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4.3 AXI Techniques

Automated X-ray inspectionInspection uncovers(AXI) is a non-destructive testing method that uses X-ray technology to penetrate components and the hiddenPrinted worldCircuit beneathBoard BGAs,(PCB), QFNs,generating images of internal structures and plated-throughhidden holes,solder joints. AXI is mandatory for products featuring Ball Grid Arrays (BGAs), Quad Flat No-leads (QFNs), and other area-array or bottom-terminated packages, where the critical connections are invisible to standard optical inspection and(AOI). in-circuitThe testinvestment can’t reach. By visualizing collapse, voiding, and hidden defects,in AXI providesis anjustified inside look at solder joint health that directly links to long-term reliability. The choice of method—fast 2D, angled 2.5D, or detailed laminography—balances speed with clarity, ensuring that every board getsby the rightrequirement levelfor ofsuperior scrutinyreliability withoutin chokingsafety-critical throughput.industries When(e.g., pairedmedical, withautomotive, calibrated setups, clear acceptance limits, and MES-linked decision rules, AXI transforms invisible risks into quantifiable, traceable data that engineers can act on.aerospace).

4.3.1 AXI invs. oneAOI: minuteThe (whyHidden weDefect bother)Trade-Off

Automated X-ray inspection sees hidden joints—BGAs/CSPs/WLCSPs, QFN thermal pads, stacked packages, even plated-through holes. AXI answersis threenot questionsa fast:

replacement
  1. Isfor something there? (missing balls, misaligned packages)
  2. DidAOI; it wet? (collapse/shape for BGAs, edge fillets on QFNs/LGAs)
  3. How gassy is it?a (voidingcomplementary percentagetechnology andused pattern)
to

Usecover AXIstructural wheredefects that AOI iscannot blind and ICT can’t reach.detect.



4.3.2 The three flavors (speed vs detail)

Mode

How it works

What it’s great at

Trade-offs

2D (single projection)

One top (and/or bottom) shot

Fast inline screening: missing balls, bridges under BGAs, big voids in QFN thermals

Overlap/“stacking” hides detail; copper shadows can trick it

Oblique / 2.5D

Slight angle sweep, a few projections

Better at spotting lifted balls, skewed rows, large void pools

Slower; still some overlap artifacts

Laminography (planar CT)

Many angled shots → reconstruct a slice at one height

Separates layers: reads QFN thermal pads cleanly; inspects one BGA plane without neighbors

Slowest inline; often used as sampling or offline root-cause

Rule of thumb: inline 2D/2.5D for every board, laminography for the tricky few (NPI, audits, or when a defect spikes).



4.3.3 BGA health: collapse, voids, and HIP clues

  • Ball collapse: good joints look like “hourglasses” (thinner middle, bonded at the pad). Flat/round “marbles” suggest no wetting or cold joints.
  • Void limits: set per ball and per package. Typical starting points (tune to customer spec):
    • Per-ball void area: ≤25% (flag >25–30%).
    • Cluster shape matters: one big void near the intermetallic interface is worse than many tiny ones.
  • HIP (head-in-pillow) signs: halo or “cap and ball” with a dark seam between paste and sphere, often non-collapsed center balls. If HIP rises, revisit TAL/soak and consider N₂ (see 9.3/9.5).



4.3.4 QFN / LFPAK thermal pads: measure what matters

  • Use laminography or at least oblique views to isolate the pad plane—2D often lies when copper stacks up.
  • Judge both total void % and pattern: lots of tiny, spread-out voids are kinder than one giant bubble under the die center.
  • Typical starting guardrails: total void ≤25–35% (customer-specific). If you’re higher: check windowed apertures (7.4) and soak profile (9.5).



4.3.5 Thick boards, heavy copper: making physics your friend

Thick FR-4, backplanes, or big heat-spreaders absorb X-rays and create overlap.

