Wettability Of Solder Research Articles

A novel method for characterization of Ultra Low Viscosity NCF layers using TCB for 3D Assembly

Abstract For die to wafer bonding of high-density interconnects and fine pitch microbumps developing and characterizing suitable underfill materials are required. In general, underfill serve to fill the spaces in-between microbumps for protection and reliability. Among the different types of underfill, Non-Conductive Film (NCF) has the advantages of fillet and volume control, and a built-in flux to aid wetting. However, challenges arise for thin dies and microbumps with fine pitches on film lamination, voiding, transparency, filler percentage, dicing compatibility and most importantly, deformation behavior and possibility to improve solder joint wetting. In a Die-to-Wafer D2W stacking with a Sn solder bump interconnect to Cu UBM, concern is high on the Cu pad oxidation due to the repeated TCB cycles that accelerate oxidation on neighboring dies. Process mitigation is needed to help reducing the oxidation. But even so, an NCF must have good embedded flux activation. Another main factor for an NCF to have efficient TCB process with good solder joint wetting, is the NCF deformation quality in which is a function of its viscosity. This parameter has direct impact on the deformation of solder bumps. High viscosity NCF would be difficult to deform, thus preventing solder contact to pad during TCB reflow temperature. High bond force is required and could lead to reduced alignment accuracy. Filler entrapment is also a subsequent concern for high filler loading, high viscosity NCF. For a low viscous NCF, careful attention in process characterization is needed in TCB with low bond force. Solder joint wetting is a problem with excessive squeeze-out due to fast and instantaneous deformation. With low viscosity, not only the bond force applied should be low, but the deformation behavior should also be understood to enable an effective NCF. We seek to demonstrate in this paper a creative methodology for Non-Conductive Film (NCF) material characterization, considering the factors of NCF viscosity, deformation, and solder squeeze-out. Characterizing NCF viscosity at fast TCB profiles is challenging considering deformation behavior of both the NCF itself and the solder bumps that shaped the solder squeeze-out and wetting. Furthermore, in this paper we use TCB tool position tracking to define the deformation curve of NCF film as a function of temperature and time at very fast profile of TCB. We use material viscosity curve as reference in relation to the actual deformation, and predict dynamic deformation based on Reynold’s equation within TCB profile duration. The experiments were performed with different heating ramp rates at target above Sn reflow of ~250C interface temperature. The deformation analysis is not limited to thin film sandwiched between parallel plates. Deformation test was performed on chips with and without microbumps and with rigid flat glass surface and its combinations. Deformation of underfill is recorded in the readout of TCB tool. As validation, we applied the optimized TCB process (force, temperature, and ramp rate) on a test vehicle with 20 and 40um pitch daisy chains and obtained close to 95% electrical yield with good joint and IMC formation. The cross-section SEM images show good wetting, revealing good activation of built-in flux when the optimized TCB profile was used.

Wetting behaviours and interfacial characteristics of Co-binder sintered polycrystalline diamond by Sn[sbnd]Ti active solder

The reactive wetting and interfacial characteristics of the Sn2Ti active solder were experimentally investigated on sintered polycrystalline diamond (PCD) substrates containing 8–18 wt.% Co binders. The initial spreading temperature of Sn2Ti on PCD was determined by the continuously elevating temperatures wetting experiment. The Sn2Ti started to spread on PCD at 500–630 °C. The higher the cobalt content, the lower the initial spreading temperature. Meanwhile, the spreading kinetics analysis through isothermal wetting experiment indicated the spreading process of Sn2Ti on PCD can be well described by the classic chemical reaction-controlled model at 650–900 °C. Both rod-like TiC carbide and faceted CoSnTi intermetallics (IMCs) were observed at the wetting triple line, with the formation morphologies strongly depending on wetting temperatures. The results obtained in this work are believed to be of importance for the optimization of brazing technique for sintered PCD, and hence the development of PCD diamond machining tools with minimized thermal damages.

Characterization of the interface between ceramics reinforcement and lead-free solder matrix

The Sn-3.0Ag-0.5Cu (SAC305) composite solder reinforced micro-sized silicon carbide and nickel-coated silicon carbide (Ni-SiC) particles were produced by a high-energy ball milling process. SiC and Ni-SiC were homogeneously mixed into SAC305 lead-free solder alloy with distinctive (0.5, 1.0, and 1.5 wt.%) percent weight, through a powder metallurgy path. The growth of nickel on ceramic SiC (1-3 μm) particles was investigated by the electroless nickel (EN) method. The relative contact perimeter (RCP) and the wettability of SAC305 composite solder were investigated with the an image analysis software. The data from image analysis software shows that the relative contact perimeter, as well as the wettability of lead-free composite solders, were significantly improved.

