Solder Paste Rheology and Viscosity Control in SMT

Solder paste is not a simple liquid, and the number printed on its datasheet is only part of the story. Paste is a suspension of metal particles in a flux vehicle, and its flow behaviour changes with the rate at which it is sheared, the temperature it sits at and the age of the material. This guide explains how gopcb uses rheology and viscosity data to control printing, and why a single viscosity figure cannot predict how a paste will behave on a fine pitch board.

Why Rheology Matters More Than Viscosity

Viscosity describes resistance to flow at one particular condition. Rheology describes how that resistance changes as conditions change, and solder paste changes a great deal. It thins when it is worked, thickens when it rests, and responds differently depending on how fast it is pushed.

That behaviour is what allows a paste to roll smoothly ahead of a squeegee, fill an aperture quickly, and then hold its shape on the pad without slumping. A paste with the right viscosity but the wrong rheology will either fail to fill or fail to hold, and no single measurement will reveal which.

The Thixotropic Behaviour of Solder Paste

Thixotropy is the property that makes paste viscosity depend on its shear history. Under the shear of a print stroke the structure breaks down, viscosity falls and the paste flows into the aperture. When the shear stops, the structure rebuilds and viscosity rises, which is what keeps the deposit in place.

Both halves of that cycle matter. A paste that thins too slowly will not fill small apertures at normal print speeds, while a paste that recovers too slowly will slump after printing and bridge on fine pitch pads. The recovery rate is often a better predictor of printing behaviour than the initial viscosity.

Shear Rate in Printing and Jetting

Shear rate is the speed gradient inside the material as it moves, and different deposition processes operate in very different ranges. Stencil printing shears paste at a moderate rate as it rolls and passes through the aperture. Jetting forces it through a small orifice at a far higher rate and in a much shorter time.

This is why a paste qualified for printing may perform badly in a jet printer, and vice versa. The rheology curve across the whole operating range is what matters, not the single point quoted on a datasheet. Our comparison of jet printing and stencil printing explains how the two processes differ.

Measuring Viscosity and Its Limits

Viscosity is normally measured with a rotational viscometer at a defined spindle speed and temperature. The result is a single number, useful for comparing lots of the same product and for confirming that a new delivery matches the previous one. It says nothing about recovery, slump or behaviour at other shear rates.

A more complete picture comes from a rheometer that sweeps shear rate and records the full curve, including the recovery profile after shearing stops. Where a paste is being qualified for a difficult application, that curve is worth far more than a viscosity reading, and it can be measured on a sample from production.

Temperature and Viscosity Drift

Viscosity falls as temperature rises, so a paste at thirty degrees behaves differently from the same paste at twenty two degrees. A jar taken straight from a refrigerator and used before it reaches room temperature will print differently from one that has been allowed to equalise, and the difference is not a material fault.

This is the practical reason for a thawing rule and for a controlled print environment. Small daily temperature swings change viscosity enough to shift deposit volume, and the effect is gradual, so it is easily attributed to other causes. Logging room temperature alongside print results makes the correlation visible.

Paste Release and Recovery

Paste release depends on adhesion between the paste and the aperture wall compared with adhesion to the pad. A paste that has recovered too quickly sticks to the wall and leaves a partial deposit, while one that recovers too slowly continues to flow after the stencil lifts and spreads onto the mask.

The recovery rate can be influenced by the print parameters as well as by the formulation. Faster separation favours pastes that recover quickly, while slower separation suits pastes that need more time to build structure. Matching the parameter to the material is more effective than searching for a universal paste.

Aging, Mixing and Slump

Paste changes with age even in a sealed jar. The flux vehicle slowly separates, the metal particles settle and the structure that keeps them suspended relaxes. Mixing restores homogeneity for a while, but repeated mixing and re-use gradually shifts the balance between metal and flux.

Slump is the visible consequence. Paste that has been left too long, or warmed too often, spreads after printing and can bridge between pads that were correctly separated in the stencil design. Inspecting deposits immediately after printing and again after a delay shows slump directly, which is a useful acceptance test for an incoming lot.

Setting Specifications That Mean Something

A paste specification should state the measurement method, the temperature and the shear conditions, not just a target value. Two suppliers can quote the same viscosity figure using different methods and produce materials that behave very differently on the same printer.

Incoming inspection should therefore repeat the agreed method rather than reading the certificate. Where a print process is sensitive, adding a slump test and a simple print trial gives a practical check that the material will behave, which a viscosity reading alone cannot provide.

Linking Rheology to Print Defects

Print defects often map directly onto rheological behaviour. Insufficient paste on small apertures suggests a paste that thins too slowly or has thickened with age. Bridging after a correct print suggests a paste that recovers too slowly and continues to flow. Smeared deposits suggest a paste that is too thin at the print shear rate.

Recording those symptoms and their timing helps separate a material problem from a machine problem. When deposits drift without any change to the printer, the paste is the first suspect, and the checks described in our solder defect notes usually confirm it.

Process Control and Verification

On a design of this kind, thixotropy 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.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

What viscosity should solder paste be? There is no single correct value, because viscosity depends on the measurement method and the paste chemistry. The useful specification states the method, temperature and shear conditions, and the value should be compared with past lots rather than with a generic target.

Why does paste print differently in the afternoon? Because temperature and shear history change its viscosity. A warmer room thins the paste, and paste that has been worked through the day has a different structure from material freshly loaded. Controlling room temperature and paste exposure keeps the difference small.

Does rheology matter for jet printing? Especially so. Jetting pushes paste through a small orifice at very high shear rate in a fraction of a second, so the high shear end of the rheology curve and the recovery behaviour determine whether droplets form cleanly.