Ball grid array packages provide a high number of electrical connections within a compact area, but their solder joints are hidden beneath the component. Because these joints cannot be checked through normal visual inspection, BGA X-ray inspection is widely used in PCB assembly, failure analysis and post-rework verification.
X-ray images reveal differences in material density, helping inspectors identify voids, solder bridges, open joints, insufficient solder and other hidden abnormalities. However, an unusual image does not automatically mean that a PCB must be rejected. Reliable decisions require product-specific acceptance criteria, process knowledge and suitable inspection equipment.
An X-ray BGA inspection system can help identify:
Voids inside solder joints
Solder bridges between adjacent balls
Missing or incomplete joints
Uneven solder-ball shapes
Package misalignment
Excess or insufficient solder
Head-in-pillow defects
PCB or package warpage indicators
The inspection result depends on detector resolution, magnification, viewing angle, sample thickness and operator experience.
For engineering analysis, sampling inspection and failure investigation, manufacturers can use an offline X-ray inspection machine. Offline systems allow operators to adjust magnification, angle and imaging parameters when examining suspicious solder joints.
A solder void is an empty area trapped inside a solder joint. Because the void is less dense than solder, it normally appears as a lighter region within the darker solder-ball image.
Common causes include:
Moisture or contamination
Volatile materials escaping from solder paste
Incorrect reflow profiles
Oxidized pads or component surfaces
Poor solder-paste storage
Via-in-pad structures
Unsuitable flux chemistry
The presence of a void alone does not determine whether a joint is acceptable. Inspectors should also evaluate its size, number, position and distribution.
A small central void may have a different reliability impact from a void located near the package or PCB interface. Therefore, factories should avoid using one universal void percentage for every product.
A useful inspection record should include the component reference, solder-joint position, void area, void location, inspection method and final decision.
A solder bridge occurs when solder creates an unintended electrical connection between adjacent joints. In an X-ray image, two solder balls may appear connected by a dark path, or the normal separation between them may disappear.
Possible causes include:
Excessive solder paste
Incorrect stencil design
Package misalignment
PCB warpage
Component movement during reflow
Excessive solder-ball collapse
Inspectors should compare suspicious joints with the surrounding BGA pattern. Normal joints usually have consistent spacing, size and shape.
However, copper traces, vias or components on the opposite side of the PCB may overlap in a two-dimensional image. Changing the viewing angle can help determine whether the dark region is a real solder bridge or an overlapping internal structure.
For complex assemblies, an industrial microfocus X-ray 3D/CT machine can separate structures by depth. This is particularly useful for double-sided PCBs, power modules, advanced semiconductor packages and multilayer assemblies.
An open joint occurs when the solder ball fails to form a complete connection with the PCB pad. This may result from:
Package or PCB warpage
Insufficient solder paste
Contaminated pads
Uneven heating
Incorrect placement height
Poor component coplanarity
Open joints are difficult to identify in a standard top-down image because the solder ball may still overlap the pad. Inspectors should look for indirect signs, including unusual ball size, inconsistent grayscale, irregular shape or repeated abnormalities near package corners.
Tilted-angle inspection often provides more useful information than a single vertical image. Electrical testing, 3D CT or cross-section analysis may still be necessary when the result is inconclusive.
A head-in-pillow defect occurs when the BGA solder ball and solder paste melt but fail to form a complete metallurgical bond. The surfaces may appear to touch while remaining partially separated.
Typical causes include:
Package warpage
Oxidation
Insufficient flux activity
Poor solder-paste transfer
Unsuitable temperature profiles
Uneven board support
Because the solder ball may still appear in the correct position, this defect can be difficult to confirm through a top-down image. Tilted X-ray inspection, CT reconstruction, electrical testing or destructive analysis may be required.
Two-dimensional X-ray inspection is fast and effective for many routine checks, but it compresses all internal structures into one image.
This can create uncertainty when inspecting:
Double-sided PCB assemblies
Package-on-package devices
Dense multilayer boards
Power modules
Overlapping electronic components
A practical inspection process should begin with a top-down image, followed by comparison with neighboring joints and tilted-angle inspection. If the defect still cannot be confirmed, 3D CT may be required to separate internal structures.
Acceptance requirements should be based on:
Applicable IPC standards
Customer specifications
Product reliability requirements
Component-manufacturer guidance
Internal process capability
Historical failure data
A consumer electronics board may not have the same acceptance limits as an automotive, medical or aerospace assembly.
Manufacturers may also establish internal warning limits that are stricter than final rejection limits. This helps identify process drift before products become nonconforming.
The purpose of x ray solder joint inspection is not only to identify defects but also to guide corrective action.
First, inspectors should confirm the indication by changing the angle or imaging settings. They should then review solder-paste records, placement data, reflow profiles and material information.
If rework is required, a controlled BGA rework station can support component removal, site preparation, optical alignment, replacement and controlled heating. The temperature profile must be developed for the specific PCB, component and solder alloy.
After rework, the assembly should be inspected again to verify that the original defect has been removed and that no new bridges, voids or open joints have been introduced.
No. Some interface defects may require tilted imaging, 3D CT, electrical testing or cross-section analysis.
No. Void size, location, product class and customer requirements must all be considered.
No. AOI checks visible features, while X-ray inspection evaluates hidden solder joints beneath the BGA package.
3D CT is useful when structures overlap in a 2D image or when the exact depth and shape of a defect must be confirmed.
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