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44 PCB007 MAGAZINE I JUNE 2021 solder particles also wet to the copper caps on the PTHs, the caps become permanently met- allurgically bonded to the metal network of the sintered paste interconnect. erefore, dur- ing lamination, the sintering conductive paste delivers a continuous and robust thermal and electrical conduction through sintered metal joints. Figure 3 illustrates the sintering paste concept of metallurgically interconnected par- ticles. [1] In Figure 1, the copper plating over the PTHs on either side of the sintering paste via can be clearly seen to be wetted by the solder alloy in the paste in the cross-section to form a contin- uous metallurgical pathway. By design, pro- cessing of sintered conductive pastes is done at temperatures compatible with standard print- ed circuit board materials and common lam- ination cycle conditions. Once sintered, as seen in the cross-section view, there are a vari- ety of metallic phases in the metallic network, however, all the phases present in the sintered material are stable in thermal exposures well above the initial process temperature and can thus withstand assembly or other subsequent thermal processing. e metal matrix does not remelt, nor is it damaged during lead-free sol- der reflow as can be seen by again referring to the cross-section in Figure 1, which has been exposed to six such solder reflow cycles. ese sintering conductive pastes overcome several of the disadvantages of copper paste materials. Of particular importance in vertical interconnection, they provide a low and sta- ble electrical resistivity on the order of 30 µW- cm. is stable electrical performance in both normal use and under environmental and ther- mal cycling is the result of the alloyed metal- lurgical web with a compliant design that ac- commodates the large shi in CTE in the Z-ax- is as the laminate is heated above its Tg, which preserves the via integrity. Furthermore, the sintered conductive paste has a high thermal conductivity of nearly 20 W/mK, [2] an order of magnitude better than most conventional conductive adhesives, which prevents thermal runaway. More critically, because the metallur- gical joint is contiguous from pad-to-pad, the high bulk thermal conductivity is not squan- dered on poor interfacial transfer between the pad and the deposit. e low bulk and inter- facial electrical and thermal resistance are also critical to current-carrying capacity and signal integrity. Transient Liquid Phase Sintering Sintering is a term that has become more fa- miliar in the electronics packaging industry with the advent of nanomaterials into joining applications, but the term sintering encom- passes many variations. Sintering is a process in which adjacent surfaces of metal powder particles are bonded by heating. Nano-phase sintering relies on the very high surface activi- ty of the nano-sized particles to cause rapid in- terdiffusion without the formation of a liquid state. Liquid phase sintering is a form of sinter- ing during which solid powder particles coex- ist with a liquid phase. Densification and ho- mogenization of the mixture occur as the met- Figure 3: Conceptual view of a sintered TLPS interconnect.