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PCB & PCBA X-Ray Inspection Services
SUGA provides X-Ray inspections of solder joints that are not visually accessible due to obscuring parts in an original equipment manufacturer (OEM) printed circuit board assembly (PCBA) build. The image data is reviewed and analyzed based on defined acceptance criteria and marked high-risk locations to support engineering and quality decisions before quote preparation.
Hidden-Joint Inspection
Defined Review Basis
Marked Image Evidence
OEM Review Support
Hidden Solder Joints Create the X-Ray Inspection Need
A PCBA unit can pass normal visual inspection, but solder joints can remain hidden from view underneath the package. In such cases, the inspection serves as documentation used to establish whether the solder-joint areas can be visually inspected or whether a density-based X-Ray method is needed because the solder-joint area is not visible from above.
Optical Access Sets the First Limit
Generally, the primary method for evaluating a solder joint through optical access includes visible solder on the surface, component position or placement, component polarity, and any visible colors or markings. If the solder joint itself is located under a ball array, beneath a bottom-termination package, beneath a shielded area, or under overlapping components, the ability to evaluate what is being soldered does not always exist.
Hidden Interfaces Change the Inspection Decision
X-Ray inspection of a PCB assembly does not serve solely to show the external cosmetic appearance of a component. It provides image data supporting identification of possible solder-joint issues where additional review is needed based on density patterns detected in the record. The resulting record can support engineering or quality decisions, but it does not replace electrical or functional testing.
When Hidden Structures Need X-Ray Review
X-Ray inspections are most appropriately conducted at a solder joint, termination, or target location where visual evaluation of the joint cannot occur from the external side of the component. Typical target locations for X-Ray inspection of electronic components include ball grid arrays (BGA), Chip Scale Packages (CSP), Land Grid Arrays (LGA), Quad Flat No-lead (QFN), Dual Flat No-lead (DFN), other Bottom Termination Components (BTCs), shielded areas, or overlapping double-sided layouts. If the inspection is not conducted at these locations, an external visual evaluation of the PCBA can appear acceptable. Until the joint condition has been reviewed using X-Ray evaluation, the actual condition of the solder joint remains unknown.
Hidden-Joint Structures Reviewed by X-Ray
| Package / Structure | Optical Access Issue | X-Ray Review Target | Record Type |
|---|---|---|---|
| BGA or CSP | Ball array blocked by package body | Void, bridge, open-risk signal, ball shape | X-Ray image record |
| QFN, DFN, or BTC | Thermal pad and bottom termination not fully visible | Void, solder spread, insufficient wetting indicator | Component-location image set |
| LGA package | Bottom-side solder interface hidden after placement | Joint shape, alignment, open-risk signal | Location-based image record |
| Shielded components | Solder area blocked from optical view | Hidden solder condition | Defined X-Ray image view |
| Dense double-sided PCBA | Overlapped solder features from top and bottom sides | Side-dependent image interpretation | Marked image or review record |
| Post-reflow suspect area | Hidden solder condition after reflow | Bridge, void, non-wet or open indicator | Decision review record |
These package structures are common evaluation triggers for X-Ray inspection, but the required inspection depth will depend on the package structure, target location, PCB side, and record requirement.
Ball Arrays and Bottom-Side Interfaces
BGA and ball-array package solder joints are located below the respective package body, so surface inspection can verify the placement outline but will not provide a direct view of the solder interface below the component. An image is sometimes needed to assess whether more investigation is warranted for the hidden solder area.
QFN, DFN, and Other Bottom-Termination Packages
QFN, DFN, and BTC package solder locations may be partly or completely hidden below the package body and therefore require image-based evaluation when the issues relate to bottom-side wetting, solder continuity, or questions regarding the post-reflow condition.
Shielded or Optically Blocked Areas
Some solder areas require X-Ray evaluation because the view of solder joint locations has been physically obstructed by a shield, enclosure, or surrounding structure. In these cases, the primary concern is not whether the solder joint is hidden behind the body of the package, but that there is limited physical access to view the solder joint.
Dense Double-Sided Layouts with Overlap Risk
In dense double-sided assemblies, overlapping board features can complicate interpretation of X-Ray images. The placement of component bodies on the top or bottom side of the same board location, when located too close to one another, requires the reviewer to use additional layer layout information, marked locations, and image angle views to avoid misinterpretation of the result.
