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3D Solder Paste Inspection
SPI Inspection Before Component Placement
SUGA uses a 3D solder paste inspection system, with board-specific settings, to check the quantity, shape, and location of the solder paste immediately after application, before component placement on the board.
Board-Specific SPI Program
Paste Volume, Height, Area, Offset
Print-Stage Paste Review
SPI Review Scope Before Placement
After Solder Paste Printing
During SMT 3D solder paste inspection, each solder paste deposit — volume, height, area, and offset — is directly measured rather than relying on solder-joint performance after assembly.
Paste-Stage Timing and Downstream Risk
SUGA’s SPI is used as a solder paste-stage inspection and, therefore, applies to paste-stage inspection rather than solder-joint inspection. Identifying solder paste risks through SPI enables corrective action prior to making solder joint-level decisions. After assembly, visual/AOI checks, X-Ray checks, FAI, or electrical test results are usually not an effective means of determining whether the root cause condition occurred during solder paste printing.
Customers may receive a determination regarding a paste quality matter based on an assessment made at the solder-joint level, X-Ray image, or verification record — all of which do not identify the failure’s source as related to the paste stage.
Program Flags and Print-Side Signals
SPI can flag issues related to solder paste geometry, alignment, bridge risk, or insufficient paste based on measurement parameters used during the measurement process. This measurement helps indicate if action is needed or if the result should be held for pattern confirmation.
Paste Volume, Height, Area, and Offset in the SPI Program
The main SPI measurements help SUGA detect variation on each solder pad. The volume, geometry, and location of the paste application on a PCB allow SUGA to identify both normal and abnormal measurements and determine whether an outlier should be further evaluated.
SPI Measurement Items
| SPI Review Item | Reference / Target | Measurement Method | Process Input | Record / Limit |
|---|---|---|---|---|
| Paste Volume | Pad-geometry-defined volume target; Cpk target when included | 3D volume measurement | Stencil opening; paste type; pad design | Measurement record; SPC chart if requested |
| Paste Height | Height target; stencil-thickness-dependent | 3D height profile | Stencil thickness; print setting | Program limit |
| Paste Area | Area limit | Area and volume algorithm | Opening shape; pad geometry; stencil wall performance | Area deviation trend |
| Paste Offset | XY offset limit; fiducial-based reference | Paste-to-pad alignment map | Fiducial condition; printer setup | Printer feedback log if connected |
| Bridge Risk | Bridge or overlap condition | 3D topology map | Compact rows; local spacing; paste spread tendency | Review, reprint, or agreed process action |
| Insufficient Paste | Low-deposit threshold | Volume threshold check | Opening release; stencil cleaning status | Reprint or engineering attention when needed |
| Slump or Smear | Shape abnormality | 3D paste shape check | Paste state from released print process | SPI image or defect record |
| Inspection Coverage | Pad coverage | Inline SPI before placement when included | Board size; panel layout; alignment marks | Program coverage record |
| Printer Window | Printer setup values from released print process; SPC when applied | SPC chart; offset compensation if connected | Stencil opening; paste state; printer setup | Printer setup record; agreed limit |
SPI limits are not universal defaults like those found in a generic assembly specification. SPI limits depend on four inputs: board files that define the geometry of the individual pads on the board; stencil data that defines how the stencil openings behave during the printing process; solder paste specification that defines the material characteristics of the solder paste; and the defined pass/fail criteria. Together, these inputs help clarify if the measurements are local flags, broader print-process signals, or items that may affect pricing.
Core Geometry Measurements
Core geometry values should be reviewed collectively, not as individual numbers. Low or fluctuating amounts of solder paste deposited can quickly shift the next question to opening release, material handling, or stencil wear. Offset is more closely associated with alignment marks, board support, printer setup, and panel stability. Therefore, the measurement is not just a number but a pointer to possible areas in the print process that can require further attention.
Paste-Level Risks
The print stage can still show bridge risk, insufficient paste, paste slump, and paste smear. Signals for corrective action should separate one-time readings from repeatable patterns based on the same part and materials over a longer evaluation period.
SPI Signals That Trace Back to Printing
SPI data becomes decision-ready when at least one of the following steps has taken place: ruling out process setup factors before making a change to the print process; or checking if the same signal crosses from a local measurement into a repeatable pattern across more than one print run.
