Reflow Cycles Research Articles

Effect of Multiple Reflow Cycles on the Shear Strength of Nano-Al2O3 Particles Reinforced Sn3.6Ag Lead-Free Solder Alloy

The effect of nano-Al2O3 particles reinforcement on shear strength of Sn3.6Ag solder joint exposed to multiple reflows was studied. The nano-composites of Sn3.6Ag solder were developed by mechanical dispersion of nanoparticles in the solder alloy. The melting, mechanical and microstructural properties of Sn3.6Ag composite solders with varying weight fractions of nano-Al2O3 particles were evaluated by subjecting them to multiple reflow cycles. The results showed an improvement in the wettability of the solder with inclusion of nano-Al2O3 particles. The composite Sn3.6Ag solders with 0.01–0.05 wt% nanoparticles reinforcement showed an improvement in the shear strength and ductility of the solder joint compared to monolithic solder alloy under multiple reflow cycles. Samples doped with 0.05 wt% nanoparticles and reflowed for two reflow cycles displayed an appreciable suppression in interfacial intermetallic compound’s growth and improvement in the solder joint shear strength. The addition above 0.1 wt% in solder showed a decrease in the beneficial effects of nano-Al2O3 particles reinforcement.

Microstructure Evolution of Sn-Cu Based Solder Paste with Different Cu Concentration Subjected to Multiple Reflows

The evolution in microstructure of Sn-Cu based solder paste with different copper (Cu) content subjected to multiple reflow cycles was investigated. In this study, the Sn-0.7Cu (SC) solder paste was used as based material. The Cu particles were added into SC solder paste to produce new Sn-4Cu and Sn-10Cu solder paste. After that, the solder paste was then reflowed on Cu-OSP surface finished and subjected to six times reflows. Characterization focuses on the bulk solder microstructure, morphology and intermetallic compound (IMC) thickness after multiple reflows. Results reveal that solder composition significantly affect the microstructure formation and growth of IMC.

Practical Evaluation of Polymer Waveguides for High-Speed and Meter-Scale On-Board Optical Interconnects

We comprehensively evaluated optical characteristics, environmental stabilities, fiber optics compatibilities, and high-speed performances of multimode polymer waveguides fabricated in small batch with a length of 946 mm. The waveguides exhibit a low transmission loss (0.046 dB/cm at a wavelength of 850 nm), low interchannel crosstalk (≤ −58 dB), and a large misalignment tolerance (±20 μm) between fiber and waveguide. The waveguides show good environmental stability and the increase of propagation loss is as small as 0.005 dB/cm and 0.004 dB/cm after a 1000-h ageing test and five solder reflow cycles, respectively. The waveguides have good compatibilities with fibers having different core diameters and index profiles that may be encountered in the real application environment. High-speed transmission performances were evaluated using both nonreturn to zero (NRZ) transmission at data rates of 25 Gb/s and 30 Gb/s, and PAM4 transmission at data rates of 20 Gb/s and 56 Gb/s, respectively. There is no obvious degradation on the eye diagram due to the insertion of the waveguide. Error free transmission were successfully obtained for NRZ transmission at data rates of both 25 Gb/s and 30 Gb/s. The BER for PAM4 transmission reached a level of 10−4 at data rates of both 10 Gbaud (20 Gb/s) and 28 Gbaud (56 Gb/s), which is well below the forward error correction limitation. Moreover, it is the bandwidth of the test equipment rather than the polymer waveguide itself that limits the high-speed transmission performances in our experiments. The results demonstrate that the polymer waveguides are good candidates for high-speed and meter-scale on-board optical interconnect applications.