Marked Locations for Focused Review
Marked critical areas allow SUGA to focus on the package or locations most important for the X-Ray review. When critical locations are marked in advance, SUGA can avoid reviewing the wrong locations or areas, particularly in dense layouts where image interpretation can be difficult.
Image Signals Behind Voids, Bridges, and Open-Risk Indicators
X-Ray solder joint inspection signals for voids, bridging, and open risk indicate that an image can provide a meaningful representation of an area that was hidden but becomes visible as a pattern interpreted against its package and circuit context. The function of the image is not to develop an extensive list of defect names. It is to distinguish routine density variation from signals that affect risk priority, review depth, or decision timing.
Density Changes Inside the Solder Mass
Density variation within the solder mass suggests possible voids or insufficient solder. A single image does not define how many voids or how much solder is present, but repeated localized density variation helps show how solder is distributed and how the solder location can influence void judgment, process reporting, or review priority.
Density Continuity Across Adjacent Hidden Pads
Where density appears to be continuous across two adjacent hidden pads, the image can show a condition where bridging risk is possible. The review should focus on whether the visible density pattern is located between pads that should remain separate, whether it is isolated or repeated, and whether net context or electrical test results are needed to assess short-circuit risk before a decision is made.
Irregular Collapse or Interface Separation
Some hidden solder joints exhibit irregular collapse, an unusual ball shape, or signs of partial interface separation. The primary concern for these joints is the uncertainty of the solder interface rather than an aesthetic surface issue. Therefore, these signs should not be taken as conclusive proof of a final electrical outcome or final judgment, but they can be used as warning signs for additional review or investigation.
Offset Patterns and Isolated Solder Features
A shifting package can create an offset pattern between the expected pad area and the visible solder image. Isolated solder features suggest stray solder balls or unexpected movement of solder near hidden interfaces. These findings help determine whether the next area of concern is alignment, clearance, or a local solder anomaly before closing the assessment.
Image signals can assist in identifying the anomaly. Still, defined acceptance criteria, circuit information, or additional verification are needed before final judgment on the solder joint.
What X-Ray Findings Need Before Acceptance Decisions
For an inspection team to use an image finding as an actionable determination, the finding and accompanying image signal must relate to a defined decision basis. This basis can include drawing notes, approved quality plans, or IPC-defined acceptance criteria. A complete acceptance decision can require circuit net information, electrical test results, computed tomography (CT), or further physical verification.
Defect Signals X-Ray Can Identify but Not Always Confirm
| Defect Signal | Likely Location | Image Indicator | Acceptance Basis | Decision Limit |
|---|---|---|---|---|
| Solder Void | BGA ball or QFN thermal pad | Low-density area inside solder mass | Approved drawing, quality plan, or IPC basis | Void size and location need defined acceptance criteria |
| Bridge Risk | Adjacent hidden pads or balls | Continuous solder density across adjacent joints | Image review with net context; electrical correlation when needed | Electrical correlation when needed |
| Open or non-wet signal | Ball-to-pad or pin-to-solder connection | Irregular collapse or separation indicator | Oblique imaging or CT review when needed | X-Ray can flag risk, not prove circuit behavior |
| Head-in-Pillow | BGA solder ball connections | Separated or waist-shaped joint indicator | Angled X-Ray or CT review when needed | Additional verification when needed |
| Package Shift | BGA or QFN underside | Ball-to-pad offset pattern | Assembly drawing/quality plan | Decision based on drawing-defined limits |
| Missing or insufficient solder | Hidden termination or bottom pad | Weak or incomplete solder density at the package-to-pad connection | Must match actual pad and component geometry | Decision tied to pad and component geometry |
| Foreign Solder Ball | Under or near package body | Isolated solder density | Image location and net-risk context | Risk determined by solder ball location and clearance |
| Suspicion of an Internal Interface | Hidden joints or internal package structures | Contrast discontinuity | Engineering assessment or inspection record | Failure analysis or cross-section when confirmation is needed |
The next step is to identify what the finding affects, which acceptance criteria apply, and which record will support the decision.
