The “Discovery” of alpha activity in lead and solder

More than 15 years ago the presence of ²¹⁰Pb activity in lead was found to create significant background in a low-background radiation detection spectrometer. Shortly thereafter, ²¹⁰Po alpha particles emitted from the surface of a solder joint were directly observed with this spectrometer. Antiquity lead was used to eliminate these sources of radioactivity. The experiments leading up to these “discoveries” are described. The magnitude of the effect of ²¹⁰Pb in lead on the experiment is discussed. The sensitivities of various techniques for measuring ²¹⁰Pb in lead are compared, and a practical limit for alpha activity in lead is proposed.     

Electromigration Behavior in Sn-37Pb and Sn-3.0Ag-0.5Cu Flip-Chip Solder Joints under High Current Density

The electromigration of conventional Sn-37Pb and Pb-free Sn-3.0Ag-0.5Cu (in wt.%) solder bumps was investigated with a high current density of 2.5 × 10⁴ A/cm² at 423 K using flip-chip specimens comprised of an upper Si chip and a lower bismaleimide triazine (BT) substrate. Electromigration failure of the Sn-37Pb and Sn-3.0Ag-0.5Cu solder bumps occurred with complete consumption of electroless Ni immersion Au (ENIG) underbump metallization (UBM) and void formation at the cathode side of the solder bump. Finite element analysis and computational simulations indicated high current crowding of electrons in the patterned Cu on the Si chip side, whereas the solder bumps and Cu line of the BT substrate had a relatively low density of flowing electrons. These findings were confirmed by the experimental results. The electromigration reliability of the Sn-3.0Ag-0.5Cu solder joint was superior to that of Sn-37Pb.     

A study in flip-chip UBM/bump reliability with effects of SnPb solder composition

This paper aims to investigate the electromigration phenomenon of under-bump-metallization (UBM) and solder bumps of a flip-chip package under high temperature operation life test (HTOL). UBM is a thin film Al/Ni(V)/Cu metal stack of 1.5 μm; while bump material consists of Sn/37Pb, Sn/90Pb, and Sn/95Pb solder. Current densities of 2500 and 5000 A/cm² and ambient temperatures of 150–160 °C are applied to study their impact on electromigration. It is observed that bump temperature has more significant influence than current density does to bump failures. Owing to its higher melting point characteristics and less content of Sn phase, Sn/95Pb solder bumps are observed to have 13-fold improvement in Mean-Time-To-Failure (MTTF) than that of eutectic Sn/37Pb. Individual bump resistance history is calculated to evaluate UBM/bump degradation. The measured resistance increase is from bumps with electrical current flowing upward into UBM/bump interface (cathode), while bumps having opposite current polarity cause only minor resistance change. The identified failure sites and modes from aforementioned high resistance bumps reveal structural damages at the region of UBM and UBM/bump interface in forms of solder cracking or delamination. Effects of current polarity and crowding are key factors to observed electromigration behavior of flip-chip packages.     

Design, assembly and reliability of large die and fine-pitch Cu/low-k flip chip package

This paper reports the design, assembly and reliability assessment of 21 × 21 mm² Cu/low-k flip chip (65 nm node) with 150 μm bump pitch and high bump density. To reduce the stress from the solder bump pad to low-k layers, Metal Redistribution Layer (RDL) and Polymer Encapsulated Dicing Lane (PEDL) are applied to the Cu/low-k wafer. Lead-free Sn2.5Ag, high-lead Pb5Sn and Cu-post/Sn37Pb bumps are evaluated as the first-level interconnects. It is found that the flip chip assembly of high-lead bumped test vehicle requires the right choice of flux and good alignment between the high-lead solder bumps and substrate pre-solder alloy to ensure proper solder bump and substrate pre-solder alloy wetting. Joint Electron Device Engineering Council (JEDEC) standard reliability is performed on the test vehicle with different first-level interconnects, underfill materials and PEDL.By integrating PEDL to the Cu/low-k chip, the reliability performance of the flip chip package has been improved by almost two times. This paper has demonstrated Moisture Sensitivity Test-Level 2 (MST-L2) qualified large die and fine-pitch Cu/low-k flip chip package. The presented results are significant for the development of flip chip packaging technologies for future advanced Cu/low-k generations.     

