2-D or 3-D X-ray Changing board topologies cloud the issue
08/01/2000
Examining the factors involved in choosing an AXI system for the post-reflow testing of soldered connections.
By Frank Silva
The process of manufacturing electronic circuits has become a complex operation prone to many variations. Electronic assemblies built for telecommunications, computer systems, optical networking, automotive and avionics applications are very sophisticated and, as such, use sophisticated area array packages, such as ball grid arrays (BGAs), flip-chip assemblies (FCAs) and chip-scale packages (CSPs). These package types pose several challenges for the test and verification of assemblies.
 Figure 1. Solder joints are analyzed and measured based upon a 3-D X-ray profile that is generated from either a 2-D transmission or a 3-D X-ray slice. |
Automated X-ray inspection (AXI) has become an essential tool for determining process quality and assembly verification in high-speed assembly lines. Of all tests for quality control, AXI itself provides the most complete solder joint test access along an entire solder contact length (e.g., heel-to-toe and side fillets). This provides the best opportunity to find otherwise difficult-to-observe defects and the most comprehensive measurements for process monitoring. Other methods, including automated optical inspection (AOI) and laser systems, are line-of-sight test methods and therefore only capable of testing a portion of any solder joint. Additionally, AOI, laser, in-circuit testing (ICT) and various other types of test systems have limited test access, and particularly cannot test assembled area array types of devices.
An AXI system combines speed and high-quality control and can provide a full spectrum of fault coverage, ranging from defects between open and shorts, including insufficient, off-position, lifted leads, tombstoning, billboarding, solder balls, voids, presence/absence and excess solder. However, what type of AXI system is most useful for the post-reflow testing of soldered connections? Is a 2-dimensional or 3-dimensional system best? This is a source of serious discussion and confusion based upon the perceived capability of each system. This article examines the disparities between the two X-ray inspection systems and discusses how each tests solder joints in a fast and consistent manner.
2-D or 3-D AXI
In practice, there are two forms of AXI X-ray inspection systems. The first form is the traditional single-plane or transmission X-ray imaging system (2-D), and the second, a more sophisticated form, is a multi-plane, or off-axis X-ray imaging system (3-D).
 Figure 2. Typical 2-D image train and transmission X-ray image showing a bridge on a QFP. |
Foremost, both the 2-D and 3-D AXI systems project X-ray images that are planar. While the 3-D version can slice or focus an image at any given height plane (offering a significant advantage over the 2-D system), the final image is a planar image, nonetheless. But, this planar image is actually seen by the computer as a three-dimensional image. Each x-y coordinate uses the gray-scale intensity of the image to create a 3-D image. This computer-generated 3-D image is called the solder joint profile (Figure 1). The profile has a measurable shape (height, length, width, thickness, fillet slope and many other features), which determines good soldered connections from defective ones on either a 2-D and 3-D AXI system.
2-D AXI is widely used for the inspection of single-sided surface mount technology (SMT) assemblies, such as automotive electronic printed circuit boards (PCBs). The traditional transmission system is imaged at a 90-degree plane to the X-ray beam and detector (Figure 2). This type of setup produces an X-ray image projected as a single plane. 2-D AXI systems can quickly image a PCB providing significant test access and fault coverage at a reasonable cost. However, on some complex PCB assemblies, tall devices (e.g., filters or large capacitors) and certain area array devices can cast shadows, posing some difficulties for inspection of insufficient or open solder joints.
As compared to a 2-D AXI system's straight-down image, 3-D AXI systems project an X-ray beam at an angle to the board. Illustrating the benefit of this form of inspection, Figure 3 shows an X-ray image taken using board rotation. By rotating the board, the X-ray beam is projected at an angle - providing the opportunity to see the solder attach point and fillet clearly for improved diagnostic resolution compared to 2-D AXI systems. These types of off-axis images provide the foundation for 3-D AXI imaging systems.
 Figure 3. Image generated using board rotation. Shows an open on a flip chip. |
Not to be confused with full 3-D reconstruction, most 3-D techniques (including computed tomography, conventional laminography and digital tomosynthesis) take 2-D X-ray images at given slice heights or planes and then, through advanced image software, assemble a completed 3-D image. Although these methods are not new (their historical origins are derived from medical imaging technology), implementing them quickly in a production environment certainly is innovative.
