A flexible electromagnetic interference shielding film coated with a hybrid solder-magnetic powder mixture on a PET substrate

Fine Microstructured In–Sn–Bi Solder for Adhesion on a Flexible PET Substrate: Its Effect on Superplasticity and Toughness

“…The corresponding EDS mapping analysis (Figure S2) revealed that there was a more localized distribution of In, Sn, and Bi elements. This was due to the slightly different solidification temperatures of each In-, Sn-, and Bi-rich phase, although the gas atomization technique using the rapid solidification process (10 3 –10 7 K s –1 ) considerably suppressed nucleate formation and phase growth by providing overcooling of the solder powders. , As a result, the surface microstructures (Figure f) of the gas-atomized solder powders consisted of intricately shaped dendrites with randomly dispersed In-, Sn-, and Bi-rich phases.…”

“…The gas atomization process was commenced by melting the solder constituents in an alumina crucible at 400 °C for 10 min. The molten liquid was then atomized to form small droplets that were rapidly solidified under He gas flowing at 75 bar. ,− Following this, the gas-atomized solder powders were collected and loaded onto a series of American Society for Testing and Materials E11 standard sieves and shaken on a sieve shaker for 5 min to obtain solder powders with a specific size range of less than 20 μm . The classified solder powders were blended with epoxy resin and polyethylene glycol at a weight percent ratio of 90:5:5 with a three-roll miller (Exakt 50 I, Exakt Technologies, Inc.) to produce solder pastes .…”

“…The classified solder powders were blended with epoxy resin and polyethylene glycol at a weight percent ratio of 90:5:5 with a three-roll miller (Exakt 50 I, Exakt Technologies, Inc.) to produce solder pastes . A roll-to-plate printer was used to prepare dot and rectangle array patterns of specimens with dimensions of 16 × 16 mm 2 on a poly­(ethylene terephthalate) (PET) substrate. ,− ,, Subsequently, the In–Sn–Bi solder pastes were reflowed at 90 °C for 1 min and at 110 °C for 1 min in a glovebox filled with an inert gas and then rinsed using sonication for 3 min after being fully submerged in anhydrous ethanol.…”

“…In flexible microelectronics packaging, there has been an increase in demand for more advanced solders with high electrical conductivity, a low melting temperature, and high ductility and toughness, which allows them to perform well in extreme environments such as those found in miniaturized microelectronic components, in which the pitch distance drops to the submicron level and there is an increasing pin count to meet high microchip loads. − In this respect, the eutectic composition of In–Sn solder, which has an intrinsically low electrical resistivity of (10.0–15.0) × 10 –6 Ω cm due to the presence of In (8.4 × 10 –6 Ω cm) and Sn (11.5 × 10 –6 Ω cm), facilitates the rapid transfer of electronic/electrical signals between microchips and conductive patterns on a substrate. , Furthermore, the In-based solder also has a low melting point of 118 °C, and on the basis of the metallurgy principle with regard to the movement toward a more eutectic temperature (usually a lower temperature), the incorporation of a certain additive can decrease the melting temperature even further, which would make it possible to reflow the solder on a plastic substrate with low thermal resistance. ,, For example, poly­(ethylene terephthalate) (PET) substrates exhibit considerable thermal damage after reflowing at temperatures even slightly higher than 110 °C, which severely limits the use of conventional solders due to the need to reflow them at the much higher temperature of 250 °C. ,,− Such high temperatures during the reflow process can also damage other heat-sensitive components, including conductive polymers, organic light-emitting diodes, polymer light-emitting diodes, and so on, and simultaneously thermal stress and strain appear in the solder. ,− Given such features, either soldering (completed melting process without residual thermal stress and strain caused by the temperature gradient) at a low melting temperature of less than 110 °C or utilizing a plastic substrate with high thermal resistance can be applied to solve these problems. ,,− , In our experiments, the addition of a small amount of Bi to the solder enabled to lower the melting point to below 110 °C, a temperature at which the PET substrate does not thermally decompose. ,,− As another solution, high-thermal resistance films, such as polyimide, polycarbonate, and so on, which are thermally resistant up to 250 °C can be used, although such substrates are several hundred times more expensive than PET and much less flexible and transparent, which is beyond the scope of this study. ,,,− Also...…”

“…7,8 Furthermore, the In-based solder also has a low melting point of 118 °C, and on the basis of the metallurgy principle with regard to the movement toward a more eutectic temperature (usually a lower temperature), the incorporation of a certain additive can decrease the melting temperature even further, which would make it possible to reflow the solder on a plastic substrate with low thermal resistance. 1,7,9 For example, poly(ethylene terephthalate) (PET) substrates exhibit consid-erable thermal damage after reflowing at temperatures even slightly higher than 110 °C, which severely limits the use of conventional solders due to the need to reflow them at the much higher temperature of 250 °C. 1,7,9−11 Such high temperatures during the reflow process can also damage other heat-sensitive components, including conductive polymers, organic light-emitting diodes, polymer light-emitting diodes, and so on, and simultaneously thermal stress and strain appear in the solder.…”