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.     

Influence of trace alloying elements on the ball impact test reliability of SnAgCu solder joints

In this work, we present ball impact test (BIT) responses and fracture modes obtained at an impact velocity of 0.8 m/s on SAC (Sn–Ag–Cu) package-level solder joints with a trace amount of Mn or RE (rare earth) additions, which were bonded with substrates of OSP Cu and electroplated Ni/Au surface finishes respectively. With respect to the as-mounted conditions, the Ni/Au joints possessed better impact fracture resistance than those with Cu substrate. Subsequent to aging at 150 °C for 800 h, multi-layered intermetallic compounds emerged at the interface of the Ni/Au joints and gave rise to degradation of the BIT properties. This can be prevented by RE doping, which is able to inhibit the growth of interfacial IMCs during aging. As for aged Cu joints, the Mn-doped samples showed the best performance in impact force and toughness. This was related to the hardened Sn matrix, and most importantly, a greater Cu₃Sn/Cu₆Sn₅ thickness ratio at the interface. Compared to Cu₆Sn₅, Cu₃Sn with a similar hardness but greater elastic modulus possessed better plastic ability, which was beneficial to the reliability of solder joints suffering high strain rate deformation if no excess Kirkendall voids formed.     

Investigation of soldering process and interfacial microstructure evolution for the formation of full Cu3Sn joints in electronic packaging

Within electronic products, solder joints with common interfacial structure of Cu/IMCs/Sn-based solders/IMCs/Cu cannot be used under high temperature for relatively low melting points of Sn-based solders (200–300 °C). However, there is a trend for solder joints to service under high temperature because of the objective for achieving multi-functionality of electronic products.With the purpose of ensuring that solder joints can service under high temperature, full Cu₃Sn solder joints with the interfacial structure of Cu/Cu₃Sn/Cu can be a substitute due to the high melting point of Cu₃Sn (676 °C). In this investigation, soldering process parameters were optimized systematically in order to obtain such joints. Further, interfacial microstructure evolution during soldering was analyzed. The soldering temperature of 260 °C, the soldering pressure of 1 N and the soldering time of 5 h were found to be the optimal parameter combination. During soldering of 260 °C and 1 N, the Cu₆Sn₅ precipitated first in a planar shape at Cu-Sn interfaces, which was followed by the appearance of planar Cu₃Sn between Cu and Cu₆Sn₅. Then, the Cu₆Sn₅ at opposite sides continued to grow with a transition from a planar shape to a scallop-like shape until residual Sn was consumed totally. Meanwhile, the Cu₃Sn grew with a round-trip shift from a planar shape to a wave-like shape until the full Cu₃Sn solder joint was eventually formed at 5 h. The detailed reasons for the shape transformation in both Cu₆Sn₅ and Cu₃Sn during soldering were given. Afterwards, a microstructure evolution model for Cu-Sn-Cu sandwich structure during soldering was proposed. Besides, it was found that no void appeared in the interfacial region during the entire soldering process, and a discuss about what led to the formation of void-free joints was conducted.     

Acceleration of the growth of Cu3Sn voids in solder joints

Soldering to Cu surface finishes usually leads to the formation of a bi-layer intermetallic structure, Cu₃Sn/Cu₆Sn₅, that provides a more robust bond than common alternatives. Occasionally, and so far unpredictably, voids may however grow within the Cu₃Sn over time and allow for premature failure of microelectronics products in service. A quantitative assessment of the reliability risk of voids observed after accelerated aging requires the knowledge of the variation of void growth with temperature and time. It is argued that in the case of realistic solder joints diffusion controlled void growth kinetics are unlikely to follow simple Arrhenius and parabolic dependencies, respectively. Nevertheless, three very different sets of samples were all shown to exhibit void growth that could be well approximated by a parabolic time dependence and an effective activation energy of 0.65–0.80 eV.     

