Reliable Additive Manufacturing Using Transient Liquid Phase Sintering

Additive manufacturing is rapidly revolutionizing the way products are designed and built. Its advantages in terms of mobile manufacturing, mass customization, part reduction, waste reduction, and just-in-time sparing are causing it to be considered for many electronics applications. Many of these applications require high thermal and electrical conductivity and high strength in harsh environments. Such applications would benefit greatly from additive manufacturing of metals. However, the number of metal systems that can be successfully manufactured this way is small, with some of the highest conductivity metals (e.g. Cu, Ag) being particularly difficult. Two ways of depositing metals dominate the market. The first is direct ink writing (DIW). The other common technique is laser sintering, including direct metal laser sintering (DMLS). While the DIW process is fine in principle, the inks typically used are not optimized for harsh applications, and produce a voided and porous layer that limits the thermal and electrical conductivity. Laser sintering must be done in vacuum and requires that the laser raise the temperature of the powder to around 500°C which can result in damage to the substrate. Furthermore, this process is expensive, lacks mobility, and consumes significant energy. This paper will discuss a new form of metal additive manufacturing that addresses the shortcomings of current direct writing and laser sintering approaches by making use of the transient liquid phase sintering process. During the TLPS process, low melting point semimetal powder of Indium will be melted at a temperature of 300°C. This liquid will then surround and diffuse with high melting temperature metal powders forming intermetallic compounds at the solid-liquid interface. These intermetallics possess a higher melting point than the low temperature semimetal. The paper will demonstrate the use of this technique to make reliable 2D lines and 3D structures. It will also discuss the deposition and sintering process and its effect on the adhesion strength, thermal conductivity, and electrical resistivity of the resulting structures.

Transient Liquid Phase Sintering of PM Steel—A Matter of the Heating Rate

Powder metallurgy (PM) offers several variants to introduce alloying elements for establishing the desired final composition. One route is the master alloy (MA) approach. The composition and the elements contained in the MA can be adjusted to obtain a liquid phase that penetrates through the interconnected pore network and thus enhances the distribution of the alloying elements and the homogenization of the microstructure. Such a liquid phase is often of a transient character, and therefore the amount of liquid formed and the time the liquid is present during the sintering are highly dependent on the heating rates. The heating rate has also an impact on the reaction temperatures, and therefore, by properly adjusting the heating rate, it is possible to sinter PM-steels alloyed with Fe-Cr-Si-C-MA at temperatures below 1250 °C. The present study shows the dependence of the melting regimes on the heating rate (5, 10, 20, 120 K/min) represented by “Kissinger plots”. For this purpose, liquid phase formation and distribution were monitored in quenching dilatometer experiments with defined heating up to different temperatures (1120 °C, 1180 °C, 1250 °C, 1300 °C) and subsequent quenching. Optimum sintering conditions for the materials were identified, and the concept was corroborated by C and O analysis, CCT diagrams, metallographic sections, and hardness measurements.

Microstructure Evolution and Shear Property of Cu-In Transient Liquid Phase Sintering Joints

Transient liquid phase sintering (TLPS) is a promising joining technology that can achieve high temperature resistant solder joints at low temperature, showing excellent potential in power electronics. In this work, Cu/Cu-In/Cu solder joints were successfully prepared by TLPS process. The effects of bonding pressure and holding time on the microstructure and shear strength of Cu-In TLPS joints at 260 and 320°C were studied. The results showed that as bonding pressure increased from 0.1–0.6 MPa, the porosity decreased and shear strength increased significantly. No obvious change was found as bonding pressure continued to increase to 1 MPa. As holding time increased at 260°C, Cu11In9 was formed and gradually transformed to Cu2In that can withstand elevated temperature. Meanwhile, the porosity decreased while shear strength increased. It was calculated that volume expansion (12.74%) occurred during the phase transition from Cu11In9 to Cu2In. When bonding temperature increased to 320°C, only Cu2In was detected and then gradually transformed to Cu7In3 with the growing holding time. As holding time reached 120 min, their porosity increased and lead to weak shear strength due to volume shrinkage (15.43%) during the phase transition from Cu2In to Cu7In3.