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JUNE 2021 I PCB007 MAGAZINE 43 nect between PTHs in two PCB subconstruc- tions, and Figure 2 is a large-view cross-section of a 72-layer PCB that has been divided into four subconstructions, and joined with TLPS paste vias layers. One of the important considerations when employing a subassembly core-to-core man- ufacturing strategy is determining an effective material for interconnecting the cores. Electri- cal performance, reliability, manufacturability, and cost are all critical factors. Interposer sheets consisting of copper-paste- filled vias drilled or laser ablated into prepreg are one option for interconnection of the cores during a conventional lamination cycle. e problem with this solution is that non-sinter- ing copper-filled epoxy pastes used in these vias do not offer adequate electrical or reliabil- ity performance. Sintering conductive pastes are a proven al- ternative technology solution that enables high performance and reliability in a core-to- core interconnection strategy for the manufac- ture of high-layer-count, high-speed and high- frequency PCBs. Sintering Conductive Paste vs. Conventional Conductive Paste Conventional conductive epoxy materials are thermosetting or thermoplastic resins load- ed with metal powders that provide electrical conductivity through percolation-based con- ductive pathways. ey can be either isotropic or anisotropic depending on the level of the conductive particle loading and the method of application. e main deficiencies of these passively loaded polymers occur with temper- ature and humidity fluctuations. ese fluctua- tions cause loss of electrical continuity due to the oxidation of the contact pads and particles as well as the expansion of the polymer. In ad- dition, no metallurgical connection is formed with the pads when using conventional con- ductive pastes. e conventional conductive paste materials have high interfacial resistance to the copper pads, have low current carrying capability, and cannot maintain an electrical shi below 10% when subjected to common reliability test requirements. e reliability and performance limitations of conventional conductive pastes are sur- mounted by sintering conductive paste materi- als. Sintering conductive pastes combine small particles of solder materials with small parti- cles of solderable metals in a self-inerting flux. During lamination of the subassembly cores into a single PCB, the solder particles melt, wet the solderable particles, and form an inter- connected metallic network. e metallic net- work 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. Because the Figure 2: A high-layer-count PCB broken into four cores and joined with sintering paste interposers. (Source: i3 Electronics)