An investigation of the reliability of solderable ICA with low-melting-point alloy (LMPA) filler

Conductive adhesives play a major role in the electronic packaging industry as an alternative to solder due to their potential advantages that include mild processing conditions and superior thermo-mechanical performance. In a conductive adhesive interconnection, adequate mechanical and electrical performance and long-term reliability are critical.In this paper, the reliability of solderable isotropic conductive adhesive (ICA) interconnections was investigated. Reliability testing was performed via thermal shock (−55 to 125 °C, 1000 cycles) and high-temperature and high-humidity tests (85 °C, 85% RH, 1000 h). The interfacial microstructure of the solderable ICA was also investigated. Additionally, the fracture mode was investigated via mechanical pull strength testing before and after the reliability test. The electrical resistance of the solderable ICA interconnection showed improved stability compared to conventional ICAs, and similar stability to conventional solder paste (Sn–3Ag–0.5Cu and Sn–58Bi) due to the metallurgical interconnection formed by the molten LMPA fillers between the corresponding metallization layers. After the reliability tests, the grown IMC layer was composed of Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase), and the scallop-type IMC transformed into a layer-type IMC. The fracture propagated along the Cu–Sn IMC/SnBi interface and the fracture surface showed a semi-brittle fracture mode mixed with cleavage and ductile tear bands.     

Non-conductive film with Zn-nanoparticles (Zn-NCF) for 40 μm pitch Cu-pillar/Sn–Ag bump interconnection

Non-conductive film with Zn nano-particles (Zn-NCF) is an effective solution for fine-pitch Cu-pillar/Sn–Ag bump interconnection in terms of manufacturing process and interfacial reliability. In this study, NCFs with Zn nano-particles of different acidity, viscosity, and curing speed were formulated and diffused Zn contents in the Cu pillar/Sn–Ag bumps were measured after 3D TSV chip-stack bonding. Amount of Zn diffusion into the Cu pillar/Sn–Ag bumps increased as the acidity of resin increased, as the viscosity of resin decreased, as the curing speed of resin decreased, and as the bonding temperature increased. Diffusion of Zn nano-particles into the Cu pillar/Sn–Ag bumps are maximized when the resin viscosity became lowered and the solder oxide layer was removed. To analyze the effects of Zn-NCF on IMC reduction, IMC height depending on aging time was measured and corresponding activation energies for IMC growth were calculated. For the evaluation of joint reliabilities, test vehicles were bonded using NCFs with 0 wt%, 1 wt%, 5 wt%, and 10 wt% of Zn nano-particles and aged at 150 °C up to 500 h. NCF with 10 wt% Zn nano-particle showed remarkable suppression in Cu₆Sn₅ and (Cu,Ni)₆Sn₅ IMC compared to NCFs with 0 wt%, 1 wt%, and 5 wt% of Zn nano-particles. However, in terms of Cu₃Sn IMC suppression, which is the most critical goal of this experiment NCFs with 1 wt%, 5 wt%, and 10 wt% showed an equal amount of IMC suppression. As a result, it was successfully demonstrated that the suppression of Cu–Sn IMCs was achieved by the addition of Zn nano-particles in the NCFs resulting an enhanced reliability performance in the Cu/Sn–Ag bumps bonding in 3D TSV interconnection.     

Electro- and thermomigration induced Cu3Sn and Cu6Sn5 formation in SnAg3.0Cu0.5 bumps