SPI Signals and Action Thresholds
| SPI Signal | What It May Indicate | Typical Threshold for Wider Check | Practical Follow-Up | Record |
|---|---|---|---|---|
| Single low deposit reading | One opening or one first-pass reading may be an isolated reading rather than a stable print problem. | Hold as a single occurrence unless it repeats in the same opening group or nearby feature set. | Compare against paste handling, opening release, and first-pass setup before changing parameters. | Measurement note if needed |
| Low deposit pattern in the same opening group | The issue could be moving beyond one opening toward material, stencil, or clean-cycle condition. | Expand review when the same group repeats across boards, panel positions, or production checks. | Move from a local opening check to stencil wear, paste condition, clean-cycle timing, or squeegee condition (blade pressure or wear). | SPI trend or SPC record if requested |
| Offset in the same board area | Printer compensation may not be the whole answer when the same area shifts repeatedly. | Broaden the check when the same row, panel area, or alignment reference repeats after compensation is checked. | Check fiducial recognition, panel stability, support, and rail setup before further printer adjustment. | XY map; printer feedback log if connected |
| Bridge-risk pattern in tight pad areas | Paste spread, local clearance, or cleanliness may need focused attention. | Check further when flags cluster around the same clearance area instead of appearing randomly. | Compare pad geometry, local spacing, paste spread, and if the flag is isolated or repeated. | SPI image or defect record |
| Height or area variation | Support, opening behavior, or paste state could be affecting deposit shape. | Extend the check when variation follows board edge, support location, or the same feature group. | Check board support, stencil wall response, paste condition, and opening response. | Height profile or area trend |
| False-call pattern | The program window could be too tight for the board state being read. | Reassess when acceptable deposits are repeatedly flagged in the same board area or feature type. | Check threshold setting, program validation, or inspection window. | Program validation record |
| Capability trend input | Process capability needs discussion when Cpk is part of the agreed requirement. | Use only when capability tracking is requested or included in the inspection record. | Treat as process-capability input, not as an automatic stop or reprint rule. | SPC record; action log if used |
A one-time reading should be separated from a recurring pattern before the next action is selected: a local check, a wider print-side investigation, or a program-window question.
Signal Context Before Process Changes
Not every SPI flag requires action as soon as it is noted. Compare one offset reading to the program window, the alignment setup, and the fiducial recognition before taking action. If an early board has a low deposit amount, or if a low deposit amount appears shortly after a change in material-handling practice, then assess if the issue came from setup or handling. If there is variation in height at the edge of a board, support setup should be evaluated before changing printer parameters.
Repeated Signals Before Action
A repeated signal moves the issue from localized review to broader process investigation. A single weak opening may remain an isolated occurrence; however, if the same pattern occurs within a group of pads, the investigation should include other pads and expand to broader printing factors. A repeated offset within the same board area moves the issue from one compensation value to questions concerning fiducial stability, panel handling, or support consistency.
Offset and Alignment Patterns
Offset is different from volume or bridge-risk signals because the cause may not be located only in the printer, stencil, or paste chain. Once printer compensation has been ruled out, the next items to check are the fiducial quality, panel accuracy, PCB dimensional stability, and verification of the support-pin position and rail setup (conveyor edge guides). For tight clearance or very dense areas, changing only print parameters may not fix alignment-driven patterns.
A Typical Print-Stage Scenario
A common print-stage example is a 0.4 mm pitch QFP area that consistently has lower volume readings at the same row of openings on early production runs. The first inclination would be to adjust the printer parameters; however, you would likely get a better indication of whether the lower readings were due to process factors if you compared the flagged row to the stencil-opening geometry, the paste’s ability to release from the stencil wall, and the timing of the cleaning cycles. If the same identified row improves after reducing the cleaning-cycle time, the cause of the lower volume reading is more likely a printing-related factor than a defective joint at the end of the soldering process.
Cpk as a Review Input
Cpk can support common discussions about process capability as long as Cpk is part of the defined requirement. Cpk cannot be used as an automatic stop-or-reprint signal unless production has defined a response to that signal.
Choosing SPI, AOI, X-Ray, FAI, or Testing
Overlapping Inspection Needs
There is often a need for more than one inspection type for a given board. A board with 0201 passive areas and a hidden BGA package could require SPI for the paste amount, AOI for the visible joint surface, and X-Ray for the hidden BGA package. The goal is not to create unnecessary duplication of records; each inspection should stay tied to the associated question.
Common Mismatches
Concerns related to paste amount should not treat post-reflow AOI as the only inspection; AOI detects the joint result rather than the initial paste deposit. FAI should not be governed by SPI; it relies on BOM, drawing revision, and approval records.
How SUGA Reads SPI Results
After identifying a signal, the next concern is whether the signal can be interpreted through a verified 3D SPI system. A recognized machine still needs proper input to produce meaningful output; without it, it may over-report a healthy deposit or miss a true directional change.
SPI results should be read together with the actual board, panel layout, alignment marks, stencil data, program settings, and reporting expectations. A 3D SPI system supports 3D paste measurement, but exact model details, speed, field of view, resolution, and board-size values should only be used when verified. Compact QFP rows or other aperture-sensitive areas may need closer program and stencil attention, but they should not be presented as default capability promises.
Equipment Brand Is Not the Whole Result
A 3D SPI machine can support repeatable manufacturing measurements; however, the machine brand does not fully define the quality of the inspection.
Equipment and Program Settings
Measurements using verified equipment and defined inspection parameters apply to a project when a 3D SPI system is configured for the production line. The program defines the coverage of various pads and the fiducial locations used in the inspection.
