Solder Paste Deposit Area Ratio Limits
An aperture releases paste when the friction between the paste and the wall is small compared with the force pulling the paste out. The two figures that describe this are the area ratio and the aspect ratio.
What the Two Ratios Describe
The area ratio is the area of the aperture opening divided by the area of its walls. The aspect ratio is the width of the aperture divided by the stencil thickness. Both describe the same competition between adhesion and release.
The area ratio is the more useful of the two on a fine pitch part. Our paste volume notes describe the deposit it produces.
Where the Limit Comes From
The limit is empirical and it depends on the paste and on the stencil wall finish. A common working figure is a minimum area ratio, with the paste supplier stating a value for their material.
Paste and stencil should be qualified together. Our paste notes describe the properties that matter.
What Happens Below the Limit
Below the limit the deposit is incomplete and variable. The paste stays in the aperture or it is transferred partially, and the variation between boards rises sharply.
The variation is what shows in the data. Our SPI notes describe the measurement.
What to Change First
The order of preference is usually: reduce the stencil thickness, improve the wall finish, increase the aperture where the pad allows, and change to a paste with a smaller powder size.
Each change has a consequence for the deposit volume. Our stencil notes describe the tooling side.
When the Pad Is Simply Too Small
Some designs place a pad below any achievable limit. The options then are to change the footprint, to accept a lower deposit and a thinner joint, or to use a different assembly method such as jetting.
The choice is a design decision and it belongs at layout review. Our design release notes list the points to confirm.
Verification
The verification is a calculated area ratio for the smallest aperture on the board, a paste and stencil pair qualified together, a deposit volume measured across a panel, and a documented decision where a pad falls below the limit.
Our first pass yield notes describe how the data is used.
Process Control and Verification
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. 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.
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
Is the area ratio or the aspect ratio the real limit? Both apply and the area ratio usually governs on a fine pitch part, because the wall area rises faster than the opening shrinks.
Does a thinner stencil always help? It improves the release and it reduces the deposit volume, which may fall below what the joint needs.
What does gopcb provide for aperture design? We provide a calculated area ratio for the smallest aperture on the board, a paste and stencil pair qualified together rather than separately, a deposit volume measured across a panel, and a documented decision where a pad falls below the achievable limit.
