Ahmed Sharif

Comparative study of the dissolution kinetics of electrolytic Ni and electroless Ni–P by the molten Sn3.5Ag0.5Cu solder alloy

Abstract Lead-free solders have high Sn content and high melting temperature, which often cause excessive interfacial reactions at the interface. Sn3.5Ag0.5Cu lead-free solder alloy has been used to identify its interfacial reactions with two-metal layer flexible substrates. In this paper we investigate the dissolution kinetics of Sn3.5Ag0.5Cu solder on electrolytic Ni/electroless NiP layer. It is found that during 1 min of reflow electroless NiP layer dissolves slightly lower than the electrolytic Ni due to the barrier layer formation between the intermetallic compounds (IMCs) and electroless NiP layer. Faster nucleation of IMCs on the electrolytic Ni layer is proposed as the main reason for higher initial dissolution. The appearance of P-rich Ni layer acts as a diffusion barrier layer between the solder and electroless NiP layer, which decreases the dissolution rate and IMCs growth rate than that of the electrolytic Ni layer, but weaken the interface and reduces the ball shear strength and reliability. After acquiring certain thickness P-rich Ni layer breaks and increases the diffusion rate of Sn and as a consequence both the IMCs growth rate and dissolution rate also increases. It is found that 3 μm thick electroless NiP layer cannot protect the Cu layer for more than 120 min at 250 °C. In electrolytic Ni shear strength does not change significantly and lower dissolution rate and more protective for Cu layer during long time molten reaction.

The effect of curing on the performance of ACF bonded chip‐on‐flex assemblies after thermal ageing

Purpose – Anisotropic conductive film (ACF) is now an attractive technology for direct mounting of chips onto the substrate as an alternative to lead‐free solders. However, despite its various advantages over other technologies, it also has many unresolved reliability issues. For instance, the performance of ACF assembly in high temperature applications is questionable. The purpose of this paper is to study the effect of bonding temperatures on the curing of ACFs, and their mechanical and electrical performance after high temperature ageing.Design/methodology/approach – In the work presented in this paper, the curing degree of an ACF at different bonding temperatures was measured using a differential scanning calorimeter. The adhesion strength and the contact resistance of ACF bonded chip‐on‐flex assembly were measured before and after thermal ageing and the results were correlated with the curing degree of ACF. The ACF was an epoxy‐based adhesive in which Au‐Ni coated polymer particles were randomly disp...

Interfacial microstructure and shear strength of Ag nano particle doped Sn–9Zn solder in ball grid array packages

a b s t r a c t Sn-9Zn solder joints containing Ag nano particles were prepared by mechanically mixing Ag nano parti- cles (0.3, 0.5 and 1 wt%) with Sn-9Zn solder paste. In the monolithic Sn-Zn solder joints, scallop-shaped AuZn 3 intermetallic compound layers were found at their interfaces. However, after the addition of Ag nano particles, an additional uniform AgZn 3 intermetallic compound layer well adhered to the top surface of the AuZn 3 intermetallic compound layer was found. In addition, in the solder ball region, fine spher- ical-shaped AgZn 3 intermetallic compound particles were observed as well as Zn-rich and b-Sn phases. With the addition of Ag nano particles, the shear strengths consistently increased with an increase in the Ag nano particle content due to the uniform distribution of fine AgZn 3 intermetallic compound par- ticles. The shear strength of monolithic Sn-Zn and 1 wt% Ag nano particle content Sn-Zn solder joints after one reflow cycle were about 42.1 MPa and 48.9 MPa, respectively, while their shear strengths after eight reflow cycles were about 39.0 MPa and 48.4 MPa, respectively. The AgZn 3 IMCs were found to be uniformly distributed in the b-Sn phase for Ag particle doped Sn-9Zn composite solder joints, which result in an increase in the tensile strength, due to a second phase dispersion strengthening mechanism. The fracture surface of monolithic Sn-Zn solder exhibited a brittle fracture mode with a smooth surface while Sn-Zn solder joints containing Ag nano particles showed a typical ductile failure with very rough dimpled surfaces.

