Solder Paste Storage, Working Life and Handling

Solder paste is a chemical product with a shelf life, a working window and a sensitivity to how it is handled. Most of the variation that appears in a print between the start and the end of a shift comes from the paste rather than from the printer.

What the Paste Has to Do

The paste must flow through the aperture, release from the walls, hold its shape on the pad, and then melt into a joint. Each of those steps is controlled by a different part of the formulation, and each is affected by time and temperature.

The flux in the paste is active only within a range of conditions. Outside that range the release changes, the slump changes, and the residue that is left after reflow behaves differently.

Storage

Paste is stored cold to slow the reactions that change its rheology, and the storage temperature is part of the specification. A jar kept at room temperature for a season is not the same product as one kept at the stated temperature.

The container should be sealed and upright. A jar that has been opened and left with the lid ajar absorbs moisture, and the moisture shows up as spatter during reflow. Our assembly notes describe the environment the process expects.

Warming and Opening

A cold jar must reach room temperature before it is opened, otherwise condensation forms on the paste. The warming time should be stated and it should be long enough that the whole jar has equalised.

Opening a jar early defeats the purpose, because the condensation the warming step was meant to prevent happens anyway. The jar should be labelled with the time it was taken out of the refrigerator.

Working Life and Reuse

Paste on the stencil is exposed to air and to the squeegee, and its solvent evaporates. The time it can stay on the stencil before the print changes is the working life, and it should be measured for the product.

Paste removed from the stencil should not be returned to the jar. The mixing of used and fresh paste produces a blend whose properties are between the two and whose history is unknown. Our paste volume notes describe how the effect is detected.

Print Parameters and the Paste

The squeegee speed, the pressure and the separation speed are set with the paste, not independently of it. A paste that is stiffer than the one the process was set up with needs a different print to give the same deposit.

A change of paste supplier is therefore a process change, and it should be treated as one, with a first article and a measured deposit rather than a trial run on production. Our paste inspection notes describe how the deposit is verified.

Batch and Traceability

The paste batch should be recorded against the boards it produced. Where a defect pattern appears across several assemblies, the batch is one of the first things to look at.

The expiry date should be checked when the jar is issued, not when the problem appears. Expired paste often prints acceptably and reflows badly, which is the most expensive combination. Our fabrication notes notes list the records that should be kept.

Verification

The paste condition is verified by the deposit measurement at the start of the run and at intervals, and by the appearance of the joints after reflow.

Where the deposit falls during a run, the cause is usually the working life or the stencil condition, and the two should be distinguished before the printing parameters are changed. Our inspection notes describe the joint check that follows.

Process Control and Verification

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. 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.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

FAQ

How long can paste stay on the stencil? It depends on the formulation and the room, so it should be established by measuring the deposit over time on the actual product rather than taken from a catalogue.

Can expired paste be used for prototypes? It can print, but the result is not evidence that the design works, so the prototype should use paste that is within its life if the result is going to be used for a decision.

What does gopcb provide for solder paste control? We provide storage and warming procedures with stated times, working life defined by measured deposit, print parameter development for the specific paste, batch traceability against the assemblies produced, expiry control at issue, and deposit verification during the run.