What to change

  • Energy & current: bump kV/µA to punch through—but avoid blasting noise into the image. Pair with longer integration or frame averaging for clarity.
  • Geometry: use magnification wisely; too high magnification on thick boards narrows the field and increases blur.
  • Angles: add oblique views to separate stacked features; flip the board when practical to examine the other side’s pads.
  • Regions of interest: don’t image the world—scan only the BGA fields, QFN thermals, and suspect zones to keep cycle time sane.



4.3.6 PTH and thick barrels (if you inspect THT)

AXI can estimate barrel fill by grayscale through the hole:

  • Judge % fill, void gaps, and solder cones.
  • On high-aspect holes in thick boards, expect longer exposures or dual-angle shots. Pair findings with wave/selective process tuning rather than tightening AOI.



4.3.7 Build a stable AXI program (library + recipe)

  • Import CAD/centroid so the tool knows pad centers, ball pitch, and row/column counts.
  • Teach per package: expected ball count, window masks for QFN thermals, and slice heights for laminography.
  • Golden images: store “good” joints for each BGA/QFN so reviewers have a reference.
  • Throughput plan: inline = 2D/2.5D with targeted ROIs; offline (NPI/FA/escapes) = laminography/CT on a handful of parts.



4.3.8 Limits & decisions (so reviews are fast)

Set three bins per feature:

  • Pass (green): below void/shape limits, ball counts OK.
  • Review (yellow): borderline void %, odd shapes, localized clusters.
  • Fail (red): missing ball / severe HIP image / gross voids or bridges.



4.3.9 Common artifacts → quick fixes

What the image showsFeature

LikelyAutomated artifactOptical Inspection (AOI)

FixAutomated X-ray Inspection (AXI)

Inspection Medium

GrainyVisible “snow,”Light lowand contrastCameras

LowX-rays exposure(penetrates or motion blur

Raise integration/avg frames; check conveyor stopmaterials)

Defects Detected

DarkMissing streakscomponents, underPolarity, copperSkew, poursSurface-level Bridging.

Shadowing/overlap

UseVoids, obliqueHidden Bridges, Head-in-Pillow (HIP), Open/Cold Joints anglebeneath or laminography; shrink ROIcomponents.

Coverage

Visible Joints only.Balls lookIneffective ovalfor inBGAs, oneQFNs, directionand shielded areas.

GeometricHidden blurJoints. atNear high100% magnification

Backstructural offcoverage magnificationof orthe refocus at pad planeboard.

“Voids” that move with angleSpeed/Cost

Part featuresFast, not(10 voids– 20 seconds/board). Lower CapEx.

SlowerLock slice(30 height;– confirm60 onseconds/board). secondHigh angleCapEx and OpEx (X-ray tube replacement, safety).

RandomBest “missing” balls near tray edgesUse

Teach/alignHigh-volume, errorhigh-speed lines; standard consumer electronics.

Re-teachHigh-reliability, package;low-volume/high-complexity verifyboards CAD(automotive, import origin/θmedical).


Strategic
Mandate:


The complexity of the board's BOM and the Cost of Failure determine the necessity of AXI. If a hidden solder void can cause a crash or patient harm, AXI is non-negotiable.

4.3.102 CalibrationDefect &Detection: GR&R (trustSeeing the picture)Unseen

AXI detects defects based on material density. Solder (containing heavy elements like Lead or Tin) absorbs more X-rays and appears dark, while voids, flux residue, and fiberglass appear lighter.

A) Critical Hidden Defects

  • Flat/dark fieldVoids: correctionTrapped ongas scheduleor (removesflux detectorresidue drift).
  • Geometricinside checksolder withjoints. aVoids referencereduce gridthe so distances and counts stay true.
  • Golden board recheck: same packages, same ROIs, same results.
  • GR&R sampling (3 ops × 3 repeats) on void % and ball count so numbers survive audits.