Effect of Multiple Reflow Cycles and Al2O3 Nanoparticles Reinforcement on Performance of SAC305 Lead-Free Solder Alloy

The effect of Al2O3 nanoparticles reinforcement on melting behavior, microstructure evolution at the interface and joint shear strength of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder alloy subjected to multiple reflow cycles was investigated. The reinforced SAC305 solder alloy compositions were prepared by adding Al2O3 nanoparticles in different weight fractions (0.05, 0.1, 0.3 and 0.5 wt.%) through mechanical dispersion. Cu/solder/Cu micro-lap-shear solder joint specimens were used to assess the shear strength of the solder joint. Differential scanning calorimetry was used to investigate the melting behavior of SAC305 solder nanocomposites. The solder joint interfacial microstructure was studied using scanning electron microscopy. The results showed that the increase in melting temperature (TL) and melting temperature range of the SAC305 solder alloy by addition of Al2O3 nanoparticles were not significant. In comparison with unreinforced SAC305 solder alloy, the reinforcement of 0.05-0.5 wt.% of Al2O3 nanoparticles improved the solder wettability. The addition of nanoparticles in minor quantity effectively suppressed the Cu6Sn5 IMC growth, improved the solder joint shear strength and ductility under multiple reflow cycles. However, the improvement in solder properties was less pronounced on increasing the nanoparticle content above 0.1 wt.% of the solder alloy.

Shielding effect of Ag3Sn on growth of intermetallic compounds in isothermal heating and cooling during multiple reflows

In this research, the shielding effect of Ag3Sn particles on Cu6Sn5 grain boundaries in isothermal heating and cooling of multiple reflows was investigated in Sn/Cu and Sn3.5Ag/Cu solder bumps by utilizing the real-time imaging technology of Shanghai Synchrotron Radiation Facility as well as scanning electron microscope. Results show that controlled by volume diffusion growth mechanism, the value of time exponent n for Sn is much closer to 1/2 with increasing reflow cycle than for Sn3.5Ag that governed by both grain boundary and volume diffusion mechanisms in isothermal heating. Consequently, IMC increment in Sn3.5Ag is larger than in Sn due to the Cu supplement effect of J*in−b in cooling during multiple reflows. The reason for the phenomena may be the shielding effect of Ag3Sn nano particles on grain boundaries in Sn3.5Ag during multiple reflow process.

Separating Temporary Carrier from Large Thin Panel with Air Jetting

Abstract As electronic components becomes thinner and more powerful than ever, substrates are being pushed to be so thin that they are flexible. Furthermore, both passive and active components are being embedded into substrates to further reduce packaging thickness. To fabricate a substrate that is too thin to be rigid and flat, a temporary carrier is applied onto the substrate to support the fabrication. However, but it must be removed upon completion, and the substrate and carrier must be bonded with very high adhesion strength during solder reflow cycles, due to a mismatch in the coefficient of thermal expansion (CTE). Such high adhesion strength typically creates more problems in the debonding process. Unlike mechanical debonding, which generates significant peeling stress on a device's surface, air jet debonding introduces air streams between the carrier and the substrate, thus lifting the carrier from underneath and compressing the substrate surface downward. This results in the most uniform stress distribution, which is best suited for larger-area debonding processes. In this paper, the air-jet debonding of a glass carrier from a 300 x 300 mm thick panel is demonstrated. The bonding adhesive Z-Coat 421G is developed to temporarily bond with panel substrate for lead-free assembly.

Geometrical outline evolution and size-inhibiting interaction of interfacial solder bubbles and IMCs during multiple reflows

By utilizing synchrotron radiation X-ray imaging technique for in situ observation, the geometrical outline evolution and size-inhibiting interaction of solder bubbles and IMCs during multiple reflows were investigated in this work. It was found that the interfacial bubbles can inhibit the growth of IMC grains caused by the hindering effect of bubbles on contact and reaction between solder and substrate; meanwhile, following the growth of IMC grains, the bubble size in return got smaller and its aspect ratio became larger with reflow cycle, which is the result of inhibiting effect of solid IMC grains on solder voids; Because of boundary segregation and thermomigration, Cu atoms tend to gather on the bubble boundary and form a Cu-rich phase, consequently resulting in the formation of IMCs at the gas/solder interface area.

