In Situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-Voltage Transmission Electron Microscopy

The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation at micro- and nano-meter resolutions. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0 wt%Ag-0.5 wt%Cu (SAC305) solder alloy and a Cu substrate in situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

Gas brazing of thin-sheet galvanized steel with aluminum solders

. In this work, the first stage of experimental research was carried out to estimate the main physicochemical processes that determine the qualitative characteristics of a brazed joint made of thin sheet galvanized steel during gas brazing with aluminum solder systems. In particular, an estimation was made of the ability of spreading and wetting of aluminum solders (AlSi5, AlSi12) on the surface of thin sheet galvanized steel ( DX56D + Z of 0.4 mm thick and zinc-coated layer of 45–65 microns) at a step-by-step increase in the heating area of the base metal in the presence and absence of flux (Al-Flux 726). The aluminum alloys was heated “not directly,” but through the base metal to maximize the preservation of the anticorrosive zinc coating at the interface between the liquid solder and the base material.

Versatile TIM Solution with Chain Network Solder Composite

ABSTRACT A novel epoxy-based SAC solder paste TIM system has been developed with the use of non-volatile epoxy flux. Cu filler was added to the solder paste, with Cu volume % of metal ranged from 17 to 60 volume % of metal. Formation of semi-continuous high melting Cu chain network was achieved, with Cu particles bridged by the CuSn IMC. This chain network, at sufficient concentration, serves as skeleton and maintains the shape of the sandwiched solder paste layer, thus prevents further spread out at subsequent SMT reflow process, and also allowed formation of TIM joint even in the absence of solderable metallization on flip chip and packaging housing. This chain network hampered the flow of liquid solder, thus restrained the expansion of outgassing, and consequently resulted in low voiding. Existence of crevices was attributed to excessive oxide brought in by Cu particles, and appeared to increase with increasing Cu filler content. Presence of ductile solder within TIM joint promises high resistance against brittle cracking under stress. The Cu content could be further optimized between 17 and 33 volume % of metal to avoid flux bleeding and maintain good epoxy adhesion between TIM phase and parts. The 20°C thermal conductivity achieved was 6.1 W/mK, and could be up to about 13 W/mK with further epoxy flux optimization.

Ultrasonic-Driven Spreading of Liquid Solder on Nonwetting Substrates

In this work, the spreading of a solder droplet on a substrate agitated by ultrasonic vibration was recorded by a high-speed camera. The dynamics and physical processes of the spreading, such as corrugate formation and atomization, were investigated. Results showed the solder droplet was able to spread on a nonwetting substrate, and it presented periodic expanding-shrinking spreading characteristics with a periodicity of dozens of acoustic periods. Corrugates formed as a result of the capillary wave propagation on the droplet, and the formation became intensive on a violently vibrating surface. Atomization preferentially occurred at the spreading front during solder expansion, where the liquid solder appeared as a film and burst on the whole droplet with strong vibration. High ultrasonic power resulted in fast spreading and a large spreading diameter. In particular, the solder droplet exhibited fast spreading and a large spreading diameter on the TC4 alloy with high characteristic impedance. The Sn-4Cu solder with large viscosity spread slowly and exhibited a small spreading diameter.

The role of physico-chemical properties of liquid solder in reactive wetting: the Cu/SnZnIn system

The measurements of surface tension, density and viscosity of liquid Sn-Zn eutectic alloys containing 0, 5, 10, 15 and 25 mole fraction of In were carried out using the sessile drop method, dilatometric technique and capillary method. The measurements were performed at temperature range between 493 and 843 K. The technique of sessile drop was applied in the measurements of wetting angles and spreading test in the SnZnIn/ Cu system. Surface tension, density and viscosity measurements were carried out in a protective argon-hydrogen atmosphere. Wettability tests were performed in air in the presence of flux Alu33, at 250?C for 2 minutes. Subsequently, the microstructure of solder and the resulting joints was studied. The addition of In to eutectic Sn-Zn alloy improved the wetting properties and causes a reduction of thickness of the intermetallic compounds layer created at the interface between the liquid solder and the Cu substrate.

Effect of Indium Additions on the Formation of Interfacial Intermetallic Phases and the Wettability at Sn-Zn-In/Cu Interfaces

The wettability of copper substrates by Sn-Zn eutectic solder alloy doped with 0, 0.5, 1, and 1.5 at.% of indium was studied using the sessile drop method, with flux, in air, at 250°C and reflow time of 3, 8, 15, 30, and 60 min. Wetting tests were performed at 230, 250, 280, 320, and 370°C for an alloy containing 1.5 at.% of indium, in order to determine activation energy of diffusion. Solidified solder/substrate couples were studied using scanning electron microscopy (SEM), the intermetallic phases from Cu-Zn system which formed at the solder/substrate interface were identified, and their growth kinetics was investigated. The ε-CuZn4 was formed first, as a product of the reaction between liquid solder and the Cu substrate, whereas γ-Cu5Zn8 was formed as a product of the reaction between ε-CuZn4 and the Cu substrate. With increasing wetting time, the thickness of ε-CuZn4 increases, while the thickness of ε-CuZn4 does not change over time for indium-doped solders and gradually disappears over time for Sn-Zn eutectic solder.