Void Judgment Needs a Defined Limit
A void signal can only be acted upon if there are defined limits for its size, location, or acceptance. The same image pattern could have different evaluations based on package type, solder-joint function, or other requirements. SUGA can use the record as a basis for review, but the acceptance decision must be based on defined acceptance criteria rather than a universal void rule.
Bridge Risk Needs Circuit Net Information
If there is a continuous density pattern between adjacent hidden pads, it suggests a bridge-risk location. The next action is to confirm the circuit net data to determine if the pads are on separate electrical nets or part of a permitted connection. When the image pattern is ambiguous, electrical correlation is sometimes necessary before treating the condition as a final circuit concern.
Open-Risk Indicators Need Functional or Electrical Check
Interface separation, irregular collapse, and head-in-pillow indicators signal open risk. The image can show that an isolated location needs to be addressed, but the image alone does not validate that the assembled board will operate as intended. When there is a concern regarding intended performance, functional or electrical test records may be required.
Destructive Proof Requires a Confirmed Scope
Cross-section review or failure analysis can provide physical proof of a condition when the image does not validate the finding and other methods are not conclusive. Because cross-sectioning and failure analysis can be destructive and outside normal review depth, these methods should be confirmed before use. The specific nature of the finding will determine which image view or verification method is used.
Matching the View to the Review Need
Various types of hidden-joint findings can require different image views. A top-down view can provide a quick way to identify density discrepancies in hidden joints, but angled or CT views are sometimes needed when overlapping joints, interface suspicion, or ambiguous image patterns affect the decision. Automated X-Ray inspection (AXI) for PCB assemblies is most useful when target locations, inspection criteria, expected records, and review parameters have been determined before performing AXI.
Imaging Method Selection for Hidden Solder Joints
| Imaging Method | Best-Fit Question | Record Produced | Constraint |
|---|---|---|---|
| 2D X-Ray | Is there a visible hidden-joint density issue? | Top-down density image | Overlap can mask interface detail |
| Oblique X-Ray | Is the joint shape or interface suspicious? | Angled solder geometry | Access and package structure matter |
| Automated X-Ray Inspection | Does a repeated hidden-joint area need program-based review? | Program-based image capture | Requires agreed target locations |
| 3D X-Ray or CT | Is overlap blocking interpretation? | Slice or reconstructed image | Not default for every lot |
| Electrical test correlation | Does the image signal relate to a short or open risk? | X-Ray image plus test result | X-Ray image is not a circuit test result |
| Cross-section escalation | Does an unresolved internal anomaly need physical proof? | Physical section record | Destructive; only when confirmed |
The chosen verification method must match the identified finding. CT, cross-section, and electrical correlation are not default test methods; they should be used only when the image question cannot be closed using a simpler technique.
Top-Down Density for First-Level Screening
The top-down X-Ray view provides first-level review of hidden solder areas. Unless the image has excessive noise, the first-level top-down review can be enough to decide whether a second-level review is needed, without immediately exploring other methods.
Angled Views for Interface Suspicion
If the area in question is not only the solder joint but also the joint shape or interface from another view, angled review can help determine whether separation, collapse irregularity, or overlap can be resolved beyond the top-down review.
Program-Based Capture for Repeated Areas
AXI can be used for program-based capture of repeated inspections of the same area where a defined inspection program is used, provided that both the inspection program and the intended review scope are well defined. Programmed inspections should be used for defined inspection programs, not as blanket inspections across all assemblies. Program-based capture requires predetermined target locations, review parameters, and reporting scope.
CT or Sectioning for Unresolved Overlap
CT or sectioning can be used to verify overlapping structures or interface anomalies that cannot be resolved with the standard view. Additional clarification is sometimes required to assess whether either method fits the required resolution. When additional views are required to resolve overlapping structures, the selected method should provide the information needed for the OEM review decision.
X-Ray Records Used in OEM Review Decisions
When X-Ray records are linked to the component location, a visible risk flag, and a mutually accepted reference for the reviewed image, those records are easier to use in the OEM review decision process. They support conversations with the OEM because they identify what was reviewed, where the anomaly is located, and what outcomes have yet to be established.
Marked Images for Location-Based Review
Marked records help identify the board location, component reference, package type, and visible pattern together in the same image. If a void is present at a hidden solder joint, a vague statement such as “void present” does not inform the reader where the void exists. Marked images make it easier to verify the review because the location, package type, and visible pattern are shown together.
