Sn phase was detected inside the SnAg solder. However, no changes were found in the electrical contact resistance in spite of severe IMC formation in the SnAg solder after five reflows. In thermally aged flip-chip samples, Cu₆Sn₅ and Cu₃Sn IMCs were found at the Cu/SnAg interface, and (Cu, Ni)₆Sn₅ IMCs were found at the SnAg/Ni interface. However, Ag₃Sn IMCs were not observed, even for longer aging times and higher temperatures. The growth of Cu₃Sn IMCs at the Cu/SnAg interface was found to lead to the formation of Kirkendall voids inside the Cu₃Sn IMCs and linked voids within the Cu₃Sn/Cu column interfaces. These voids became more evident when the aging time and temperature increased. The contact resistance was found to be nearly unchanged after 2000 h at 125°C, but increases slightly at 150°C, and a number of Cu/SnAg joints failed after 2000 h. This failure was caused by a reduction in the contact area due to the formation of Kirkendall and linked voids at the Cu column/Cu₃Sn IMC interface.     

Effect of Stand-Off Height on Microstructure and Tensile Strength of the Cu/Sn9Zn/Cu Solder Joint

To investigate the effect of stand-off height (SOH) on the microstructure and mechanical behavior of certain solder joints, Cu/Sn9Zn/Cu solder joints with a SOH of 100 μm, 50 μm, 20 μm, and 10 μm were prepared and studied. It was found that, as the SOH is reduced, the Zn content in the solder bulk experiences a rapid decrease due to consumption by metallurgical reaction in the reflow process; hence, the microstructure of the solder bulk is changed significantly from a Sn-Zn eutectic structure to a hypoeutectic structure. By contrast, Cu content in the solder bulk experiences a rapid increase with reducing SOH, and this leads to more dissociative intermetallic compounds (IMCs) in the solder bulk. These compositional and microstructural changes induced by reducing the SOH correlate closely with the mechanical properties of the solder joints. In our study it is found that, as SOH is reduced, the tensile strength of the solder joints decreases, the fracture path of the solder joint transfers from the solder bulk into the interface between the IMC layer and the solder bulk, and the fracture mode tends to change from ductile to brittle. These findings point to a probable way to improve the mechanical properties of miniaturized solder joints by controlling the composition and dissociative IMCs in the solder bulk.     

Influence of Bonding Parameters on the Interaction Between Cu and Noneutectic Sn-In Solder Thin Films

Thermocompression bonding of through-layer copper interconnects is of great interest for fabrication of three-dimensional (3D) integrated circuits. We have investigated interactions of Cu films with noneutectic Sn-In at length scales of 1 μm to 5 μm. The effects of bonding time, bonding temperature, and post- bonding annealing temperature on intermetallic compound (IMC) formation, joint microstructure, and shear strength were investigated using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometry (XRD), and shear testing. It is shown that bonding temperature plays an important role in increasing the true contact area, while the postbonding annealing temperature affects the formation of a single IMC, the η-phase [Cu₆(Sn,In)₅]. Both of these phenomena were found to contribute to the shear strength of the joints. It is shown that two-step bonding processes, involving short bonding times and longer postbonding annealing, can be used to optimize the bond formation for increased throughput.     

The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 μm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 μm)/Ni/Ti UBM, and it was confirmed to be the (Au_1−x−yBi_xNi_y)Sn₂ phase by XRD. The intermetallic compounds grown form the Au (0.1 μm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 μm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.     

