Solder Paste Working Life and Replenishment

Solder paste is a mixture of alloy powder, flux and solvent, and it changes from the moment the container is opened. Solvent evaporates, the flux absorbs moisture, the powder oxidises and the viscosity rises. Working life is the period over which those changes stay small enough that the printing process still behaves, and it is counted differently depending on whether the paste is in the jar, on the stencil or in a printed deposit.

What Working Life Means in Practice

The figure on the data sheet usually refers to the paste on the stencil, at a stated temperature and humidity, and it is a period during which the deposit volume and the print quality remain within a defined band. It is not an absolute limit at which the paste stops working; it is the point at which the maker will no longer guarantee the behaviour. Used beyond it, the paste still prints, and the deposits get progressively smaller and more variable.

The practical definition for a shop should be its own: a working life measured on its own printer, at its own temperature and humidity, and ending when the deposit volume drifts beyond the control limit. That figure is more useful than the data sheet value because it accounts for the environment the paste actually sits in. It should be re-measured when the season changes, since a room that holds 45 % relative humidity in winter can hold 65 % in summer.

Open Time, Stencil Life and Working Life

Three different clocks run at once. Open time is the period the jar may be open before the contents are used, and it matters mainly for the solvent loss from the bulk. Stencil life is the period a paste may remain on the stencil and be worked, which is shorter than the open time because the paste is spread into a thin layer and exposed to the air over a large area. Working life is the period from printing to reflow during which the deposit is still good.

Confusing the three leads to the wrong corrective action. A board with slumped deposits and a long time between printing and reflow has a working life problem and needs a faster route to the oven. A board with dry, crumbly paste on the stencil has a stencil life problem and needs more frequent replenishment. A jar that has formed a skin has an open time problem and needs better storage. Each is a different fix and each has a different record.

Viscosity Change and Its Measurement

Viscosity rises as the solvent leaves and as the flux chemistry changes, and the rise is not linear. Paste that has been on the stencil for an hour may be almost unchanged, and the same paste an hour later may be unusable. The measurement that matters is the one taken in the printer, because a viscosity measured in a jar in the lab does not describe the paste on the stencil.

A practical proxy is the deposit volume from a controlled print, measured by the printer’s own inspection system or by a weight measurement on a test board. Printing a defined pattern and weighing the deposits gives a number that can be trended, and the trend shows the viscosity change before it shows as a defect. Where a viscometer is available, the reading should be taken at the same temperature and shear rate each time, because both change the result by more than the ageing does.

Replenishment Practice on the Printer

Paste on the stencil should be replenished in small quantities rather than being allowed to spread thin across the whole aperture area. The standard practice is to keep a rolling bead in front of the squeegee, adding a small amount at defined intervals and removing the material that has been worked past its limit rather than mixing it back in. Removing the old material is the part that is most often skipped, and it is the part that matters.

The interval should be set from the measured working life, not from a habit. Where the working life on the stencil is 60 minutes, replenishment at 15 to 20 minutes keeps the paste within its window while it is being worked. The removed material should be discarded rather than returned to the jar, because it has already lost solvent and returning it contaminates the fresh paste. A small waste allowance per shift is cheaper than a batch of variable deposits.

Cold Storage, Warm-Up and Condensation

Paste stored cold has a much longer shelf life, and the temperature should follow the maker’s figure, typically between 2 °C and 10 °C. The jar must be allowed to reach room temperature before it is opened, because opening a cold jar condenses moisture onto the paste, and the water changes the flux chemistry and can cause solder balls during reflow. The warm-up period is usually two to four hours for a 500 g jar, and it should be written into the procedure rather than estimated.

The jar should be stirred gently rather than shaken, and only for the time the maker recommends. Shaking introduces air, which produces voids and inconsistent deposits, and over-stirring heats the paste. Where the paste has separated, gentle stirring restores it; where it has skinned, the skin should be removed and discarded rather than stirred back in. The storage procedure should state the temperature, the warm-up time and the stirring rule for each paste used.

Mixing Old and New Paste

Mixing fresh paste with material that is near the end of its working life is common and is a false economy. The mixture has a viscosity that is between the two, so it prints acceptably at first and then ages faster than the fresh paste would, because the older fraction is already partway through its change. The result is a working life that is unpredictable and shorter than the datasheet figure.

Where mixing is unavoidable, the proportion should be limited and the mixture should be treated as having the working life of the older component. A cleaner rule is not to mix at all, and to size the replenishment quantity so that the paste on the stencil is consumed before it ages out. That requires the printer to be set up with a small bead and a defined interval, which is the same discipline as the replenishment practice described above.

Signs That Paste Has Passed Its Window

The signs are visible before they become defects. The paste on the stencil stops rolling and starts to stick to the squeegee; the print leaves a rough surface instead of a smooth one; the deposits start to look dry at the edges; and the stencil does not clean as easily during the wipe. Each is a sign that the viscosity has risen and that the release is changing.

In print, the first measurable symptom is a fall in deposit volume, usually in the smaller apertures first because they are the most sensitive to viscosity. That is why the deposit volume check should include a fine-pitch aperture rather than only a large one. The paste volume record, kept per print or per defined interval, is what turns the observation of these signs into a number that the operator can act on.

Records and Acceptance Limits

The record should carry the paste type and lot, the date and time the jar was opened, the time it reached room temperature, the periods it spent on the stencil and the deposit volume checks. With those entries, a defect that appears in a batch can be compared against the paste history for that batch, and the question of whether the paste was out of its window can be answered rather than debated.

Acceptance limits should be set for the deposit volume and for the working life, and both should be reviewed when the paste or the product changes. The paste inspection check should be scheduled at the start and end of each working period, because the change within a period is what the limit is designed to catch. Where the paste is changed to a different type, the limits should be re-established from a trial rather than carried over.

Additional Considerations for This Build

Practical attention to paste replenishment 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 paste replenishment explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Can paste be returned to the jar at the end of a shift? It can, but the returned material has already lost solvent and will shorten the working life of what is left in the jar. The usual practice is to discard the material on the stencil and return only what is still in the sealed portion of the syringe or the unopened part of the jar.

Does refrigeration extend the time on the stencil? No. Cold storage extends the shelf life before opening. Once the paste is on the stencil it is at room temperature, and its working life is governed by the room conditions rather than by how it was stored.

Is a viscosity measurement needed if the deposits look right? Not routinely, but it is a useful check when a paste change is made or when a deposit volume drift cannot be explained. The measurement needs a controlled temperature and shear rate, and without those it produces numbers that cannot be compared.