Root Cause Investigation of Lead-Free Solder Joint Interfacial Failures After Multiple Reflows

Solder joint interconnects in three-dimensional (3D) packages with package stacking configurations typically must undergo multiple reflow cycles during the assembly process. In this work, interfacial open joint failures between the bulk solder and the intermetallic compound (IMC) layer were found in Sn–Ag–Cu (SAC) solder joints connecting a small package to a large package after multiple reflow reliability tests. Systematic progressive 3D x-ray computed tomography experiments were performed on both incoming and assembled parts to reveal the initiation and evolution of the open failures in the same solder joints before and after the reliability tests. Characterization studies, including focused ion beam cross-sections, scanning electron microscopy, and energy-dispersive x-ray spectroscopy, were conducted to determine the correlation between IMC phase transformation and failure initiation in the solder joints. A comprehensive failure mechanism, along with solution paths for the solder joint interfacial failures after multiple reflow cycles, is discussed in detail.

Rapid Solidification of Sn-Cu-Al Alloys for High-Reliability, Lead-Free Solder: Part II. Intermetallic Coarsening Behavior of Rapidly Solidified Solders After Multiple Reflows

Controlling the size, dispersion, and stability of intermetallic compounds in lead-free solder alloys is vital to creating reliable solder joints regardless of how many times the solder joints are melted and resolidified (reflowed) during circuit board assembly. In this article, the coarsening behavior of Cu x Al y and Cu6Sn5 in two Sn-Cu-Al alloys, a Sn-2.59Cu-0.43Al at. pct alloy produced via drip atomization and a Sn-5.39Cu-1.69Al at. pct alloy produced via melt spinning at a 5-m/s wheel speed, was characterized after multiple (1-5) reflow cycles via differential scanning calorimetry between the temperatures of 293 K and 523 K (20 °C and 250 °C). Little-to-no coarsening of the Cu x Al y particles was observed for either composition; however, clustering of Cu x Al y particles was observed. For Cu6Sn5 particle growth, a bimodal size distribution was observed for the drip atomized alloy, with large, faceted growth of Cu6Sn5 observed, while in the melt spun alloy, Cu6Sn5 particles displayed no significant increase in the average particle size, with irregularly shaped, nonfaceted Cu6Sn5 particles observed after reflow, which is consistent with shapes observed in the as-solidified alloys. The link between original alloy composition, reflow undercooling, and subsequent intermetallic coarsening behavior was discussed by using calculated solidification paths. The reflowed microstructures suggested that the heteroepitaxial relationship previously observed between the Cu x Al y and the Cu6Sn5 was maintained for both alloys.

Alternative Metallurgies for MEMS Lid Seal

Abstract MEMS devices are a segment of the electronics industry enjoying explosive growth. With the advent of the internet of things (IoT), the trajectory for growth will continue to be steep and price pressures on both the devices and packaging will be intense. Current MEMS packaging schemes rely heavily on an architecture that employs a metal-can or laminate “lid” attached to the device package with a solder ring. This ring serves to connect the package elements together mechanically and to protect the device from noise (electronic or acoustic) and environmental damage. Like most modern electronic devices, MEMS devices are subject to the challenges imposed by the requirement for lead-free packaging. As with many devices, solder connections formed within the package have a similar composition, and therefore a similar reflow profile, to the solder used to attach the packaged components to a substrate. Devices that yield well at the manufacturer are thus susceptible to field failure at the assembly site induced by remelting of the solder within the package during assembly reflow. Unlike in SIP packages where the solder joints in the module are often encased in mold compound, the ring seal in MEMS devices is exposed and unreinforced resulting in failures such as lid shift, solder flow or flux contamination in the device cavity, and package deconstruction - particularly during rework. These type of post-fabrication yield losses can be very costly. Transient liquid phase (TLPS) sintering paste compositions are a new class of solder replacement materials that can be processed at typical reflow temperatures, but which do not remelt when subjected to subsequent thermal excursions. TLPS pastes combine solder alloy particles and reactive metal particles in proportions such that the solder ‘thermosets’ during a typical solder reflow cycle. This ‘thermosetting’ behavior results in a joint that does not remelt at the original reflow temperature, and thus enables the highly reliable electrical interconnect essential for assemblies that will undergo subsequent reflow excursions. TLPS pastes are similar to solders in many respects. The electrical, thermal and mechanical performance is similar to conventionally used tin-based solders. The TLPS pastes are stored and applied like solder pastes. The reflow cycles used to form the TLPS paste joints are also similar to those for forming solder joints. However, unlike solder, TLPS materials do not change shape during reflow or wet substrates beyond the deposition footprint. Also unlike solder, the metals within the bulk of the TLPS interconnect react with one another resulting in a stable interconnect with a thermally robust bonding structure to joining surfaces, even after subsequent thermal cycles. These attributes present both unique opportunities and unique challenges for addressing MEMS lid seal applications. This paper will explore the key material and engineering challenges for implementation of TLPS paste in MEMS lid seal applications.

