Fluid Degradation
A rheological property change in printed circuit adhesives describes the loss of the paste’s ability to recover its thick structure after shear stress has been removed. Printing processes are affected by thixotropic index decay when the paste remains on the stencil under constant action of the squeegee. This decay represents a change in the internal structure of the material that reduces its ability to resist slumping.
Slump Prevention
When this index drops, the printed solder paste loses its print definition and starts to spread laterally. This spreading leads to solder bridging between adjacent pads on fine-pitch components during the reflow stage. This failure mechanism is especially common when the paste is used beyond its specified work life on the stencil.
Material Monitoring
Assembly factories run regular checks on the printed solder paste deposits to identify when this property begins to degrade. By measuring the print height and comparing it to the stencil thickness, inspectors can determine when to replace the paste. This control preserves the print quality and minimizes the risk of shorts during reflow.
If the paste continues to degrade, the printing process must be stopped to clean the stencil and replenish the paste reservoir with fresh material.
