4.3 AXI Techniques

Automated X-ray Inspection (AXI) is a critical, non-destructive testing method that uses X-ray technology to penetrate components and the Printed Circuit Board (PCB), generating images of internal structures and hidden 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 (AOI). The investmenthigh inCapEx of AXI is justified by the requirement for superior superiorstructural reliability in safety-critical industries (e.g., medical, automotive, aerospace).industries.

4.3.1 AXI vs. AOI: The HiddenCost Defectof Trade-OffCoverage

AXI is not a replacementstructural fortest AOI;that itcomplements isAOI, afocusing complementary technology used to cover structuralon defects that AOIlie cannotbeneath detect.components, where optical access is impossible.

Feature	Automated Optical Inspection (AOI)	Automated X-ray Inspection (AXI)
Inspection Medium	Visible Light and Cameras	X-rays (penetrates materials)
Defects Detected	Missing components, Polarity, Skew, Surface-levelSurface Bridging.	Voids, Hidden Bridges, Head-in-Pillow (HIP), Open/Cold Joints beneathinternal components.opens.
Coverage	VisibleSurface Joints only. Ineffective for BGAs,area-array QFNs, and shielded areas.packages.	Hidden Joints. Near 100% structural coverage of the board.coverage.
Speed/Cost	Fast (10 – 20 seconds/board). Lower CapEx.	Slower (30 – 60 seconds/board). High CapEx and OpExOpEx. (X-ray tube replacement, safety).
Best Use	High-volume, high-speed lines; standard consumer electronics.	High-reliability, low-volume/high-complexity boards (automotive, medical).

Strategic Mandate:Strategy: TheAXI complexitymust ofbe applied selectively to high-risk components (BGAs, QFNs) to maintain line throughput, while AOI handles the board'shigh-volume BOMvisual 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.checks.

4.3.2 DefectAXI Detection:Technology: Seeing2D vs. 3D Laminography

AXI systems utilize the Unseenproperty

AXIthat detectsdense defects based on material density. Solder (containing heavy elementsmaterials, like Leadsolder, or Tin) absorbsabsorb more X-raysray andenergy appearsthan dark,lighter whilematerials voids,(fiberglass, fluxsilicon, residue,air). andThis fiberglasscreates appearcontrast lighter.in the resulting image.

A) Critical2D HiddenAXI DefectsLimitations

• Voids:2D AXI Trapped gas or flux residue inside solder joints. Voids reduce the joint's mechanical strength and thermal conductivity, leading to localized overheating.

  • Control Limit: The acceptable voiding limit is typically ≤ 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.
• Head-in-Pillow (HIP): Verification of complete solder ball collapse and full fusion of the component ball with the paste on the pad.

B) Technology Types

• 2D AXI: Providesprovides a single, top-down transmission image. UsedWhile primarilyuseful for initialchecking defectbasic detectionpresence (and large voids, grossits shorts).primary Limitedlimitation effectivenessis image overlap on double-sided boardsboards. The solder joints from the top and bottom interfere, making accurate, quantitative analysis of internal joints ambiguous or impossible.

  B) 3D Laminography (Planar CT)

  Laminography (also known as imagesPlanar fromComputed bothTomography, sidesor PCT) is the solution for eliminating image overlap. It is the core technology behind modern

• 3D AXI.
  (Planar CT/Tomography):
  • Mechanism: UsesThe obliqueX-ray viewing anglessource and imagedetector move in synchronized paths (oblique angles) relative to the board, capturing multiple images. Software then uses mathematical reconstruction to generate virtualcreate virtual cross-sections or "slices" of individualthe solderboard.
  • Function: joints.Laminography allows the inspector to focus digitally on a specific layer (e.g., the component side joint plane) while blurring out the overlapping features from the opposite side.
  • Application: This technologyis allowsmandatory for clearreliable isolationinspection of double-sided BGAs and measurementstacked packages, providing clear, quantifiable measurements of defects onlike double-sided boardsvoiding and provides the clearest view of BGA ball shape and HIP defects.HIP.

  4.3.3 AXIDefect ProtocolDetection and Process Control

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

  1. A) Critical Hidden Defects

     Programming:Defect	Mechanism and Location	Reliability Consequence	Control Limit (Mandate)
     Voids	Gas (flux volatiles) trapped within the solder joint, visible as light areas in the dark solder mass.	Reduces thermal conductivity AXI does not rely on aand Goldenmechanical Boardstrength.	Standard limit is ≤ 25% of joint area. High-reliability thermal pads often require ≤ 15%.
     Head-in-Pillow (HIP)	The BGA ball and printed paste fail to fully fuse. Requires oblique angle viewing (Chapteror 2.5)3D becauseLaminography itfor seesclear throughdetection.	Latent material.defect Theleading programmingto usesintermittent failure in the field.	AXI must verify complete ball collapse and fusion.
     Hidden Bridges	Unintended solder connection between adjacent pads beneath area-array packages.	Short circuits invisible to AOI.	AXI must confirm 100% separation between adjacent balls/pads.

     B) Process Loop

     AXI provides critical data for tuning the CADReflow dataProfile:

     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).
     • Inconsistent Joint Collapse (HIP risk) – Requires adjustment of Time Above Liquidus (TAL) or Peak Temperature adjustment (Chapter 3.2).
  3. Safety and OpEx: Strict X-ray safety protocols are mandatory. High operational costs necessitate strategic use; AXI should be applied selectively on the high-risk components (BGAs, QFNs) rather than inspecting 100% of all components on the entire board to maintain line throughput.

  Final Checklist: AXI Implementation

  Requirement	Control Point	Quality/Cost Focus
  Technology	Laminography (3D AXI) used for all double-sided boards and critical BGA inspection.	Provides clean cross-section images, eliminating overlap error.
  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 acceptability.acceptability for reliable heat dissipation.
  Process Loop	DataAXI on excessive voidingdata must trigger an immediate review of the Reflow Profile and Paste Selection (Chapter 3).	Drives continuous process improvement upstream.