Putting X-Rays to the Test:

Inspecting for Solder-Joint Failure

By Ron Iscoff, Editor When a German physicist, Wilhelm Conrad Roentgen, accidentally discovered a form of radiation in 1895, he named it "x-ray" because of its unknown nature.1

While x-rays (electromagnetic radiation operating in wavelengths from 100 ª-0.01 ª) have become most familiar as tools for diagnosing human ailments, they are also finding increasing use in the electronics industry, mainly for the detection of anomalies on PC board assemblies (PCBA). The vast majority of PCBA defects are solder-joint related, according to most experts.

Solder Bond

Figure 1. The HP 5DX Series II integrates a time-lapse, rotated image of the PC board from an electronically steerable x-ray source.

One area of ongoing concern in chip-scale packaging, which strongly and directly addresses the bottom line, is the quality of the solder bond between the mounted CSP and the PC board. X-ray inspection equipment, when integrated into the PCBA manufacturing flow, can reveal many solder-joint anomalies, including excess or insufficient solder, bridging, no reflow, voids, missing balls, and skewed placement.2 Available systems may cost anywhere from $50,000 to $500,000 depending on the level of automation, handling options and computing power. Manual systems require the operator to load and unload the samples to be inspected. They also require the operator to employ a degree of skill in reading the sample's x-ray "signature" from the video monitor and deciding whether a sample has failed.

Figure 2. The Nicolet Imaging Systems NXR-14101 is designed for manually inspecting electronic components and assemblies.

Automated x-ray inspection equipment, which ranges from about $150,000 up, removes the judgment call from the operator and examines the x-ray image pixel-by-pixel using specialized algorithms. While a manual system provides more rough data, this intelligence is provided at the expense of throughput. With an automated system, throughput is dependent on the density of the component layout of the PCBA. Although in-circuit testing (bed of nails) is effective at detecting opens, shorts and defective components on the PCBA, ICT requires a physical contact. With the continuing trend toward high density interconnect substrates and double-sided assemblies, reduced ICT access is becoming commonplace. Moreover, ICT yields no reliability data for the solder connection, just the electrical function at the time of the ICT.3

Figure 3. The CR Technology XRV system combines x-ray and visual inspection.

In addition to being classified as either manual or automated, x-ray inspection technology is separated between 2-dimensional and 3-dimensional. The 2D technology is represented by most of the available systems, which employ transmission x-rays. The x-ray image provides information about solder mass and solder distribution. As the x-ray beam is transmitted through the circuit board, the entire solder joint is imaged, inside and out.4

X-Ray Laminography

Figure 4. X-Tek's VTX system offers a total system magnification of 1800x.

"These defects are typically 80-90 percent of the total," Charette says. The test coverage is independent of the packaging technology used and the density of the circuit board. "The x-ray test can reveal de-fects that are hard, if not impossible to find with other methods. For example, the x-ray will easily reveal insufficient solder placed on a CSP" Charette says automated transmission x-ray (2D) systems are useful for testing the solder joints on single-sided boards, but not effective for double-sided boards.

However, Craig Rahn, western regional sales manager for competitor Nicolet Imaging Systems, San Diego, argues that 2D systems provide "a great deal of information" and that the additional data provided by laminography is typically "not commensurate with the added expense and effort needed to capture the image."5 Figures 2-4 show representative 2D systems.

Figure 5. X-ray image (Nicolet)

Aside from its effectiveness in testing solder joint integrity, x-ray inspection can be effective for looking into the internal connections of chip-scale or other advanced packages. "The internal interconnections of microelectronic packages have shrunk to 1/10th the diameter of a human hair," observes Joe Owren, general manager of X-Tek LLC, San Jose. "At this size, high resolution and high magnification x-ray systems have become virtually the only way to non-destructively inspect these tiny packages before continuing with a destructive method." Additionally, says Owren, x-ray testing may eliminate or reduce the need for further testing because it provides substantial data.

In addition to disclosing solder joint defects on populated PC boards, as shown in Figure 5, x-ray inspection can reveal numerous flaws, including die attach voids (Figure 6), faulty seals and component placement defects .

"Advances in x-ray equipment design, such as fast image processing, programmable routines and quick component loading and unloading offer the user the great benefit of time savings. In today's market, where component design changes regularly, saving time in bringing a reliable package to market is very attractive," Owren emphasizes.

Figure 6. X-ray image (X-Tek)

With the continued squeeze on PC board real estate, resulting in more tightly packed PCBAs, solder-joint defects represent a major and costly irritant in the manufacturing process—and a likely entrÚ for the growth of x-ray inspection systems.

References

1. J. Suchy and E. Watson, "X-Ray," Encarta '98.
2. C. Rahn, "X-Ray Inspection Applications for Chip-Scale Packages," Chip Scale Review, July-August 1998, p. 54.
3. F. Silva, untitled, Nicolet Imaging Systems, nd.
4. Ibid
5. Rahn, p. 49.