Microstructure control of the Sn-Ag-Cu-X solder alloy system through nucleation catalysis of Sn

Tin, the major constituent in Sn-Ag-Cu (SAC) lead-free solders often has difficulty in nucleating during solidification. This often results in the formation of large, brittle pro-eutectic intermetallic (IMC) phases, particularly Ag3Sn, in addition to reduced coupled eutectic volume. This results in weaker, less reliable joints. This research seeks to catalyze tin nucleation at lower undercooling, thereby maximizing the eutectic volume while preventing large pro-eutectic phase formation to promote SAC solder joint reliability. To accomplish this, a near eutectic (NE) SAC alloy is modified with a fourth element (X) selected to favor substitution for Cu in Cu6Sn5. While unadulterated Cu6Sn5 is a poor catalyst for β-Sn, a Cu6Sn5 lattice strained by X may improve this. X candidates (Co, Ni, Fe, Mn, Zn, and Al) were selected based on Darken-Gurry criteria for having a similar atomic radius and electronegativity relative to copper. Undercooling was measured using DSC with fluxed Cu pans at cooling rates common in industry, thereby mimicking real processing conditions. These calorimetric solder joints were then cross-sectioned and analyzed. It was discovered that some X additions increase the undercooling relative to the base SAC alloy while others decrease it. Of the selected X elements, Zn and Al, both with larger atomic radii than Cu, substitute into Cu6Sn5 and result in significantly reduced undercooling. Their corresponding microstructures are favorable and include a high eutectic volume and no embrittling Ag3Sn precipitates

Comparative Study of the Sn-Ag and Sn-Zn Eutectic Solder Alloy

The present work aimed at investigating the properties of Sn-Zn and Sn-Ag solder alloys and extensively focuses on the microstructure, thermal and mechanical properties. The compositions at their eutectic temperatures were weighed carefully in the weighing machine and were prepared after melting the binary system in the furnace at much higher temperatures than their respective eutectic temperatures. The microstructures of both the solder alloys were investigated using a Scanning Electron Microscope (SEM) and optical microscope. The composition and phase analysis of the solder alloys was done using Energy dispersive X- ray spectroscopy (EDS) and X- ray diffraction respectively. Differential scanning calorimetry (DSC) was carried out to find out the melting temperatures of the alloys. Fractography was done to find out the type of fracture under impact testing. Microhardness of the solder alloys were also found out and analysed. The wettability of the samples was observed under Scanning Electron Microscope(SEM) after soldering the alloys on the Copper circuit board. A thorough analysis was done after the experiments were conducted to find out a better solder alloy out of the two

Effect of Addition of Al to Sn-Zn Solder Alloys

Conventional solders consist of Lead, that was found to be toxic and carcinogenic. Hence, restrictions were put on its use by the industrially developed nations. To counter the use of Lead, active research was pursued into the development of Lead-free solders. In our project, we fabricated alloys of composition 3:15:82, 7:43:50 and 10:80:10 (in terms of Aluminium, Zinc and Tin respectively), under furnace cooled and air-cooled conditions. The use of Aluminium was made so as to increase the resistance of the solder to atmospheric corrosion, and also to improve the wettability of the samples. Optical micrographs were obtained for each sample so as to analyze their microstructures. For a deeper understanding, SEM images of each sample were obtained, and EDX analysis was performed side-by-side so as to understand the elemental composition of different phases present in the sample. DSC and TG tests were conducted to determine the melting point of the solder alloy, and the weight gain in the alloy on oxidation respectively. The wettability of each sample was also analyzed. We recorded and plotted down the trends in each case. We then tried to evaluate the most effective solder composition on the basis of the above tests. The near-eutectic composition was considered so as to avoid the formation of a pasty phase that will cause disruption in electrical work

Low Melting Temperature Solder Materials for Use in Flexible Microelectronic Packaging Applications