Two important trends in the microelectronics business are the development of three dimensional packaging solutions which increase the number electronics components on the same area, and the application of VLSI electronics under harsh environment conditions. Both trends lead to a growing importance of intermetallic compound (IMC) formation in Sn based solder joints. Due to miniaturization a growing part of the solder joint volume is transformed into IMCs and finally the reflow process becomes a transient liquid phase soldering (TLPS) process. For harsh environment applications TLPS enables the transformation of low melting Sn contacts into high melting IMC joints. In both cases a model for the prediction of migration-induced IMC formation is required for the fabrication of IMC joints.For the general prediction of the migration induced IMC formation the related material parameters are needed. Against this background the Cu₃Sn and Cu₆Sn₅ formation was observed during temperature storage tests on Amkor® Package-on-Package packages (12 × 12 mm) with SnAg_3.0Cu_0.5 ball grid arrays. A mathematical model was developed to calculate the average mass flux of Sn and Cu during the stress tests. Based on the mass flux values the activation energies and diffusion constants for Cu and Sn in Cu₃Sn and Cu₆Sn₅ were determined. Afterwards the temperature storage was combined with an AC and a DC current load to investigate thermo- and electromigration-related phenomena. Based on the IMC formation speed during the AC and the DC tests the heat of transport Q^* and the effective charge of the moving ion Z^* were calculated. An interpretation of the material parameters is given in consideration of the high defect density in Cu₃Sn and Cu₆Sn₅.     

Power electronic assemblies: Thermo-mechanical degradations of gold-tin solder for attaching devices

The eutectic Au₈₀Sn₂₀ solder alloy has been applied in semiconductor assemblies and other industries for years. Due to some superior physical properties, Au/Sn alloy gradually becomes one of the best materials for soldering in electronic devices and components packaging but the voids growth in AuSn solder joints is one of the many critical factors governing the solder joint reliability. Voids may degrade the mechanical robustness of the die attach and consequently affect the reliability and thermal conducting performance of the assembly. Severe thermal cycles [− 55 °C/+175 °C] have highlighted degradations in AuSn die attach solder. The inspection of as-prepared die-attachments by X-ray and SEM (observation of cross-section) shows that the initial voids sizes were increased and a propagation of transverse cracks inside the joint between voids has appeared after ageing, it was featured also the existence of the IMC typical scallop-shape morphology with the phase structure of (Ni, Au)₃Sn₂ on as-reflowed joints. In this paper, we evaluate the origin of these degradations and ways to address them.     

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.     

Effects of self-aligned electroplating Cu pillar/Sn-xAg bump on dense Al lines for chip-to-package connection

Self-aligned electroplating is applied to form the Cu pillar/Sn-Ag bump for semiconductor device packaging, while passivation SiN cracks are usually observed at the bump edge on the bump of the array (BOA). In this paper, the simulation method was used to investigate the mechanism of SiN cracks and then, the bump process was optimized to improve the mechanical properties of the Cu pillar/Sn-Ag bump. It was found that higher reflow rounds could improve the shear strength due to the large degree of contact between the rugged scallop-like shape of the Cu₆Sn₅ and the Sn-Ag solder. The fracture plane cleaved between the Sn-Ag and Cu₆Sn₅ interface is consistent with the simulation results. The hardness of the Sn‒Ag solder is proportional to the reflow rounds, and the amount of Ag₃Sn phase precipitation within the Sn-Ag solder contributes to the hardness value. In contrast, the disadvantage is that thermal residual stress could deteriorate the SiN crack, especially for a BOA structure The study concludes that an optimal bump process, including Sn-2%Ag solders at 260 °C for 30 s, could obtain a high shear strength and appropriate solder hardness without passivated SiN cracking.     

Effect of thermal aging on the interfacial structure of SnAgCu solder joints on Cu

Interfacial structure plays a great role in solder joint reliability. In solder joints on Cu, not only is Kirkendall voiding at the solder/Cu interface a concern, but also the growth of interfacial Cu–Sn intermetallic compounds (IMCs). In this work, evolution of microstructure in the interfacial region was studied after thermal aging at 100–150 °C for up to 1000 h. Special effort was made during sample preparation to reveal details of the interfacial structure. Thickness of the interfacial phases was digitally measured and the activation energy was deduced for the growth of Cu₃Sn. Kirkendall voids formed at the Cu/Cu₃Sn interface as well as within the Cu₃Sn layer. The thickness of Cu₃Sn significantly increased with aging time, but that of Cu₆Sn₅ changed a little. The interfacial Cu₃Sn layer was found growing at the expense of Cu₆Sn₅. Evolution of the interfacial structure during thermal aging is discussed.     

