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SEPTEMBER 2022 I PCB007 MAGAZINE 67 conductive pastes. e Holy Grail of conduc- tive pastes has been copper-based or carbon- nanotube based. But even the best nanoparti- cle pastes exhibit a bulk resistivity more than 20 times that of bulk plated copper. e lat- est to be developed is a nano-copper particle that is coated to prevent oxidation, allowing air handling of the paste. By mixing the nanoparti- cles with micron-sized copper particles, resis- tivities of only six times that of bulk copper have been achieved 2 . Transient Liquid Phase Sintering Pastes e most electrically and thermally conduc- tive metallic pastes are those that are TLPS. ese are metallic pastes that combine small particles of solder materials (many nanoparti- cles) with small particles of solderable metals in a fluxing polymer binder. Aer sintering and during lamination of the subassembly cores or blind vias into a single PCB, the solder particles melt, wet the solderable particles, and form an interconnected metallic mesh. e metal- lic mesh formed has a melting point higher than the original solder alloys and is therefore stable through subsequent thermal excursions such as lead-free assembly operations. e sol- der particles also wet to the copper caps on the PTHs, the caps become permanently metal- lurgically bonded to the metal mesh of the sin- tered paste interconnect, and deliver a contin- uous and robust thermal and electrical con- duction through sintered metal joints. Table 1 shows the testing of this type of paste. Via Filling Experiments Even though the resistivity of these nano- pastes is higher than bulk plated copper, the ideal application is for filling blind vias, as they are small and less sensitive to the resistivity. Table 2 shows the results of tests conducted on 125 um (0.005") diameter blind vias of various depths drilled into different substrates and sub- sequently filled with the nano-copper conduc- tive pates and sintered. e samples were both etched into pads and le as copper-clad panel- plated specimens. Sintering was conducted either by an 808 nm diode laser, photonic flash tube (BB flash) or baked at a reduced tempera- ture in a reducing environment.