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46 PCB007 MAGAZINE I JUNE 2021 als diffuse into one another and potentially form new alloy and/or intermetallic species. In transient liquid phase sintering (TLPS) of powders, the liquid phase only exists for a short period of time as a result of the homoge- nization of the metals to form a mixture of sol- id alloy and/or intermetallic species. e liq- uid phase has a very high solubility in the sur- rounding solid phase, thus diffusing rapid- ly into the solid to form the metallic reaction products. Diffusional homogenization cre- ates the final composition without the need to heat the mixture above its equilibrium melting point. Aer cooling, subsequent temperature excursions, even beyond the original (LMP) alloy melt temperature, do not reproduce the original melt signature of the mixture. is is the "signature" of a typical low-temperature transient liquid phase sintered metal mixture. is signature can be seen in Figure 4 where the paste is initially sintered at 190°C and then subsequently cycled to 280°C multiple times to simulate multiple reflow cycles. As can be seen in the DSC scan, the original endothermic al- loy melt is largely gone aer the initial cycle and there is some continued interdiffusion in the first simulated reflow cycle, but the micro- structure composition is very stable through the remaining reflow cycles. TLPS technology is used to produce con- ductive compositions that include metal powder(s), solder alloy powder(s), and a self- inerting flux system. TLPS compositions are used to form interconnects by creating a pat- terned deposition of the TLPS composition, and then sintering the metallic components in the composition by heating to relatively low temperature. During heating, the self-in- erting flux cleans the metal powders, allow- ing TLPS to occur. Aer heating, the self-in- erting flux chemically binds the resultant met- al oxides, rendering them harmless. For this reason, these compositions provide good elec- trical and thermal conductivity with little op- portunity for conductivity deterioration due to oxidation, corrosion, or thermal expansion and contraction. TLPS compositions do not require nanoparticles, precious metals or poly- mer binders to create robust interconnects un- der standard lamination conditions. Figure 4: DSC signature of a TLPS reaction.