Suppression of Cu6Sn5 in TiO2 reinforced solder joints after multiple reflow cycles

In the current generation of 3D electronic packaging, multiple reflows are often required during soldering. In addition, electronic packages may be subjected to additional solder rework or other heating processes. This paper investigates the effects of multiple reflow cycles on TiO2 reinforced Sn–0.7Cu solder fabricated by a powder metallurgy microwave sintering technique. Compared to TiO2-free equivalents, a relative suppression of the Cu6Sn5 phase, both as primary crystals and as an interfacial layer was observed. The likely mechanism relates to the TiO2 nanoparticles promoting nucleation and decreasing the amount of time that liquid is in contact with the interfacial layer. The TiO2 particles appear to stabilise the interfacial Cu6Sn5 layer and result in a more planar morphology. The suppression of Cu6Sn5 results in TiO2 reinforced solder joints having a higher shear strength after multiple reflow cycles compared to Sn–0.7Cu solder joints.

Retained ratio of reinforcement in SAC305 composite solder joints: effect of reinforcement type, processing and reflow cycle

Purpose This paper aims to systematically study the effect of reinforcement type, processing methods and reflow cycle on actual retained ratio of foreign reinforcement added in solder joints. Design/methodology/approach Two kinds of composite solders based on SAC305 (wt.%) alloys with reinforcements of 1 wt.% Ni and 1 wt.% TiC nano-particles were produced using powder metallurgy and mechanical blending method. The morphology of prepared composite solder powder and solder pastes was examined; retained ratios of reinforcement (RRoR) added in solder joints after different reflow cycles were analysed quantitatively using an Inductively Coupled Plasma optical system (ICP-OES Varian-720). The existence forms of reinforcement added in solder alloys during different processing stages were studied using scanning electron microscope, X-ray diffractometry and energy dispersive spectrometry. Findings The obtained experimental results indicated that the RRoR in composite solder joints decreased with the increase in the number of reflow cycles, but a loss ratio diminished gradually. It was also found that the RRoR which could react with the solder alloy were higher than that of the one that are unable to react with the solder. In addition, compared with mechanical blending, the RRoRs in the composite solders prepared using power metallurgy were relatively pronounced. Originality/value Present study offer a preliminary understanding on actual content and existence form of reinforcement added in a reflowed solder joint, which would also provide practical implications for choosing reinforcement and adjusting processing parameters in the manufacture of composite solders.

Optimisation of industrial production of low-force sensors - adhesive bonding of force-centring ball

This work addresses the issue of attaching the force-centring part (a round ball) to the load cell of a force sensor, a piezoresistive thick-film Wheatstone bridge deposited onto a ceramic cantilever. As the current soldering process requires expensive metallisation steps for both the ball and the cantilever, and subjects the solder pads used for mounting the cantilever to an additional reflow cycle, an alternative adhesive bonding process was developed, allowing both simpler production and the use of other ball materials such as ceramic and glass. The selfcentring action of solder capillary forces was ensured by structuring the adhesive so as to form a mechanical cuvette allowing centring of the ball by gravity. The selected adhesive materials exhibited good printability and bonding, as well as surviving the subsequent soldering and cleaning process steps.

