Solder Paste Storage and Handling: A Process Guide

Solder paste is a mixture of metal powder and flux that begins to change the moment it is opened, and most printing defects that are blamed on the stencil can be traced back to how the paste was stored, thawed and handled before it reached the machine.

What Is in the Jar

A paste contains solder powder, a flux vehicle, a rheology modifier and an activator, and the balance between them is what keeps the paste printable. The powder is classified by particle size and by the spread of that size, and a paste rated for fine pitch contains smaller and more uniform particles so that the stencil apertures fill completely.

Because the metal is denser than the flux, the paste separates slowly while it stands. That separation is reversible by stirring, but the flux that has migrated to the surface dries during the time it is exposed, and the dried vehicle cannot be restored by mixing.

Storage Temperature and Shelf Life

Paste is normally stored refrigerated at a temperature that slows the chemical activity of the activator without freezing the vehicle, and the labelled shelf life assumes that temperature is held. A jar stored in a warm room will lose usable life in weeks rather than months.

The date on the label is the date the manufacturer will stand behind, not the date the paste becomes unusable. A paste that is past its date can still print acceptably, but it should be qualified by a print test on the production stencil rather than assumed to be interchangeable. Our solderability notes describe how the wetting behaviour is verified.

Thawing and the Water Problem

A cold jar attracts condensation, and water in the paste causes spitting and voids during reflow. The correct procedure is to leave the jar sealed until it has reached room temperature, which for a standard jar takes a couple of hours, and never to accelerate the process with heat or warm water.

Opening a jar early is one of the most common causes of a defect that appears as a scatter of small voids under a joint. The paste looks normal and prints normally, so the cause is rarely suspected until the pattern of voids is mapped against the batch of paste.

Stirring and Consistency

Paste that has separated should be stirred gently until it is uniform, and the stirring should be done with a clean tool in a way that does not fold air into the mixture. Machine mixing is more consistent than hand mixing, and it is worth using where the volume justifies it.

The consistency that matters is the one the process needs, not the one the jar had when new. A paste that has been open for several hours becomes tackier as the solvent evaporates, and adding thinner is not a fix, because the thinner changes the flux chemistry as well as the viscosity.

Open Time and the Working Window

Once a jar is open, the clock starts. The working window depends on the room’s humidity and temperature, and it is shortened sharply by moving air, so a stencil printer sitting under an air-conditioning diffuser will see its paste dry out far sooner than one in still air.

The practical control is to dispense only what the shift will use and to return the remainder to a sealed container rather than leaving it on the stencil. Paste left on the stencil at the end of the day should be discarded rather than scraped back into the jar, because it carries the dried fraction with it.

Printing Parameters That Depend on Paste

The printing parameters are set for a particular paste, and changing the brand without re-qualifying the process is a common source of trouble. Squeegee speed, squeegee pressure, separation speed and the interval between printing and placement all interact with the paste’s rheology.

Separation speed is the parameter most often overlooked. A paste that pulls cleanly at one separation speed will leave strings or smear at another, and the difference is visible only under magnification. Our solder paste inspection notes describe how the printed deposit is measured.

How Paste Goes Wrong in the Stencil Aperture

Two classic print defects come directly from paste condition. Poor release, where the deposit is short or pulled from the pad, appears when the paste is too tacky, when the aperture walls are not clean, or when the area ratio is too low. Slump, where the deposit spreads after printing, appears when the paste has absorbed moisture or when the room is too warm.

Both defects are correctable at the printer, but the correction is wasted if the paste itself is at fault. Checking the jar’s history before adjusting the machine saves a shift of work.

Paste Selection for the Application

The alloy is chosen for the temperature the assembly will see, the flux chemistry for the cleanliness requirement, and the powder size for the finest feature on the board. A no-clean flux is appropriate where the residue is acceptable, while a water-washable flux gives a cleaner board at the cost of a washing step and a risk of residue under components that the water cannot reach.

The powder size should be matched to the aperture rather than to fashion. Using a fine powder everywhere reduces the print window for coarse features and costs more, so the choice is usually made per product rather than per factory. Our solder defect notes link the powder classification to the defects that appear when the match is wrong.

Traceability and Records

Because paste has a limited life, the batch number and the opening time should be recorded against the product that used it. Without that link, a defect that appears in one shipment cannot be tied to the material that caused it, and the investigation stops at the printer.

The record also supports the decision to extend or discard. A paste that has been open for six hours and is still printing within specification is a data point that only exists if the opening time was written down. Our fabrication notes guidance describes how the same discipline applies on the fabrication side.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

Should paste be stirred before use? Yes, gently, until it looks uniform. The separation is normal and reversible, but the stirring must not whip air into the paste, because trapped air produces voids in the printed deposit that reflow cannot remove.

How many times can paste be reused after printing? Paste that has been on the stencil should not be returned to the jar at all. Where reclaiming is unavoidable, the recovered paste should be kept separate, used only for non-critical features, and never mixed into virgin paste.

What does gopcb check on solder paste? We check the batch number and the expiry date, the thawing record, the room temperature and humidity at the printer, and the print result through the inspection system. Any deviation from the qualified window is treated as a process change and re-qualified before the product is released.