Paste Stirring and Deaeration Before Printing
Paste stirring is the step that returns a solder paste to the condition it had when it was manufactured, and skipping it changes how the whole stencil prints. A paste that has stood in a jar separates slightly, and the first deposits taken from it are usually the outliers that nobody can explain afterwards.
The preparation routine is short but has to be done the same way every time. Thawing, stirring, checking the appearance and recording the lot number take a few minutes and remove most of the day-to-day variation in print volume.
Why Stirring Matters
Stirring redistributes the flux and the alloy powder that have separated during storage. Solder paste is a suspension, and gravity plus time moves the heavier fraction downwards, so the top of the jar becomes richer in flux while the bottom becomes drier and stiffer.
Printing from an unstirred jar therefore starts with a soft, tacky paste and ends with a stiff one, and the transition is invisible unless print volume is measured. The amount of stirring that is useful is limited, because excessive shear warms the paste and can damage the flux chemistry, so the aim is a uniform material rather than one that has been worked too hard. Stirring also re-incorporates the powder that has settled around the rim of the jar, which is the part of the paste that dries out first and forms the lumps that block a fine aperture.
Deaeration and Entrapped Air
Air enters the paste during filling, during transfer from the jar to the stencil and during the print stroke itself. A bubble that sits inside a deposit becomes a void in the reflowed joint, and a void under a thermal pad reduces the area that carries heat.
Deaeration is the removal of that air before printing. Slow stirring with a flat blade, a short dwell after mixing, and handling that avoids folding the paste over itself all reduce the amount of air that reaches the stencil. A bubble that is 50 micrometres across is invisible on the stencil and becomes a void that covers several percent of a thermal pad area.
Manual Stirring Practice
Manual stirring is done with a clean spatula or a plastic blade, using slow circular strokes for one to two minutes. The motion should fold the paste rather than whip it, because whipping introduces air faster than the material can release it.
The tool matters as much as the motion. A metal spatula that has been used on another paste, or one that has been left dirty on the bench, contaminates the jar with dried particles that later block an aperture. A plastic blade is preferred because it cannot scratch the jar and cannot transfer metal particles from a previous paste.
Automatic Paste Mixers
Automatic mixers run the jar on two axes and produce a more repeatable result than a hand routine. A typical cycle runs one to three minutes at a low speed, and the machine removes the difference between two operators preparing the same paste.
The setting should be recorded and tied to the paste type rather than to the operator preference. A paste with a high metal load needs a different cycle from a fine-pitch paste with a lower viscosity, as discussed in the notes on solder paste viscosity control. A cycle that is too long warms the paste and lowers its viscosity, which appears as a spread deposit rather than as a uniform one.
Thawing, Temperature and Viscosity
Frozen paste has to reach room temperature before it is opened, because condensation forms on cold paste and adds water to the chemistry. A jar taken from the freezer needs two to four hours at 20 to 25 °C, depending on its size.
Opening a cold jar is a common cause of a paste that spits and skips during printing. Once thawed, the paste should be stirred immediately before use and then kept sealed when the printer is idle, as described in the storage and thaw guidance. Paste that has been thawed and refrozen should be discarded, because the freeze cycle separates the flux from the alloy in a way that stirring cannot undo.
Open Time on the Stencil
Paste on the stencil has a limited open time, and the limit is set by the solvent loss rather than by the alloy. A paste that stays on the foil through a long break dries at the edges of the aperture, and the next deposits are short and irregular.
The practical rule is to stir the paste on the stencil at defined intervals and to remove and replace it after a break of more than half an hour. Open time behaviour is covered in the notes on paste open time. The interval depends on the paste chemistry and on the printer enclosure, and it should be measured rather than assumed from a supplier note.
Signs of Poor Paste Preparation
Poor preparation leaves visible evidence in the print. Deposits with ragged edges, a paste that rolls ahead of the squeegee instead of rolling evenly, and a stencil that leaves a film behind after the stroke all point to a paste that was not ready.
Volume data shows the same problem numerically. A standard deviation that is twice the normal value on the first panel of a shift, falling back to normal on the second, describes a paste that needed more stirring rather than a machine that needs adjustment. A paste that has been standing for two hours on an open stencil prints small and dry, and the deposits look dull rather than glossy.
Verification Before Release to the Line
A short check before the paste goes onto the printer catches most preparation faults. The appearance should be uniform, the paste should hold a peak when lifted with a spatula, and there should be no skin or crust on the surface.
Where the line runs high-value product, the first printed panel can be measured with SPI and compared against the band established for that paste lot. gopcb records the lot number, the thaw time and the mixing cycle with the panel data so the preparation step can be reviewed later. The check takes under a minute and should be recorded, because the same observation made twice by different operators carries more weight than a single opinion.
Records and Consumable Control
Records should carry the paste part number, the lot, the thaw start and end times, the mixing cycle used and the date the jar was opened. A paste that has been opened and closed repeatedly ages faster than one used in a single shift.
Consumable control is the other half of the story. A jar that is topped up from an older jar, or a stencil that is wiped with a solvent-soaked cloth, both reintroduce the variation that careful preparation removes.
FAQ
How long should solder paste be stirred? One to two minutes by hand with a slow folding motion, or one to three minutes in a two-axis mixer at low speed, using the setting agreed for that paste type.
Does stirring introduce air into the paste? It can, which is why the motion folds rather than whips the material and why a short dwell follows the mixing step before the jar is used.
Why does the first panel of a shift print differently? Usually because the paste was taken from a jar that had separated or had not fully thawed, and both conditions change viscosity without changing any printer setting.
