{"id":2178,"date":"2026-07-24T17:38:03","date_gmt":"2026-07-24T23:38:03","guid":{"rendered":"https:\/\/sbcgroup.com.mx\/?p=2178"},"modified":"2026-07-24T17:38:05","modified_gmt":"2026-07-24T23:38:05","slug":"inspeccion-por-rayos-x-axi-en-electronica-analisis-de-juntas-ocultas","status":"publish","type":"post","link":"https:\/\/sbcgroup.com.mx\/en\/2026\/07\/24\/inspeccion-por-rayos-x-axi-en-electronica-analisis-de-juntas-ocultas\/","title":{"rendered":"X-ray Inspection (AXI) in Electronics: Hidden Joint Analysis"},"content":{"rendered":"

Automated X-ray Inspection (AXI) Technologies for BGA Components<\/h1>\n\n\n\n

In the era of extreme miniaturization, modern electronics manufacturing faces an invisible challenge: verifying the structural integrity of connections that the human eye and conventional cameras simply cannot see. With the widespread adoption of packaging such as Ball Grid Array (BGA), Quad Flat No-lead (QFN), and Chip Scale Package (CSP), solder joints have become completely hidden beneath the component body. Given this physical limitation of Automated Optical Inspection (AOI), Automated X-ray Inspection (AXI) has established itself as the definitive technology for ensuring reliability in high-density assemblies.<\/p>\n\n\n\n

AXI technology not only allows you to \u201csee through\u201d silicon and the FR-4 substrate, but also provides precise quantitative data on solder volume, voids, and internal alignment. In this technical analysis, we will break down the operating principles of X-ray inspection, the differences between 2D and 3D architectures, the acceptance criteria under the IPC-A-610 standard, and how this technology has become the first line of defense against the proliferation of counterfeit integrated circuits in the global supply chain.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Limitations of Optical Inspection (AOI) for Hidden Joints<\/h2>\n\n\n\n

Automated Optical Inspection (AOI) uses structured lighting and high-resolution cameras to evaluate surface features based on light reflectivity. It is exceptionally fast and effective for verifying component presence, polarity, character reading (OCR), and visible solder fillet formation on packages such as SOICs or passive components (0402, 0201). However, the optical technology requires a direct line of sight.<\/p>\n\n\n\n

When a BGA component is placed on a printed circuit board (PCB), its hundreds of solder balls are trapped between the component substrate and the board. An AOI camera, even with angled mirrors, can only marginally inspect the outer row of balls. The internal connections, which often carry critical high-speed signals or handle the main heat dissipation, remain in a complete blind spot. This limitation makes AOI inadequate for detecting catastrophic defects such as head-in-pillow (HiP), internal short circuits, or excessive voids in the center of the BGA array.<\/p>\n\n\n\n

Operating Principles: AXI 2D vs 2.5D vs 3D CT<\/h2>\n\n\n\n

X-ray inspection in electronics operates on the principle of differential attenuation. When X-ray photons (typically between 30 and 160 keV) pass through a PCBA, the materials absorb them according to their atomic number (Z) and density. Lead-free solder alloys, rich in tin (Z=50), silver (Z=47), and copper (Z=29), exhibit high attenuation and appear as bright white areas in the image. Conversely, the FR-4 substrate and the silicon die (Z=14) have very low attenuation, appearing dark. This difference in contrast, governed by Beer-Lambert's Law, is what allows visualization of the internal structure of the boards.<\/p>\n\n\n\n

Technical Parameter<\/th>AXI 2D (Transmission)<\/th>AXI 2.5D (Laminography)<\/th>AXI 3D (Computed Tomography)<\/th><\/tr><\/thead>
Methodology<\/strong><\/td>Single-angle orthogonal projection.<\/td>Multiple projections at oblique angles.<\/td>Complete volumetric reconstruction (CT).<\/td><\/tr>
Typical Resolution<\/strong><\/td>3 to 5 micrometers.<\/td>2 to 4 micrometers.<\/td>0.5 to 2 micrometers.<\/td><\/tr>
Inspection Speed<\/strong><\/td>High (60-200 plates\/hour).<\/td>Average (30-80 plates\/hour).<\/td>Low (5-20 plates\/hour).<\/td><\/tr>
Layer Differentiation<\/strong><\/td>Poor (the characteristics overlap).<\/td>Good (separates upper\/lower layers).<\/td>Excellent (isolated cross-sections).<\/td><\/tr>
Precision in Voids<\/strong><\/td>\u00b1 3% to 5%.<\/td>\u00b1 2% to 3%.<\/td>\u00b1 1% to 2%.<\/td><\/tr>
Main Application<\/strong><\/td>High-volume inline inspection, simple PCBs.<\/td>Double-sided PCBs (Double-sided SMT).<\/td>Failure analysis, R&D, complex multilayer boards.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The choice between these architectures depends on production volume and assembly complexity. While 2D systems are ideal for single-sided boards due to their speed, modern boards with BGA components mounted on both sides (top\/bottom) require 2.5D or 3D systems to prevent the images of the top and bottom spheres from overlapping, which would make automated analysis impossible.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

BGA Component Analysis and Critical Defect Detection<\/h2>\n\n\n\n

The analysis of Ball Grid Array components is the primary driver of AXI technology adoption. Image processing algorithms evaluate multiple geometric and density parameters for each individual sphere in the array. The inspection verifies sphere circularity (ensuring uniform collapse during reflow), precise alignment between the sphere and the PCB pad, and the overall board diameter.<\/p>\n\n\n\n

