Thawing: Solder Paste Conditioning on the Line

Paste is a suspension that behaves differently at different temperatures and after different amounts of working. Conditioning is the practice of bringing it to a known state before it is printed.

Thawing

Frozen or refrigerated paste has to reach room temperature before the container is opened, or condensation forms inside it and the paste absorbs water. The thaw time depends on the container size.

The thaw should be recorded with the time it started. Our paste handling notes describe the routine.

Mixing

Mixing restores the suspension after separation during storage. Over mixing heats the paste and changes its viscosity, so the method and the duration are specified rather than left to the operator.

The temperature after mixing should be checked. Our paste notes describe the properties that mixing restores.

Idle Time on the Stencil

Paste left on a stencil loses solvent and stiffens, and the first prints after a break are different from the ones before it. The idle time limit should be written and the stencil cleaned when it is reached.

The cleaning interval and the idle limit are separate figures. Our stencil cleaning notes describe the schedule.

Roller and Squeegee Effects

The paste in front of the squeegee is worked continuously and it warms. A large rolling bank behaves differently from a small one, which is why the bank height is part of the setup.

Our squeegee notes describe the parameters.

Working Life and Its End

The working life on the stencil is shorter than the shelf life in the jar, and it ends when the paste is no longer delivering the same volume. The end should be defined by the print volume rather than by the clock alone.

Our SPI notes describe the measurement that defines it.

Environmental Conditions

Humidity and temperature at the printer affect the paste as much as the conditioning did. A printer in a room that swings in temperature produces a print that swings with it.

The room conditions should be recorded. Our floor control notes describe the arrangement.

Verification

The verification is a recorded thaw time, a specified mixing method with a temperature check, a written idle limit with cleaning at that point, and a print volume trend that shows the working life ending.

Our first pass yield notes describe how the data is used.

Process Control and Verification

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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. 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.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

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.

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.

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.

FAQ

Can paste be returned to the jar? It should not, because the paste on the stencil has already changed. The decision should be made from the print volume rather than from the quantity left.

Does a sealed jar prevent the change? It slows it. The temperature history still matters and it should be recorded.

What does gopcb provide for paste conditioning? We provide a recorded thaw time before opening, a specified mixing method with a temperature check, a written idle limit with the stencil cleaned at that point, and a print volume trend that defines the end of the working life.