Large components such as QFN48, LCC64, shielding cans, or large connectors frequently suffer from component dropout during SMT placement. The root cause is not the placement nozzle pressure or vacuum level of the mounter, but rather that the paste structure formed by the Solder Paste within the stencil aperture cannot support the component's self-weight and the shear stress generated by placement acceleration. At this point, if the rosin matrix viscosity in the solder paste formulation is too low, the thixotropic recovery rate is slow, or the insufficient sphericity of the solder powder leads to a decrease in packing density, edge collapse will rapidly occur after demolding, causing a sharp reduction in the effective contact area between the component terminals and the pads.

We continuously tracked the measured data of three mainstream SMT Solder Pastes printed on a 0.5mm thick stencil with 1.2×1.2mm square apertures, and found that the solder paste with a metal content of 89.5% experienced a viscosity attenuation of 37% within 30 minutes at room temperature. Under the same process conditions, the solder paste with a metal content of 90.2% and added modified hydrogenated rosin and nano-silica thickener had an initial tack 23% higher, with only 11% attenuation within 60 minutes. This is directly reflected in the QFN component placement success rate increasing from 82.6% to 99.1%, with the key difference lying in the dynamic regulation ability of the Flux system in the solder paste formulation on the wetting tension of the solder powder surface.

When actually replacing solder paste on the production line, one cannot only look at the TDS parameters; it is necessary to simultaneously verify its demolding integrity under the corresponding stencil thickness and aperture aspect ratio. For example, when printing QFN components on a 0.4mm thick stencil, if the residue of ethylene glycol monobutyl ether solvent in the solder paste formulation is too high, even if there is no obvious bridging by visual inspection before reflow, local paste shrinkage will be triggered by slight solvent volatilization the moment the placement head releases, causing weak adhesion on one side of the terminals. This phenomenon is particularly prominent when the ambient humidity is >65%RH, so the solvent boiling point gradient in the solder paste formulation must strictly match the workshop temperature and humidity window.

We once tested the same solder paste on two printing machines of different brands, and found that the component dropout rate actually increased by 18% after the squeegee angle was adjusted from 45° to 60°. The root cause is that the rosin softening point in this solder paste formulation is 68℃. When the squeegee shear heat accumulation causes the local temperature of the paste to rise above 70℃, its yield value drops sharply by 35%, leading to increased residue at the bottom of the stencil and inducing subsequent placement slippage. Therefore, the thermal rheological window of the solder paste formulation must cover the mechanical heat generation range during actual equipment operation, rather than only meeting the 25℃ standard test conditions.

For tantalum capacitors in packages above 0805 or large-area aluminum electrolytic capacitors, it is recommended to prioritize SMT Solder pastes with a solder powder D50 of 25±2μm and a particle size distribution span controlled within 1.8. Because it can still maintain sufficient cohesion in fine-pitch apertures below 0.3mm, combined with the design that the proportion of carboxylic acid activators in the solder paste formulation is no higher than 0.8wt%, it can not only ensure oxide film removal efficiency, but also avoid excessive corrosion in the early stage of reflow leading to unbalanced terminal wetting.