Electromigration Behavior of Low-Silver Sn-0.3Ag-0.7Cu-1.6Bi-0.2In Solder Joints

In typical service environments, high current densities pass through the interconnection leads of solder joints. The electromigration reliability of solder joints is thus of great significance. The microstructure, crystal orientation, and electromigration reliability of Sn-0.3Ag-0.7Cu-1.6Bi-0.2In solder were studied in this work. The results showed that the melting point of Sn-0.3Ag-0.7Cu-1.6Bi-0.2In solder is 212.3°C, with good wettability. The addition of Bi and In reduced the melting point and undercooling of the solder, with improved solder wettability. Compared with Sn-3.0Ag-0.5Cu solder, the microstructure of the remelted solder was well distributed with fine grains. Compounds formed by substituting Sn atoms with In atoms. Among the linear solder joints, those made of Sn-0.3Ag-0.7Cu-1.6Bi-0.2In showed easier formation of polycrystalline structure, and the c-axis orientation of β-Sn grains was more disordered. The addition of Bi and In to form Sn-0.3Ag-0.7Cu-1.6Bi-0.2In affected the formation and growth of intermetallic compounds and improved the electromigration reliability of solder joints.

Relationship between the Reduction Behavior of Copper and Tin Oxide Films by Formic acid and Solder Wettability

Relationship between the Reduction Behavior of Copper and Tin Oxide Films by Formic acid and Solder Wettability

Microstructure Characterization and Interfacial Reactions between Au-Sn Solder and Different Back Metallization Systems of GaAs MMICs.

GaAs monolithic microwave integrated circuits (MMICs) with different back metallization systems (TiW/Au and Au/Ti/Au) exhibit different problems in the automatic Au-Sn eutectic bonding process, such as edge breakage or excessive voids. In this study, the formation mechanism of the edge breakage and excessive voids were investigated to prevent the damage of the MMICs in mass production scenarios. The microstructure and elemental distribution were studied using a scanning electron microscope and energy-dispersive spectroscopy. The void contents of the brazed region were measured with three-dimensional computed tomography. The top Au layer of the TiW/Au metallization partially dissolved in the melting An-Sn solder. Consequently, liquidus temperature of the solder increased, leading to isothermal solidification with the formation of ζ-Au5Sn in the scrubbing process, which was the reason for the edge breakage. The terminal Au film of the Au/Ti/Au metallization completely dissolved in the melting An-Sn solder. The metallurgical combination was achieved by the formation of the TiAu4 intermetallic compound between the Au-Sn solder and the Ti layer. The wettability of Au-Sn solder on Ti layer should be improved to prevent the formation of the excessive voids.

A Study on Polyvinylidene Difluoride (PVDF) Anchoring Polymer Layer (APL) Solder Anisotropic Conductive Films (ACFs) for Fine-Pitch Flex-on-Flex (FOF) Interconnection

Fine-pitch flex-on-flex (FOF) assembly has been introduced for various electronic packaging applications because electronic products require higher functionality, flexibility, and I/O density. In this article, novel polyvinylidene difluoride (PVDF) anchoring polymer layer (APL) solder anisotropic conductive films (ACFs) were investigated for fine-pitch FOF applications. The capture rates of conductive particles and the insulation rates between neighboring electrodes have been significantly enhanced using PVDF APL solder ACFs as a result of suppressing the movement of conductive particles by the high tensile strength of the PVDF APL layer. For good solder wetting on electrodes, flux activators were added into the PVDF APL layer to remove the solder oxide layer. However, the tensile strength of the PVDF layer decreased as the flux activator content in the APL increased. The capture rates of conductive particles decreased as more flux activators were added because of the lower tensile strength of PVDF. Furthermore, optimized flux activator content in ACFs with 0.5 wt% in the APL and 2-wt% flux activators were also added in non- conductive films (NCFs) to obtain good solder wetting. Using the optimized APL solder ACFs, excellent contact resistances and insulation resistances were obtained before and after a pressure cooker test (PCT) and 85 °C/85% relative humidity (RH) reliability tests. As a conclusion, the PVDF APL solder ACF assembly can provide an excellent interconnect solution for fine-pitch FOF assemblies used for portable electronic product applications.

