Hiren Kotadia

A review: On the development of low melting temperature Pb-free solders

Pb-based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb free solders such as SAC105 and SAC305. Although this poses a great challenge to researchers around the world, significant progress is being made in developing new solder alloys with promising properties. In this review, we discuss fundamental research activity and its focus on the solidification and interfacial reactions of Sn-based solder systems. We first explain the reactions between common base materials, coatings, and metallisations, and then proceed to more complex systems with additional alloying elements. We also discuss the continued improvement of substrate resistance to attack from molten Sn which will help maintain the interface stability of interconnections. Finally, we discuss the various studies which have looked at employing nanoparticles as solder additives, and the future prospects of this field.

Massive spalling of Cu-Zn and Cu-Al intermetallic compounds at the interface between solders and Cu substrate during liquid state reaction

The interfacial intermetallic compound (IMC) formation between Cu substrate and Sn-3.8Ag-0.7Cu-X (wt.%) solder alloys has been studied, where X consists of 0–5% Zn or 0–2% Al. The study has focused on the effect of solder volume as well as the Zn or Al concentration. With low solder volume, when the Zn and Al concentrations in the solder are also low, the initial Cu-Zn and Al-Cu IMC layers, which form at the solder/substrate interface, are not stable and spall off, displaced by a Cu6Sn5 IMC layer. As the total Zn or Al content in the system increases by increasing solder volume, stable CuZn or Al2Cu IMCs form on the substrate and are not displaced. Increasing concentration of Zn has a similar effect of stabilizing the Cu-Zn IMC layer and also of forming a stable Cu5Zn8 layer, but increasing Al concentration alone does not prevent spalling of Al2Cu. These results are explained using a combination of thermodynamic- and kinetics-based arguments.

Electronics Assembly and High Temperature Reliability Using Sn-3.8Ag-0.7Cu Solder Paste With Zn Additives

In this paper, we report a comparison of interfacial reactions of Sn-3.8Ag-0.7Cu (SAC 387) and SAC (0-1.5 Zn) solder pastes on Cu (organic solderability preservative finish) and Au/Ni-P/Cu [electroless Ni immersion gold (ENIG)] substrate metallizations with Ni/Sn and Cu/Sn plated component leads. Zn added to the paste in the form of surface-coated micrometer-sized particles dissolves into the solder during reflow. High-temperature aging (150 °C and 185 °C), thermal cycling experiments ( -20°C to 175 °C for FR4 substrate, -40°C to 185 °C for ENIG polyimide substrate), and shear testing of the solder joints were carried out. At a Cu interface, adding Zn to the solder joint improves the shear strength and suppresses Cu3Sn and overall interfacial intermetallic compound (IMC) and Kirkendall void formation . However, above this temperature, the presence of Zn accelerates IMC growth. At a Ni interface, IMC suppression with Zn was noted at all temperatures. The amount of IMC suppression depends on the Zn concentration in the IMCs, which in turn depends on the geometry of joint as well as the original concentration of Zn in the solder.

Remedies to control electromigration: Effects of CNT doped Sn-Ag-Cu interconnects

Electromigration is a critical reliability problem in electronic industry, especially with the shrinkage and downscaling of microelectronic feature size, which results in gradual increase of current density. Carbon nanotube(CNT) doping is adopted in this paper. CNT has demonstrated high electromigration resistance. In our work, CNT doping is combined with SAC interconnects. A CNT after surfactant will be incorporated into SAC solder interconnection. Best percentage of CNT doping is found from this experiment, and better electromigration reliability can be observed from this work by SEM image. Moreover, the shear stress distribution is improved using computational study, which shows better mechanical properties. The combination of experimental and numerical study is highlighted in this work.

Multi-physics computer simulation of the electromigration phenomenon

Previous works on electromigration in microelectronics devices have been reviewed, and a multi-physics EM simulation method that combines electric, thermal, atomic diffusion, and stress analysis has been described. The proposed method can be used to predict the atomic vacancy concentration distribution and void formation in metals or alloys that are subject to current loading.

Comments on electromigration analysis methods

Failures caused by the electromigration (EM) are becoming a primary reliability concern of integrated circuits and electronics packaging designers and manufacturers. In the foreseeable future, the trend of greater scale integration and further miniaturization in the microelectronics industry is expected to continue and this will make the metallization in electronics devices more susceptible to EM failures. In the last few decades, various methods and techniques have been developed and used in EM analysis and EM-aware designs, and numerical modeling in particular has become a very important tool that makes it easier to accurately understand and predict the behavior of EM under realistic environmental conditions. In this paper, a review has been carried out on EM analysis techniques and models ranging from early empirical models to recent numerical simulation method capable of solving three-dimensional EM problems.

Electromigration aware design for nano-packaging

The physical phenomenon electromigration (EM) and computer simulation methods of EM in microelectronics devices have been reviewed. A multi-physics EM simulation method which can be used to predict voids appearance in conductors has been described and its relevant challenges have been discussed. The optimizing methods for nano-packaging are discussed in this work and shunt structure for solder joint is proposed and been verified to have a significant potential to resist EM by our model.

Effect of Al and Zn alloying elements on Sn-3.8Ag-0.7Cu and Sn-3.6Ag solder reaction with Cu and Ni(P) substrate

Sn-Ag-Cu (SAC) and Sn-Ag alloys are considered to be the most promising Pb-free solders for electronics applications. The properties of these solders can be improved by the addition of minor alloying elements to control the intermetallic compounds (IMCs) that are formed during soldering and high temperature storage/cycling. Additional alloying elements into the solder alloys change the solidification path and reaction products. In this study, the effects of Al addition in the range 0.5 to 2wt.% and Zn in the range 0.5 to 1.5wt.% were studied, together with the effect of varying solder volume. The interfacial reaction studies were carried out on Cu and Ni(P) substrates. The resultant solder joint microstructure after reflow and isothermal aging at 150°C up to 500h were investigated under Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray analysis (EDX) for phase identification and Optical microscopy (OM) for qualitative and quantitative analysis. Our experimental results have confirmed that addition of Al and Zn alloys forms Al2Cu and Cu5Zn8 on Cu substrate and Al3Ni and Ni5Zn21 on Ni(P) substrate respectively. These IMC layers have previously been found to act as barrier layers preventing excessive IMC growth. Additional effects of the additives on the eutectic structure of the solder and its properties are also reported. It should be noted that Al and Zn are currently avoided in reflow soldering processes due to their adverse effects on solder wetting. Given the optimum concentrations of Al and Zn found in the present study for barrier layer formation, future work will concentrate on avoiding the wetting problems by injecting the reactive elements (Al, Zn) into the solder in the form of nanoparticles, which release the active element after the initial wetting processes, are complete.