Paste Kneading and Print Stability Over a Shift

Solder paste is a suspension, and a suspension separates. Kneading is the operation that restores the mixed state before printing, and the stability of the print over a shift depends on it being done consistently rather than quickly.

What Happens Without Kneading

The metal particles settle and the vehicle rises, so the paste at the top of the jar is thinner than the paste at the bottom. A print made from the top of a jar is a print made from a different material from the one at the bottom.

The effect is largest after a period of storage and it is present within a shift on a jar that is left standing. Our SPI notes describe how the deposit shows it.

How Kneading Is Done

The paste is stirred with a spatula until it is uniform in appearance, and the operation is done in a folding motion rather than a whipping one. Whipping introduces air, which forms voids in the deposit and in the joint.

A mechanical mixer is used on a larger pot and it has the same requirement: mix without aerating. The parameters for the mixer belong in the instruction. Our paste volume notes describe the deposit target that the mixing supports.

Temperature and the Thaw

A paste that is stored refrigerated is brought to room temperature before the jar is opened, so that condensation does not form on the surface. The thaw time is set by the jar size and it is measured rather than estimated.

Opening a cold jar introduces water, and water in the paste produces spatter and voids. That is the entire reason the thaw is specified, and it is also the reason the jar is not warmed artificially. Our board quality notes describe how the resulting joints are judged.

The deposit volume drifts over a shift as the paste on the stencil dries and as the jar is worked. The drift is measurable, and the usual behaviour is a slow fall in volume as the paste stiffens.

Checking the volume at intervals rather than only at the start of the shift is what detects the drift. A printer that is set up and left is a printer that is printing something different after four hours.

The interval for adding fresh paste is set from the drift rather than from a habit. Where the volume is stable for two hours, the interval is two hours; where it drifts in forty minutes, a smaller quantity is added more often.

Adding paste on a schedule that is not measured produces a volume that oscillates rather than one that is stable. Our first pass yield notes describe how the oscillation appears in the data.

The record is the paste lot, the thaw start and end, the mixing time and the interval between additions. Those four explain a drift that a single volume reading cannot.

The paste on the stencil also ages, and it is removed and replaced rather than diluted. A stencil that is replenished by adding fresh paste to old paste accumulates the older material at the edges.

Acceptance and Its Evidence

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

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. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. 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.

Verification and Records

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

Can a jar be used straight from the refrigerator? It can, and the condensation it introduces is a defect source, which is why the thaw is specified.

Does a mechanical mixer eliminate the need for a thaw? No. Mixing distributes what is in the jar and it does not remove water that has already formed.

What does gopcba provide for paste handling? We provide a thaw time set from the jar size and measured, a mixing instruction that avoids aeration, additions made on a measured interval rather than a habit, paste removed from the stencil rather than diluted, and records of lot, thaw, mixing and additions.

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. 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. 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.