X-ray approach depends on application

Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 9/26/2006 7:56:00 AM

In our July 2006 "Machine-Vision & Inspection" newsletter, Paul Groome, manager of Teradyne's automated x-ray product group, described his company's decision to forego offering AOI inspection systems in favor of x-ray. I recently contacted him again to discuss his perception of the relationship between alternate x-ray techniques and their target applications.

Q: How does x-ray inspection in general fit into an overall test strategy?

A: In-circuit test is still the first line of defense, but it is no secret that access to board nodes is becoming more difficult. X-ray complements the fault coverage we still get with in-circuit test. Between the two, we expect to find all manufacturing defects. The in-circuit tester will identify a bad 1-kohm resistor. Placement or solder-joint quality of a 201 device is left for the x-ray system—even if the 201 is obscured by the board or another device.

Q: What kinds of defect rates do you encounter?

A: Failure rates are much lower than they used to be. A great deal depends on the nature of the application. Manufacturing processes for cell phones and automotive modules must deliver high-quality products in high volumes. Companies expect less than 50 defects per million opportunities (DPMO), although they often experience closer to 500 DPMO. At that level, a densely populated board almost never passes first time. Our goal is to deliver a final defect rate lower than 10 DPMO. We think we can do that.

Q: How do manufacturers decide between two-dimensional and three-dimensional x-ray solutions?

A: Both 2-D and 3-D technologies offer advantages. On boards with relatively low-profile components, the speed of the 2-D approach often represents a better choice. Our 2-D systems achieve inspection speeds of up to 6 sq.in./s, adequate for high volumes. Many automotive modules, for example, measuring 6 in. by 7in., can be inspected in less than 16 s including handling time.

But consider boards containing high-profile devices such as ball-grid arrays (BGAs), land-grid arrays (LGAs) and package-on-packages (PoPs). In those cases, 2-D is fast but 3-D methods allow more comprehensive results. Being able to perform 2-D and 3-D analysis in a single pass seems the ideal solution.

[Editor's note: Instead of conventional pins, LGA devices contain pads of bare gold-plated copper that make direct contact with the board surface. Device manufacturers contend that LGAs provide a larger contact surface than pins do, permitting cleaner signals and higher frequencies. In addition, the devices' greater node densities accommodate more power pins and therefore supply more stable power to the chip. With PoPs, two packages rest on top of one another. A standard interface routes signals between them. Typically, a memory subchip resides on top of a logic package to save board space, especially for products where size is a critical factor.]

Q: What determining factors do you see encouraging the wider acceptance of x-ray inspection?

A: One issue is ease-of-use. We don't want to require a higher skill level from operators on the manufacturing floor than we have to. And, of course, cost. As the technology's costs decline and the value of its benefits increases, more people will adopt it.