Transient Liquid Phase Sintering Pastes as Solder Alternatives in High Temperature Applications

High operating temperature electronics are a growing market in the electronics industry. Most of the components and substrates necessary to support the production of these harsh environment devices are now available, but the interconnect materials for component, module and board level assembly are lagging in development. Currently the high operating temperature markets are being served by lead-bearing solders and expensive alloys such as gold-tin or gold-germanium. Lead has been banned from the majority of electronics applications in many areas of the world, but so far has been exempted in the high temperature solder applications due to the lack of an adequate replacement. Even without the impending regulatory restrictions, the melting temperature of the lead containing solders is marginal for the next generation of high operating temperature electronics. Transient liquid phase (TLPS) sintering paste compositions are a new class of solder replacement materials that can be processed at typical reflow temperatures, but which do not remelt when subjected to subsequent thermal excursions. TLPS pastes combine solder alloy particles and reactive metal particles in proportions such that the solder ‘thermosets’ during a typical solder reflow cycle. This ‘thermosetting’ behavior results in a joint that does not remelt at the original reflow temperature, and thus enables the highly reliable electrical interconnect essential for high operating temperature applications. TLPS pastes are similar to solders in many respects. The electrical, thermal and mechanical performance is similar to conventionally used tin-based solders. The TLPS pastes are stored and applied like solder pastes. The reflow cycles used to form the TLPS paste joints are also similar to those for forming solder joints. However, unlike solder, TLPS materials do not change shape during reflow or wet substrates beyond the deposition footprint. Also unlike solder, the metals within the bulk of the TLPS interconnect react with one another resulting in a stable interconnect with a thermally robust bonding structure to joining surfaces, even after subsequent thermal cycles. Though not a practical replacement for solder in common assembly operations, TLPS technology is an attractive solder alternative for specialty interconnect applications. In this paper, the nature and characteristics of TLPS pastes will be will be explored in comparison to solder paste materials commonly used in high operating temperature applications.

Adhesion energy of printed circuit board materials using four-point-bending validated with finite element simulations

Modern printed circuit boards (PCB) are high performance products consisting of metal and dielectric materials in a multi-layered structure. Due to this build-up different failures, such as cracking or delamination, may occur during manufacturing and use leading to failure of the entire electronic device. The mismatch in the thermal expansion coefficients leads to stresses in the structure during temperature change, e.g., during the reflow process. To improve device reliability, it is critical to understand the delamination between different layers and to know the adhesion energy of the interfaces in a PCB. The adhesion energy in test PCBs was determined using four point bending (4PB) experiments before and after 15 reflow cycles. The investigations show that 4PB is applicable for determining the adhesion energy in samples made of halogen free pre-pregs and copper sheets with standard manufacturing processes. Furthermore, the applicability of the analytical adhesion energy calculation in the presence of non-linearities was examined by finite element simulations. It was found that friction between the sample and the pins of the loading device has an influence on the reaction force used for the calculation of the critical energy release rate. Plastic deformation of the 4PB sample, especially in the ductile copper layers, also will affect the analytically determined critical energy release rate. The role of both factors on the analytical approach to measure adhesion energies of PCB interfaces with 4PB will be shown and discussed.

Formation, processing and characterization of Co–Sn intermetallic compounds for potential integration in 3D interconnects