The effect of intermetallic compound morphology on Cu diffusion in Sn-Ag and Sn-Pb solder bump on the Ni/Cu Under-bump metallization

The eutectic Sn-Ag solder alloy is one of the candidates for the Pb-free solder, and Sn-Pb solder alloys are still widely used in today’s electronic packages. In this tudy, the interfacial reaction in the eutectic Sn-Ag and Sn-Pb solder joints was investigated with an assembly of a solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. In the Sn-3.5Ag solder joints reflowed at 260°C, only the (Ni_1−x,Cu_x)₃Sn₄ intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb solder reflowed at 225°C for one to ten cycles, only the (Ni_1−x,Cu_x)₃Sn₄ IMC formed between the solder and the Ni/Cu under-bump metallization (UBM). Nevertheless, the (Cu_1−y,Ni_y)₆Sn₅ IMC was observed in joints reflowed at 245°C after five cycles and at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, and elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role in the formation of the (Cu_1−y,Ni_y)₆Sn₅ IMC. In addition, the diffusion behavior of Cu in eutectic Sn-Ag and Sn-Pb solders with the Ni/Cu UBM were probed and discussed. The atomic flux of Cu diffused through Ni was evaluated by detailed quantitative analysis in an electron probe microanalyzer (EPMA). During reflow, the atomic flux of Cu was on the order of 10¹⁶−10¹⁷ atoms/cm²sec in both the eutectic Sn-Ag and Sn-Pb systems.     

Correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps

The correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps has been studied. Upon solid-state aging, a diffusion-controlled process was observed for the interfacial Ni-Sn compound formation of the Sn/Ni(P) reaction couple and the activation energy is calculated to be 42 KJ/mol. For the Sn0.7Cu/Ni(P), in the initial aging, a needle-shaped Ni-Sn compound layer formed on Ni(P). Then, it was gradually covered by a layer of the Cu-Sn compound in the later aging process. Hence, a mixture layer of Ni-Sn and Cu-Sn compounds formed at the interface. For the Sn3.0Cu/Ni(P), a thick Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted in a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. In contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu solder bumps is relatively weak and exhibits poor resistance against the aging process. Upon the reflowing process, the gap formation at the Ni(P)/Cu interface caused a fast degradation in the interfacial strength for Sn solder bumps. For Sn0.7Cu and Sn3.0Cu solder bumps, Ni₃P formation was greatly retarded by the self-formed Cu-Sn compound layer. Therefore, Sn(Cu) solder bumps show better shear strengths over the Sn solder bump.     

Microstructure and shear strength evolution of Sn-Ag-Cu solder bumps during aging at different temperatures

Flip-chip devices with Sn-3.8Ag-0.7Cu solder on electroless Ni (EN) without immersion Au were studied after aging at different temperatures. The (Cu,Ni)₆Sn₅ intermetallics (IMCs) growth was volume diffusion controlled and Kirkendall voids were found in the Ni₃P layer even at the initial stage of high-temperature aging due to the faster diffusion of Ni in the Ni₃P layer via its column structure boundaries. The Ag₃Sn IMCs were distributed in the bulk solder, existing as plate- or lamella-like phases or as small particles around the β-Sn dendrites, and the (Cu,Ni)₆Sn₅ IMCs existed as facet-like phases. The plate- and lamella-like Ag₃Sn phases break up into small parts and these broken parts, together with small Ag₃Sn particles, coarsen into pebble-like phases during high-temperature aging. Shear tests showed that all the solder bumps fractured in the bulk solder. The shear strength of solder bumps decreases at the initial stage of aging at 150°C and 175°C, and the strength degradation during aging may be caused by the coarsening of small Ag₃Sn particles.     

