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16 SMT Magazine • October 2014 nanOcOPPer-baseD sOLDer-Free eLectrOnic assembLY materiaL continues FEATurE ered, the first restriction on available materials is traditionally the melting point. The material must be processable at a temperature that will not compromise the electronic components or the substrate. Hence, low melting metals and alloys are the first candidates, while materials that are more cost effective like copper or alu- minum or have excellent electrical and ther- mal properties such as gold, silver, copper, and aluminum are immediately passed over due to their high melting points. However, in this in- stance, nanotechnology allows access to more of the periodic table. It is well documented that as a metal particle size shrinks to the nanometer scale, the temperature at which these particles will join together drops significantly below the melting point of the bulk material 9 . It should be possible, then, to develop a pure copper solder by forming sufficiently small copper nanoparti- cles such that the melting point is reduced from the bulk value of 1084°C to a traditional elec- tronics processing temperature around 200°C. Developing such a solder paste must address a number of requirements including: 1. Sufficiently small nanoparticle size 2. A reasonable size distribution 3. Reaction scalability 4. Low cost synthesis 5. Oxidation and growth resistance at ambient conditions 6. Robust particle fusion when subjected to elevated temperature. This paper outlines the development of a novel nanocopper material—from synthesis to board level integration—as a solder-free elec- tronic assembly material. Copper was chosen because it is already used throughout the elec- tronics industry as a trace, interconnect, and pad material, minimizing compatibility issues. It is cheap (one-quarter the cost of tin; 1/100th the cost of silver, and 1/10,000th the of gold), abundant, and has 10x the electrical and ther- mal conductivity compared to commercial tin- based solder. nanocopper synthesis Oxide-free copper nanoparticles with di- ameters in the 5–25 nm range were synthe- sized via solution chemistry (Figure 2). An in- expensive copper salt precursor is the basis of the reaction. The salt is dispersed in a suitable solvent, and subsequently reduced with sodi- um borohydride (NaBH4). The resulting cop- per atoms agglomerate into larger entities and are stabilized by the addition of surfactants to the reaction mixture. Quick cooling, after the reaction is complete, helps to arrest particle growth. The reaction output is then repeated- ly washed to isolate the nanocopper particles and to remove side-products. Throughout the synthesis process, temperatures, color chang- es, solution viscosities, and times are moni- tored closely. None of the chemicals used in the synthesis are rare or prohibitively expen- sive. Figure 2: reaction progression from a soluble copper salt (A-C) to reduction to copper nanoparticles (d).