Solder paste is a classic example of a non-Newtonian fluid with thixotropic behavior and a pronounced so-called yield stress.
What does this mean in practical terms?
At rest, solder paste exhibits a high internal structural strength.
This ensures that the paste remains stable, does not slump, and retains its shape once it has been printed.
Only when a certain shear stress is exceeded—such as the pressure applied by the squeegee—does the paste begin to flow.
This point is referred to as the yield stress.
Once this threshold is reached, the viscosity of the paste changes significantly.
Under shear stress, the paste becomes soft, smooth, and readily flowable.
This behavior is deliberately utilized during the stencil printing process.
As the squeegee moves across the stencil, high shear forces act on the paste.
The paste locally liquefies, flows in a controlled manner through the stencil apertures, and completely fills them.
It is essential that the paste flows uniformly—independent of squeegee speed, squeegee pressure, or aperture geometry.
A critical aspect is the release behavior from the stencil.
After printing, the paste must detach cleanly from the apertures.
If the rheological balance is not correct, paste may adhere to the stencil walls or tear during release.
The result is incomplete or distorted solder paste deposits.
Once the squeegee has passed and the shear stress is removed, something crucial happens:
the paste rebuilds its internal structure.
Within a very short time, the viscosity increases and the printed deposit becomes dimensionally stable.
On the printed circuit board, this means that the paste remains exactly where it was deposited.
It does not spread, does not wet adjacent pads, and retains a defined height and geometry—even for fine-pitch structures.
This behavior also plays a central role during component placement.
The paste must provide sufficient tack to securely hold components in place, while avoiding lateral displacement or being squeezed out from underneath the component.
At the same time, the flux system within the paste becomes active.
Already during the preheating phase, it begins removing oxide layers from pads and component terminations.
Simultaneously, it protects the metallic surfaces from re-oxidation—an essential prerequisite for good wetting during the subsequent melting process.
In the reflow oven, the demands increase once again.
The paste must not spatter, foam, or outgas uncontrollably during heating.
A stable rheological behavior ensures that the solder coalesces in a controlled manner and forms a uniform, homogeneous solder joint.
As the solder melts, the paste deliberately loses its previous structure.
After cooling, only the metallic solder joint remains, while the flux components largely evaporate or are chemically converted.
Ideally, the process leaves no residue or only minimal residue behind.
Especially in no-clean systems, this is critical for electrical reliability, optical quality, and long-term performance of the assembly.
In summary:
Within just a few minutes, solder paste must exhibit completely opposing properties.
It must flow under pressure, remain stable at rest, react actively during heating, and ultimately almost completely disappear.
This exceptional combination of physics, chemistry, and process control makes solder paste one of the most demanding materials in SMT manufacturing.