Three-Dimensional Thermoelectrical Simulation in Flip-Chip Solder Joints with Thick Underbump Metallizations during Accelerated Electromigration Testing

In flip-chip solder joints, thick Cu and Ni films have been used as under bump metallization (UBM) for Pb-free solders. In addition, electromigration has become a crucial reliability concern for fine-pitch flip-chip solder joints. In this paper, the three-dimensional (3-D) finite element method was employed to simulate the current-density and temperature distributions for the eutectic SnPb solder joints with 5-μm Cu, 10-μm Cu, 25-μm Cu, and 25-μm Ni UBMs. It was found that the thicker the UBM is the lower the maximum current density inside the solder. The maximum current density is 4.37 × 10⁴ A/cm², 1.69 × 10⁴ A/cm², 7.54 × 10³ A/cm², and 1.34 × 10⁴ A/cm², respectively, when the solder joints with the above four UBMs are stressed by 0.567 A. The solder joints with thick UBMs can effectively relieve the current crowding effect inside the solder. In addition, the joint with the thicker Cu UBM has a lower Joule heating effect in the solder. The joint with the 25-μm Ni UBM has the highest Joule heating effect among the four models.     

Effects of Solder Volume on Formation and Redeposition of Au-Containing Intermetallics in Ni/Au-SnAgCu-Ni(P) Solder Joints

SnAgCu is one of the most promising candidates for lead-free solders to replace conventional eutectic SnPb solders. The effects of solder volume on interfacial reactions and microstructure evolution in Ni/Au-SnAgCu-Ni(P) solder joints have been investigated under soldering and thermal aging conditions. The results show that solder volume has a strong effect on the formation of Au-containing intermetallic compounds (IMCs) and their redeposition at the interfaces. The size and volume fraction of Au-containing IMCs dispersed in the solder bulk increased with decreasing solder joint dimensions. For the small solder joint with 300-μm solder ball, the (Au,Ni)Sn₄ IMCs redeposited to the interfaces after thermal aging at 150°C for 9 days, but this was not observed for the other two solder joints with large solder volume. These results also indicate that the redeposition of (Au,Ni)Sn₄ is closely associated with the migration of Cu toward the interfaces.     

Interface Formation in the US-Wedge/Wedge-Bond Process of AlSi1/CuNiAu Contacts

Interface formation between 25-μm AlSi1 wire and flash-Au substrate metallization during a bonding time of 50 ms has been investigated. Only a few milliseconds after the ultrasonic power is switched on, intermetallic phase growth starts, continuing until the end of the wire-bonding process. During the entire bonding time, the fraction of the interface covered with Au₈Al₃ increases, and at the end of the bonding time, the interface is nearly completely covered with that phase. Finite-element modeling (FEM) of the temperature in the interface region indicates maximum temperatures well below 100°C, thus making solely thermally activated intermetallic phase growth impossible. However, it is demonstrated that the phase growth observed during the ultrasonic wire-bonding process could result from an accelerated diffusion process caused by a higher vacancy concentration. The accelerated diffusion process would have an activation energy Q of 0.36 eV.     

Effect of Al-Trace Width on the Electromigration Failure Time of Flip-Chip Solder Joints

The effect of Al-trace width on electromigration (EM) in flip-chip solder joints was investigated experimentally. EM tests were performed on eutectic Sn-Ag solders with 40-μm- and 100-μm-wide Al traces. Under the same stressing conditions (0.5 A at 165°C), the failure time was 44.1 h for solder joints with 40-μm-wide traces and 250.1 h for solder joints with 100-μm-wide traces. The Al-trace width influenced both the current crowding and the Joule heating effects. Thus, both effects are responsible for the significant difference in failure time. Finite-element analysis was used to examine the current crowding effect in solder bumps with Al traces of the two different widths. The results showed that the current crowding effect was slightly higher in joints with 40-μm-wide traces. In addition, the temperature coefficient was used to measure the real temperatures in the solder bumps during EM. The results indicated that the width of the Al traces had a substantial influence on the Joule heating effect. The measured temperature in the solder bump was 218.2°C and 172.2°C for the bump with 40-μm- and 100-μm-wide Al traces, respectively. This difference in the Joule heating effect plays a crucial role in causing the difference in the failure time of solder joints with the two different widths.     

