Paste Agitation: How Much Mixing Solder Paste Needs

Paste agitation is the mixing that solder paste receives before and during printing, and it is either the thing that makes a stiff batch printable or the thing that ruins a good one. Paste is a suspension of alloy powder in a flux vehicle, and it separates slowly as it stands. A little movement restores the suspension; too much movement adds air and drives off solvent.

The line between the two is narrower than most operators assume. This note covers what agitation does to the material, where each kind of mixing belongs, and the signs that mixing has gone too far.

What Paste Agitation Does to Solder Paste

Solder paste is thixotropic, which means its viscosity falls while it is sheared and recovers when it rests. Agitation applies shear, so it lowers the viscosity of the paste at the moment of use. That is useful when a batch has stiffened in storage, because the shear lets it flow into apertures that a stiffer paste would bridge.

The same shear also changes the structure of the suspension. Alloy powder settles during storage, and gentle mixing returns the particles to a uniform distribution. Violent mixing does the opposite: it whips air into the vehicle, and each air pocket becomes a void in the reflowed joint.

Shear Thinning and Recovery

Shear thinning is reversible, but the recovery rate is a property of the flux chemistry. Paste that recovers quickly holds a crisp deposit and resists slump. Paste that recovers slowly keeps flowing after the stencil lifts, which shows up as rounded deposits and, on fine pitch, as bridging.

Mixing changes where the paste sits on that curve. Immediately after stirring, the paste is at its thinnest. Printing within a few minutes of mixing therefore uses a material that has not returned to its design viscosity, and the print will differ from one taken later in the shift. The measurement methods are described in our notes on solder paste rheology.

Stirring a Jar: What Helps and What Hurts

Stirring is intended to undo settling, and the correct tool is a clean spatula used slowly and gently. Ten to twenty slow folds through the paste will even out a separated jar without folding in air. A power mixer run at speed adds air and warms the paste through friction, which shortens its working life.

Paste that has been refrigerated must reach room temperature before it is opened, because condensation on cold paste adds water to the flux. Once the jar is open, the surface skins over within minutes in a dry room. Covering the jar and returning it to the shelf is part of paste handling and matters more than the stirring itself.

Machine Agitation on the Stencil

On the printer the paste is agitated continuously by the squeegee. The roll in front of the blade is a mixing chamber, and the shear it applies is what makes the paste flow into apertures. That agitation is deliberate, continuous and part of the design of the process rather than an operator intervention.

Its intensity is set by print speed and by squeegee pressure. Running a very fast stroke with a hard blade shears the paste hard and repeatedly, which is fine for a short run and less fine over a long one, because solvent loss at the roll surface accelerates with each pass. The mechanics of the roll are covered in our notes on the solder paste roll.

Aeration and Void Risk

Air entrained in the paste arrives at the pad as a void. Small voids are normal and usually acceptable in a reflowed joint, but a distribution shifted towards large voids changes the thermal and mechanical behaviour of the joint. The visible sign on the deposit is a frothy or porous appearance after printing.

Once aerated, paste does not easily shed the air. Standing time lets some bubbles rise, and a slow fold can release more, but the reliable cure is prevention. Avoid high speed mixing, do not let a jar stand open with a mixer in it, and do not return paste to the pot if it has been dropped.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/High-Speed-Printed-Circuit-Board.jpg" alt="Enclosed print head chamber during a print run on an SMT line” />

Agitation in an Enclosed Print Head

An enclosed print head changes the picture, because the paste is held in a sealed chamber and pushed out by pressure. The paste in the chamber is not exposed to air, so solvent loss and skinning are much slower, and mixing happens as material is forced through the chamber and past the blades.

The risk shifts to the cartridge and to the pressure setting. A cartridge that has been shaken before fitting carries air that ends up in the chamber, and it cannot escape through a sealed system. Fitting cartridges gently, purging the first paste and keeping the chamber topped up are the practical controls.

Rules for Automatic Stirrers

Where a stirrer is used, it should be a slow planetary or rotating type set to the supplier recommendation, which in turn follows the printing standards published by IPC, and the time should be written down. Fast impeller mixers, paint shakers and ultrasonic baths are all wrong for solder paste. Each of them either adds air, heats the paste or both.

Record the mixing time and the paste lot with the print data, so that a volume change can be attributed to a mixing change. Where the paste is mixed in the jar, keep the same jar upright and covered between uses, and dispose of any paste that has been through more than the recommended number of cycles, as set out in our notes on paste drying warning signs.

Checks Before Release

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Verification and Records

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Points to Confirm at First Article

The sequence of operations is part of the specification, because a different order produces a different result from the same steps. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Should paste be stirred before every print run? No. A jar that was mixed once and used within its open time does not need a second mix. Repeated mixing exposes the paste to air each time and reduces the number of hours it can usefully print.

Does agitation restore paste that has started to dry? It can appear to. Mixing a dried surface layer back into the bulk hides the loss of solvent for a short time, and the print fails again within a few dozen boards. Paste that has skinned over in the jar should be discarded rather than mixed.

Is vacuum mixing worth the cost? Vacuum mixing removes entrained air more effectively than any open vessel method and is used where void limits are tight. It needs dedicated equipment and a sealed container, so it suits a high volume line rather than a job shop.