Particle Free Copper Inks

Copper is the preferred material for future electronics: high thermal and electric conductivity, durable and ductile, widely available on the global marked and fully recyclable. Therefore, it is the material of choice for electrical wiring and printed electronics. In addition, all copper electrical interconnects are presently in the research focus due to the advantages of single-component metal systems, i.e. copper is used for connecting components onto copper pads. With the progress of additive manufacturing copper pastes and inks have a high potential. The pastes contain copper in three different states, i.e. ball milled copper flakes, copper nanoparticles and copper salts. Very often the pastes are hybrid, i.e. they contain copper flakes and nanoparticles or copper flakes and salts or even all three copper sources. In the presentation, an overview of the materials will be presented. Afterwards, the focus will be on the copper particle free inks, which are fully based on metal salt. The dominating salt for copper particle free inks is Cu-format (CuF). By different complexing agents, e.g. Amino-2-Propanol, 2-Amino-2-Methyl-1-Propanol or Hexylamine, the decomposition temperature can be reduced to low temperature as 100°C. By the decomposition Cu atoms are released and nanoparticles form which sinter and form the conducting traces. High conductivity can be reached – 50% of bulk copper is reported in literature – and thin fine structured layers are realized. However, the key is to control the nanoparticle formation before the sintering. This is achieved by additives which hinder the growth of the particles when they are formed, realizing homogeneous sized small nano-particles and the speed of the subsequent sinter process, i.e. the heat up rate. The mechanisms of nanoparticle formation and the sinter process itself are discussed, i.e. low temperature sintering in an oven versus laser sintering. Results are presented which show that the particle free ink and the sinter process is ready to be transferred to industrial applications. In addition, first results are presented using the ink for bonding components onto substrates.