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4.3 AXI Techniques

2D / 3D / laminography, BGA voiding limits, and what to do when the board is thick and the copper is bossy.

Automated X-ray inspection (AXI) isuncovers the behind-the-sceneshidden detectiveworld forbeneath jointsBGAs, thatQFNs, camerasand plated-through holes, where optical inspection and in-circuit test can’t see,reach. revealingBy what’svisualizing happeningcollapse, under BGAs, QFN thermal pads,voiding, and other hidden connections.defects, DependingAXI onprovides thean technique—inside look at solder joint health that directly links to long-term reliability. The choice of method—fast inline 2D, angled 2.5D, or detailed laminography—itbalances speed with clarity, ensuring that every board gets the right level of scrutiny without choking throughput. When paired with calibrated setups, clear acceptance limits, and MES-linked decision rules, AXI transforms invisible risks into quantifiable, traceable data that engineers can flagact missing balls, poor collapse, excessive voiding, or signs of head-in-pillow long before electrical tests show problems. Image quality depends on managing X-ray energy, angle, and focus to work around thick boards and heavy copper, which can cause shadows and blur. Clear, per-feature limits for void percentage, collapse shape, and defect patterns make reviews quick and decisions consistent, especially when tied to MES tracking and “golden” reference images. With calibration routines, good CAD data, and targeted regions of interest, AXI becomes a repeatable process control tool rather than a slow, reactive checkpoint—turning hidden joints into just another well-understood quality metric.

on.

4.3.1 AXI in one minute (why we bother)

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

  1. Is something there? (missing balls, misaligned packages)
  2. Did it wet? (collapse/shape for BGAs, edge fillets on QFNs/LGAs)
  3. How gassy is it? (voiding percentage and pattern)

Use AXI where AOI is blind and ICT can’t reach.



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 shows

Likely artifact

Fix

Grainy “snow,” low contrast

Low exposure or motion blur

Raise integration/avg frames; check conveyor stop

Dark streaks under copper pours

Shadowing/overlap

Use oblique angle or laminography; shrink ROI

Balls look oval in one direction

Geometric blur at high magnification

Back off magnification or refocus at pad plane

“Voids” that move with angle

Part features, not voids

Lock slice height; confirm on second angle

Random “missing” balls near tray edges

Teach/align error

Re-teach package; verify CAD import origin/θ



4.3.10 Calibration & GR&R (trust the picture)

  • Flat/dark field correction on schedule (removes detector drift).
  • Geometric check with a reference grid 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 smooth TAL, add N₂ if marginal; verify VIPPO is filled/cap-plated; improve board support.
  • QFN voiding → adjust windowing and soak; N₂ can help; avoid more peak unless needed.
  • Random missing balls → feeder/pick shock or board warp; check placement logs and supports.
  • Uniform high voids across a lot → paste/lot issue or profile drift; A/B with a fresh jar and re-profile.

(Ties back to Chapter 9’s defect playbook and Chapter 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 ball and QFN thermal set (customer-aligned)
  • 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)



Bottom line: use 2D/2.5D

When AXI toprograms keepare tuned with the beltright movingimaging technique, stable calibration, and laminographywell-defined whenreview youthresholds, needthey cleanshift slices.from Judgereactive collapsetroubleshooting to proactive quality control. The benefit is consistent visibility into hidden joints, fewer latent escapes, and voidsconfidence withthat clearwhat limits,can’t tunebe profilesseen andby apertureseye whenis datastill saysunder so, and keep programs/grids calibrated. Do that, and hidden joints stop being a mystery—just another set of checks your line passes, board after board.

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