Lead-Free, Fluxless Solder Joints to Synthetic Diamond

With the recent trends towards smaller heat sources and higher heat fluxes, conduction spreading resistance can become a bottleneck to efficient heat dissipation from temperature-sensitive components such as advanced microelectronics and high power light-emitting diodes (LEDs). However, the use of a high thermal conductivity heat spreader such as a synthetic chemically vapor deposited (CVD) diamond results in an additional interface between two materials. The near elimination of thermal interface resistance at material boundaries is critical for this application to be viable from a thermal standpoint. A lead-free, fluxless soldering process for joining metal substrates to a CVD diamond heat spreader was sought in this study to achieve the low thermal interface resistance and Restruction of Hazardous Substances Directive (RoHS) compliancy desired for this application. A typical titanium-platinum-gold metallization was applied to the surfaces of the diamond to enable it to bond with metal solders. Both indium solder and a popular tin-silver-copper solder (SAC305) were examined in this work. The indium was cold-welded to the substrates under pressure. The SAC305 solder was reflowed in a nitrogen furnace with variable pressure at the joint. The resulting solder thicknesses and joint quality were assessed using a scanning electron microscope (SEM). Lastly, a prototype of high-heat flux source with a CVD diamond spreader and forced convection with air was built to illustrate the thermal advantages of properly incorporating a diamond spreader with a fluxless solder bond.