Solder Paste Metal Load and Powder Selection

Solder paste is a mixture of metal powder and flux, and the ratio between them is the metal load. It affects the print, the reflow and the residue, and it is one of the few paste properties that can be compared directly between products.

What the Metal Load Is

The metal load is the percentage of the paste mass that is the alloy powder. The rest is flux, solvent and rheology modifiers, and it is the part that has to be removed or left behind.

A high metal load prints with a lower slump, reflows into a joint with less shrinkage and leaves less residue. It also flows less readily through a small aperture. Our area ratio notes describe the aperture that limits the flow.

Powder Size and Distribution

The powder is classified by the size and the distribution of the particles. A finer powder passes through a small aperture more easily and it has a larger surface area, which means more oxide and a need for more flux activity.

The type number refers to the size range, and a stencil with fine apertures needs a finer powder. Using a coarse powder on a fine aperture produces a blocked aperture rather than a short deposit. Our paste notes describe the handling that keeps the powder in suspension.

Effect on the Print

A high metal load gives a deposit that holds its shape and a print that resists slump between printing and reflow. That matters on a fine pitch part where a slumped deposit bridges.

The same high metal load needs a longer or a slower separation from the stencil, because the deposit sticks to the wall more firmly. The print parameters must move with the paste. Our print notes describe how the parameters are developed.

Effect on Reflow

During reflow the flux volatilises and the deposit collapses to a fraction of its printed height. A high metal load leaves more metal and less shrinkage, so the joint that forms is closer to the volume that was printed.

The collapse also produces the self-alignment that corrects a small placement error. A paste with a very high metal load moves less, so the placement accuracy has to be better. Our placement notes describe the accuracy that is required.

Residue and Cleanability

Less flux in the paste means less residue, which suits a no-clean process. The residue that remains is still there, and it still has to be acceptable for the application.

Where the board is cleaned, a low residue paste removes more easily because there is less material to dissolve. Our cleanliness notes describe how the result is verified.

Comparing Products

Two pastes with the same metal load can behave differently, because the alloy, the powder shape and the flux chemistry all differ. The metal load is one of several parameters and it should not be used as the only comparison.

Where a paste is changed, the print parameters, the profile and the residue should all be re-established rather than transferred from the previous product.

Verification

The verification is the deposit measurement with the new paste at the smallest aperture, the joint appearance after reflow and the residue against the requirement.

The measurements should be taken on the product rather than on a test vehicle, because the aperture geometry decides the result. Our paste inspection notes describe the measurement.

Process Control and Verification

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.

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.

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.

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

FAQ

Is a higher metal load always better? It is better for slump and residue and it is harder to print through a small aperture. The optimum follows the aperture size on the product.

Does a finer powder always print better? It passes a small aperture more easily and it oxidises faster, so it has a shorter working life on the stencil.

What does gopcb provide for paste selection? We provide paste selection by metal load and powder type for the aperture on the product, print parameter development that follows the paste, profile adjustment for the collapse and the flux, residue verification against the cleanliness requirement, and re-qualification of the print and the profile after a paste change.