Solder Paste Open Time and Work Life on the Line
Solder paste changes from the moment the jar is opened. The solvent evaporates, the flux chemistry reacts with the air and the viscosity rises, so a paste that printed correctly in the morning behaves differently in the afternoon. Open time and work life describe how long the material remains usable, and controlling them is one of the simplest ways to keep a print process stable.
What the Two Terms Describe
Open time is the period during which an exposed deposit or an open jar remains usable. Work life is the period during which the paste can be worked on the stencil, printed and replenished without a loss of print quality. The two are related but not identical, because a deposit on a board and a mass of paste on a stencil age at different rates.
Both are properties of the specific formulation rather than universal constants. A paste intended for a long production run has a longer working life than one intended for fine pitch work, and the two cannot be substituted without revalidating the process.
What Changes as Paste Ages
The viscosity rises as the solvent leaves, which reduces the ability of the paste to fill and release from an aperture. The flux becomes more concentrated and its activity changes, which affects how it performs in the reflow. The particles can agglomerate, which affects the print resolution and the tendency to clog.
The practical symptoms appear in the print first. The deposit volume falls, particularly on small apertures, the paste does not release cleanly and leaves a rough surface, and the deposits become inconsistent across the board. Each of these is measurable, and the paste condition is one of the first things to check when a print problem develops during a shift.
Viscosity, Rheology and Print Quality
The paste has to shear thin as the squeegee rolls over the aperture so that it fills the opening, and then recover its viscosity quickly so that it does not slump. As the paste ages, the recovery time increases and the deposit slumps or the release becomes incomplete.
The measurement of this behaviour is a rheology test, but the practical check on the line is the print itself. Where a deposit measurement is available, the volume distribution widening during a shift is the clearest indication that the paste has aged. The measurement method is the same one described in solder paste inspection and applied to the deposit data over time.
Stencil Replenishment Practice
Replenishment is where the working life is managed. Paste should be added in small amounts at defined intervals, and the paste that has been on the stencil should be removed rather than mixed with the fresh material. Mixing produces a blend whose properties reflect neither the fresh nor the aged material, and the blend changes continuously as more is added.
The interval between replenishments should be short enough that the paste on the stencil never reaches the end of its work life. Where the interval is too long, the print degrades toward the end of each interval, which produces a repeating pattern of defects rather than a gradual drift. The pattern is a useful diagnostic because it points directly at the replenishment practice. Paste mixing is one of the handling practices described in solder paste volume and stencil design.
Open Time on the Board
A printed deposit begins to dry as soon as it is on the board. The rate depends on the ambient humidity and temperature, the deposit size and the paste formulation. A small deposit on a fine pitch pad has a high surface to volume ratio and dries faster than a large one, which is why a fine pitch assembly is more sensitive to the delay between printing and placement.
The delay should be defined and controlled. Where the line stops, the boards that are already printed have to be treated as suspect, because the paste may have dried beyond the usable point. The decision to use or reject them should follow a rule rather than an opinion, and the rule should be based on a deposit measurement after the delay.
Thawing, Mixing and Jar Handling
Frozen paste has to be thawed in its sealed container before use, and the time required depends on the jar size. Opening a jar before it has fully thawed allows condensation to form inside, which changes the paste. The thaw should be recorded, and the jar should not be warmed artificially.
Mixing is another point where practice varies. A paste that is designed to be used without mixing can be damaged by stirring, which introduces air and changes the rheology. Where mixing is required, the supplier instruction should be followed rather than general practice, and the effect on the print should be verified.
Paste Type and Its Working Behaviour
Different paste formulations behave differently over time, and the working life is one of the properties that distinguishes them. A water soluble paste often has a shorter work life than a no clean one because its chemistry is more sensitive to evaporation, while a paste designed for high throughput balances the working life against the print speed and the aperture size it has to fill.
The choice should therefore follow the production pattern. A line that runs long batches with infrequent changes benefits from a longer working life, while a line that changes product often may accept a shorter one because the paste is replaced with the job. Where the paste is changed for a cost reason, the working life should be checked rather than assumed to be equivalent, and the print quality verified on the actual product.
Measuring the Effect on Print Quality
The effect is measurable through the deposit data. Printing a run and measuring the volume at the start, the middle and the end of the working life shows how much the paste changes, and where the change is significant the working life is the limiting factor for the interval. The same test after a paste change shows whether the new material behaves the same way.
The measurements should include the smallest aperture on the board, because it is the first to suffer. A paste that still prints well on a large pad and poorly on a fine pitch aperture has a working life that is shorter than the average suggests, and the practical limit should be set from the worst case. The general acceptance logic for print defects is described in solder defect assessment.
Controls and Records
The controls are a defined thaw routine, a defined working life on the stencil, a defined replenishment interval and a rule for printed boards that have waited. Each of them should be recorded with the paste batch and the product, so that a print problem can be related to the material condition rather than investigated as a machine fault.
The gopcb assembly group records the jar opening time, the batch and the interval to the last accepted print for each production run. Where a print problem appears, that record shows whether the paste was within its work life, and it separates a material issue from a machine issue before any adjustment is made.
FAQ
Can aged paste be revived by adding solvent? No. Adding solvent changes the flux chemistry and produces a paste with unpredictable behaviour. The material should be discarded.
Does the working life reset when fresh paste is added? Only for the fresh material. The paste already on the stencil keeps ageing, which is why the old material should be removed rather than mixed.
How does humidity affect open time? Low humidity dries the paste faster, so a process that is stable in a humid season can develop print problems in a dry one without any other change.
