Solder Paste Age and Lot Change Control on the Line

Paste age is not one number but three: the refrigerated shelf life, the time since the jar was thawed, and the time the paste has spent open on the stencil. A printer can be running paste that is well inside its shelf life and still be outside its working window, which is why age has to be tracked in stages rather than as a single date.

Lot change is the second half of the same control. When a new jar is opened, the rheology of the material on the stencil changes, and the first boards after that change are the ones most likely to show a print defect. Treating the change as a process event rather than as a stock action is what keeps the print volume stable.

What Paste Age Actually Means

Refrigerated shelf life is stated by the paste supplier and typically runs from four to six months from the date of manufacture when the jar is stored between 2 and 10 °C and has never been opened. The clock is a storage clock, not a use clock, and it says nothing about the state of the material once the seal is broken.

After opening, the relevant limits are the thaw time, the working time on the stencil and the total number of hours the paste has been exposed to ambient air. A jar that has been opened and closed four times has a different history from one that was used in a single shift, even if both carry the same manufacture date.

Refrigerated Storage and Shelf Life

Storage temperature is the first control point. A refrigerator that cycles between 2 and 12 °C ages the paste faster than one holding a steady 5 °C, and a jar stored against a freezer wall may be partly frozen at one end and warm at the other.

Labelling is the second. Each jar should carry the manufacture date, the receipt date and the open date, and the oldest stock should be consumed first. Without that discipline the paste that gets used is the jar that happens to be at the front, which is rarely the one closest to its limit.

Thaw Time and Temperature Control

Frozen or refrigerated paste must be brought to room temperature before it is opened, because opening a cold jar condenses moisture into the material. A 500 g jar typically needs two to four hours in still air at 20 to 25 °C, and the time depends on the jar size rather than on the age of the paste.

Accelerating the thaw with hot water, an oven or a microwave is a common and damaging shortcut. The outer layer reaches temperature first and the centre stays cold, and the resulting separation of flux and powder shows up as a print that is inconsistent from the first board to the last, as covered in the thaw guide.

Open Time on the Stencil

Once the jar is open, the material begins to lose solvent and to thicken. A typical no-clean paste holds its print window for four to eight hours on the stencil, and the usable time shortens with a low room humidity, a high stencil temperature and a long idle period between prints.

The practical check is the print itself rather than a clock. Where deposit volume has started to fall on the smallest apertures, or where the paste no longer rolls cleanly ahead of the squeegee, the material is at the end of its working life whatever the label says, which is the subject of the open time discussion.

Lot Change and First Print Verification

A lot change is a process change and should be treated like one. When the jar on the machine is replaced, the printer should be set to a known state, the paste should be stirred to its specified condition, and the first three boards should be printed and inspected before the line is allowed to run at full speed.

The verification is a print volume measurement against the qualified mean, not a visual check. The viscosity printing window for the new lot is the same as for the old one, but only if the material has been conditioned in the same way, which is what the thaw and open time records are for.

Rheology Drift With Age

Paste is a suspension of solder powder in flux, and the flux is the part that changes with time. Solvent loss raises viscosity, and the effect is not uniform across the deposit because the paste at the top of the roll experiences more air than the paste inside it.

At the printing shear rate, an aged paste can still measure inside its viscosity specification and yet release poorly, because the yield behaviour and the tack have moved. This is why viscosity alone is a weak predictor of print quality on old material and why the print result has to be part of the acceptance evidence.

Print Defects That Point to Old Paste

Old paste announces itself in a recognisable pattern. Deposits become shorter, the printed area shrinks at the corners of large apertures, and the paste shows a dry skin on the surface of the roll rather than a glossy, cohesive shape.

On fine pitch apertures the failure is more abrupt: deposits are missing or ragged, and the stencil underside carries smears that require more frequent cleaning. Where those symptoms appear together with a long open time on the stencil, the material is the first suspect, as described in the paste life notes.

Disposal, Return and Segregation Rules

Paste that has reached its open time limit should not be returned to the jar, because the aged material on the stencil would contaminate the fresh material in the jar. Lines that follow this rule usually keep a separate scrap container and record the quantity, which also gives a useful measure of how much paste is being wasted.

Jars that have been opened should be segregated from sealed stock, and the segregation should be visible at a glance. Where a line cannot maintain that discipline, the practical fallback is to issue paste in smaller packages so that a jar is finished within a single shift.

Records That Support the Decision

The record for each lot should carry the manufacture date, the receipt date, the thaw start and end times, the open time on the stencil and the print volume result from the first boards. Together these five values explain most paste-related defects without any further investigation.

A printed label on the jar is the simplest way to keep the data with the material, since the operator handling the paste is the same person who should be recording the times. Records kept in a computer system that the operator cannot reach at the machine tend to be reconstructed at the end of the shift.

FAQ

Can paste that is inside its shelf life still be too old to use? Yes. Shelf life applies to sealed, refrigerated stock, and once a jar is opened the working window is measured in hours on the stencil rather than in months in the refrigerator.

How long should a jar be left to thaw? Two to four hours in still air at 20 to 25 °C suits most jar sizes, and the jar is ready when its outside no longer feels cool to the touch and the label shows no condensation.

Is it safe to top up a jar with paste from the stencil? No. Returns carry material that has already been exposed to air, and mixing them into sealed stock shortens the working life of every board printed from that jar afterwards.