X-ray Inspection of Area Array Solder Joints

An area array package has its joints under the body, so the only practical way to look at them without destroying the assembly is with X-ray. The image shows a lot and it does not show everything.

What the Image Shows

A two dimensional image shows the projected solder mass. A joint that is present and round appears as a bright disc; a joint with a void appears with a darker region inside it.

It also shows bridging between adjacent joints, missing balls, and displacement of the whole row. Those are the defects that the method is best at. Our void notes describe the measurement that is usually taken from the image.

What the Image Does Not Show

A joint that is present at the top and open at the pad can project the same as a sound joint. An open at the interface is invisible in a two dimensional projection unless the geometry happens to reveal it.

A crack that runs parallel to the projection is also hidden. The method confirms the presence of solder rather than the integrity of the connection. Our joint criteria notes describe the criteria that apply to what is visible.

Resolution and Coverage

The resolution has to be sufficient to see the smallest feature that matters, which is usually the void inside the smallest joint rather than the joint itself.

A high resolution over a large board takes time, so the inspection is typically applied to a sample or to a region rather than to every joint. The sampling decision should be explicit. Our sampling notes describe how the plan is set.

Angle and Oblique Views

An oblique view separates the layers that a straight down view superimposes, which makes an open at one interface visible as a shift in the projected shape.

The oblique view takes longer and it is used where the straight view is inconclusive or where the joint class is critical. It is the standard way to look for a head in pillow defect.

Three Dimensional Methods

Computed tomography reconstructs the volume and shows the joint in section without destroying it. It is the most informative method and the slowest, so it is used for failure analysis rather than for production screening.

The reconstruction should be calibrated against a known structure, because the interpretation depends on the threshold chosen. Our failure analysis notes describe when it is used.

Interpretation and Consistency

The interpretation depends on the settings and on the operator. The voltage, the current and the filter change the image, so a criterion expressed only in words is applied differently by different people.

Reference images produced from the same machine and the same settings remove most of the ambiguity. Our training notes describe how the references are used.

Complementary Checks

X-ray is used together with the process control that produced the joints, because the image cannot confirm the interface that matters most.

The paste volume, the profile and the placement accuracy are what make the interface sound, and the X-ray confirms that the joint is present. Our paste inspection notes describe one of the controls that carries the rest of the assurance.

Process Control and Verification

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage. 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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Can X-ray prove a joint is good? It proves the solder is present and shaped as expected. The interface at the pad is the part it cannot confirm.

How much void is acceptable? The limit comes from the thermal and mechanical load through the joint, and it should be measured to a defined method rather than estimated from the image.

What does gopcb provide for X-ray inspection? We provide inspection with resolution matched to the smallest feature, an explicit sampling plan, oblique views where the straight view is inconclusive, computed tomography for failure analysis, reference images from the same machine and settings, and process controls that cover the interface the image cannot show.