Rheological Characterization
A material testing method in rheology applies increasing sinusoidal deformation to a solder paste sample to determine the linear viscoelastic region and structural strength. Material laboratories perform an oscillatory strain sweep to measure the transition of the paste from an elastic solid to a flowing liquid. This analysis determines how much stress the material can endure before its internal microstructure begins to break down.
Yield Point
The point where the storage modulus falls below the loss modulus indicates the yield stress of the material. This measurement predicts the behavior of the paste during the high-stress print stroke and the subsequent zero-stress rest period. Paste with a high yield point resists slump and maintains deposit shape on the board pads, preventing bridging defects during reflow.
Paste Evaluation
Formulators use these measurements to compare the stability of different solder paste chemistries during storage and use. This test ensures that the paste maintains its structural integrity when subject to the mechanical forces of high-speed printing. The test results allow engineers to select the most stable paste formulation for high-density boards.
By identifying pastes that breakdown too easily, the factory avoids using materials that would run or slump during assembly, thereby reducing the risk of solder bridging and pad-to-pad shorts.