Wetting and Soldering Behavior of Eutectic Au-Ge Alloy on Cu and Ni Substrates

Au-Ge-based alloys are interesting as novel high-temperature lead-free solders because of their low melting point, good thermal and electrical conductivity, and high corrosion resistance. In the present work, the wetting and soldering behavior of the eutectic Au-28Ge (at.%) alloy on Cu and Ni substrates have been investigated. Good wetting on both substrates with final contact angles of 13° to 14° was observed. In addition, solder joints with bond shear strength of 30 MPa to 35 MPa could be produced under controlled conditions. Cu substrates exhibit pronounced dissolution into the Au-Ge filler metal. On Ni substrates, the NiGe intermetallic compound was formed at the filler/substrate interface, which prevents dissolution of Ni into the solder. Using thin filler metal foils (25 μm), complete consumption of Ge in the reaction at the Ni interface was observed, leading to the formation of an almost pure Au layer in the soldering zone.     

Interfacial Intermetallic Growth and Strength of Composite Lead-Free Solder Alloy Through Isothermal Aging

The effects of particle reinforcement of Sn-4.0wt.%Ag-0.5wt.%Cu (SAC405) lead-free solder on interfacial intermetallic layer growth and strength of the ensuing joints through short-term isothermal aging (150°C) were studied. Composite solders were prepared by either incorporating 2 wt.% Cu (3 μm to 20 μm) or Cu₂O (∼150 nm) particles into SAC405 paste. Aggressive flux had the effect of reducing the Cu₂O nanoparticles into metallic Cu which subsequently reacted with the solder alloy to form the Cu₆Sn₅ intermetallic. While all solders had similar interfacial intermetallic growth upon reflow, both of the composite solders’ growth rates slowed through aging to reach a common growth rate exponent of approximately 0.38, considerably lower than that of the nonreinforced solder (n = 0.58). The nanoscale reinforced solder additionally exhibited the highest tensile strength in both the initial and aged conditions, behavior also attributed to its quick conversion to a stable microstructure.     

Interaction Between Ni and Cu Across 95Pb-5Sn High-Lead Layer

Ni/95Pb-5Sn/Cu ternary diffusion couples were used to investigate the cross-interaction between Ni and Cu across a layer of 95Pb-5Sn solder. High-lead solder layers with a thickness of 100 μm or 400 μm were electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited high-lead solder surface. The diffusion couples were then aged at 150°C to 250°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high-temperature process such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that massive spalling also occurred during aging even though reflow was not used. The massive spalling began with the formation of microvoids. When the microvoids had congregated into large enough voids, intermetallic compounds (Cu₃Sn) started to spall from the interface. This spalling phenomenon occurred sooner with increasing temperature and decreasing solder volume.     

Effect of Pd Thickness on the Interfacial Reaction and Shear Strength in Solder Joints Between Sn-3.0Ag-0.5Cu Solder and Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Surface Finish

Intermetallic compound formation at the interface between Sn-3.0Ag-0.5Cu (SAC) solders and electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish and the mechanical strength of the solder joints were investigated at various Pd thicknesses (0 μm to 0.5 μm). The solder joints were fabricated on the ENEPIG surface finish with SAC solder via reflow soldering under various conditions. The (Cu,Ni)₆Sn₅ phase formed at the SAC/ENEPIG interface after reflow in all samples. When samples were reflowed at 260°C for 5 s, only (Cu,Ni)₆Sn₅ was observed at the solder interfaces in samples with Pd thicknesses of 0.05 μm or less. However, the (Pd,Ni)Sn₄ phase formed on (Cu,Ni)₆Sn₅ when the Pd thickness increased to 0.1 μm or greater. A thick and continuous (Pd,Ni)Sn₄ layer formed over the (Cu,Ni)₆Sn₅ layer, especially when the Pd thickness was 0.3 μm or greater. High-speed ball shear test results showed that the interfacial strengths of the SAC/ENEPIG solder joints decreased under high strain rate due to weak interfacial fracture between (Pd,Ni)Sn₄ and (Cu,Ni)₆Sn₅ interfaces when the Pd thickness was greater than 0.3 μm. In the samples reflowed at 260°C for 20 s, only (Cu,Ni)₆Sn₅ formed at the solder interfaces and the (Pd,Ni)Sn₄ phase was not observed in the solder interfaces, regardless of Pd thickness. The shear strength of the SAC/ENIG solder joints was the lowest of the joints, and the mechanical strength of the SAC/ENEPIG solder joints was enhanced as the Pd thickness increased to 0.1 μm and maintained a nearly constant value when the Pd thickness was greater than 0.1 μm. No adverse effect on the shear strength values was observed due to the interfacial fracture between (Pd,Ni)Sn₄ and (Cu,Ni)₆Sn₅ since the (Pd,Ni)Sn₄ phase was already separated from the (Cu,Ni)₆Sn₅ interface. These results indicate that the interfacial microstructures and mechanical strength of solder joints strongly depend on the Pd thickness and reflow conditions.     

