Abstract: The emergency of micro/nanoelectronic systems and miniaturized portable devices raises urgent demand on miniaturized and integrated multi-layer ceramic capacitors (MLCC). For MLCC fabrication, tape-casting technology is one of the key processes, where routinely using toluene-ethanol solution of polyvinyl butyral (PVB) as a binder. Thus, the solution properties and their gelation play crucial roles in suppressing the flaws (e.g., pinhole and local heterogeneity) in the dielectric layer. However, the effects of volume ratio of toluene to ethanol have yet been investigated in detail. This work decouples the effects of toluene and ethanol on the intermolecular, intramolecular and PVB-solvents interactions, and further understands the effects of solution’s compositions on gelation process. It is found that ethanol is a good solvent of PVB, through the expansion of the glassy region as structured by the packing of alkyl side chains, while toluene can effectively reduce the friction between PVB chains and solvents. Upon being exposed to atmosphere, PVB in solutions undergoes coarsening and coalescence through "nucleation-growth" mechanism. When the relative humidity is low in atmosphere, the compositional trajectory cannot cross the binodal curve and thus produce dense morphology, while high relative humidity will cause defects because the compositional trajectory crosses the binodal or spinodal curves. The defects almost do not affect the effective modulus of PVB film and thermomechanical properties. This work provides reference significance for screening PVB polymer, solvent system and film forming conditions in MLCC casting process.