Cobalt exhibits good wetting by lead-free solders (Humpston, 2010) [1], generates Sn-rich intermetallic compounds (IMC’s) and has lower dissolution in solders than Cu and Ni, thus potentially being a more effective solder diffusion barrier with improved electro- and thermo-migration resistance in interconnect applications (Ishida et al., 1991; Okamoto et al., 2006; Wang et al., 2013; Kanatzidis et al., 2005; Liu et al., 2004; Labie, 2007) [5–7]. In this work, we extend prior data (Vakanas et al., 2013; M.O. et al., 2012; Vakanas et al., 2014) on formation, processing, characterization and quantification of Co–Sn IMC phases generated upon deposition, reflow and aging of electroplated and solder paste screen printed samples. Cobalt oxides growth (vs. Cu) was characterized by X-ray Photoelectron Spectroscopy (XPS) as a function of multiple reflows. Cobalt oxides cleaning by microwave Ar/H2 plasma was also demonstrated. Co–Sn IMC growth rate (vs. Cu–Sn and Ni–Sn) was quantified after one reflow cycle and aging in solid-state temperature range of 180–220°C. Relevant kinetic parameters were calculated assuming a diffusion-driven, parabolic growth model. IMC phases were identified by Energy-Dispersive X-ray Spectroscopy (EDS) verifying Sn-rich Co IMC’s. Young’s modulus (E) and hardness (H) were measured by nanoindentation indicating that Co–Sn IMC’s are more compliant and softer than Cu–Sn IMC’s. Implications of the findings for material selection in 3D micro-bumping and stacking integration are discussed with a future outlook and recommendations to improve interconnect reliability.

Z-Interconnect Technology - A Reliable, Cost Efficient Solution for High Density, High Performance Electronic Packaging

Common themes across all segments of electronic packaging today are density and performance. High density interconnect (HDI) technology is one of the most commonly utilized methods for electronic package density improvement, while many different areas have been investigated for performance improvement, from low loss dielectric and conductor materials, to via design and via stub reduction. Electrical performance and density requirements are sometimes complementary, but often times, conflicting with one another. This paper will describe the design, materials, fabrication, and reliability of a new Z-Interconnect technology that addresses both high density and high performance demands simultaneously. Z-Interconnect technology uses an electrically conductive adhesive to electrically interconnect several cores (Full Z) or sub-composites (Sub Z) in a single lamination process. Z-Interconnect technology will be compared and contrasted to other commonly used solutions to the performance and density challenges. HDI or sequential build-up technology is a pervasive solution to the density demands in semiconductor packaging and consumer electronics (e.g. Smart phones), but has not caught hold in HPC or A&D printed wiring board (PWB) applications. One solution for PWB electrical performance enhancement is plated through hole (PTH) stub reduction by “back drilling” the unwanted portion of the PTH. Pb-free reflow and Current Induced Thermal Cycling (CITC) test results of product coupons and specially designed test vehicles, having component pitches down to 0.4mm, will be presented. Z-Interconnect test vehicles have survived 6X Pb-free (260C) reflow cycles, followed by greater than 3000 cycles of 23C–150C CITC cycles. Test vehicle and product coupons also easily survive 10 or more 23C–260C CITC cycles.

Interfacial microstructure and hardness of nickel (Ni) nanoparticle-doped tin–silver–copper (Sn–Ag–Cu) solders on immersion silver (Ag)-plated copper (Cu) substrates

Sn–Ag–Cu composite solder has been prepared by adding Ni nanoparticles. Interfacial reactions, the morphology of the intermetallic compounds (IMC) that were formed, the hardness between the solder joints and the plain Cu/immersion Ag-plated Cu pads depending on the number of the reflow cycles and the aging time have all been investigated. A scallop-shaped Cu6Sn5 IMC layer that adhered to the substrate surface was formed at the interfaces of the plain Sn–Ag–Cu solder joints during the early reflow cycles. A very thin Cu3Sn IMC layer was found between the Cu6Sn5 IMC layer and the substrates after a lengthy reflow cycle and solid-state aging process. However, after adding Ni nanoparticles, a scallop-shaped (Cu, Ni)–Sn IMC layer was clearly observed at both of the substrate surfaces, without any Cu3Sn IMC layer formation. Needle-shaped Ag3Sn and sphere-shaped Cu6Sn5 IMC particles were clearly observed in the β-Sn matrix in the solder-ball region of the plain Sn–Ag–Cu solder joints. Additional fine (Cu, Ni)-Sn IMC particles were found to be homogeneously distributed in the β-Sn matrix of the solder joints containing the Ni nanoparticles. The Sn–Ag–Cu–0.5Ni composite solder joints consistently displayed higher hardness values than the plain Sn–Ag–Cu solder joints for any specific number of reflow cycles–on both substrates–due to their well-controlled, fine network-type microstructures and the homogeneous distribution of fine (Cu, Ni)–Sn IMC particles, which acted as second-phase strengthening mechanisms. The hardness values of Sn–Ag–Cu and Sn–Ag–Cu–0.5Ni on the Cu substrates after one reflow cycle were about 15.1 and 16.6 Hv, respectively–and about 12.2 and 14.4 Hv after sixteen reflow cycles, respectively. However, the hardness values of the plain Sn–Ag–Cu solder joint and solder joint containing 0.5 wt% Ni nanoparticles after one reflow cycle on the immersion Ag plated Cu substrates were about 17.7 and 18.7 Hv, respectively, and about 13.2 and 15.3 Hv after sixteen reflow cycles, respectively.

