Issue link: https://iconnect007.uberflip.com/i/390433
20 SMT Magazine • October 2014 metric analysis (TGA), and high-resolution transmission electron microscopy (HRTEM). The primary form of quality control consists of SEM and EDS. These techniques prove to be quick and low-cost while providing data that can be easily interpreted. For each reaction, a sample of the as-synthesized material and a fused sample (heated to 210°C with a nine- minute ramp and a four-minute soak under constant N2 flow) are subjected to SEM imag - ing and EDS analysis from which initial par- ticle size, material purity and the fusion char- acteristics (necking, growth, porosity) can all be determined (Figure 4). High quality nano- copper material fuses into a continuous net- work with grains clearly exhibiting crystalline facets. Porosity can also be determined using a combination of ion-beam milling and SEM image analysis. Porosity of the fused material is an important metric in that it has a marked ef- fect on the electrical, thermal, and mechanical properties of the final material. HRTEM imaging revealed fringe patterns from which crystal structures and phases could be determined. Additionally, images were ana- lyzed to confirm particle sizes and the arrange- ment of surfactant layers. Fused particles were examined to understand grain and grain bound- ary development. nanocopper mechanical and thermal Properties Before attempting to build electronic boards, a significant effort was undertaken to establish conditions that yielded high nanocopper bond strength. Specifically, the goal was to be compa- rable to eutectic Sn63Pb37 solder and other high temperature tin-lead solders qualified for use in space. Raw nanocopper material from synthesis was formulated into pastes with additives de- signed to improve suspension, dispersion, flow properties and fusion. The process for testing formulations was iterative and provided quan- titative data on the tensile strength for each for- mulation. A non-standard tensile test had to be developed since the material does not lend itself to casting bulk dog-bone specimens of consis- tent quality as ASTM test procedures require. Formulations were made by washing nano- copper powder several times with various chem- icals, such as solvents, surfactants, and thicken- ers using a variety of methods, such as manual stirring, sonication, and homogenization. The viability of these formulations was quickly screened by performing a quick fusion test on Al substrates and making a qualitative measure of material strength and hardness. Promising formulations were further investigated through SEM and assembly of small LED boards to test electrical performance. To quantify the tensile strength of a for- mulation, a unique tensile test specimen was developed. Tensile specimens consisted of two copper bars that were fused together with nano- nanOcOPPer-baseD sOLDer-Free eLectrOnic assembLY materiaL continues FEATurE Figure 4: SEM micrographs of copper nanopar- ticles isolated from the synthesis (A) and material after fusing at 210°C for four minutes (B). The scale bars represent 100 nm and 500 nm for pan- els A and B, respectively.