Solder Paste Mixing: Preparation, Placement and Process Control

Solder paste arrives in a jar as a thixotropic suspension of alloy powder, flux and solvent, and it is never perfectly uniform after storage. Powder settles under its own weight, solvent migrates to the surface, and the rheology a printer sees at the start of a shift is not the rheology the paste had when it was packed. Bringing the material back to a repeatable state before it reaches the stencil is one of the least glamorous and most effective controls on an assembly line.

Why Mixing and Degassing Matter

A printed deposit has to be the same on the first board of a batch and on the thousandth. That consistency depends less on the printer than on the material presented to it, so every source of variability in the paste has a direct effect on deposit volume and on the defects that follow.

Air in the paste is the quiet offender. It does not show up on a deposit weight check, it does not appear in incoming inspection, and it only reveals itself as voids in the reflowed joint or as random volume variation across a panel.

Viscosity and Rheology in Plain Terms

Paste is shear thinning: it flows when the squeegee pushes it and holds its shape once the aperture is filled and the stencil lifts. That behaviour is what makes printing possible, and it is also what makes the material so sensitive to how it has been handled.

Paste that has sat undisturbed has a high apparent viscosity and a low working viscosity, and a short mechanical work cycle brings the two together. Temperature has a comparable effect, which is why the room and the paste itself both need to be inside a defined window before work starts.

Mixing Practice on the Floor

Automatic mixers that rotate the jar about two axes are the usual choice because they work the whole volume rather than the top layer, and because they do it without folding air into the mass. Hand stirring with a spatula tends to do the opposite: it aerates the surface and leaves the lower half of the jar untouched.

Time and speed come from the supplier and should be written on the process sheet rather than left to operator judgement. After mixing, most pastes need a short rest so the bubbles that were introduced can rise out, and that rest belongs in the cycle time instead of being treated as an optional extra.

Degassing Methods

Vacuum degassing and centrifugal degassing both remove entrained air, and the choice depends on the material and on the volume being handled. A vacuum chamber pulls dissolved gas and trapped bubbles out over minutes, while a centrifuge drives them to the surface in seconds but can separate the constituents of a soft paste if it is run too hard.

Whatever the method, the useful measure is not the setting on the machine but the result: the same working viscosity, the same slump behaviour and the same printed volume from jar to jar. When paste degassing is treated as a variable rather than a step, deposit weight tends to drift through the shift.

Deposit Quality and Stencil Printing

Well mixed paste releases from the aperture walls cleanly, leaving vertical sidewalls and a flat top surface. Paste carrying trapped air or a separated flux phase leaves ragged edges, residues on the stencil web, and a deposit that slumps before placement.

Because these effects are visible, they are also controllable. Weighing a deposit or measuring it on a paste inspection machine turns the mixing step into something that can be verified, which is what stencil printing needs from its incoming material. Our article on paste volume and stencil design covers the geometry side of the same problem.

How Entrained Air Behaves in Reflow

Air that survives printing is carried into the joint. During heat up the flux volatilises and the gas expands, and if the escape path through the alloy is blocked the void stays in the joint. Fine pitch joints and thermal pads are where this shows first.

The reflow profile matters here as well. A ramp that brings the paste to melting quickly can trap volatiles under a skin of alloy, whereas a profile with a controlled soak gives them time to leave before the alloy is fully liquid. Mixing and profile are two parts of one problem.

Process Control and Verification

The control plan should state the mixing method, the time, the rest period, the pot life and the temperature window, and it should say who checks each one. Recording a mixing cycle is trivial and it is the only way to separate a material problem from an equipment problem when a defect appears.

Verification can be as simple as a deposit weight check plus a periodic viscosity reading, or as formal as a printability test at incoming goods. Either way the data should be reviewed alongside print defect rates, because that is what ties the material step to the yield figures the business cares about. The wider framework is set out in our article on solder paste qualification.

Common Defects and Their Causes

Bridging, solder balling and insufficient paste are the three defects most often linked to material handling. Bridging usually points to a low working viscosity and slumping, balling to moisture picked up from a cold jar opened too early, and insufficient paste to a jar whose top layer has dried during a long shift.

The response should follow the evidence. Adjusting the printer before checking the paste is a common mistake, and it often moves a good process away from its settings while the real cause sits in the jar. Our article on stencil cleaning and storage describes the housekeeping that prevents part of this.

Storage, Pot Life and Changeover

Paste should be stored cold, warmed in its sealed jar to room temperature before opening, and used within the pot life printed on the label. Opening a cold jar condenses moisture onto the paste, and no amount of mixing will remove that water afterwards.

At changeover, partial jars should not be quietly topped up with fresh material, because the mixed pot life is unpredictable and traceability is lost. Our article on paste storage and handling covers the routine that keeps the material step under control, and the feeder side of the same discipline is described in our article on placement program optimisation.

FAQ

How long should paste be mixed? The time comes from the supplier and is usually one to three minutes for an automatic mixer. Longer is not better, because overworking a soft paste raises its temperature and shortens the working window.

Can a mixed jar be returned to the refrigerator? It can be stored again, but the pot life clock keeps running once the jar has been opened, so the remaining time should be tracked and the jar should never be combined with fresh material.

Does degassing remove the need for a soak in the profile? No. Degassing reduces the air the paste carries in, but the flux still releases volatiles during reflow, so the profile needs a stage that lets them escape before the alloy is fully liquid.