Effect of nano-Ag3Sn additions on wettability, interfacial intermetallic growth and mechanical properties of Zn–30Sn–1Ge solders on Cu substrates

The influences of different addition (0, 0.2, 0.4, 0.6, 0.8, and 1.0 wt%) of Ag3Sn on the wettability, the morphology of interfacial intermetallic compounds (IMCs), and the mechanical properties of Zn30Sn1Ge (ZSG) solder on Cu substrate were investigated. The Ag3Sn nanoparticles with an average particle size of approximately 30–45 nm were dispersed in the α-Zn phase and IMC layer. The added Ag3Sn nanoparticles can remarkably improve the wettability of ZSG solder and inhibit the formation and growth of the interfacial IMCs between the nanocomposite solders and Cu substrates. The enhancement of wettability and inhibition of interfacial IMC increased with the increase of nano-Ag3Sn addition amount under low concentration. However, the excessive addition of Ag3Sn nanoparticles in ZSG solder will degrade the wettability and inhibition of interfacial IMCs. When the amount of Ag3Sn nanoparticles is 0.6 wt%, the ZSG–0.6Ag3Sn solders possess the best wettability. In addition, the ZSG–0.4Ag3Sn solder exhibits the most prominent effect in retarding the formation and growth of interfacial IMCs. Meanwhile, the addition of Ag3Sn nanoparticles increases the shear strength and Vickers hardness of the solder joint. When the amount of Ag3Sn nanoparticles is 0.4 wt%, the shear strength of the welded joint is increased by 13.3%. Moreover, the macroscopic hardness of ZSG–0.6Ag3Sn solder is 49.507 HV, which is 57.5% higher than that of the solder without added nanoparticles (31.38 HV). Results show that the addition of Ag3Sn nanoparticles plays an important role in the interfacial reaction and mechanical properties between the ZSG solder and Cu substrate.

Vpliv dodatka nano-ZnO na omočljivost in strukturo na fazni meji pri spajkanju aluminija s spajkami na osnovi Sn brez dodatka škodljivega svinca

Vpliv dodatka nano-ZnO na omočljivost in strukturo na fazni meji pri spajkanju aluminija s spajkami na osnovi Sn brez dodatka škodljivega svinca

Plastic Flow Verification in a Tool Cavity for Production of Test Sample for Wettability Solders Measurement

The paper describes the structural design, simulation and proper manufacture of a tool for manufacturing of test samples in spherical form. This shape is essential for an innovative assessment of solder wettability, since the small sphere by its ideal shape represents the total non-wettability of substrate surface. In order to verify the design of the shaping tool, a simulation software, type Deform was used, which allowed the FEM analysis of plastic flow in the tool cavity. Just after successful computer simulation, a shaping tool was designed and then manufactured in laboratory conditions, thus the fabrication of test samples in spherical form was successfully tested.

Miniaturization, Triple-Line Effects, and Reactive Wetting of Microsolder Interfaces.

While reactive microsolder joints are of ubiquitous importance in modern electronics, the effects of joint miniaturization on wetting behavior remain largely unexplored. We elucidate this fundamental question of scalability by investigating the wettability of eutectic SnPb solder on Cu and Ni-electrodeposited metallization strips of varying widths. Contact angles are presented in dependence of the metallization width which is varied from 3 mm down to ∼100 μm. The measured angles clearly increase with decreasing metallization width. Based on the measurements and by modifying Young's equation, it is shown that the behavior of the wetting angle can be quantitatively understood with an "effective" triple-line energy of ϵt = (753 ± 31) × 10-9J/m for SnPb on Cu. The interpretation of this energy term is discussed in relation to the forming intermetallic phase and the ensuing surface roughness. A remarkable similarity between the experimentally observed size dependence and the crossed-groove perturbation model of Huh and Mason demonstrates that the rough intermetallic phase induces wetting hysteresis such that it is quantitatively well described by an effective triple-line energy.

