Solder Paste Viscosity: Thixotropic Index and Shear Rate

Solder paste is not a simple liquid. Its viscosity falls when it is stirred or squeezed and recovers when it is left alone, which is what allows it to be pushed through a stencil aperture and then stay in place on the pad. Paste viscosity is therefore quoted together with the shear rate at which it was measured, and the thixotropic index describes how strongly the material behaves this way.

Why Paste Viscosity Is Specified

The printing process subjects the paste to two very different conditions. Under the squeegee it is sheared hard and must flow into the aperture quickly; on the pad, a second later, it must hold its shape without slumping. A paste whose viscosity does not change with shear rate cannot do both.

Viscosity also changes with temperature and with age. A paste that has been left on a warm stencil is thinner than the specification, and one that has begun to dry is thicker. Comparing two measurements only makes sense when both were taken at the same temperature and after the same preparation, and a supplier data sheet without those details cannot be used to judge a delivery.

Shear Rate and Thixotropic Index

Rheometers measure viscosity across a range of shear rates and produce a curve rather than a single figure. The thixotropic index is a convenient reduction of that curve: it is the ratio of the viscosity measured at a low shear rate to the viscosity measured at a high one, typically ten times higher, and higher values indicate a more shear-thinning paste.

A high index gives good aperture fill and good shape retention, but too high a value means the paste recovers slowly, so it does not hold its shape immediately after the squeegee passes and may slump before the stencil is lifted. The published index should be treated as a guide for the paste family rather than as a specification to be met exactly.

Measurement Methods and Their Differences

Two instruments are common. A rotational rheometer with a cone and plate geometry gives a full viscosity curve control, while a simple viscometer with a spindle gives one or two points. Both can be used to compare a paste with its specification, but only the first shows the behaviour across the shear rates the printing process uses.

Sample preparation and temperature are the dominant sources of disagreement. The paste must be brought to the test temperature, stirred in a defined way and loaded without introducing air, or the reading will differ between laboratories by more than the difference between two suppliers. The measurement procedure should be recorded with the result.

Slump and Its Consequences

Slump is the tendency of a printed deposit to spread after the stencil is removed. A paste that slumps too far moves towards the neighbouring pad and increases the risk of a bridge after reflow; a paste that does not slump at all may be too stiff to fill the aperture completely.

The measurable consequence appears at reflow as paste bridging between adjacent pads and as solder balling at the edges of a deposit. Both are visible in the paste volume and area measurement before reflow, since a slumped deposit has a larger area and a lower height than the aperture intended.

Working Window and Paste Age

The working window is the period during which the paste behaves within its specification after the jar is opened. All pastes change with time: the flux reacts with the powder, solvent evaporates and the viscosity rises. The rate depends on the paste chemistry and on the conditions in the printing area.

A paste near the end of its working window can still print, but its behaviour is closer to the edge of its window and less tolerant of a marginal aperture. This is why the working window should be recorded and enforced rather than treated as advisory, and why a return to specification after a pause cannot be assumed.

Temperature and Viscosity

Viscosity falls as temperature rises, and the effect is large enough to matter over a workshop range. A paste measured at 25 C in a laboratory and printed at 30 C in a workshop that is warm in summer will behave as a thinner material than the specification describes.

Temperature control in the printing area, and temperature equalisation before measurement, are therefore part of paste control rather than laboratory housekeeping. Chilled paste taken straight from storage should be allowed to reach room temperature in its sealed container before it is either printed or measured.

Adjusting the Print to the Paste

Print parameters are set against the paste rather than the other way round. A paste with a high index and a slow recovery suits a slower separation and a lower squeegee speed, while a thinner paste may need a higher speed to avoid leaving excess material behind.

The stencil is the other half of the match. A thin, low-area ratio aperture needs a paste that flows readily and releases cleanly, while a thick aperture with coarse features tolerates a stiffer material. Apertures that fill and hold paste between prints indicate that the paste is too stiff for the geometry.

Setting Limits and Rejecting Paste

Limits should be set on the properties that affect the print, with the measurement method and temperature stated. A common approach is to accept a range of viscosity measured at one shear rate and to use the index only to compare families, because the index is sensitive to the method used and to the geometry of the measuring system.

Paste should be rejected when it is outside its stated working window, when it has been left on the stencil beyond the manufacturer’s limit, or when a print study at the specified parameters no longer produces an acceptable deposit. Returning paste to the jar after it has been on the stencil mixes aged material with fresh and makes the whole jar unpredictable.

Records and Supplier Comparison

The record for each paste lot should carry the type, the metal load, the measured viscosity with its method and temperature, the date the container was opened and the working window in use. With that information, a change in print behaviour can be attributed to the paste rather than to the printer.

Where a supplier is changed, the comparison should be made on a print study rather than on the data sheet alone, because the two pastes may meet the same figures and still print differently. The viscosity curve and the thixotropic index are useful for screening, but the deposit volume across a range of parameters is what decides whether the change is safe.

FAQ

What viscosity should solder paste have? The figure depends on the measurement method and shear rate, so it is quoted together with both. The useful test is whether the paste produces an acceptable deposit across the print parameter window, not whether it matches a single number.

What does a high thixotropic index mean? That the paste thins strongly under shear and thickens again when the shear stops. It fills apertures well but recovers slowly, so it can slump if the separation is too quick.

Why does paste print differently in summer? Because viscosity falls with temperature, and an uncontrolled printing area can be several degrees warmer. Temperature equalisation before measurement and temperature control in the area both matter.