Solder Paste Rheology And Viscosity Control
Solder paste is not a liquid that flows in the ordinary sense. It is a suspension of metal particles in a flux vehicle, and its viscosity changes with the rate at which it is sheared and with the time that has passed since it was sheared. That behaviour is what allows a squeeze of paste to flow into a small aperture under the moving blade of a squeegee, hold the shape of the aperture when the stencil is lifted, and then resist slumping until the reflow begins. Without it, printing as it is practised would not work.
This article explains the rheological properties that matter, how they are measured, what changes them, and how the measurement is used in production.
What Rheology Means For Paste
Viscosity is the resistance of a material to flow, and for paste it is not a single number, because the material thins as the shear rate rises. That behaviour is called shear thinning, and it is the property that lets the paste roll ahead of the squeegee at high shear and then stop moving when the shear is removed. Thixotropy is the time dependent part of the same behaviour: after the shear stops, the structure recovers over a period of seconds or minutes rather than instantly.
Two further quantities describe the behaviour. The yield stress is the stress below which the paste does not flow at all, and it is the property that keeps a printed deposit from spreading under its own weight after the stencil is lifted. The thixotropic index is a single number derived from the viscosity at two shear rates, and it is the figure that most suppliers quote and most production lines track, even though it is only a summary of a curve.
Why The Behaviour Matters In Printing
The squeegee stroke applies a high shear rate to the paste, which lowers its viscosity and lets it fill the aperture completely, including the corners. When the stroke ends, the paste has to recover enough structure to hold the shape of the aperture as the stencil separates from the board, and it has to release from the aperture wall rather than staying in it. A paste that recovers slowly slumps after printing and bridges to a neighbour, while a paste that recovers too quickly does not fill the aperture and leaves a deposit with a missing corner.
The printing parameters interact with the rheology rather than overriding it. A higher squeegee speed raises the shear rate and lowers the viscosity further, and a lower separation speed gives the paste more time to recover and to release cleanly. That is why a paste with a wide thixotropic range tolerates a wider window of speed and pressure, and why a change of paste supplier is a change of the process rather than a substitution of one consumable for another.
Measuring The Properties
The measurement is made with a rotational viscometer at a controlled temperature, and the result depends on the geometry of the spindle, on the shear rate, and on the temperature, so the method has to be fixed before the numbers can be compared. A common approach is to measure the viscosity at a low and a high shear rate and to quote the ratio, which is the thixotropic index, together with the absolute value at the higher rate.
Two process oriented tests support the laboratory figures. A slump test prints a pattern of deposits and measures how far they spread after a defined time, which is a direct measure of the behaviour that causes bridging. A tack test measures the force required to pull a probe away from a printed deposit, which describes the adhesion of the paste during placement. Both are performed on a sample from the batch and both are more closely related to the print result than the viscosity figure alone.
What Changes The Behaviour
The formulation sets the baseline. A higher metal load raises the viscosity and the yield stress, a finer powder raises the surface area and therefore the viscosity for the same load, and the flux chemistry determines how strongly the structure recovers after shear. The temperature is the most immediate influence on the line, since a paste that is warming on the stencil thins progressively through a shift.
Time and exposure change the paste as well. The vehicle loses solvent and absorbs moisture while the jar is open, so the viscosity rises and the recovery behaviour changes, and the paste on the stencil has a working life measured in hours rather than days. Mixing and thawing are part of the same picture: a paste that was thawed and not brought to room temperature evenly prints differently from one that was, and a paste that has been stirred to a different degree has a different structure when it is applied.
Using The Measurement In Production
The practical use is as a receiving and control check. The incoming batch is measured against a specification, the paste on the stencil is checked at intervals or when a print problem appears, and the result is compared with the print quality that the line is producing. Where the correlation is established, the measurement becomes a leading indicator: a reading that drifts before the deposits do allows the paste to be changed before the defects appear.
The measurement cannot replace the print inspection, because the same viscosity can produce different results in different conditions. What it can do is remove one variable from the diagnosis. When a bridging defect appears, a viscosity reading that is inside the specification points to the stencil, the support, or the separation speed, and a reading outside it points to the paste. The ink used for solder mask is judged by the same properties, which are described under solder mask ink thixotropy, the design rules for printing under manufacturable design guidelines, and the fabrication sequence under PCB design and fabrication.
Process Control and Verification
On a design of this kind, yield stress is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, yield stress is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
FAQ
What is a good thixotropic index? It depends on the printing process and on the aperture size, and the specification should come from the paste supplier and from the print result on the product rather than from a general figure.
Why does the paste thin during a shift? Because its temperature rises towards the ambient of the room and because the vehicle loses solvent and absorbs moisture while it is exposed on the stencil.
Can water or thinner be added to correct a thick paste? It should not be. Adding a solvent changes the flux chemistry and the residue, and a paste that is outside its viscosity specification should be replaced rather than adjusted.