Soldering-induced Cu diffusion and intermetallic compound formation between Ni/Cu under bump metallization and SnPb flip-chip solder bumps

Nickel-based under bump metallization (UBM) has been widely used as a diffusion barrier to prevent the rapid reaction between the Cu conductor and Sn-based solders. In this study, joints with and without solder after heat treatments were employed to evaluate the diffusion behavior of Cu in the 63Sn-37Pb/Ni/Cu/Ti/Si₃N₄/Si multilayer structure. The atomic flux of Cu diffused through Ni was evaluated from the concentration profiles of Cu in solder joints. During reflow, the atomic flux of Cu was on the order of 10¹⁵–10¹⁶ atoms/cm²s. However, in the assembly without solder, no Cu was detected on the surface of Ni even after ten cycles of reflow. The diffusion behavior of Cu during heat treatments was studied, and the soldering-process-induced Cu diffusion through Ni metallization was characterized. In addition, the effect of Cu content in the solder near the solder/intermetallic compound (IMC) interface on interfacial reactions between the solder and the Ni/Cu UBM was also discussed. It is evident that the (Cu,Ni)₆Sn₅ IMC might form as the concentration of Cu in the Sn-Cu-Ni alloy exceeds 0.6 wt.%.     

Microstructure and shear strength of Sn37Pb/Cu solder joints subjected to isothermal aging

The effects of isothermal aging on the microstructure and shear strength of Sn37Pb/Cu solder joints were investigated. Single-lap shear solder joints of eutectic Sn37Pb solder were aged for 1–10 days at 120 °C and 170 °C, respectively, and then loaded to failure in shear with a constant loading speed of 5 × 10⁻³ mm/s. The growth of the interfacial Cu–Sn intermetallic compounds (IMC) layer (Cu₆Sn₅ + Cu₃Sn) of Sn37Pb/Cu solder joints subjected to isothermal aging exhibited a linear function of the square root of aging time, indicating that the formation of Cu–Sn IMC was mainly controlled by the diffusion mechanism. And the diffusion coefficient (D) values of IMC layer were 1.07 × 10⁻¹⁷ and 3.72 × 10⁻¹⁷ m²/s for aged solder joints at 120 °C and 170 °C, respectively. Shear tests results revealed that as-reflowed solder joint had better shear strength than the aged solder joints and the shear strength of all aged solder joints decreased with increasing aging time. The presence of elongated dimple-like structures on the fracture surfaces of these as-reflowed or aged for short time solder joints were indicative of a ductile failure mode. As aging time further increased, the solder joints fractured in the mixed solder/IMC mode at the solder/IMC interface.     

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.     

Lead-Free Bumping Using an Alternating Electromagnetic Field

A novel lead-free bumping technique using an alternating electromagnetic field (AEF) was investigated. Lead-free solder bumps reflowed onto copper pads through AEF have been achieved. A comparison was conducted between the microstructures of the lead-free solder joints formed by the conventional thermal reflow and AEF reflow. Keeping the substrate temperature lower than that of the solder bumps, AEF reflow successfully created metallurgical bonding between the lead-free solders and metallizations through an interfacial intermetallic compound (IMC). The AEF reflow could be finished in several seconds, much faster than the conventional hot-air reflow. Considering the morphology of the interfacial Cu₆Sn₅ IMC, a shorter heating time above the melting point would be a better choice for solder joint reliability. The results show that AEF reflow is a promising localized heating soldering technique in electronic packaging.     

New lead-free,Sn-Zn-In solder alloys

In view of the need for a lead-free, drop-in replacement for the widely used 40Pb-60Sn near-eutectic solder (m.p. ~183°C), new Sn-Zn-ln based alloys with substantially the same melting point have been developed. It is shown that the alloying additions of In to the Sn-Zn binary system result in a suppression of the melting point to 175-188°C, and at the same time significantly improve the wetting characteristics. While a relatively active flux may be required for good solderability in air atmosphere, the recent manufacturing trend of using inert atmospheres is likely to allow acceptable manufacturability using less active fluxes in the future.     