Effect of carbon nanotubes and their dispersion on electroless Ni–P under bump metallization for lead-free solder interconnection

Electroless Ni–P under bump metallization (UBM) has advantages of even surface, low cost and simplicity to deposit, but their mechanical strength, corrosion resistance and stability still face challenges under high soldering temperature. Incorporating carbon nanotubes (CNTs) into electroless Ni–P UBM might be expected to provide Ni–P–CNT composites with high mechanical strength and stability. Ni–P–CNT composite coatings as well as Ni–P coatings were fabricated by electroless plating process. In order to homogeneously disperse CNTs in composite coatings, acid pre-treatment and surfactant dispersant were introduced. During composite electroless plating, the ultrasonic agitation was also employed. In this study, scanning electronic microscopy (SEM) was used to observe the morphology and the CNTs were proved to be uniformly distributed in Ni–P–CNT coatings by SEM and atomic force microscopy. It was verified that the surface of the composite was quite smooth and continuous; CNTs are equably embedded in the matrix, which is advantageous for conductivity, mechanical strength and corrosion resistance. Shear tests were conducted to evaluate the effect of CNT reinforcement on the mechanical properties of joints, and the joints with CNT additions exhibited higher shear strength at different reflow cycles. Moreover, deposition mechanism of CNTs with Ni was analyzed and confirmed by transmission electron microscopy. Factors that affecting plating process was also discussed, and the optimum plating condition was suggested in this study.

Effect of 1 wt% ZnO nanoparticles addition on the microstructure, IMC development, and mechanical properties of high Bi content Sn–57.6Bi–0.4Ag solder on Ni metalized Cu pads

In the present study, we investigate the performance of 1 wt% ZnO nanoparticles addition to Sn–57.6Bi–0.4Ag lead-free solder and thereby study the effect of nanoparticle addition on the hardness and the inter-metallic compound (IMC) growth, under different reflow conditions. The interfacial morphology of both the plain Sn–57.6Bi–0.4Ag solder and the composite Sn–57.6Bi–0.4Ag/1 % ZnO solder containing nanoparticles on Ni metalized Cu pads ball grid array substrates and the distribution of nanoparticles on the composite solder were characterized metallographically by using scanning electron microscope (SEM). Prior research efforts by others has found strong evidence that the thickness of the IMC layer increases substantially with the increase in the number of reflow cycles. At the interfaces between solder ball and substrate, scallop-shaped Sn–Ni–Cu IMC layer was found in both the plain solder and the composite solder. Meanwhile the SEM results indicated that the growth of IMC of the composite solder joint was different compared to the plain solder with same number of reflow cycles and reflow time. During the shear test, the shear strength of the composite solder samples containing nanoparticle reinforcement were consistently higher than the plain solder joints due to the fact that the IMC growth has been retarded substantially by the nanoparticle addition. The fracture surface of plain solder exhibited a brittle fracture mode with a relatively smooth surface while doped solder joints showed typical ductile failures with very rough dimpled surfaces.