4.3.11 What to do when AXI finds trouble (smallest fix first)

  • BGA HIP/poor collapse → extend and smoothjoint's TAL,mechanical add N₂ if marginal; verify VIPPO is filled/cap-plated; improve board support.
  • QFN voiding → adjust windowingstrength and soakthermal conductivity, leading to localized overheating.
    • Control Limit:; N₂The canacceptable help;voiding avoidlimit moreis peaktypically unless≤ needed.25% of the total joint area for standard BGAs, but high-reliability specifications often require ≤ 15% on thermal pads (QFN/power components).
  • Hidden Bridging/Shorts: Unintended solder connections between adjacent balls or leads under BGAs or QFNs.
  • RandomHead-in-Pillow missing balls(HIP): →Verification feeder/pickof shock or board warp; checkcomplete placementsolder logsball collapse and full fusion of the component ball with the paste on the pad.

B) Technology Types

  • 2D AXI: Provides a single, top-down transmission image. Used primarily for initial defect detection (large voids, gross shorts). Limited effectiveness on double-sided boards as images from both sides overlap.
  • 3D AXI (Planar CT/Tomography): Uses oblique viewing angles and image reconstruction to generate virtual cross-sections of individual solder joints. This technology allows for clear isolation and measurement of defects on double-sided boards and provides the clearest view of BGA ball shape and supportsHIP defects.

4.3.3 AXI Protocol and Process Control

AXI data must be used for process optimization, not just final inspection.

  1. Programming: AXI does not rely on a Golden Board (Chapter 2.5) because it sees through material. The programming uses the CAD data to define inspection regions. Experienced programmers are required to set accurate pass/fail limits for voiding and density.
  2. Process Feedback: AXI data provides direct feedback to the reflow process:
    • Excessive Voiding – Requires Reflow Profile tuning (longer soak/preheat to vent flux) or an atmosphere change to Nitrogen (Chapter 3.3).
    • UniformInconsistent highJoint voids across a lotCollapse →– Requires paste/lotTime Above Liquidus (TAL) issue or profilePeak drift;Temperature A/Badjustment with(Chapter a fresh jar3.2).
  3. Safety and re-profile.OpEx: Strict
    4. X-ray

    safety protocols are mandatory. High operational costs necessitate strategic use; AXI should be applied selectively on the high-risk components (TiesBGAs, backQFNs) rather than inspecting 100% of all components on the entire board to Chaptermaintain 9’sline throughput.

Final Checklist: AXI Implementation

Requirement

Control Point

Quality/Cost Focus

Application

AXI used for 100% of hidden joints (BGAs, QFNs, etc.).

Ensures structural integrity of non-visible connections.

Technology

3D AXI used for double-sided boards and critical BGA inspection.

Provides clean cross-section images, eliminating overlap error.

Limits

Voiding limits are defined by specification (e.g., 25% IPC maximum) and locked in the AXI program.

Quantifies defect playbookacceptability.

Process Loop

Data on excessive voiding must trigger an immediate review of the Reflow Profile and ChapterPaste 7’s printing.)



4.3.12 Pocket checklists

Program & setup

  • CAD/ROIs loaded; packages taught (ball counts, slice heights)
  • Energy/integration set for board thickness; oblique views defined for hidden fields
  • Golden images attached for quick review

Limits & flow

  • Void % limits per ballSelection and(Chapter 3).QFN

Drives thermalcontinuous setprocess (customer-aligned)improvement upstream.

  • Pass/Review/Fail routing to MES with image save
  • Inline = 2D/2.5D; laminography reserved for NPI/audits/escapes

    • Health

    • Flat/dark & geometric calibration current
    • GR&R spot check scheduled; sample runs plotted
    • Image artifacts list posted (so reviewers fix causes, not widen limits)



    When AXI programs are tuned with the right imaging technique, stable calibration, and well-defined review thresholds, they shift from reactive troubleshooting to proactive quality control. The benefit is consistent visibility into hidden joints, fewer latent escapes, and confidence that what can’t be seen by eye is still under control.


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