Interfacial reaction and joint reliability of fine-pitch flip-chip solder bump using stencil printing method

This study was focused on the formation and reliability evaluation of solder joints with different diameters and pitches for flip chip applications. We investigated the interfacial reaction and shear strength between two different solders (Sn-37Pb and Sn-3.0Ag-0.5Cu, in wt.%) and ENIG (Electroless Nickel Immersion Gold) UBM (Under Bump Metallurgy) during multiple reflow. Firstly, we formed the flip chip solder bumps on the Ti/Cu/ENIG metallized Si wafer using a stencil printing method. After reflow, the average solder bump diameters were about 130, 160 and 190 μm, respectively. After multiple reflows, Ni₃Sn₄ intermetallic compound (IMC) layer formed at the Sn-37Pb solder/ENIG UBM interface. On the other hand, in the case of Sn-3.0Ag-0.5Cu solder, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were formed at the interface. The shear force of the Pb-free Sn-3.0Ag-0.5Cu flip chip solder bump was higher than that of the conventional Sn-37Pb flip chip solder bump.     

Electromigration in the Flip Chip Solder Joint of Sn-8Zn-3Bi on Copper Pads

Electromigration in Sn-8Zn-3Bi flip chip solder bumps on Cu pads has been studied at 120°C with an average current density of 4 × 10³ A/cm² and 4.5 × 10⁴ A/cm². Due to the polarity effect, the thickness of the intermetallic compound Cu-Zn (γ-phase) formed at the anode is much greater than that at the cathode. The solder joint fails after 117 h of stressing at 4.5 × 10⁴ A/cm², and void formation at the cathode can clearly be seen after polishing. However, it is the melting at the edge of the bump that causes the solder joint to fail. A simulation of the current density distribution indicates that the current density is not distributed uniformly, and current crowding occurs inside the bump. The results indicate that the increase of current density associated with Joule heating has affected melting and enhanced damage in the solder joint during electromigration.     

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.     

Formation of Highly Crystallized β‐PbO Thin Films by Cathodic Electrodeposition of Pb and Its Rapid Oxidation in Air

The process of electrodeposition of β‐PbO thin films from aqueous solutions of Pb^II salts has been studied in detail. Contrary to the mechanism assumed in previous studies, thin films of crystalline β‐PbO are obtained after cathodic electrolysis in aqueous solutions of various soluble salts of Pb^II (Pb(NO₃)₂, Pb(ClO₄)₂, and Pb(CH₃COO)₂), and in both the presence and the absence of O₂, thus indicating no contribution of OH^– generation by electroreduction of NO₃^– and/or O₂ to the formation of β‐PbO. A gradual color change is noted: a freshly electrodeposited gray film turns yellow as it dries in air. Drying of the films under controlled atmosphere (Ar or O₂), combined with scanning electron microscopy (SEM) observation and X‐ray diffraction (XRD) measurement, has revealed that freshly deposited films are of metallic Pb, which are oxidized and converted into β‐PbO. Such a reaction is operative only when a freshly electrodeposited activated wet Pb film is in contact with gaseous O₂. Despite the rapid conversion of a solid material, the resultant β‐PbO thin films are highly crystallized and possess highly ordered internal nanostructure. Elongated nanoparticles (30 nm × 100 nm) are assembled in a regular alignment to compose a large platelet (greater than 10 μm in size) with single‐crystalline character, as revealed by transmission electron microscopy (TEM) observation and selected‐area electron diffraction (SAED) measurement.     

Effects of palladium and solder aging on mechanical and fatigue properties of tin-lead eutectic solder

The effects of (a) 0.5 wt.% of Pd addition, and (b) aging on mechanical and fatigue properties of eutectic solder (63Sn37Pb) were investigated. The creep rate of eutectic solder at room temperature is not affected by Pd addition. However, at 80°C, solder containing Pd creeps slower than Sn-Pb eutectic. Strain rate dramatically affects yield and tensile stress of eutectic solder with Pd as it does for the binary solder. Isothermal fatigue life of solder at 25°C is essentially not changed by Pd addition. The microstructure of Pd-containing solder consisted of polyhedral grains of (Pb), (Sn), and a dispersion of PdSn₄ intermetallic. Significant microstructural changes and interphase interface phenomena take place during creep deformation at 25 and 80°C. Ambient aging for seven years leads to solder softening and to moderate increase in isothermal fatigue life.     