Formation of Cu6Sn5 phase by cold homogenization in nanocrystalline Cu–Sn bilayers at room temperature

Solid state reaction between nanocrystalline Cu and Sn films was investigated at room temperature by depth profiling with secondary neutral mass spectrometry and by X-ray diffraction. A rapid diffusion intermixing was observed leading to the formation of homogeneous Cu₆Sn₅ layer. There is no indication of the appearance of the Cu₃Sn phase. This offers a way for solid phase soldering at low temperatures, i.e. to produce homogeneous Cu₆Sn₅ intermediate layer of several tens of nanometers during reasonable time (in the order of hours or less). From the detailed analysis of the growth of the planar reaction layer, formed at the initial interface in Sn(100 nm)/Cu(50 nm) system, the value of the parabolic growth rate coefficient at room temperature is 2.3 × 10^− 15 cm²/s. In addition, the overall increase of the composition near to the substrate inside the Cu film was interpreted by grain boundary diffusion induced solid state reaction: the new phase formed along the grain boundaries and grew perpendicular to the boundary planes. From the initial slope of the composition versus time function, the interface velocity during this reaction was estimated to be about 0.5 nm/h.     

Mechanical and corrosion resistances of a Sn–0.7 wt.%Cu lead-free solder alloy

Sn–Cu alloys are interesting lead-free solder alternatives, with particular interest in the eutectic/near-eutectic compositions. However, little is known about the corrosion responses of these solders while subjected to corrosive environments. The present study examines the Sn–0.7 wt.%Cu solder alloy and the experimental results include a range of cooling rates and growth rates during solidification, metallography with comprehensive characterization of the distinct dendritic and cellular regions and the resulting corrosion and tensile mechanical parameters (potential, corrosion rate, polarization resistance, tensile strength). A β-Sn phase having a dendritic morphology is shown to characterize regions that are associated with higher cooling rates during solidification, while for lower cooling rates (<0.9 K/s) the prevalence of eutectic cells is observed. It was found that lower corrosion resistance and higher mechanical strength are associated with a microstructure formed by an arrangement of very fine dendritic branches and Cu₆Sn₅ fibrous intermetallic compound (IMC).     

Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

Solder joint integrity has long been recognized as a key issue affecting the reliability of integrated circuit packages. In this study, both experimental and finite element simulation methods were used to characterize the mechanical performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with different standoff heights (h, varying from 500 to 100 μm) and constant pad diameter (d, d = 480 μm) and contact angle under shear loading. With decreasing h (or the ratio of h/d), results show that the stiffness of BGA solder joints clearly increases with decreasing coefficient of stress state and torque. The stress triaxiality reflects the mechanical constraint effect on the mechanical strength of the solder joints and it is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of h has very limited influence on the stress triaxiality under shear loading. Moreover, when h is decreased, the concentration of stress and plastic strain energy along the interface of solder and pad decreases, and the fracture location of BGA solder joints changes from near the interface to the middle of the solder. Both geometry and microstructure greatly affect the shear behavior of joints, the average shear strength shows a parabolic trend with decreasing standoff height. Furthermore, the brittle fracture of BGA solder joints after long-time isothermal aging was investigated. Results obtained show that, under the same shear force, the stress intensity factors, K_I and K_II, and the strain energy release rate, G_I, at the Sn–3.0Ag–0.5Cu/Cu₆Sn₅ interface and in the Cu₆Sn₅ layer obviously decrease with decreasing h, hence brittle fracture is more prone to occur in the joint with a large standoff height.     

