Solder Paste Thawing: Design Rules and Process Limits

Solder paste is a mixture of solder powder, flux and a solvent system, and its behaviour changes with temperature and with time. Most printing problems that are blamed on the printer or the stencil can be traced to paste that was used too cold, left open too long or stored outside the specified range. The correct handling routine is simple and it is the cheapest process control available.

Why Temperature Matters

The viscosity of paste falls as the temperature rises. Paste that is used straight from a refrigerator is thick, it does not roll properly on the stencil and it does not fill small apertures, which produces deposits that are short and irregular.

Paste that is too warm is thin and it slumps after printing. The deposit spreads beyond the pad, the walls of the deposit collapse and the paste is more likely to bridge to a neighbouring pad or to slide under the mask.

The target is the room temperature at which the paste was specified, usually around 22 to 25 degrees. The temperature that matters is the paste itself, not the room, and a jar that has been standing beside a cold wall will take longer to reach it than one on a bench.

Thawing Procedure

The solder paste thawing procedure is to remove the jar from the refrigerator, keep the lid on, and leave it to reach room temperature undisturbed. A defined warm up time should be written into the work instruction rather than left to judgement.

Opening the lid before the paste has warmed allows condensation to form on the surface. Water in the paste changes the flux chemistry, and it also produces spitting during reflow as the moisture turns to vapour, so the rule is to wait and then open.

Accelerating the warm up with a warm bath or a hot plate is not a safe shortcut. The outside of the jar warms first and the centre lags, so the paste is at an unknown temperature when it is opened, and the jar that is warmed repeatedly is exposed to a temperature cycle that the supplier did not specify.

Open Time and Working Window

Once the jar is open, the paste begins to dry. The solvent evaporates from the surface, the paste thickens and the flux activity falls. The open time is the period the supplier allows before the paste should be discarded or replaced.

On the stencil the same process occurs more quickly, because the paste is spread into a thin layer with a large surface area. The printing window is shorter than the jar open time, and a paste that has been rolled on the stencil for several hours will print differently from one that was loaded ten minutes earlier.

A practical control is to record the time the paste was loaded and the time it was removed, and to replace the roll after the defined window rather than when the deposits start to look wrong. The visual check always confirms the problem after it has begun.

Mixing and Consistency

Paste separates during storage, with the flux rising and the metal settling. A gentle stir before use restores the consistency, and the supplier instruction should be followed on how much mixing is appropriate.

Mechanical mixing with a high speed tool can introduce air, and entrapped air becomes voids in the deposit and in the joint. Where a mixer is used it should run slowly and the paste should be allowed to settle before it is transferred to the stencil.

The consistency should be judged against a reference rather than by feel. A simple check is to compare the roll on the squeegee with the previous batch, and a more objective check is a viscosity measurement on a sample at a controlled temperature.

Storage Conditions

The storage temperature should be within the range the supplier specifies, which for most paste is a refrigerator between 2 and 10 degrees. A domestic refrigerator with a freezer compartment is not suitable, because the paste can freeze at the back and freezing damages the powder.

The shelf life is quoted from the date of manufacture and it applies only when the storage condition has been met. A jar that has been stored warm for a week has aged by more than a week, and the date code alone does not describe its condition.

The rule that follows is to record the receipt date, to rotate the stock so that the oldest paste is used first and to keep the jars sealed until they are needed. The record is what makes a failure traceable to a particular batch rather than to the process in general.

Paste and the Print Result

Every paste property changes the deposit. Viscosity affects the fill of the aperture, the tackiness affects whether the deposit holds the component before reflow and the metal content affects the volume of solder that remains after the flux has burned off.

A paste that is at the end of its open time will usually show a lower deposit volume, a rougher surface and a tendency to stick to the stencil rather than release from it. The symptoms look like a stencil problem, which is why the paste record should be checked before the tooling is blamed.

Comparing two pastes by their appearance is unreliable. The comparison should be made on the paste volume data from the inspection system, on the defect rate and on the reflow result, with one variable changed at a time.

Dealing With Leftovers

Paste that has been on the stencil should not be returned to the jar. It has been exposed to air, it may contain contamination and it has begun to dry, and mixing it with fresh paste changes the properties of the whole jar.

Leftover paste in a jar that has been opened and not used should be evaluated against the open time rule rather than kept indefinitely. Where the paste is expensive, a defined and recorded re-use procedure is better than an informal one, because an informal rule will be applied differently by each operator.

The stencil and the tooling should be cleaned at the same time, since dried paste on the aperture walls is a reservoir that contaminates the next batch. Cleaning the stencil with the correct solvent, and removing the residue rather than spreading it, is part of the handling routine.

Practical Rules

Thaw with the lid on for the specified time, open only when the paste has reached room temperature, and record the load and removal times on the stencil. Rotate the stock and keep the storage temperature inside the supplier range.

Record the batch and the timings with the build records and the defect history, and review the jetting paste rules and the paste volume data whenever the deposit volume moves without a printer setting change.

FAQ

Why must the lid stay on while the paste thaws? Condensation forms on cold paste when the lid is opened. Water in the paste alters the flux chemistry and causes spitting during reflow.

How long can paste stay on the stencil? Less than the jar open time. Spread thin on the stencil it dries quickly, and the printing window should be defined and recorded per product.

Can leftover paste be put back in the jar? No. Paste that has been on the stencil has been exposed to air and may be contaminated, and mixing it with fresh paste changes the properties of the whole jar.