Solder Paste Shelf Life: Cold Chain, Thaw and Working Time
Solder paste is a suspension of solder powder in flux, and it is chemically active from the moment it is manufactured. Its shelf life is the time over which that activity stays inside a usable range, and every step from the supplier to the stencil shortens it.
The practical control is a chain of limits: a storage temperature, an unopened shelf life, an opened shelf life, a thaw time and a working life on the stencil. Each of them is quoted by the supplier for a formulation, and each has to be verified on the line rather than assumed.
Why Paste Has a Shelf Life at All
Flux contains activators, solvents and rheology modifiers, and those materials react slowly even at room temperature. The activators are designed to work at soldering temperature, but they also react with the solder powder at a much lower rate, and the reaction thickens the paste and consumes the activity that is needed later.
Temperature is the main accelerator, which is why cold storage extends the life and why a jar left on a bench over a weekend is a different material on Monday. Oxygen, moisture and light contribute as well, and the powder itself coarsens as the oxide layer around each particle grows.
Cold Storage and the Cold Chain
Paste is normally stored between 2 and 10 °C, with the supplier stating an unopened shelf life of six to twelve months from the date of manufacture. The temperature has to be held throughout, because a jar that has been warm for a day and then chilled again has already aged.
The cold chain therefore includes transport and receiving, not just the store. A delivery that sat on a loading dock in summer, or a store with a door left open, shows up as paste that behaves normally for a week and then thickens suddenly, long before its labelled date.
Thawing Without Condensation
A jar taken from the refrigerator must be brought to room temperature before it is opened, which takes two to four hours for a 500 g jar. Opening it cold draws condensation onto the paste and onto the inside of the lid, and that water changes the flux.
The jar should be thawed upright and unopened, and it should reach within about 3 °C of the room before the lid comes off. Thaw time can be shortened by moving the jar to a warm water bath, but any method that heats the paste above room temperature risks separating the flux from the powder.
Working Life on the Stencil
Once the paste is in the printer it begins to lose solvent from the exposed surface, and its viscosity rises. Working life is the time over which the print still meets the volume specification, and for most no-clean and water-soluble formulations it is four to eight hours on the stencil under normal room conditions.
The figure applies to paste that is being printed, because the roll is continuously re-mixed by the squeegee. Paste that sits still in a corner of the stencil skims over and dries much faster, and it should be removed rather than folded back into the roll, as described in the notes on paste working life.
Viscosity, Slump and Print Volume Over Time
Viscosity is the property that carries the process, and it is measured at a defined shear rate so that results can be compared. A paste that has aged by one step in viscosity prints a smaller volume at the same pressure and speed, and it slumps less in the seconds after the stencil lifts.
Slump is not always bad, since a little flow helps the paste wet the pad, but a paste that has thickened will not slump at all and leaves a deposit with square corners and a narrow contact area. The print window and the measurement method are covered in the notes on viscosity and the printing window.
Reuse and Re-mixing Rules
Paste taken off the stencil at the end of a shift should not be returned to the jar. It carries dried particles and stencil debris, and mixing it with fresh paste produces a blend whose properties are not the properties of either component.
Some shops allow a limited amount of used paste to be kept separately and used for coarse work, and that practice has to be written down with a time limit and a volume limit if it is allowed at all. The simplest rule, that paste leaves the jar once and is discarded at the end of the shift, is also the easiest to audit.
Labels, FIFO and Traceability
Every jar should carry the manufacturer date code, the lot, the date it was opened and the date it must be used by. A jar without a date is a jar that will be used beyond its life, because nobody can prove otherwise.
First in, first out is the control that makes the labels useful, and it needs to be enforced physically rather than trusted to habit. The paste lot should also be recorded against the assemblies printed with it, so that a defect found later can be traced to the material, which is the same discipline used for evidence of drying on the stencil.
What a Bad Jar Looks Like in the Data
The first sign is usually a fall in print volume on the smallest apertures while the large ones stay in specification. The paste also becomes harder to roll, and the roll breaks up instead of turning smoothly in front of the blade.
The inspection data shows this as a downward drift in the mean volume with the distribution intact, which distinguishes aged paste from a stencil problem. A stencil problem moves a group of apertures together; ageing moves the whole board in one direction.
Setting Limits for a Specific Product
The limits should be written for the product rather than taken from the supplier sheet, because a fine pitch board uses the paste at a smaller volume than a coarse one and reaches its limit sooner. A working life of eight hours may be perfectly safe on one product and too long on another.
Verification is a printing trial at the end of the intended life, with the volume measured and the deposits reflowed to inspect the joints. That trial costs a few boards and a few hours and gives the line a working life it can defend. Room conditions should be part of the limit, since 35 °C and 20 percent relative humidity will shorten it, and the process settings that compensate are covered in the notes on print speed and pressure.
FAQ
How long can solder paste be stored? Six to twelve months unopened at 2 to 10 °C, as stated by the supplier. Once the cold chain has been broken the remaining life is shorter and cannot be recovered by returning the jar to the refrigerator.
Can paste be re-frozen after thawing? No. Freezing and thawing separates the flux from the powder and drives condensation into the material, and the damage is not reversible by mixing.
How long does paste last on the stencil? Four to eight hours for most formulations under normal room conditions, but the figure should be verified on the product by printing at the end of the intended life and inspecting the joints.