Effect of nano Al2O3 additions on the microstructure, hardness and shear strength of eutectic Sn–9Zn solder on Au/Ni metallized Cu pads

Abstract Nano-sized, nonreacting, noncoarsening Al 2 O 3 particles have been incorporated into eutectic Sn–Zn solder alloys to investigate the microstructure, hardness and shear strength on Au/Ni metallized Cu pads ball grid array substrate (BGA). In the plain Sn–Zn solder joint and solder joints containing Al 2 O 3 nano-particles, a scallop-shaped AuZn 3 intermetallic compound layer was found at the interfaces. In the solder joints containing Al 2 O 3 nano-particles, a fine acicular-shaped Zn-rich phase and Al 2 O 3 nano-particles were found to be homogeneously distributed in the β-Sn matrix. The shear strengths and hardness of solder joints containing higher percentage of Al 2 O 3 nano-particles exhibited consistently higher value than those of plain solder joint and solder joints containing lower percentage of Al 2 O 3 nano-particles due to control the fine microstructure as well as homogeneous distribution of Al 2 O 3 nano-particles acting as a second phase dispersion strengthening mechanism. The fracture surfaces of plain Sn–Zn solder joints exhibited a brittle fracture mode with smooth surfaces while Sn–Zn solder joints containing Al 2 O 3 nano-particles showed a typical ductile failure with very rough dimpled surfaces.

Interfacial reactions of Sn-Cu and Sn-Pb-Ag solder with Au/Ni during extended time reflow in ball grid array packages

Lead-free solders with high Sn content cause excessive interfacial reactions at the interface with under-bump metallization during reflow. The interface formed after reflow affects the reliability of the solder joint. For this paper, we investigated the interfacial reactions of Sn0.7Cu and Sn36Pb2Ag solder on electrolytic Ni layer for different reflow times. The traditionally used Sn36Pb2Ag solder was used as a reference. It was found that during as-reflowed, the formation of Cu-rich Sn–Cu–Ni ternary intermetallic compounds (TIMCs) at the interface of Sn0.7Cu solder with electrolytic Ni is much quicker, resulting in the entrapment of some Pb (which is present as impurity in the Sn–Cu solder) rich phase in the TIMCs. During extended time of reflow, high (>30 at.%), medium (30-15 at.%) and low (<15 at.%) Cu TIMCs formed at the interface. The amount of Cu determined the growth rate of TIMCs. Cu-rich TIMCs had higher growth rate and consumed more Ni layer. By contrast, the growth rate of the Ni–Sn binary intermetallic compounds (BIMCs) in the Sn36Pb2Ag solder joint was slower, and the Ni–Sn BIMC was more stable and adherent. The dissolution rate of electrolytic Ni layer for Sn0.7Cu solder joint was higher than the Sn36Pb2Ag solder joints. Less than 3 m of the electrolytic Ni layer was consumed during molten reaction by the higher Sn containing Sn0.7Cu solder in 180 mi na t 250 °C. The shear strength of Sn–Pb–Ag solder joints decreased within 30 mi no f reflow time from 1.938 to 1.579 kgf due to rapid formation of ternary Ni–Sn–Au compounds on the Ni–Sn BIMCs. The shear strength of Sn0.7Cu solder joint is relatively stable from 1.982 to 1.861 kgf during extended time reflow. Cu prevents the resettlement of Au at the interface. The shear strength does not depend on the thickness of intermetallic compounds (IMCs) and reflow time. Ni/Sn–Cu solder system has higher strength and can be used during prolonged reflow.

Structural transition and its effect in La, Zr co-substituted mono-domain BiFeO3

A new approach was employed in explaining the weak ferromagnetic behavior of conventionally synthesized Zr4+ modified Bi0.8La0.2FeO3. Rietveld refinement of XRD patterns revealed a polar-to-non-polar R3c → Pnma structural transition in Zr4+substituted samples. Magnetic properties were discovered to be remarkably enhanced, with extracted coercivity and remanence as high as 14 kOe and 0.2 emu/g, respectively. More importantly, an answer to the essential question of the magnetic domain state of the samples has been put forward. Our analysis established, nearly without doubt, the presence of grains consisting of a single magnetic domain. Separated ferromagnetic and anti-ferromagnetic components of the total M-H curves helped to reveal an immense effect of the structural transition on the shape of the hysteresis loops. The orthorhombic magnetocrystalline anisotropy of the Pnma phase has been primarily deemed responsible for the high coercivity and remanence of the Zr4+ modified samples. The effect of the grain...