Risk Flags for Engineering Decision
When there is a void, bridge-risk, open-risk, offset, or isolated solder-feature signal, the record can be marked to indicate potential risk. Quality records made during the review document the difference between what was observed, accepted, rejected, and forwarded for further verification. The image illustrates why the solder joint requires further review; the final determination will be based on acceptance criteria, circuit requirements, or other documented records.
X-Ray Records Used for OEM Review Decisions
| Record or Input | Used To Decide | Required Basis | Record Format | Missing Risk | Next Action |
|---|---|---|---|---|---|
| Hidden-joint image record | BGA or QFN acceptance review | Approved drawing or quality plan | Marked image record | Unclear acceptance or review basis | Engineering decision |
| Void image record | Thermal pad or BGA void review | Drawing, quality plan, or IPC-based limit | Void review record | Void judgment not tied to acceptance basis | Process feedback when repeated |
| Bridge-risk image | Short-risk screening | Net context and package location | Risk flag | Image signal not linked to electrical risk | Electrical test correlation |
| Open-risk image | Intermittent or non-wet concern | Failure symptom or inspection plan | Suspect joint record | Hidden joint concern remains unresolved | Oblique, CT, or FA escalation |
| BOM or CPL package list | Hidden-joint scope planning | Package type and reference designator | Package risk list | BGA, QFN, LGA, or CSP scope missed | Hidden-joint parts identified before review |
| Gerber, ODB++ layout data, and assembly drawing | Location and side confirmation | Current design file and approved drawing | Inspection location map | Wrong-side or wrong-area review | Current revision and critical-area confirmation |
| Reported review coverage | Review documentation | Agreed review scope | Coverage summary | Report does not support the required review | Clarify coverage before the next lot |
Covered and Not-Covered Scope for Review
A defined review scope is necessary when a coverage summary only includes selected areas, selected packages, or selected suspect areas. The coverage summary helps distinguish between local review results and full-board inspection coverage, while providing applicable information for OEM decisions. The reporting requirements and parameters also help define the review scope before quote preparation.
What Affects an X-Ray Inspection Quote
Submissions can have similar descriptions, such as X-Ray inspection, yet require different levels of effort due to the number of hidden-joint locations, the amount of image evaluation required, and the documentation required after image review. These factors define the quote basis before approval.
What Changes the Review Effort
When the inspection changes from reviewing a limited number of similar hidden-joint locations to varying package types, side-dependent locations, or image quality requiring angled verification views, the review effort changes. Adding similar site locations can change the inspection coverage area, while modifying the inspection procedure has a greater impact on how the work is planned.
To define the review scope, three factors must be considered: the total number of site locations requiring X-Ray inspection for image evaluation, the inspection level needed, and whether any unresolved anomalies need to be verified. Using the inspection requirement rather than a general description gives the discussion a more direct link to the inspection itself.
What Changes the Report Output
The expected record type affects the quote basis. If the submitted requirements include marked images, risk notes, or a coverage summary, the documentation requirements need to be more clearly defined than for a limited image review.
All expected deliverables from the inspection and the associated results should be included in the submitted information. When target locations or expected records are unclear, SUGA may need to ask additional questions before providing a quote.
Files That Define the X-Ray Review Scope
When SUGA generates marked images, risk notes, and review records, there should also be sufficient information to establish the scope of the solder joint, package area, or suspicious area inspection. Proper identification of the inspected areas, board sides, board revisions, and expected records establishes the review scope and supports quote preparation.
BOM and CPL for Hidden-Joint Parts
SUGA uses BOM and CPL reference designators to identify parts such as BGA, CSP, LGA, QFN, or DFN. SUGA aligns the BOM with the CPL to obtain a more targeted review based on package-level risk rather than evaluating the assembly as a whole and potentially missing a target package during the review.
Gerber or ODB++ for Board Location
If SUGA performs review only for selected targets or areas within the assembly, Gerber or ODB++ layout data can help limit the chance of misinterpretation, mismatched side selection, or unclear top-side and bottom-side features in a single image.