Planning Values Need Context
The program, stencil basis, alignment reference, and confirmed reporting scope should match those of the actual board for a meaningful assessment.
Reference Values for 3D SPI Planning
These reference values, such as Cpk 1.33 / 1.67, stencil thickness of 80-150 µm, and 0.4 mm pitch / 0.5 mm pitch, are to be used for planning purposes and not to be construed as acceptance criteria, requirements, or definitive process settings.
Model-Specific Equipment Details
Before model-specific specifications are used in a project, verify that the model name, speed, resolution, field of view, or associated board size has been identified in the verified equipment record. If a request for an in-line 3D SPI system is made, SUGA will confirm that the line setup is properly described as part of that request.
Board Conditions Affecting SPI Scope and Cost
Various board conditions can change how SPI scope and cost are defined. Complexity levels help identify project scope, but the final scope is still based on submitted project files, the line setup, and project reporting requirements.
Standard, Medium, and High Complexity Conditions
Standard boards usually include a single board or simple panel, clear fiducials, normal pad spacing, and no special reporting request. Basic paste-stage review is typically enough when the SPI program can read the main deposits clearly.
Medium-complexity boards can include compact QFP rows, multi-panel layout, local sensitive deposits, or board-edge areas that need additional attention. These factors can require feature-level review, clearer coverage expectations, and confirmation of alignment or support requirements.
High-complexity boards can include mixed pitch, weak fiducials, repeated findings, SPC requests, or added reporting requirements. These factors can change setup review, documentation, reporting, and pricing.
Files Needed for SUGA SPI Fit Review
Prepare the following files so SUGA can review SPI fit with fewer follow-up questions.
SPI File Submission Checklist
| Submission Item | Status | File / Format | Why It Matters | If Missing |
|---|---|---|---|---|
| Board files | Required | Gerber or ODB++ | Supports pad geometry and program setup. | SUGA will need the files before confirming SPI applicability. |
| Panel layout | If applicable | Gerber, ODB++, or PDF drawing | Affects board fit and alignment review. | Panel-related setup cannot be confirmed. |
| Fiducial locations | Required when SPI applicability is reviewed | Gerber or drawing mark-up | Supports alignment checking. | Alignment risk may need follow-up. |
| Stencil data | Recommended | Stencil drawing or aperture file | Supports opening and release review. | Deposit findings may be harder to interpret. |
| Solder paste specification | Recommended | Material specification or approved paste note | Helps read material-related deposit characteristics. | Material handling context may need follow-up. |
| BOM or assembly drawing | Recommended for pricing | XLSX, CSV, or PDF | Helps match component package areas with inspection concerns. | SUGA may rely on marked board files instead. |
| Sensitive feature mark-up | If applicable | Marked PDF, screenshot, or drawing note | Shows areas that need closer site-specific attention. | SUGA may ask which features should receive separate review. |
| Reporting expectations | If applicable | Email note or quality requirement | Defines SPC, image, record, or equipment-detail needs. | SUGA may need follow-up questions before pricing. |
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How SUGA Checks Submitted Files
To verify that the submitted documents have been received correctly, SUGA checks them against the SPI setup discussion for board size, panel layout, and alignment marks, and reviews the control information specified in the stencil inputs, sensitive features, and reporting requests.
SPI Fit Confirmation After File Review
SPI fit confirmation will be included after the file review, in addition to any recommendation concerning action needed for the stencil or board conditions in question. SUGA can also clarify which stencil or board conditions are of concern and indicate whether AOI, X-Ray, FAI, or testing reports should be treated separately.
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SPI Inspection FAQ
When measurement of solder paste volume, dimension, or position within specified constraints is needed, such as within tight clearance or at a particular position, SPI technology can provide verified measurements of solder paste deposits.
The process involves stencil printing, SPI, result evaluation, and then component placement. If SPI detects a problem before placement, then it may indicate a one-time event, a localized area of the board, or a pattern of suspected solder paste deposits requiring further investigation.
SPI technology can detect excessive or insufficient solder paste deposits, bridge risk, offset or position discrepancies, solder paste displacement, or slump. When reviewing flagged defects, check if the defect appears again in a tight-clearance area or across multiple panel positions. This review can help confirm which board area may be affected and support the finding.
There is not a single IPC standard that applies to every SPI measurement. Some exceptions apply when the project documentation, contract, or other quality requirements specify a deposit limit in writing. Such specifications should be reviewed with the project documents.
It is helpful to have indicated SPI volume, height, area, and offset, but each measurement must be considered in conjunction with its relative location to the printed feature group and other local features or reporting criteria that could apply.
If the first flag for an identified print defect was isolated, follow-up on the same pattern would be required to clarify if the reason for the defect was material handling, stencil state, setup, support, or alignment.
SPI data does not necessarily eliminate instances of rework after a reflow process, but it may help decrease future rework by providing a means to detect an inappropriate deposit pattern during the print process, assuming the PCB design and process allow examination at the SPI stage.