Small components challenge inspection throughput
Any discussion of printed-circuit-board inspection must address the tradeoffs between methods and process points.
By Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 4/1/2008
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| Josh Petras YESTech |
Q: How do you maximize throughput with ever-smaller devices?
A: Consider a typical AOI sensor configuration consisting of a digital camera with constant resolution and a fixed-focus lens. The smallest detectable inspection feature depends on lens magnification. At some point, shrinking components and solder features will become too small to analyze accurately. Increasing magnification reduces the field of view, so the AOI system would have to acquire more images for the same area. Increasing the camera’s resolution with a larger sensor but equivalent frame rate costs more and typically requires higher quality optics and more post-processing PC hardware.
Q: And if you can’t justify the expense?
A: In that case, you can outfit a single system with several cameras and lenses. This allows the AOI operator to select resolution on-the-fly in software. You might mount a 25-micron (pixel-size) camera/lens configuration for lower-resolution bar codes and to verify component placement and mount a 12.5-micron configuration to inspect 0201s and other fine-pitch devices, thereby providing the best balance of magnification and speed.
Q: How does x-ray fit into the picture?
A: Although there is some overlap, AOI gives a fast picture of component placement and other visible features, while x-ray provides the best inspection of solder joints, especially on ball-grid arrays, chip-scale devices, and other devices with hidden joints.
Maintaining two individual machines allows you to fine-tune each to balance detection and inspection speed. In addition, you can place the AOI system either directly in front of x-ray at post-reflow or at pre-reflow. Either way, results pass to the rework operator as a unified report.
Q: When would such a dual-system solution be most advantageous?
A: You would generally adopt it with high-reliability boards where throughput is important and where you can justify the cost of repair. Repairing high-reliability boards tends to be easier than repairing cellphones and similar products because they don’t generally include as many small components.
Q: What happens when repair is difficult or not cost effective?
A: In those cases, we advocate you “inspect early and often.” Process monitoring is more important because you will need to scrap most defective boards. The goal is to identify the defect trends and modify the process to prevent them.
Q: Should you inspect before or after reflow?
A: It depends. Suppose a component is slightly off-pad. The pre-reflow step identifies it as a failure, but the liquefaction of the solder during reflow causes it to correct itself. If you repair based only on pre-reflow results, some repairs may be unneeded. The beauty of the dual-system approach is it lets you keep x-ray at post-reflow but move AOI to where it will best address your production problems.
