Issue link: https://iconnect007.uberflip.com/i/1383248
JUNE 2021 I PCB007 MAGAZINE 47 e microstructure of processed TLPS com- positions looks like a network of particles of metal, each bearing one or more "shells" of the newly formed alloy/intermetallic composi- tions, which are in turn interconnected by the non-reactive portion of the original solder al- loy. Open areas of the metallic network struc- ture are generally filled with the self-inerting flux in small pockets. e interconnected met- al forms 84 to nearly 100 volume percent of the sintered paste depending on the specific layup and lamination conditions. Reaction between the metal and the reac- tive element(s) of the solder alloy may result in either partial or complete incorporation of the metal particles into the newly formed alloy and/or intermetallic species. e number and nature of the new alloy and/or intermetallic species that form is dependent on the selection of metallic constituents in the TLPS compo- sition, their relative proportions, the particle size distribution, and the process temperature. e composition of the residual components of the original solder alloy is likewise depen- dent on these factors; therefore, cross-sections of the processed TLPS compositions will ex- hibit a number of distinct phases that vary in proportion according to both the formulation and process conditions. Transient liquid phase sintering (TLPS) pastes have been successfully used for over two decades in printed circuit board (PCB) in- terconnects—printed jumper wire and Z-axis layer-to-layer connections—that are subject- ed to subsequent solder reflow. In these appli- cations, TLPS pastes are sintered during com- mon PCB fabrication processes such as lami- nation, and will not remelt in subsequent as- sembly operations. Billions of interconnects have been made with TLPS pastes in these PCB applications. Oen, the PCBs fabricat- ed with TLPS interconnects are for extreme- ly complex, reliability-critical, and relative- ly niche applications such as supercomputing and aerospace. e increasing demands from the cloud compute and 5G telecom markets are removing the "niche" categorization from that characterization. Cost Analysis vs. Conventional PCB Fabrication Methods e value proposition for the use of blind and buried vias in the fabrication of high-lay- er-count PCBs depends heavily on the specif-