A Modular Solder System with Hierarchical Morphology and Backward Compatibility

Abstract

A modular solder system with hierarchical morphology and micro/nanofeatures in which solder nanoparticles are distributed on the surface of template micropowders is reported. A core-shell structure of subsidiary nanostructures, which improved the intended properties of the modular solder is also presented. In addition, polymer additives can be used not only as an adhesive (like epoxy resin) but also to impart other functions. By combining all of these, it is determined that the modular solder system is able to increase reflowability on a heat-sensitive plastic substrate, oxidation resistance, and electrical conductivity. In this respect, the system could be readily modified by changing the structure and composition of each constituent and adopting backward compatibility with which the knowledge and information attained from a previously designed solder can offer feedback toward further improving the properties of a newly designed one. In practice, In-Sn-Bi nanoparticles engineered on the surface of Sn-Zn micropowders result in pronounced reflowing on a flexible Au-coated polyethylene terephthalate (PET) substrate even at the low temperature of 110 °C. Depending on their respective concentrations, the incorporation of CuO@CeO nanostructures and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) polymers increases oxidation resistance and electrical conductivity of the modular solder.

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

“…8,12−14 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. 1,7,[9][10][11]15 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. 1,7,9−11 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.…”

“…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...…”

“…However, in lowering the melting temperature of the solder through the addition of a supplementary element to the In-based alloy system, a major challenge that arises is the formation of new intermetallic compounds (IMCs) with an intrinsically brittle nature, which can cause the degradation and mechanical failure of the adhesion of the solder joint in a flexible micro-/nanoelectronic circuit. ,− Thus, a suitable additive is required not only to suppress the formation of ductility-reducing IMCs through effective solder design but also to induce microstructure transformation and improve the thermomechanical properties of the solder for utilization on flexible substrates, especially in wearable devices . Keeping the previous point in mind, ductility is a crucial parameter related to the reliability of a solder joint on a flexible substrate because it must be highly capable of frequent bending during use of the device .…”

“…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.…”

Microstructural Evolution and Tensile Testing of a Bi–Sn (57/43) Alloy Processed by Tube High-Pressure Shearing

“…For this alloy, it is rather difficult for dislocations to pass through the interfaces at phase boundaries. Moreover, boundary sliding at the phase boundaries is also very difficult in the as-cast Bi-Sn alloy as the Bi phase and the Sn phase lamellar structures are closely interlocked within each other [31][32][33][34]. It is reasonable to expect, therefore, that the ductility of the as-cast Bi-Sn alloy will be poor due to its lamellar structure, and this is confirmed in these experiments where the as-cast alloy had an elongation of only around 130% under a strain rate of 1.0 × 10 −4 s −1 .…”