Microstructures and properties of Sn–0.3Ag–0.7Cu solder doped with graphene nanosheets

Low silver Sn–0.3Ag–0.7Cu (SAC0307) solders with different amounts of graphene nanosheets (GNSs) prepared by powder metallurgy were used to investigate the impacts of GNSs on the microstructures, melting temperature, wettability, and tensile strength of the SAC0307–xGNSs (x = 0, 0.01, 0.03, 0.05, 0.07, and 0.09 wt%) solders. The experimental results indicate that the wettability of SAC0307 solder can be improved significantly by adding GNSs and the melting temperature of the solder sample ranges from 219.6 to 222.7 °C. It is also revealed that with increasing GNSs content, the morphology of GNSs in the solder matrix changed from the initial dispersion distribution to the agglomeration, while the diameter and depth of the dimples of the solder joint fracture showed a tendency of increasing first and then decreasing. Doped with 0.07 wt% of GNSs, the solders showed the largest tensile strength. When GNS addition exceeds 0.07 wt%, it will induce negative effects on tensile property of the solder.

Influence of kaolin geopolymer ceramic additions to the wettability and electrical properties of Sn-3.0Ag-0.5Cu (SAC305) lead free solder

The effect of kaolin geopolymer ceramic addition to the wettability and electrical resistivity of Sn-3.0Ag-0.5Cu (SAC305) lead free solder was successfully explored. Powder metallurgy with microwave sintering method was used to fabricate SAC305 composite solder. Five different weight percentage (wt.%) of kaolin geopolymer ceramic (0, 0.5, 1.0, 1.5 and 2.0 wt.%) were used in this study. The wettability of composite solder revealed the optimum contact angle was achieved at 1.0 wt.% with 20.8 °. Then, the electrical resistivity of composite solder showed significant change in the resistivity value.

Effect of surface finish on the wettability and electrical resistivity of Sn-3.0Ag-0.5Cu solder

The effect of different surface finish with Sn-3.0Ag-0.5Cu (SAC305) solder was successfully investigated. The SAC305 solder was fabricated by using casting method and solder was placed on copper substrate that coated with different surface finish. The soldering process was carried out by using F4N reflow oven followed up with the mounted and metallographic steps. Wettability of SAC305 solder was observed through contact angle formed between solder and four different surface finish located on the copper substrate. Subsequently, the electrical resistivity of solder was studied by conducted the four-point probes. The results of wettability test was found to be in the accepted range which is below 45° for all different surface finish. In terms of electrical resistivity, the results showed that the ImAg surface finish had enhanced the electrical conductivity of SAC305 lead-free solder.

Wettability, interfacial reactions, and impact strength of Sn–3.0Ag–0.5Cu solder/ENIG substrate used for fluxless soldering under formic acid atmosphere

Fluxes have a negative impact on the environment; as a result, fluxless soldering has become a promising method in electronic packaging. However, detailed studies on fluxless soldering are very rare, especially those involving the use of electroless nickel immersion gold (ENIG) substrate under formic acid (FA) atmosphere that can effectively reduce the majority of solders. In this work, the characteristic parameters of FA reflow soldering performed by combining Sn–3.5Ag–0.5Cu (SAC305) solder and ENIG substrate are compared with the soldering conducted using liquid rosin mildly activated (RMA) flux. It is found that the wettability of FA-exposed solder is greater than that of RMA-containing solder because the former spreads across the interfacial layer of intermetallic compounds (IMCs) produced before the melting of SAC305. Additionally, the interfacial reactions of FA-exposed solder resemble those of RMA solder before and after the thermal aging at 150 °C. Therefore, the impact strengths of these two solders are almost the same due to the similarity of their microstructures and close growth rates of (Cu,Ni)6Sn5 IMC layers during thermal aging. The findings of this study suggest that FA reflow soldering is a promising environmentally friendly technique for electronic packaging.

Effect mechanism of Ag and Bi elements on the corrosion process in Sn–9Zn–1Ag and Sn–9Zn–3Bi solders