Assembly Drawing for Orientation and Side
Assembly drawings provide specific details for orientation, side, component location, polarity markings, and critical component placement. This information allows SUGA to relate the image back to the actual board position when evaluating critical components, hidden areas, or suspicious areas.
Inspection Notes for Expected Records
Inspection notes define the expected records. Expected records include a standard image inspection, marked image record, detailed void evaluation, risk statement, or coverage summary. SUGA needs to understand the expected records so the review type can be identified as a limited image review or a broader OEM review record. Including BOM and CPL, layout data such as Gerber or ODB++, assembly drawings, and inspection notes helps establish the review scope and reduce excessive back-and-forth communication.
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PCB X-Ray Inspection FAQ
PCB X-Ray inspection captures images of solder joints or designated target locations where optical viewing is limited or obstructed by overlapping components, shields, or board structures. PCB X-Ray inspection allows identification of hidden solder joints or solder-related defects in the PCB assembly. However, PCB X-Ray does not verify electrical or functional circuit operation, since these tasks require additional testing.
X-Ray review provides imaging and evaluation of soldered areas that would not otherwise be visible through optical inspection alone. PCB X-Ray creates image-density contrast between solder, pads, components, and PCB structures. This enables review of solder-mass inconsistencies, possible bridging, incorrect component placement, or solder-joint interface discrepancies. Clear images can support decision-making, while unclear images may require electrical testing, circuit information, or physical verification.
PCB X-Ray inspection can support review of image characteristics related to voids, insufficient solder volume, possible bridging, open-risk clues, component misalignment, and isolated solder features. These are image characteristics, not definitive defect classifications by themselves. Handling may vary depending on where the finding is located, what acceptance criteria apply, and how the part functions electrically.
Yes. PCB X-Ray helps identify solder-joint concerns related to BGAs because the image can show solder-density patterns in areas where voids exist, how solder is distributed, whether bridging risk is possible, and whether BGA collapse or interface concerns are visible. These observations are made through the image rather than direct visual access to the underside of the package.
After reflow, X-Ray inspection is used to identify hidden solder conditions that are not visible from the package surface. Common indicators include voids, bridge-risk signals, irregular solder collapse, possible interface separation, placement offset, and isolated solder balls. This information helps indicate what requires action before a part is accepted, escalated, or correlated with another verification method.
PCB X-Ray inspection is most effective when the solder interface or target area is not optically visible. Common examples include ball grid arrays, bottom termination components, shields, and densely populated double-sided layouts with overlapping components. Suitability is influenced by package type, board side, target location, and review purpose, so not all components require the same level of inspection.
No. PCBAs are tested functionally or electrically, while X-Ray inspection produces image records. An image can flag concern with a given solder joint, but it does not confirm that a circuit will function. The two checks can support the same quality decision, but they measure different types of data.
X-Ray inspection results can provide OEMs with information regarding visible risk and highlighted images for areas where additional inspection may be necessary. Although X-Ray records can support quality decisions, these records cannot be used alone to determine whether an OEM will accept an item unless a defined acceptance basis has been established.
Related Inspection and Testing Services
Different inspection or testing needs require different types of inspections or tests. These options help match quality concerns with the appropriate review or test approach. X-Ray inspection can be one part of the overall quality plan.
PCB & PCBA Inspection Methods
An engineer who needs to compare inspection methods can use SUGA to compare SPI, AOI, X-Ray, FAI, and visual inspections instead of using one method as the only source for inspection or testing decisions.
3D Solder Paste Inspection
3D solder paste inspection is used to confirm solder paste characteristics such as volume, height, area, or offset before component placement. This refers to the printed solder paste condition before reflow, not the hidden solder-joint condition after reflow.
3D Automated Optical Inspection
3D AOI inspection is used to evaluate visible solder, component position, polarity, marking, height, or tilt from surface image records. It does not replace X-Ray review when the main concern is a hidden interface.
First Article Inspection
First Article Inspection is used to determine whether the first assembled PCB matches the agreed build information. If the main concern is hidden solder geometry, that would not be the primary intent of FAI.
Visual Inspection
Visual inspection is used to evaluate external appearance and workmanship quality where the product characteristics can be evaluated without X-Ray access.
PCBA Testing
PCBA testing is used to confirm whether the assembled board powers up, communicates, and functions as designed. Test records are separate from X-Ray inspection image records.