Elastic Component
Viscoelastic properties of polymer resins and solder pastes describe their ability to store and release deformation energy under stress. The storage modulus g prime represents the elastic, solid-like behavior of the material during handling and printing. High values indicate that the paste or resin resists deformation, maintaining its structure when at rest.
In solder paste, this property prevents the printed deposits from sagging or slumping on the circuit board before reflow occurs.
Structural Stability
Rheological testing of wet solder paste helps manufacturers predict printability on the production line. When a printed solder brick is left on the pad, the storage modulus g prime ensures the deposit does not spread out or bridge with neighboring pads. If the modulus is too low, the solder paste behaves more like a liquid than a solid, leading to print slump and subsequent solder bridging defects.
Rheometers measure this property by applying small oscillatory forces that do not disrupt the internal structure of the paste. This measurement allows production engineers to screen out defective paste batches before they reach the stencil printer.
Phase Transition
Resin flow during lamination depends on the balance between elastic and viscous behavior. As temperature rises in the press, the material softens, and the storage modulus decreases until the resin flows to fill voids between circuit traces.