Numerical and experimental analysis of the Sn3.5Ag0.75Cu solder joint reliability under thermal cycling

The Sn3.5Ag0.75Cu (SAC) solder joint reliability under thermal cycling was investigated by experiment and finite element method (FEM) analysis. SAC solder balls were reflowed on three Au metallization thicknesses, which are 0.1, 0.9, and 4.0 μm, respectively, by laser soldering. Little Cu–Ni–Au–Sn intermetallic compound (IMC) was formed at the interface of solder joints with 0.1 μm Au metallization even after 1000 thermal cycles. The morphology of AuSn₄ IMC with a small amount of Ni and Cu changed gradually from needle- to chunky-type for the solder joints with 0.9 μm Au metallization during thermal cycling. For solder joints with 4 μm Au metallization, the interfacial morphology between AuSn₄ and solder bulk became smoother, and AuSn₄ grew at the expense of AuSn and AuSn₂. The cracks mainly occurred through solder near the interface of solder/IMC on the component side for solder joints with 0.1 μm Au metallization after thermal shock, and the failure was characterized by intergranular cracking. The cracks of solder joints with 0.9 μm Au metallization were also observed at the same location, but the crack was not so significant. Only micro-cracks were found on the AuSn₄ IMC surface for solder joints with 4.0 μm Au metallization. The responses of stress and strain were investigated with nonlinear FEM, and the results correlated well with the experimental results.     

Microstructure characterization of Sn-Ag solder joints between stud bumps and metal pads

The microstructure of the flip-chip solder joints fabricated using stud bumps and Pb-free solder was characterized. The Au or Cu stud bumps formed on Al pads on Si die were aligned to corresponding metal pads in the substrate, which was printed with Sn-3.5Ag paste. Joints were fabricated by reflowing the solder paste. In the solder joints fabricated using Au stud bumps, Au-Sn intermetallics spread over the whole joints, and the solder remained randomly island-shaped. The δ-AuSn, ε-AuSn₂, and η-AuSn₄ intermetallic compounds formed sequentially from the Au stud bump. The microstructure of the solder joints did not change significantly even after multiple reflows. The AuSn₄ was the main phase after reflow because of the fast dissolution of Au. In the solder joints fabricated using Cu stud bumps, the scallop-type Cu₆Sn₅ intermetallic was formed only at the Cu interface, and the solder was the main phase. The difference in the microstructure of the solder joints with Au and Cu stud bumps resulted from the dissolution-rate difference of Au and Cu into the solder.     

The Feasibility of Au Ball Bonding on Sn-Plated Cu

The feasibility of thermosonic gold wire bonding on Cu coupons with Sn/Cu metallizations was studied by evaluating shear strength and microstructure of balls bonded on different Sn metallization samples. The 0.85 ± 0.08 μm and 5.34 ± 0.21 μm thick metallizations were produced by dipping the Cu coupon in 250°C molten Sn solder for 1 s (sample 1) and 30 s (sample 2), respectively. Cu₆Sn₅ intermetallic compounds are formed during dipping. After wire bonding, Au-Cu-Sn layers are found on the ball-coupon interface of both samples. The highest ball shear force observed was 40 gf (1 gf = 9.81 mN) and was achieved on sample 1 using 520 mW and 40 gf of ultrasonic power and bonding force, respectively. The shear fracture goes through the Au ball. The Sn is squeezed out of the contact zone during wire bonding and forms flashes that extend 5 μm and 25 μm beyond the contact zone for samples 1 and 2, respectively.     

