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October 2014 • SMT Magazine 21 copper. A metal fixture with a Teflon collar was used to align the copper bars and apply mod- erate pressure during the heating cycle (Figure 5A and B). The copper bars were weighed be- fore and after fusion to determine the amount of nanocopper in each joint. Tensile tests were then completed using a tensile test machine, and the fracture surfaces (Figure 5C) were ana- lyzed by SEM and EDS. Many formulations were considered, creat- ed, tested, and down-selected. In total, 41 dif- ferent sets of tensile tests were performed on nearly 350 tensile specimens. The maximum load held by a nanocopper joint was 1,155 lb or 7,683 psi for a 0.4375 inch diameter bar. Several sets achieved consistent bond strength above 4,400 psi which is the minimum ten- sile strength of the lowest strength SnPb-based solder alloys currently used for flight hardware assembly. To compare the results of this non- standard test to other materials, a number of baseline test specimens were generated using SnPb solders in place of nanocopper. These samples showed a tensile strength of 11,000 psi. SEM imaging of the fracture surface of test- ed tensile specimens indicated that the above tensile strengths were achieved with less than 10% cohesive ductile failure. Extrapolation of this data suggests that just 20% cohesive duc- tile failure should produce joints with strengths equal to that of eutectic SnPb solder. The SEM images showed cup-cone features typical of duc- tile metal failure, which indicates nanoparticle fusion and formation of strong metallic bonds (Figure 5D). Fusion of nanocopper material to the bulk copper rod was also seen in many sam- ples. This was an important finding as it showed that nanocopper exhibits sufficient activity to react with and bond to a thermodynamically stable bulk copper surface. Thermal conductivity was tested according to ASTM D5470 yielding a value of 140 W/mK, 35% that of bulk copper, but three times that of SnPb (35–50 W/m*K depending on composi- tion). electronic board manufacture and Other applications A commercial camera board was chosen as the first demonstration build because the as- sembly required bonding of the most important electronic component types. Each board con- sisted of four surface mount resistors, 14 surface mount capacitors, three five-pin surface mount voltage regulators, four through-hole jumper connections, one 26-pin through hole connec- tor, and one 48-contact pad QFN surface mount sensor chip. Additionally, successful camera board manufacture could be easily demonstrat- ed via still and live image capture. nanOcOPPer-baseD sOLDer-Free eLectrOnic assembLY materiaL continues FEATurE Figure 5: A–E: A) A single copper bar used for tensile testing is nested within a Teflon collar. B) A tensile specimen loaded in a fixture ready for fusion. C) photograph of a fracture surface fol- lowing tensile testing. d) SEM image of a fracture surface after tensile testing. The scale bar repre- sents 5 μm. E) A surface mount resistor bonded to a Sn surface with nanocopper. The resistor failed in the ceramic rather than the nanocopper when stressed. The scale bar represents 1 mm.