Material Characterization
Dynamic viscoelastic testing measures the internal structure and flow behavior of solder pastes and underfills. Process engineers use oscillatory rheometry to determine how these materials deform and recover during high-speed squeegee printing or dispensing operations. The technique subjects a material sample to sinusoidal shear stress to record its elastic and viscous responses.
This measurement reveals the material yield point and its ability to maintain stencil print definition without slumping. The resulting storage modulus and loss modulus curves guide the selection of paste formulations for fine-pitch components.
Testing Sequence
A thin layer of the paste is placed between a flat plate and a rotating cone that oscillates at specific frequencies and strain amplitudes. Low strain amplitudes measure the undisturbed structure of the paste, while higher strain sweeps simulate the high-shear environment of the squeegee stroke. When the squeegee passes, the paste must flow easily, and then recover its solid-like behavior quickly to prevent solder bridging.
Process Impact
Paste formulations that fail to recover their viscosity after printing cause solder defects such as bridging and mid-chip beads. If the recovery is too slow, the printed bricks slump and spread, which leads to short circuits during reflow. Analyzing these characteristics allows paste developers to adjust solvent and flux concentrations for optimal printability.
