X-Ray Inspection of BGA and QFN Joints
Why X-Ray Is Needed
A ball grid array hides every joint underneath the package, and a quad flat no-lead part hides its joints under the body edge. Optical inspection sees the package and nothing else, and functional test sees only the nets that happen to be exercised. X-ray inspection looks through the component and images the solder itself, which is why it has become the standard method for verifying these packages. It is not a substitute for the process controls that make the joints, but it is the only way to see what was actually formed.
How the Image Is Formed
An X-ray image is a shadow, and its contrast comes from how much radiation each material absorbs. Solder absorbs far more than the laminate or the package, so the joints appear dark against a lighter background. That simple mechanism explains both the strengths and the limits of the method: a joint is visible only if there is enough material in the beam path to create contrast, and the image is a projection through the whole board unless the system can rotate. A two-dimensional system shows the pattern of balls, their diameter and their relative position, which is enough to find missing balls, bridges, misalignment and gross shape problems.
Voiding and Acceptance Criteria
Voids appear as lighter circular regions inside a dark joint, because gas replaces solder in the beam path. They form from flux volatiles, from paste that was printed with an excess of flux, or from the surface finish, and they are present in almost every joint to some degree. The industry standard treats a void as acceptable up to a percentage of the joint area, with a much tighter limit for joints that carry thermal load, because a void under a thermal pad removes contact area exactly where the heat enters the board. The percentage is measured in the projected area, which means a 2D image reports the largest void cross section through the joint rather than the true volume fraction, and a designer reading a report should know which was measured.
Head-in-Pillow and the Limits
Head-in-pillow is the defect that best illustrates the limits of radiography. The solder ball sits on top of the paste, which reflowed and then solidified before the ball collapsed, so the two are touching but not joined. From above, through the package, the joint appears complete: the ball is present, its diameter is normal and there is no obvious gap. The electrical connection is intermittent and appears only under thermal or mechanical load. X-ray alone cannot reliably distinguish it, and the practical detection route is a combination of X-ray, in-circuit test with resistance monitoring and, where the risk is high, destructive cross sectioning of a sample.
QFN and Bottom-Termination Parts
QFN packages are harder to inspect than BGA because the joints are smaller, they are at the edge of the body, and the large thermal pad in the centre tends to hold a high void fraction. A two-dimensional image at an oblique angle can show the peripheral joints and their fillets on the side of the package, and it can show whether the thermal pad has wetted to the board. Where the system permits it, a tilted view is more informative than a straight top-down view for these parts, because a joint that has not wetted at the edge may still appear filled from directly above.
Two-Dimensional, Oblique and Computed Tomography
A 2D system is fast, cheap and adequate for the majority of production monitoring: it catches missing and bridged balls, gross voids and displacement. An oblique view improves the visibility of individual joints in a dense array by separating the planes of the image. Computed tomography reconstructs slices through the board and gives a true cross-sectional view of each joint, which resolves questions that a projection cannot, at the cost of a much longer scan and a lot more data. The choice is usually driven by the defect rate: use 2D for routine monitoring and reserve CT for failure analysis and for qualifying a new process or a new package.
What It Cannot See
X-ray cannot see a joint that has not formed electrically but looks complete physically, which is the head-in-pillow case, and it cannot see a poor metallurgical bond inside a joint that is the right shape. It also struggles where a joint is shielded by another dense structure in the beam path, such as a heavy ground plane or a metal stiffener. For these reasons an X-ray pass should be regarded as one part of a verification strategy that also includes in-circuit test, functional test and, for new processes, destructive analysis of samples.
Building the Inspection Plan
Inspection finds defects; it does not stop them, so the plan should be built around the defects the process is actually capable of producing. For a new package or a new paste, start with a sample-based X-ray and destructive cross section to establish that the joint geometry is right, then move to routine monitoring once the process is stable. For production, the routine check should cover the failure modes that would escape electrical test: voids above the acceptance level on thermal pads, missing or displaced balls, and tilt of a package relative to its footprint. After a reflow profile change, a paste change or a stencil change, the sample check should be repeated, because these are the changes that move the joint geometry. Keeping a record of the images alongside the parameters makes a later failure analysis far easier, since the question is usually whether the process changed or the design did.
FAQ
What does X-ray inspection detect? Missing, bridged, displaced and grossly malformed joints, plus voids within the solder and misalignment of the package to the footprint.
What void level is acceptable? The industry standard permits a modest percentage of the joint area, with much tighter limits where the joint is part of a thermal path.
Can X-ray find head-in-pillow? Not reliably on its own, because the joint looks complete in projection even when the two surfaces are not joined.
When should CT be used? For failure analysis and for qualifying a new package or process, since it is much slower than a 2D scan.
Is X-ray enough on its own? No. It complements, rather than replaces, in-circuit test, functional test and occasional destructive cross sectioning.
Conclusion
X-ray is a projection, and knowing what a projection can and cannot show is the difference between using it well and trusting it too far. Use 2D inspection for routine monitoring, oblique views for dense arrays and QFN edges, and CT where a single joint has to be resolved. Set the void criteria from the function of the joint, and combine the radiographic result with electrical test. Inspection capability sits alongside PCBA testing, the process that forms the joints is described in SMT PCB assembly, and the verification discipline is part of quality management. New packages are normally validated during prototype PCB assembly in 2026.
