When we encounter BGA soldering defects on the production line, our first reaction is not to disassemble and rework the board, but to immediately send the board to an X-Ray inspection machine to view the solder joint imaging. This is because only X-Ray inspection can penetrate the FR-4 substrate and plastic package, truly reproducing the wetting morphology, void distribution, and presence of microcracks at the interface between the solder ball and the pad. In contrast, AOI can only capture surface images, and ICT is limited by test point layout, making it impossible to see the actual bottom connection state.

In practical operation, we have found that when an X-Ray image shows multiple circular black spots with a diameter exceeding 0.15mm in the central area of a BGA, it can be basically determined as void aggregation caused by insufficient volatilization of Flux residues. At this point, it is necessary to simultaneously check whether the peak reflow temperature is too low, whether the nitrogen purity is below 99.99%, and whether the stencil aperture size is 8% smaller than the pad. Any slight deviation in these variables will hinder the exhaust of Solder Paste during the melting phase.

For 0.4mm pitch QFN devices, X-Ray inspection must use a tube voltage of ≥100kV combined with a microfocus X-ray source; otherwise, obvious blurring will appear at the image edges, making it difficult to distinguish the pad extension and solder paste climb height. We once mistakenly used the 80kV setting and passed three batches of cold solder joint boards, until cross-section verification revealed that no effective metallurgical bond was formed at the solder fillet. Therefore, X-Ray inspection parameter settings are not a fixed template and must be dynamically adjusted according to device thickness, solder joint density, and PCB lamination structure.

When Jia Jinyuan's X-Ray equipment automatically identifies solder joint defects, its judgment logic for bridging relies on the grayscale gradient change rate of adjacent solder joints. If the grayscale difference between two adjacent balls is below 12% and the transition zone width exceeds 30% of a single ball diameter, the system marks it as suspected bridging. However, in actual production lines, we have found that this algorithm will generate false alarms when the pad plating is severely oxidized or the solder paste printing offset is >0.05mm. Therefore, engineers must manually review the continuity of metal density in the image and cannot fully rely on software prompts.

Before performing X-Ray inspection, it must be confirmed that the board surface is free of adhesive residue, fingerprint stains, and warpage; otherwise, artifacts will appear in the image and interfere with judgment. Especially when inspecting 0.3mm pitch BGA, even a local warpage of 0.02mm will cause the bottom solder joints to lose focus. In this case, a vacuum fixture must be used to flatten the board before scanning, rather than forcibly applying a sharpening filter to remedy the blurriness.

We do not wait for customer complaints to initiate X-Ray inspection; instead, we include key BGA boards in the first-article mandatory inspection list for each batch, and use SPC to track the trend of void area proportion. Once the void rate at the same position on five consecutive boards exceeds 18%, we immediately stop the line to check the stability of the reflow oven temperature zone chain, because data proves that this threshold is approaching the critical point of solder joint thermal fatigue failure.