Solder Paste Viscosity Measurement and Control

Viscosity is the property of solder paste that everyone quotes and almost nobody measures under the conditions the printer actually uses. A single figure on a certificate tells you that the batch is within a range, not that it will roll, release and slump the way the process needs.

Understanding what the number describes makes the difference between a useful incoming check and a ritual. Paste is a suspension of alloy powder in flux, and its flow behaviour depends on how fast it is sheared, how long it has been sitting and how warm it is.

Why Viscosity Is Not One Number

A paste that is sheared quickly flows more easily than the same paste sheared slowly, which is the opposite of a simple liquid. That behaviour is what lets a paste stand up on a pad after printing and still pass through a small aperture during the print stroke.

The consequence is that the measured value depends entirely on the test conditions. Two laboratories can report very different viscosities for the same jar and both be correct, so a specification is only meaningful when the method, the shear rate and the temperature are stated with it.

Shear Rate and Thixotropy

Shear rate describes how fast the material is being deformed, and it rises as the squeegee speed rises and as the aperture gets smaller. Printing a fine pitch device subjects the paste to a far higher shear rate than printing a large pad, and the paste must behave acceptably across that whole range.

Thixotropy is the time dependent part of the behaviour, describing how the structure of the paste breaks down under shear and rebuilds when the shear stops. A highly thixotropic paste recovers its structure slowly, which helps print definition but can leave the paste too stiff for the next stroke if the print cycle is short.

Measurement Methods

The rotational viscometer with a cone and plate fixture is the reference method for most specifications, because it gives a controlled shear rate and a small sample volume. The reading is taken after a defined dwell, at a defined temperature, usually around 25 degrees Celsius.

The spiral viscometer, which measures how far a weighted spindle descends through a sample in a fixed time, is used on the line because it is quick and needs little cleaning. The two methods are not interchangeable, and a limit written for one should never be applied to the other without a correlation study.

Sample Preparation and Temperature

Paste taken straight from a refrigerator is not at the temperature assumed by the method and will read high. The jar should be allowed to reach room temperature in its sealed state so that no condensation forms on the surface, which usually takes two to four hours depending on the volume.

Mixing is the second variable. A jar that has separated should be stirred gently and consistently, and the same stirring routine should be used before every measurement so that the readings are comparable. Stirring vigorously heats the paste and entrains air, both of which move the result.

Reading the Result

A single reading that is inside the specification is evidence of a batch that was made correctly, not of a paste that will print correctly. What matters more is the trend across shifts, because a paste that drifts steadily toward the upper limit will fail while every individual reading still passes.

Recording the value against the jar, the date opened and the hours on the printer turns the readings into a curve. When a print defect appears, the curve shows whether the paste changed or whether something else in the process did, and that distinction saves hours of investigation.

Slump Test and Print Quality

The slump test is a functional check rather than a physical one. A line of paste is printed through a stencil, heated gently, and examined for the amount the deposit spreads. A paste that slumps badly will bridge between adjacent pads during reflow even though the printed pattern looked clean.

Because slump is functional, it correlates better with print quality than viscosity does. Where a supplier changes the flux or the powder size distribution, the slump test is the quickest way to see whether the change matters, and it can be run on the production stencil without any extra equipment.

Change Over Time

Paste on the stencil loses solvent from the moment the jar is opened, and the loss is faster with a warm room and a large exposed area. The rise in viscosity that follows is what makes the last boards of a shift print differently from the first ones.

Rolling the paste back into the jar at the end of a shift, covering the stencil during stops and keeping the room within the specified range all slow the change. None of them stop it, so the practical control is to limit the hours a jar spends on the machine and to replace it on a schedule rather than when a defect appears.

Adjusting Paste on the Line

Thinning an old paste with flux or solvent is a common habit and a poor one, because the added material changes the flux to metal ratio that the paste supplier set. The result is a deposit that wets differently and leaves more residue, and the change is not visible in the viscosity reading.

Where the paste is genuinely too stiff for the process, the correct response is to change the stencil aperture or the print speed, or to move to a paste with a different powder loading. Those are design and process decisions, and they are made with the same evidence that supports the choices in manufacturing tolerances.

Additional Considerations for This Build

Practical attention to viscosity measurement pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating viscosity measurement explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, thixotropy is the item that decides how the rest of the board is arranged. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

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

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Does a certificate value predict printing behaviour? Only weakly. It confirms the batch is in family, while the print depends on the shear behaviour, the stencil and the environment together.

How often should the line reading be taken? At the start of each shift and after any long stop, with the same instrument, the same sample volume and the same temperature so the numbers can be compared.

Is a higher viscosity paste always worse for fine pitch? No. The powder size matters more for fine apertures, and a paste with large particles will fail to release cleanly whatever its viscosity.

Can paste be recovered after it has dried on the stencil? It can be returned to the jar if it has not formed a skin, but dried material should be discarded rather than mixed back into usable paste.