Comparison of Sn2.8Ag20In and Sn10Bi10In solders for intermediate-step soldering

We chose Sn−2.8Ag−20In and Sn−10Bi−10In (numbers are in weight percentages unless specified otherwise) as Pb-free solder materials for intermediate-step soldering. We then investigated how the two solders reacted with the under bump metallurgy (UBM) of Au/Ni (Au: 1.5 μm and Ni: 3 μm) at 210°C, 220°C, 230°C, and 240°C for up to 4 min. All, of the Au UBM was dissolved into the solder matrix as soon as the interfacial reaction started. The reaction formed Au(In, Sn)₂ in the case of SnAgIn, and it formed Au(Sn, In)₄ and Au(In, Sn)₂ in the case of SnBiIn. The formation mechanism of the intermetallic phases is explained thermodynamically. The exposed Ni layer reacted with the solder and formed Ni₂₈Sn₅₅In₁₇ in case of SnAgIn, and formed Ni₃(Sn, In)₄ in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni₃(Sn,In)₄ layer in the SnBiIn/UBM is thicker than the Ni₂₈Sn₅₅In₁₇ layer in the SnAgIn/UBM. Because of the thicker intermetallic compound layer, the SnBiIn solder joint has weaker shear strength than the SnAgIn solder joint.     

The Influence of an Imposed Current on the Creep of Sn-Ag-Cu Solder

The work reported here included preliminary tests on the influence of an imposed current on the creep rate of the Pb-free solder Sn-Ag-Cu 305 (Sn-3Ag-0.5Cu in wt.%). The samples employed were double-shear specimens that contained paired solder joints, 400 μm × 400 μm in cross-section, 200 μm in thickness on Cu. Three tests were done. In the first, samples were tested under stress at room temperature with imposed current densities that ranged from 1 × 10³ A/cm² up to 6.5 × 10³ A/cm². As expected, because of Joule heating, the results show a sharp increase in creep rate with the imposed current density. A second set of tests was done to determine whether Joule heating fixed the creep rate. The steady-state temperature of the solder joints was measured under current, and samples were creep-tested at that temperature. Surprisingly, the creep rate under current was significantly below that measured in isothermal tests at the same temperature. The third set of tests studied the influence of microstructure. Samples were prepared with three starting microstructures: as cast, thermally aged by long-term isothermal exposure, and current aged by long-term exposure to a fixed current density. The three microstructures were then tested with and without current at two ambient temperatures. The different microstructures had very different creep rates in the absence of current but, surprisingly, imposing a current (5.5 × 10³ A/cm²) increased the creep rate by very nearly the same factor (~7×) in every case. Neither of these results is well understood at this time.     

Effect of burn-in on shear strength of 63Sn-37Pb solder joints on an Au/Ni/Cu substrate

A study was performed to examine the effect of burn-in and Au-plating thickness on the shear strength of 63Sn-37Pb solder joints in ball grid array (BGA) packages. The Au-plating thicknesses of 0.3 μm, 0.6 μm, 0.9 μm, and 1.4 μm were evaluated. An isothermal aging temperature of 150°C was employed to simulate burn-in conditions. The evolution of the bulk solder microstructure and intermetallic compounds at the solder joint interface were characterized and correlated to the measured shear strength. The strength of the solder joints with 0.3-μm Au plating was approximately three times higher than the thicker platings after aging. Solder joints with 0.3-μm Au plating failed within the solder matrix, and their strength was dependent upon the bulk solder microstructure and composition. The weakness of the solder joints with thicker Au platings was attributed to the formation of a brittle AuSn₄/Ni₃Sn₄ interface and a ductile Pb-rich layer at the interface.