The increasing application of heat-sensitive microelectronic components such as a multitude of transistors, polymer-based microchips, and so on, and flexible polymer substrates including polyethylene terephthalate (PET) and polyimide (PI), among others, for use in wearable devices has led to the development of more advanced, low melting temperature solders (<150°C) for interconnecting components in various applications. However, the current low melting temperature solders face several key challenges, which include more intermetallic compound formation (thus become more brittle), cost issues according to the addition of supplementary elements to decrease the melting point temperature, an increase in the possibility of thermal or popcorn cracking (reliability problems), and so on. Furthermore, the low melting temperature solders are still required to possess rapid electronic/electrical transport ability (high electrical conductivity and current density) and accompany strong mechanical strength sustaining the heavy-uploaded microelectronic devices on the plastic substrates, which are at least those of the conventional melting temperature solders (180–230°C). Thus, the pursuit of more advanced low melting temperature solders for interconnections is timely. This review is devoted to the research on three methods to improve the current properties (i.e., electrical and thermomechanical properties) of low melting temperature solders: (i) doping with a small amount of certain additives, (ii) alloying with a large amount of certain additives, and (iii) reinforcing with metal, carbon, or ceramic materials. In this review, we also summarize the overall recent progress in low melting temperature solders and present a critical overview of the basis of microscopic analysis with regard to grain size and solid solutions, electrical conductivity by supplementation with conductive additives, thermal behavior (melting point and melting range) according to surface oxidation and intermetallic compound formation, and various mechanical properties

Properties and behaviour of Pb-free solders in flip-chip scale solder interconnections

Due to pending legislations and market pressure, lead-free solders will replace Sn–Pb solders in 2006. Among the lead-free solders being studied, eutectic Sn–Ag, Sn–Cu and Sn–Ag–Cu are promising candidates and Sn–3.8Ag–0.7Cu could be the most appropriate replacement due to its overall balance of properties. In order to garner more understanding of lead-free solders and their application in flip-chip scale packages, the properties of lead free solders, including the wettability, intermetallic compound (IMC) growth and distribution, mechanical properties, reliability and corrosion resistance, were studied and are presented in this thesis. [Continues.

Interactions between liquid Sn-Bi based solders and contact metals for high temperature applications

Liquid solder interconnects are promising as an alternative approach to conventional high melting point solder interconnects for applications where the operating temperature is likely to exceed 125°C. In order to ensure that a liquid solder interconnect remains in contact with the terminations on the component and the substrate, and that electrical contact between them remains unbroken, there must be some growth of an intermetallic compound (IMC) at the interfaces between the solder and the contact metallizations. However, given that IMC growth is generally much faster when the solder is liquid, the growing IMC must act as a strong diffusion barrier to suppress further IMC growth. This paper presents preliminary studies of liquid-phase Sn-Bi based solders that result in stable interfaces between the solders and three common contact metallizations, consisting of electroless Ni(P)/Au, of Cu and of Ti-W. Small quantities (1 or 2 %) of an additional element, including Cr, Si, Zn, Ag, Au, Al and Cu, have been alloyed with the eutectic Sn-Bi composition to find an effective inhibitor additive that can achieve a strong IMC diffusion barrier. IMCs and their growth rates, as well as the consumption rates of the three contact metallizations in contact with the molten solders, were investigated. Storage temperatures of 200°C and 240°C were used, with storage times ranging between two hours and one month. Results to date show that suitable additives can significantly reduce IMC growth rates for both the Ni(P)-Au and Cu contact metallizations, while no appreciable IMC growth is observed for Ti-W in contact with both the original and the various alloyed Sn-Bi based solders. Based on the current results, criteria to further assist the design of feasible molten liquid solder – contact metallization systems have been deduced

Comparative Study of the Sn-Ag and Sn-Zn Eutectic Solder Alloy

The present work aimed at investigating the properties of Sn-Zn and Sn-Ag solder alloys and extensively focuses on the microstructure, thermal and mechanical properties. The compositions at their eutectic temperatures were weighed carefully in the weighing machine and were prepared after melting the binary system in the furnace at much higher temperatures than their respective eutectic temperatures. The microstructures of both the solder alloys were investigated using a Scanning Electron Microscope (SEM) and optical microscope. The composition and phase analysis of the solder alloys was done using Energy dispersive X- ray spectroscopy (EDS) and X- ray diffraction respectively. Differential scanning calorimetry (DSC) was carried out to find out the melting temperatures of the alloys. Fractography was done to find out the type of fracture under impact testing. Microhardness of the solder alloys were also found out and analysed. The wettability of the samples was observed under Scanning Electron Microscope(SEM) after soldering the alloys on the Copper circuit board. A thorough analysis was done after the experiments were conducted to find out a better solder alloy out of the two