Boride Coatings on Steel Protecting it Against Corrosion by a Liquid Lead-Free Solder Alloy

The goal of this research is to study the applicability of the diffusion boriding process as a high-temperature thermochemical heat treatment to enhance the lifetime of steel selective soldering tools. The main purpose of the work is to discuss the behavior of double-phase (FeB/Fe2B) iron-boride coating on the surface of different steels (DC04, C45, CK60, and C105U) against the stationary SAC309 lead-free solder liquid alloy. The boride coating was formed on the surface of the steel samples through the powder pack boriding technique. The microstructure of the formed layer was examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The borided samples were first cut in half and then immersed into a stationary SAC309 lead-free solder liquid alloy (Sn–3Ag–0.9Cu) for 40 days. Microstructure examinations were performed by SEM with energy-dispersive spectroscopy and an elemental distribution map after the dissolution test. Excessive dissolution/corrosion of the original steel surface was observed at the steel/SAC interfaces, leading also to the formation of Fe–Sn intermetallic phases. This was found to be the major reason for the failure of selective soldering tools made of steel. On the contrary, no dissolution and no intermetallic compounds were observed at the FeB/SAC and at the Fe2B/SAC interfaces; as a result, the thicknesses of the FeB and Fe2B phases remained the same during the 40-day dissolution tests. Thus, it was concluded that both FeB and Fe2B phases show excellent resistance against the aggressive liquid solder alloy. The results of the dissolution tests show a good agreement with the thermodynamic calculations.

Effect of SiO2 nanoparticle addition on growth of interfacial Ag3Sn intermetallic compound layers between lead-free solder and silver conductor

This study investigated the effects of silver powder modification on intermetallic compound (IMC) formation and silver leaching during soldering at high temperatures. Silica nanoparticles (NPs) were deposited onto a silver powder surface to inhibit silver leaching, which can lead to soldering joint failure during high-temperature soldering. The NPs were deposited through hydrolysis and a condensation reaction of tetraethyl orthosilicate (TEOS) based on the Stöber method. Fourier transform infrared spectroscopy and scanning electron microscopy were used to observe the microstructures of silver powders after the deposition of silica NPs with various TEOS concentrations and various deposition times. As the deposition time increased, the amount of silica NPs on the surface of the silver powder increased. The transmission electron microscopy results show that silica NPs were located at the IMC grain boundaries, which can hinder the dissolution of IMCs by lead-free solder melt along grain boundaries during soldering, retarding silver leaching. The growth kinetics and mechanism of IMCs during soldering were investigated. The results show that the growth of IMCs is mainly dominated by bulk diffusion. The activation energy for IMC growth increased and the growth rate decreased with increasing silica NP addition and deposition time.

Microstructure and Phase Investigation of Sn-58Bi-xCu Lead-Free Solder After Immersion in Sodium Chloride Solution

The changes in microstructure and phase of tin-bismuth-copper (Sn-58Bi-xCu) were investigated after immersion in 3.5 wt. % sodium chloride (NaCl) at variations of Cu micro-alloying at 0.25, 0.50, 0.75, 1.00 and 1.25 wt. %. The morphological observation revealed that the long crystal grains of the Cu-rich phase were produced as the amount of Cu increased. The phase analysis shows that at 0.5 wt. % Cu additions, the intermetallic compound od Cu6Sn5 began to form and dominate the microstructure. After immersion in NaCl, a porous structure was seen covering the surface of the ternary solder, indicating the formation of a defective corrosion protection layer. The predominance of Cu6Sn5 is believed to boost the galvanic corrosion coupling potential of the ternary solder. As a result, the more electrochemically reactive phase was pushed to be eliminated during immersion in 3.5 wt.% NaCl solution. Thus the black spots were formed. The presence of Cu6Sn5 was seen to be detrimental to the electrochemical performance of Sn-58Bi-xCu.

Effect of Aging on the Properties of Intermetallic Layers in SAC+Bi Solder Joints

A major problem faced by electronic packaging industries is the poor reliability of lead free solder joints. One of the most common methods utilized to tackle this problem is by doping the alloy with other elements, especially bismuth. Researches have shown Bismuth doped solder joints to mostly fail near the Intermetallic (IMC) layer rather than the bulk of the solder joint as commonly observed in traditional SAC305 solder joints. An understanding of the properties of this IMC layer would thus provide better solutions on improving the reliability of bismuth doped solder joints. In this study, the authors have used three different lead free solders doped with 1%, 2% and 3% bismuth. Joints of these alloys were created on copper substrates. The joints were then polished to clearly expose the IMC layers. These joints were then aged at 125 °C for 0, 1, 2, 5 and 10 days. For each aging condition, the elastic modulus and the hardness of the IMC layers were evaluated using a nanoindenter. The IMC layer thickness and the chemical composition of the IMC layers were also determined for each alloy at every aging condition using Scanning Electron Microscopy (SEM) and EDS. The results from this study will give a better idea on how the percentage of bismuth content in lead free solder affects the IMC layer properties and the overall reliability of the solder joints.

Effect of Mechanical Cycling at Elevated Temperatures on The Constitutive Properties and Microstructure of Lead Free Solder Alloys

Solder joints in electronic packages often experience fatigue failures due to cyclic mechanical stresses and strains in fluctuating temperature environments. These stresses and strains are induced by mismatches in coefficients of thermal expansion, and lead to damage accumulation that contributes to crack initiation, crack propagation, and eventually to failure. In this study, we have tried to compare the effects of elevated mechanical cycling on SAC305 and SAC+Bi (SAC_Q). Initially, small uniaxial cylindrical samples of both alloys were prepared and reflowed in a reflow oven. These samples were then mechanically cycled for various durations at testing temperatures of 100 °C. The measured cyclic stress-strain curves were used to characterize the evolution of the hysteresis loop properties (peak stress, hysteresis loop area, and plastic strain range) with high temperature mechanical cycling. In addition, uniaxial tensile tests and creep tests were also conducted on specimens that had been previously mechanically cycled for various durations (e.g 0, 50, 100, 200, and 300 cycles) at an elevated temperature. This allowed us to study the evolution of the constitutive behavior of the solder alloys that occurred during the high temperature mechanical cycling due to the fatigue damage that builds up in the specimens. The reductions in the properties that occur during high temperature mechanical cycling were also correlated with the corresponding changes in the microstructure of the specimens. Rectangular cross-sectioned samples of the two lead free solder alloys were polished and selected regions indented to track the changes in the microstructure of a fixed region with mechanical cycling at T = 100 °C. Using the results of this study, we are working to develop better fatigue criteria for lead free solders which are subjected to variable temperature applications.