Microstructure and shear strength evolution of SnAg/Cu surface mount solder joint during aging

The effect of aging at 150°C on the microstructure and shear strength of SnAg/Cu surface mount solder joint has been investigated with comparison to 62Sn36Pb2Ag/Cu. It is found that the diffusion coefficient of intermetallic compounds at SnAg/Cu interface is smaller than that of intermetallic compounds at SnPbAg/Cu interface at 150°. The shear strength of SnAg solder joint is higher and decreases at a smaller rate during aging compared to that of SnPbAg solder joint. The fracture surface analysis shows that as the aging time increases, the fracture takes place along the solder/Cu₆Sn₅ interface with an extension toward the Sn−Cu intermetallic layer.     

Solid-state annealing behavior of two high-Pb solders, 95Pb5Sn and 90Pb10Sn,on Cu under bump metallurgy

The solid-state annealing behavior of two high-lead solders, 95Pb5Sn and 90Pb10Sn (in wt.%), was examined. After reflow, Cu₃Sn intermetallics formed on the Cu under bump metallurgy (UBM) for both solder alloys. However, solidstate annealing produced significantly different reaction morphologies for the two solder compositions. The Cu₃Sn intermetallics spalled off faster at higher temperatures in the 95Pb5Sn solder. In the case of 90Pb10Sn solder, the Cu₃Sn intermetallics continued to grow even after 1500 h at 170°C. The difference was explained by a two-step phenomenon—Sn diffusion from the bulk solder region to the solder/Cu₃Sn interface (J_Sn), and subsequent intermetallic formation (I_Cu3Sn) by interdiffusion of Cu and Sn. For 95Pb5Sn, the relation, J_Sn < I_Cu3Sn was postulated because of insufficient supply of Sn. The relation, J_Sn > I_Cu3Sn was suggested for the continuous intermetallic growth of the 90Pb10Sn solder. Although a small difference was expected between the two quantities in both solder alloys, the difference in the solid-state annealing behavior was dramatic.     

Microstructural evolution in the Sn-Cu-Ni and Pb-Sn solder joints with Cu and Pt-Ag metallized Al2O3 substrate

The growth mechanism of intermetallics between solders and metallized substrates, after thermal aging, are investigated. The solders used in this study are unleaded Sn-Cu-Ni solder and eutectic Pb-Sn solder. The Pt-Ag/Al₂O₃, Cu block and the electroless Cu/Pt-Ag/Al₂O₃ are employed as the metallized substrates. Microstructure evolution of the interfacial morphology, elemental, and phase distribution are probed with the aid of electron-probe microanalyzer (EPMA) and x-ray diffractometry. Two kinds of intermetallics, Cu₃Sn and Cu₆Sn₅, are formed at the solder/Cu interface. However, for the solder/Pt-Ag system, only Ag₃Sn is observed at the interface. The thickness of Cu₃Sn, Cu₆Sn₅, and Ag₃Sn compound layers for all solder/metallized substrate systems shows at t^0.5 dependence at 100, 125, 150 and 170 C. According to the calculated activation energy and diffusion constant, the growth rate of Cu₃Sn and Cu₆Sn₅ intermetallics in the electroless Cumetallized substrate is relatively higher than that for Cu block one at the range of 100 C to 170 C. However, the growth rate of Cu₆Sn₅ and Ag₃Sn is reduced in the Sn-Cu-Ni solder with respect to the eutectic Pb-Sn solder. On the other hand, the Sn-Cu-Ni solder system exhibits a thicker Cu₃Sn intermetallic layer than the eutectic Pb-Sn solder after various aging times at 100 C. The thickness of Cu3Sn in the eutectic Pb-Sn solder is, however, thicker than that for Sn-Cu-Ni solder at 170 C.     