Effects of pre-bump probing and bumping processes on eutectic solder bump electromigration

This paper investigates the electromigration reliability of flip chip packages with and without pre-bump wafer probing via high temperature operation life test (HTOL) using printed and electroplated bumps. Under bump metallization (UBM) of printed and electroplated bumps is a thin film of Al/Ni(V)/Cu and Ti/Cu/Ni, respectively, while the bump material consists of eutectic Sn/Pb solder. Current densities from 7380 to 20 100 A/cm² and ambient temperatures at 100, 125 and 150 °C are applied in order to study their impact on electromigration. The results reveal that the bump temperature has a higher influence than the current density when it comes to bump failures. The observed interconnect damage is from bumps with electrical current flowing upward into the UBM/bump interface (cathode). Identified failure sites and modes reveal structural damage at the region of the UBM and UBM/bump interface, in the form of solder voiding and cracking. The effects of current polarity, current crowding, and operation temperature are key factors to electromigration failures of flip chip packaging. The maximum allowable current density of the electroplated bumps is superior to the printed bumps by a factor of 3.0–3.7 times. Besides, the median time to failure (MTTF) of without-underfill packaging is preferred to that of with-underfill packaging by 1.5–2.2 times. Furthermore, the differences in MTTF between pre-bump and without pre-bump probing procedures are 2.0–19.4% and 1.6–10.3% for printed and electroplated bumps, respectively.     

Interfacial reaction between multicomponent lead-free solders and Ag,Cu, Ni,and Pd substrates

The interfacial reaction between two prototype multicomponent lead-free solders, Sn-3.4Ag-1Bi-0.7Cu-4In and Sn-3.4Ag-3Bi-0.7Cu-4In (mass%), and Ag, Cu, Ni, and Pd substrates are studied at 250°C and 150°C. The microstructural characterization of the solder bumps is carried out by scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis. Ambient temperature, isotropic elastic properties (bulk, shear, and Young’s moduli and Poisson’s ratio) of these solders along with eutectic Sn-Ag, Sn-Bi, and Sn-Zn solders are measured. The isotropic elastic moduli of multicomponent solders are very similar to the eutectic Sn-Ag solder. The measured solubility of the base metal in liquid solders at 250°C agrees very well with the solubility limits reported in assessed Sn-X (X=Ag, Cu, Ni, Pd) phase diagrams. The measured contact angles were generally less than 15° on Cu and Pd substrates, while they were between 25° and 30° on Ag and Ni substrates. The observed intermediate phases in Ag/solder couples were Ag₃Sn after reflow at 250°C and Ag₃Sn and ζ (Ag-Sn) after solid-state aging at 150°C. In Cu/solder and Ni/solder couples, the interfacial phases were Cu₆Sn₅ and (Cu,Ni)₆Sn₅, respectively. In Pd/solder couples, only PdSn₄ after 60-sec reflow, while both PdSn₄ and PdSn₃ after 300-sec reflow, were observed.     

Materials interfaces in flip chip interconnects for optical components; performance and degradation mechanisms

Flip chip solder joint reliability is dependant on final microstructure. The interface between the solder and the underlying metallization is of primary concern since the majority of joint failures occur at or near the interface. We report the results of our investigation on the structure-property relationships of the as deposited metallization and electroplated Pb/Sn solder, as they affect failure mechanisms. Samples investigated were prepared by electron beam evaporation of a Cr/Cu/Au metallization. Electroplated solders were prepared using a high tin solder bath and current density of 3.2mA/cm².It is shown that interdiffusion and intermetallic formation between gold and copper occurs during evaporation deposition. It is also shown that a significant amount of interdiffusion and intermetallic formation between the tin in the solder and the seed layer metals (Cu and Au) occurs during the electroplating operation. It is believed that this is due largely to resistive heating of the samples. The degree of interdiffusion in the as deposited state determines the ultimate reliability of the flip chip solder bumps in an optical configuration.