The most critical defects detected using AXI include:<\/p>\n\n\n\n

Short circuits (Solder Bridges):<\/strong> They appear as bright white bands connecting two or more adjacent pads. X-ray technology is the only non-destructive method capable of detecting bridges under BGA components or metal radio frequency shields (RF shields).<\/p>\n\n\n\n

Head-in-Pillow (HiP):<\/strong> This is one of the most insidious defects in SMT manufacturing. It occurs when the BGA sphere rests on the molten solder paste but fails to fuse metallurgically, often due to oxidation or warpage of the component during thermal profiling. In an oblique X-ray image, a HiP is identified by a distinctive "waist" shape or a clear parting line in the solder profile, indicating an intermittent electrical connection that would go undetected in initial functional testing.<\/p>\n\n\n\n

Open Joints:<\/strong> A complete physical separation between the termination and the pad. In the X-ray image, it appears as a perfectly round sphere, smaller than its neighbors, indicating that the solder did not wet the pad or collapse properly.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Calculation of Voiding Percentage and IPC-A-610 Criteria<\/h2>\n\n\n\n

Voids are pockets of gas trapped within the solder joint during the reflow process, typically caused by flux outgassing. A high percentage of voids drastically reduces the thermal conductivity and mechanical strength of the connection, which can lead to premature failure due to thermal fatigue in the field.<\/p>\n\n\n\n

The AXI software calculates the percentage of voids using the following formula in a 2D projection: Void Percentage = (Total void area within the sphere \/ Projected area of the welding sphere) \u00d7 100<\/strong>. It is crucial to understand that the void morphology matters as much as the percentage. \u201cMacrovoids\u201d (large bubbles in the center) behave differently from \u201cPlanar microvoids\u201d (small bubbles aligned at the intermetallic interface, often associated with the ENIG surface finish).<\/p>\n\n\n\n

Industry standards, specifically IPC-A-610 and the IPC-7095D process guide, establish rigorous acceptance criteria. For general assemblies (Class 2), the commonly cited acceptance limit is that voids should not exceed 25% of the projected area of any individual sphere. It is a common misconception that IPC publishes different void percentages for Class 3 (high reliability); the base numerical limit is the same. What changes drastically in Class 3 is the inspection requirement (typically 100% of AXI coverage rather than sampling) and the surrounding workmanship criteria. For QFN components, where the center pad acts as the primary heat sink, voids in the thermal pad generally should not exceed 30% for Class 2, and are often restricted to less than 20% in Class 3 applications by customer-specific drawing notes.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Through-Hole Component Inspection (THT) and Barrel Filling<\/h2>\n\n\n\n

Although AXI technology is famous for its application in SMT, it is equally vital for inspecting through-hole technology (THT) components, especially on thick, multi-layer boards used in servers, telecommunications, and military applications. The critical parameter here is the "Barrel Fill.".<\/p>\n\n\n\n

During wave soldering or selective soldering, the molten alloy must rise by capillary action through the metallized hole (PTH). The IPC-A-610 standard requires that the solder fill at least 75% of the vertical height of the barrel for Class 2 assemblies. For high-reliability (Class 3) applications, this requirement is often increased to 100% of fill (with no visible voids) to ensure structural integrity under severe vibration. The AXI 2.5D and 3D systems can accurately measure this vertical fill level, which is impossible to verify visually once the component is flush with the base plate.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Counterfeit Analysis (Counterfeit IC Detection)<\/h2>\n\n\n\n

Beyond solder quality, X-ray inspection has become an indispensable tool for supply chain security. The proliferation of counterfeit integrated circuits (ICs) represents a multi-billion dollar risk to the aerospace, medical, and automotive industries. Counterfeiters often take low-cost, off-the-shelf chips, sand the surface (blacktopping), and laser-mark them as high-value military-grade or industrial components.<\/p>\n\n\n\n

AXI technology enables non-destructive internal analysis of the die and lead frame. By comparing an X-ray image of a suspected component to a reference image of a known genuine component (golden sample), engineers can identify critical discrepancies. X-ray indicators of counterfeiting include inconsistent die sizes, different wire bond routing patterns, broken or missing wire bonds, and lead frame structures that do not match the original manufacturer's specifications. This level of internal scrutiny is essential for meeting counterfeit mitigation standards such as AS6171.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Integration of AXI into the Production Line: Inline vs Offline<\/h2>\n\n\n\n

The implementation of AXI technology in the manufacturing environment is divided into two main architectures, each with distinct operational advantages.<\/p>\n\n\n\n

AXI Inline Systems:<\/strong> These systems are integrated directly into the SMT line, typically positioned immediately after the reflow oven and before the In-Circuit Testing (ICT). Leading manufacturers such as Viscom and Nordson have developed inline systems capable of processing boards in cycle times as low as 4 to 10 seconds, using 3D architectures that can scan multiple boards simultaneously. The main advantage of inline AXI is real-time feedback; if the void percentage starts to increase, process engineers can immediately adjust the oven's thermal profile or the stencil's printing pressure, preventing the production of defective batches.<\/p>\n\n\n\n

AXI Offline Systems:<\/strong> These are standalone inspection stations where an operator manually loads the plates. While they don't offer the high-volume throughput of inline systems, offline equipment (such as YXLON or Nikon Metrology high-resolution computed tomography systems) provides unparalleled flexibility. They allow for multi-axis plate manipulation (tilt and rotate) for in-depth flaw analysis, non-prototype inspection (NPI), and detailed quality audits. They are the preferred tool in reliability laboratories and for high-mix, low-volume small-batch inspection.<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Learn more<\/h2>\n\n\n\n

To learn more about quality standards and inspection technologies in electronic manufacturing, we recommend exploring the following technical resources:<\/p>\n\n\n\n