Sn whisker evaluations in 3D microbumped structures

Sn whiskering remains a reliability concern in electronic applications. Despite extensive research on growth rates and mitigation strategies, no predictive theory is in place. Literature data are available for Cu/Sn-based films and coatings as well as for board-level and flip-chip solder bumps but data are scarce for scaled-down solder volumes and for higher intermetallic-to-solder ratios. The current work investigates whiskers in “isolated geometries” for 3D solder-capped Cu microbumps with >2 orders of magnitude smaller solder volumes compared to state-of-the-art. To the best of the authors’ knowledge, this is the first time Sn whisker growth is reported in isolated solder volumes (e.g. <8 μm-side cube). Whiskers propensity was evaluated using JEDEC industrial specifications. The tested structures were: 5/3.5 μm-thick Cu/Sn films and 15 μm-diameter electroplated solder capping (Sn, SnAg, SnCu) on Cu microbumps (as-plated vs. reflowed). Selected Sn whiskers and “whisker-like” features were analysed and identified experimentally with SEM, EDX and FIB. In the absence of a predictive model, first-order and “what if” calculations based on IMC molar volume and oxide cracking hypotheses were carried out. This approach quantifies “figures of merit” for Sn whisker propensity with (1) different bump-limiting metallization (BLM) cases e.g. Cu, Ni, Co and (2) further microbump scaling. Future research recommendations are outlined to mitigate manufacturing risks by controlling “sit time” between bumping and stacking.     

The formation and growth of intermetallics in composite solder

The formation and growth of intermetallics at the solder/substrate interface are factors affecting the solderability and reliability of electronic solder joints. This study was performed to better understand the diffusion behavior and microstructural evolution of Cu−Sn intermetallics at the composite solder/copper substrate interface for eutectic solder and solder alloys containing particle additions of Cu, Cu₃Sn, Cu₆Sn₅, Ag, Au, and Ni. Annealing temperatures of 110 to 160°C were used with aging times of 0 to 64 days. The copper-containing composite solders generally formed thinner Cu₆Sn₅ layers, but thicker Cu₃Sn layers than were formed by the eutectic solder alone. These copper-containing additions, therefore, resulted in increased activation energies for Cu₆Sn₅ formation and decreased activation energies for Cu₃Sn formation as compared to the eutectic solder. The activation energy for Cu₃Sn formation decreased relative to eutectic solder for silver and gold composite solders even though less Cu₃Sn was formed at the substrate interface. Nickel and palladium drastically reduced the Cu₃Sn thickness and increased the Cu₆Sn₅ thickness. However, the Cu₆Sn₅ contained a substantial volume fraction of voids close to the copper substrate. We propose two mechanisms to explain the effects of the copper-containing and silver particles on the kinetics of intermetallic formation. First, the particles act as tin-sinks which remove tin from the solder and decrease the amount of tin available for reaction at the solder/substrate interface. Second, the particles reduce the cross-sectional area available for tin diffusion, which also reduces the amount of tin available at the interface for reaction.     

Interfacial Reactions of Sn-3.0Ag-0.5Cu Solder with Cu-Mn UBM During Aging

Cu under bump metallurgy (UBM) has been widely used in flip-chip technology. The major disadvantages of Cu UBM are fast consumption of copper, rapid growth of intermetallic compounds (IMCs), and easy formation of Kirkendall voids. In this study we added two different contents of Mn (2 at.% and 10 at.%) to Cu UBM by sputtering to modify the conventional Cu metallization. For the higher Mn concentration in the Cu-Mn UBM, a new Sn-rich phase formed between Cu₆Sn₅ and the Cu-Mn UBM, and cracks formed after aging. For the lower Mn concentration, growth of Cu₃Sn and Kirkendall voids was significantly suppressed after thermal aging. Kinetic analysis and x-ray elemental mapping provided evidence that Mn diffusion into Cu₃Sn slowed diffusion of Cu in the Cu₃Sn layer. The Mn-enriched Cu₃Sn layer may serve as a diffusion barrier to reduce the interfacial reaction rate and Kirkendall void formation. These results suggest that Cu-Mn UBM with low Mn concentration is beneficial in terms of retarding Cu pad consumption in solder joints.