Sn–9Zn lead–free solder is widely used in low melting temperature welding field due to its low melting point (198.5 °C), good connection reliability, and low cost. However, its poor wettability and corrosion resistance weaken its performance. According to previous studies, addition of 1% Ag and 3% Bi to Sn–9Zn alloy, respectively, can optimize the wettability and mechanical properties of solder. Nonetheless, the effect of added elements on corrosion process has not been determined. In this study, the effect mechanism of Ag and Bi elements on the corrosion process in Sn–9Zn–1Ag and Sn–9Zn–3Bi solders was analyzed. The changes of the surface state of each solder in the corrosion process were detected by electrochemical polarization experiment. The microstructural compositions of solders in different corrosion stages were compared through the immersion test. The results indicated that compared to Sn–9Zn solder, Sn–9Zn–1Ag solder exhibited the existence of a finer Zn-rich phase, which improved the pitting corrosion resistance of solder by blocking the entry of corrosive solution. In passivation film forming process, Ag–Zn compounds were formed on the surface of Sn–9Zn–1Ag solder, which reduced the content of Zn-rich phase, leading to the decrease of the overall corrosion rate. After the formation of passivation film, Ag compounds were precipitated on the surface of passivation film of solder, which prevented further destruction of the passivation film. However, Sn–9Zn–3Bi solder produced a large number of loose structural corrosion products on the surface of the corroded sample, thus the substrate could not be effectively protected. Furthermore, Sn–9Zn–3Bi solder showed a larger Zn-rich phase, thus it was easier for the corrosive solution to enter the sample, resulting in severe corrosion in the interior of sample. In this study, the effect mechanism of Ag and Bi addition on the corrosion process of Sn–9Zn–1Ag and Sn–9Zn–3Bi solders was revealed, which provides a theoretical basis for the study on improving the corrosion resistance of Sn–9Zn solder.

Wettability and printability of SAC305-xTiO2 Pb-free solder paste on Cu substrate

This paper was aimed to study on effects of TiO2 nanoparticles on wettability and printability of SAC305-xTiO2 Pb-free solder on Cu substrate. TiO2 nanoparticles at various concentrations (0.00, 0.05, 0.10, 0.50, and 1.00 wt%) were introduced to SAC305 solder paste by mechanical mixing. Wettability of the solder on Cu substrate was measured in terms of contact angle while printability was calculated from the ratio of printed volume on Cu substrate to ideal volume of solder on the substrate. Results showed that the introduction of TiO2 nanoparticles decreases the contact angle and 0.33 wt% TiO2 is the optimal concentration providing the lowest contact angle within the experimental range. Moreover, the addition of TiO2 nanoparticles reduced the printability of solder paste on Cu substrate. The higher amount of nanoparticles gave the lower printability. Simultaneous optimization on wettability and printability was also performed to find the optimal TiO2 concentration.

Wettability and interfacial morphology of Sn–3.0Ag–0.5Cu solder on electroless nickel plated ZnS transparent ceramic

ZnS has been widely used as infrared windows in defense applications and the ZnS glass with high purity has shown high transmittance in mid-infrared and near-infrared lights. The fabrication temperature of the ZnS ceramic was suggested to be reduced to avoid phase transformation of ZnS and sacrifice its good transparency. Similarly, the joining process of ZnS should be at a lower temperature. Traditionally, mechanical assembly and spicing techniques were used to assembly ZnS transparent windows with metal frames at the aircraft. Nevertheless, the assembly strength was still too low and might increase the risk for the safety of aircraft. Recently, low temperature bonding or soldering process has gradually become an opinion to reduce the safety risk. In this study, we utilized a pre-metallization method coating Ni–P on ZnS transparent ceramics and soldered with Sn–3Ag–0.5Cu at a low temperature. In this paper, we evaluated the solder wettability, bonding temperature, holding time and coating thickness of Ni–P layer in terms of the interfacial morphology and joining mechanism. Finally, the most failures of mechanical tests happened at Ni–P/ZnS interface due to long time reaction and weak interface coating forces between Ni–P/ZnS.

Influences of doping Ti nanoparticles on microstructure and properties of Sn58Bi solder

In this paper, the influences of adding Titanium (Ti) nanoparticles on melting characteristics, wettability, shear properties and the growth of interfacial intermetallic compounds (IMC) of Sn58Bi solder were investigated. The results show that the addition of Ti nanoparticles improved the wettability and shear strength of Sn58Bi solder, and the optimum additive content was 0.1 wt%. The microstructure of the Sn58Bi solder was refined obviously with the addition of Ti nanoparticles. The thickness of interfacial IMC reduced significantly by adding Ti nanoparticles. However, doping Ti nanoparticles had the slight effect on the melting temperature of Sn58Bi solder. Moreover, the thickness of IMC at the Sn58Bi/Cu interface was distinctly larger than that of Sn58Bi-0.1Ti/Cu solder after multiple reflows, which means that the addition of Ti nanoparticles could suppress the growth of IMC at solder/Cu interface during multiple reflows.

The importance of FEM simulation in design of a forming tool for test sample preparation to measure solder wettability

The importance of FEM simulation in design of a forming tool for test sample preparation to measure solder wettability