Solder Paste Volume Measurement with SPI
Paste volume measurement turns the printer into a measurable process. Instead of judging a deposit by its appearance, the system reports a number for every aperture, which can be compared with a target and tracked over time.
The value is in the feedback rather than in the rejection. A measurement that is used only to reject boards costs money, while one that is used to correct the printer prevents the boards from being made.
How the Measurement Works
An inspection system projects a pattern onto the deposit and reconstructs its height, from which the volume is calculated. The measurement is fast enough to run in line and covers every printed aperture.
The accuracy depends on the calibration and on the surface, so the system should be calibrated with a known artefact on a schedule. The result is a relative measure as much as an absolute one.
Setting Limits
Limits are usually expressed as a percentage of the target volume, with a tighter band for a critical aperture and a wider one for a coarse feature. A single limit across the board produces either false calls or missed defects.
The limits should be derived from the joint requirement rather than from the distribution of the process, because a process that is consistently wrong is still wrong.
Volume, Area and Height
The three measurements answer different questions. Height indicates the stencil thickness and the release, area indicates the aperture and the registration, and volume combines them.
Where a defect appears, the three together point to the cause: a low volume with a correct area indicates a release problem, while a correct volume with a displaced area indicates an alignment problem.
Trends Rather Than Snapshots
A single measurement shows the state of one board, while a trend shows the direction of the process. The trend is what allows a correction before the limits are reached.
The trend should be plotted against the paste lot, the stencil life and the squeegee life so that a step change can be attributed. Without that context, a trend is a line without a cause.
The Feedback Loop
The measurement is only useful if it reaches the printer, and the loop should close within the shift. A report that is reviewed the next day corrects yesterday’s boards rather than today’s.
The practical arrangement is a display at the printer with the current trend and the limits, so that the operator can act on it. The correction should be recorded.
Coverage and Sampling
In line systems measure every aperture on every board, which is useful for a statistical view. Where the system is offline, the sample should cover the range of aperture sizes and positions on the board.
A sample taken from one region only will miss the effect of board support and of squeegee wear, both of which vary across the panel.
Correlation With the Oven
The volume predicts the joint, and the prediction should be confirmed with a periodic reflow and inspection. Where the volume is correct and the joints are not, the cause is downstream.
The correlation should be reviewed when the paste, the stencil or the profile changes, since all three alter the relationship between volume and joint.
Records
The records should include the volume statistics, the limits and the corrections made, linked to the product and the batch. Together they describe the print process for that period.
They belong with the process evidence described for manufacturing processes, and with the printing defects they are intended to prevent, as described for printing defect causes.
Additional Considerations for This Build
Practical attention to SPI pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating SPI explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Deliberate attention to inspection limits pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating inspection limits explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, trend is the item that decides how the rest of the board is arranged. 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.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
Process Control and Verification
On a design of this kind, trend is the item that decides how the rest of the board is arranged. 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 first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Does the system replace visual inspection? It replaces the volume judgement and not the checks for bridging, contamination or placement, which are different defects.
How often should the system be calibrated? On a defined schedule and after any mechanical change, using a traceable artefact.
Can limits be widened to reduce false calls? They can, and doing so hides the process drift that the measurement was installed to reveal.
What is the most useful single figure? The trend of volume against the target, because it predicts the joint before a defect appears.
