X-Ray Inspection of SMT Solder Joints

Most solder joints on a modern board can be inspected optically, but the ones that matter most usually cannot. A ball grid array hides its joints under the package body, a land grid array hides them under the component, and a bottom terminated component hides them under a thermal pad. The only practical way to look at those joints from outside the board is with X-rays.

X-ray inspection is not a simple pass or fail tool. It produces a projection through the whole assembly, so a defect at one layer can be masked by material above or below it. Getting useful results depends on the equipment settings, the viewing angles and a clear understanding of what the images can and cannot show.

How the Image Is Formed

The board is placed between an X-ray source and a detector. Denser materials absorb more of the beam, so solder, copper and the gold in a component termination appear dark while the laminate and the plastic package appear light. The contrast in the image therefore depends on the difference in absorption between the joint and the material around it, and on the energy of the beam.

Beam energy is the first setting to get right. A beam that is too soft is absorbed by the board and produces a high contrast image of the top surface with the joints beneath invisible. A beam that is too hard penetrates everything and produces a flat, low contrast image. The useful range depends on the thickness and the material of the assembly, and it has to be set for each product rather than carried over from the last one.

Resolution and Magnification

Geometric magnification is obtained by placing the board close to the source and the detector far away, which enlarges the projection. The resolution limit, however, is set by the spot size of the source and by the pixel pitch of the detector, and enlarging the image does not improve either. A system with a small spot and a fine detector can resolve a void a few tens of micrometres across; a system with a large spot cannot, however much the image is magnified.

Where the whole board has to be inspected quickly, a lower magnification is used and the system relies on the detector resolution. Where a specific joint must be examined, the board is moved closer and the image is magnified at the cost of speed. Production inspection plans typically combine both: a fast scan at moderate magnification to find anomalies, and a detailed examination of the flagged joints at high magnification.

What X-Ray Can and Cannot Show

A void inside a solder ball is visible because the void contains no material and therefore absorbs less than the solder around it. Bridging between adjacent joints is visible in the same way. Missing balls, misaligned packages, insufficient solder and excess solder are all detectable, and the position of a joint relative to the pad can be measured directly from the image.

A crack is more difficult. A crack that is closed and in the plane of the beam may not produce a visible contrast difference, because there is no gap for the beam to pass through. Cracks are best detected by viewing from an angle, which changes the path length through the joint and makes the separation visible. This is why the ability to tilt the source and detector, or to rotate the board, is a significant capability rather than a convenience.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Test-of-Box-Build-Assembly-1.jpg" alt="Angled X-ray view revealing a crack in a solder joint” />

Void Criteria and Their Basis

Voiding is specified as a percentage of the joint area, and the acceptable figure depends on the joint and the application. A large void under a thermal pad raises the thermal resistance and can cause a hot spot, while voids in a ball of a ball grid array have less effect on the electrical connection but reduce the mechanical strength. The criteria in documents such as the IPC standards distinguish between these cases rather than applying a single number.

Measuring the percentage correctly matters as much as the criterion. The void area is measured on a projection, so it depends on the viewing angle and on the slice selected. For a through hole joint a void at the top of the barrel may not be visible from directly above. Where the criterion is tight, measurements should be taken with a defined procedure, and the same procedure should be used for the qualification samples and for production so that the numbers are comparable.

Setting Up a Production Inspection Plan

The inspection plan should start from the failure modes the process can produce. A reflow profile that is marginal produces different defects from a stencil that is worn, and the inspection should target the joints that are most sensitive to each. Sampling is usually adequate for a stable process, with a higher sample rate after a change of paste, stencil, oven profile or component supplier.

The plan also has to define what happens when a defect is found. A single void slightly over the limit is a different situation from a systematic pattern across a panel, and the response should reflect that difference. Recording the images alongside the process data allows a trend to be seen before the defect rate rises, which is the point at which an inspection programme pays for itself. Together with the other checks in a quality control system, this is what makes a reflow process predictable rather than merely monitored.

Additional Considerations for This Build

Practical attention to image resolution pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating image resolution explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, voiding is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Does X-ray inspection damage the board? It does not, at the energies used for inspection. The dose is far below the level that would affect the components or leave residual charge.

Can a good X-ray image replace a cross section? No. The image shows a projection, while a cross section shows the actual material structure at one plane. A cross section is still needed to confirm what a suspicious image means.

How often should the machine be verified? Its resolution and contrast should be checked with a reference sample whenever the tube is replaced or the settings are changed significantly, and at a defined interval during production.