Reliable Additive Manufacturing Using Transient Liquid Phase Sintering

Abstract 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.