Synthesis and characterization of indium doped cadmium sulfide nanoribbons

This paper reports the synthesis and optical characterization of indium doped cadmium sulfide (CdS) nanoribbons (NRs) by the metal-catalyzed vapour–liquid–solid growth method. The NRs had a hexagonal wurtzite structure, lengths of up to 100 µm, widths of 0.6–15 µm and thicknesses of 30–60 nm. Indium doping into the CdS lattice was identified and characterized by low-temperature photoluminescence and photoconductivity measurements. Temperature-dependent photoluminescence (PL) measurements showed that the PL spectra of the In-doped CdS NRs have three emission peaks at 9 K, which can be attributed to band edge emission (2.510 eV) and shallow donor levels (2.398 eV and 2.366 eV) due to doping, respectively. Photoconductive characteristics including spectral response and light intensity response of the doped CdS NR were also investigated. The dark current in the doped CdS NRs was found to be one order higher in magnitude than that of the intrinsic, which can be attributed to effective incorporation of the indium dopant in the CdS crystal lattice and, subsequently, the reduction in the recombination barrier in the nanostructures. The current jumped from 1.8 × 10−9 to 2.1 × 10−5 A upon white light illumination with a power density of 2 mW cm−2. The presented results show promising application of these materials in the field of optoelectronics as photo detectors.

Study of thin film metallization adhesion in ceramic multichip module

The concept of Multichip module (MCM) packaging is popularized for evaluating prototypes of novel devices and materials in extreme environmental conditions. The packaging for these types of electronics must be mechanically well-built and the metallization must be low in thermal conductivity in order to prevent the heat transfer from the external environment to the protected device. Strong bonding between metal and ceramic has been an issue of concern for many years due to delamination tendencies at the interfaces. This paper evaluates the adhesion characteristics of sputtered Copper (Cu) thin films to alumina (Al2O3) substrates. The relationship between the surface roughness of the Al2O3 substrate with appropriate surface treatment and the adhesion with Cu thin film is established experimentally. An adhesion layer with lower thermal conductivity and smaller coefficient of thermal expansion (CTE) mismatch such as Titanium (Ti) or Chromium (Cr) was also inserted in between to investigate the adhesion improvement of the metal-substrate. We have seen that Ti is essentially used as an adhesion layer between the bonding of Cu and Al2O3 for electronic applications. With higher adhesion strength, and smaller CTE mismatch with Cu and Al2O3, insertion of Ti adhesion layer with optimum substrate surface roughness could lead to a more reliable packaging solution.

Review on advances in nanoscale microscopy in cement research.

With the rapid advancement of nanotechnology, manipulation and characterization of materials in nano scale have become an obvious part of construction related technology. This review will focus on some of the nanoscopy techniques that are most frequently used in current research of cement based nanostructured materials. In particular scanning electron microscopy, transmission electron microscopy, atomic force microscopy, scanning tunneling microscopy, tomography, scanning transmission X-ray microscopy and laser scanning confocal microscopy are addressed. A number of case studies related to microscopic characterization of nano materials utilizing the aforementioned techniques from the published literature are discussed. While these approaches are beginning to yield promising insight, continued progress will definitely provide a potential sustainable solution for the design, development and promotion towards nanoscale engineering of cementitious materials.

Study on the properties of Zn–xNi high temperature solder alloys

Zn based lead free solder for high temperature applications was developed by the addition of Ni. The effect of Ni content on the characteristics of the solder such as change in microstructure, melting behavior, thermal properties and mechanical properties were investigated. The microstructures of the solders changed significantly on increasing Ni content. Optical and SEM micrographs showed a dramatic increase in both size and quantity of Zn–Ni intermetallic particles with the Ni content. Moreover the area fraction of these intermetallics (IMCs) was also quantified with an image analyzing software. The formation of γ phase rather than δ phase was observed in the microstructure of Zn–Ni systems. The structural changes improved the mechanical properties like tensile strength and hardness of the newly developed binary alloys. Fascinatingly melting behavior of the solder alloys studied by DTA analysis revealed that the melting temperature of the composite solders remained unchanged with Ni addition. TMA analysis revealed that the co-efficient of thermal expansion decreased on increasing Ni content.