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SPI testing: measure solder paste before placement
Interpret solder paste volume, height, area and offset against a defined measurement reference. Use approved printing limits and preserve the evidence behind each process adjustment.
Share the product revision, required checks and expected records. Early questions are welcome.
Is 100% displayed volume always the correct deposit?
The percentage depends on the configured reference volume. It does not describe offset, bridges or the distribution of paste across a pad. Review the measurement definition and the approved process window together with the deposit image.
What is SPI in SMT assembly?
Solder paste inspection, usually called SPI, evaluates the printed paste deposits before components are placed. Its purpose is to identify printing conditions that need correction while the board is still at an early process stage. Depending on the system and recipe, useful observations include deposited volume, height, area, offset and bridging between neighboring deposits.
SPI in this manufacturing context means solder paste inspection, not the Serial Peripheral Interface used in electronic circuits. It also differs from post-reflow inspection: a paste deposit is an input to soldering, while the final joint forms after component placement and thermal processing. Passing SPI does not establish the final joint quality or the assembled product鈥檚 function.
The value comes from a controlled feedback loop. A repeated low-volume pattern can prompt investigation of aperture release or cleaning. A systematic offset can prompt a review of registration or board support. The data localizes a question; it does not identify one root cause automatically. Preserve the printing and measurement context before changing the process.
Volume, area, height and position are separate measures
| Measure | Useful meaning | Interpretation caution |
|---|---|---|
| Volume | Integrated deposited paste above a defined reference | Requires a measured surface and a defined segmentation method |
| Area | Projected footprint of the detected deposit | A spread deposit can have a large area without sufficient useful volume |
| Height | Height above the selected board or pad reference | Maximum, mean and other height statistics are not interchangeable |
| Offset | Deposit position relative to the reference aperture or pad | Specify axis, origin, units and whether the value is signed |
| Bridging or shape | Potential merging, smear or abnormal deposit geometry | Review the local spacing, image and paste boundary definition |
describes why reconstructed surface information matters for deposit-volume measurement. That explanation supports the measurement principle; it does not establish that a particular equipment configuration or accuracy is available for your order.
Do not calculate actual irregular paste volume by multiplying a reported footprint by its maximum height. That treats the deposit as a uniform block. A real SPI system integrates its reconstructed surface using its own measurement reference and segmentation rules. The engineering calculator below uses aperture dimensions only to calculate a nominal comparison volume.
Keep the reference surface consistent when comparing boards. Solder-mask height, pad finish, local board shape and the selected zero plane can affect the meaning of a height value. A useful measurement-system review checks repeat readings on representative deposits and locations, then investigates systematic differences before treating them as a printing trend. Document the measurement definition with the recipe so another engineer can reproduce the comparison.
Define the nominal reference before choosing limits
For a rectangular aperture, nominal volume equals length multiplied by width multiplied by the local stencil thickness. For a circular aperture, nominal volume equals pi multiplied by diameter squared, divided by four, then multiplied by thickness. When all dimensions are in millimeters, the result is in cubic millimeters.
The nominal cavity volume is not the amount of paste guaranteed to transfer. Aperture wall condition, paste characteristics, release, board support, separation and printing parameters can affect deposition. Step stencils also require the thickness at the actual aperture, not an average across the foil. Rounded or irregular apertures require their true area rather than a bounding rectangle.
Percentage displays need a documented denominator. If the program compares measured volume with an aperture-based nominal, record that definition. If it uses another reference, use that reference when interpreting the result. Changing nominal values merely to center the process can hide a problem unless the engineering basis is reviewed and approved.
Solder paste nominal-volume and ratio calculator
Enter a supported aperture shape and local stencil thickness. Add a measured SPI volume if available. The calculator reports nominal volume and the measured-to-nominal ratio. Optional lower and upper percentage limits come from your approved process specification; no default IPC acceptance window is inserted.
SPI Deposit Volume Reference
The example is a mathematical illustration, not a recommended aperture or production limit. A deposit inside your entered volume window can still have unacceptable offset, bridging or distribution. A deposit outside the window needs review under the process procedure; the tool does not authorize cleaning, reprinting or product release.
For segmented exposed pads, assess the individual apertures and the intended total pattern. One total-volume number can conceal a local missing deposit or an uneven distribution. Keep the image or per-aperture measurements available when investigating a QFN or thermal-pad concern.
Use SPI feedback without losing process control
- Confirm the release. Match the board, stencil, paste and inspection recipe to the intended build revision.
- Measure the print. Verify reference alignment, board support and the measurement setup before interpreting a trend.
- Review the call. Inspect the actual deposit and distinguish a printing condition from a measurement artifact.
- Control the response. Follow the approved disposition for held boards and process adjustments.
- Verify the result. Recheck after the intervention and preserve a record of what changed.
Separate product acceptance limits from statistical process-control limits. Acceptance limits define permitted product characteristics under the agreed requirements. Control limits describe observed process behavior under an appropriate statistical model. A process can be statistically stable and still fail a product requirement.
This page therefore does not generate a process-capability index from a handful of deposits. A defensible capability study needs a suitable measurement system, representative sampling, stable process conditions and the correct statistical assumptions. More decimal places do not compensate for missing context.
Turn a deposit pattern into a focused investigation
| Observed pattern | Questions to investigate | Record to preserve |
|---|---|---|
| Repeated low volume at the same aperture | Aperture release, local stencil condition, pad geometry, support and recipe | Aperture ID, images and print sequence |
| Board-wide position shift | Registration, fiducials, clamping and data alignment | X/Y reference and program revision |
| Intermittent smear or bridge | Cleaning interval, separation, paste condition and local spacing | Before/after images and intervention |
| Unexpected height variation | Board reference, warpage, support and reconstruction quality | Location, measurement settings and reference surface |
Coordinate printing changes with PCB stencil selection and the relevant aperture design. For bottom-terminated devices, the QFN stencil-design guide provides a more specific geometry discussion. A thinner stencil is not a universal solution because the rest of the assembly may require different deposited volumes.
IPC guidance and the production agreement
concerns solder paste printing. IPC-7525 addresses stencil design, while J-STD-005 concerns soldering paste requirements. These documents have different roles. Post-assembly acceptance under IPC-A-610 should not be used as an unexplained substitute for a defined printing-control plan.
Specify the applicable revision and customer requirements, the measurement definitions, allowed limits, response to a call and authority to release held boards. Use the actual component and paste guidance where it adds product-specific requirements. This page explains the planning questions; it does not reproduce licensed acceptance tables.
A useful SPI report connects a board or panel to the recipe, stencil revision, inspection time or lot, deposit location, measurement, limit and disposition. Agree whether the delivery requires exception images, trend summaries or complete measurement exports. Storage and traceability expectations belong in the scope and quotation.
Move next to AOI for the placed and soldered assembly and, where hidden-joint evidence is needed, X-ray inspection. The first-piece release review can reconcile printing observations with the rest of the first build. Return to PCB Assembly to discuss the complete manufacturing request, or review quality-assurance evidence before supplier approval.
SPI report fields that explain a printing decision
Use these fields to scope the report with your quotation. The table is a planning example, not a 吃瓜tv test result or a promise that every record is included by default.
| Record item | Information to include | Why it matters |
|---|---|---|
| Recipe identity | Board, stencil, paste and inspection-program revisions | Use the local stencil thickness for a stepped region. |
| Reference | Aperture geometry or another documented nominal basis | Changing the denominator changes the displayed ratio. |
| Observation | Deposit location, measured volume, offset and image | A summed total can hide one missing aperture. |
| Response | Approved limits, reviewed call, intervention and verification | Keep process changes tied to the affected print sequence. |
Walk through an engineering example
Illustrative calculation: a 0.50 脳 0.30 mm rectangular aperture in 0.10 mm foil has a nominal cavity volume of 0.015 mm鲁. A measured deposit of 0.0135 mm鲁 is 90% of that reference. This arithmetic does not decide acceptance: the approved window, position and deposit distribution still need review.
Continue to review stencil thickness and aperture decisions.
The email link opens a draft for you to complete. Contact is a separate form; these pages do not send messages automatically.
Questions engineers and buyers ask
Does SPI inspect solder joints after reflow?
SPI normally evaluates printed paste before component placement. Post-reflow joint inspection is a separate task involving optical, X-ray or other appropriate methods.
What is a good SPI volume percentage?
Use the approved reference definition and product-specific process limits. There is no single percentage window on this page that applies to every package, stencil and paste combination.
Can area and height replace measured volume?
Not generally. Irregular deposits require integration of the measured surface. Multiplying footprint by maximum height is a bounding approximation, not an SPI volume measurement.
Can SPI prevent every solder defect?
No. Placement, thermal processing, component condition and other factors still influence assembly quality after paste printing.
Turn the inspection question into an agreed scope
Send the current files, product revision, quantity and the requirements that matter to your application. Identify any unfinished criteria so the engineering review can address them explicitly.
Request the method, coverage, acceptance basis and report format with your quotation. Tool briefs can support the discussion; the Contact form does not automatically receive this page鈥檚 entered data.
Engineering references reviewed September 2026. Illustrations explain methods; equipment photographs do not establish product-specific coverage, inspection results or certification.