Laminography
Figure 4 shows the principle of laminography as a synchronized motion between an X-ray source and a detector. Laminography generates a geometric focus plane, which forms an X-ray slice image of that plane. Objects in the focal plane are always projected at the same position on the detector. On the other hand, those objects that are not in the focal plane are projected to other positions of the detector and therefore are blurred out as the source and detector move in a complete circle. Laminographic imaging is always in motion and requires high reliance on repeatable and accurate mechanics to produce meaningful images.
 Figure 5. A set of four off-axes images. |
While laminographic systems can "shift" overlapping solder connections, tall devices and BGA shadows reduce the effective test coverage because of a blurring effect, which also works to limit fault detection. Laminography, as a result of these blurring effects, has reduced contrast. This limits the detection of certain open and insufficient solder, especially on BGA, FCA and CSP packages. This higher-than-desired false failure rate, inherent in laminography, is a source of frustration for many manufacturers.
Digital Tomosynthesis
For the inspection of dense PCBs, the ability to find the most demanding defects (such as opens) without incurring a high false-call rate is a key condition to maintaining an efficient test operation. The image quality produced by digital tomosynthesis provides the clearest conditions for finding and detecting difficult defects.
Digital tomosynthesis is a computational technique, which combines multiple images taken from multiple off-axis X-ray projections (Figures 5 and 6). By combining these single projections, any cross-sectional image plane can be produced. This technique incorporates a steerable X-ray beam that allows precise electronic control over the angle and location of the X-ray beam projection onto a large format image detector. Because this technique is digital and contains more image information than the analog laminographic technique, enhanced contrast and reduced image artifact is achievable and allows both greater test access and fault coverage.
AXI Test Strategies and Applications
Which test strategies are best suited for monitoring the SMT process? This is a not just a matter of economics or production throughput; ultimately it is a concern of appropriate test access, fault coverage, and proper measurements necessary to monitor the process and test the assembly. Key to the test equipment selection, or as part of the consideration for a complementary test strategy, these factors determine how well SMT process variations can be managed.
It is no secret that AXI systems are beginning to capture more attention as a complement to (and, in some cases, a replacement for) ICT testers. AXI systems must therefore work to find defects that are traditionally covered by ICT. The types of defects most manufacturers are concerned with are opens and shorts. As previously discussed, short and excess solder conditions are easy to find. The detection of opens and insufficient solder poses the greatest challenge for all test systems.
 Figure 7. Image slices of CBGA packages at the mid-plane. The left is done using tomosynthesis and the right uses laminographic techniques. Note the clarity. |
2-D AXI systems are most effectively used on single-sided SMT assemblies providing 100-percent pin test access. As compared to leaded devices, such as gullwing ICs and BGA devices, inspection is more challenging to test for insufficients and opens. Generally, it is easier to find bridging, missing balls and voiding using the conventional 2-D X-ray inspection test techniques, but 2-D AXI systems can have problems detecting subtle opens on certain BGA connections. Finding opens becomes complicated by the shadow effect of the diameter of the ball compared to the fillet because the ball diameter obscures the fillet. Yet overall, 2-D X-ray inspection is providing manufacturers with effective solutions for all test packages, including BGAs, that are easier to implement and maintain than 3-D AXI.
For certain types of plastic (eutectic) BGA packages, the fillet shadow effect is not problematic for the 2-D AXI system. Standard BGA devices normally produce a visible inner and outer ring. In these cases, making diameter measurements can provide good results for detecting BGA opens. Today, many high-end telecommunications PCBs are successfully inspected with 2-D AXI for nearly half the price of a 3-D AXI. That being said, not all plastic BGA packages are alike and neither are the substrates to which they attach.
On the other extreme, the most challenging assemblies to test are the ceramic ball grid array (CBGA) package (non-eutectic) or column grid array devices. These types of assemblies require a 3-D AXI solution to provide the image capability necessary to generate images at the pad/solder interface to evaluate and measure the solder. Typically, these types of high-melting-point packages do not provide clear solder joint profiles that conclusively yield detection. The ability to produce high-resolution and high-contrast images is critical to the inspection of the CBGA device. However, this alone may not result in 100-percent detection of hard-to-find defects. The technology to provide a more conclusive test for opens on non-eutectic or high-melt area array devices will likely require the generation and final computation of images in vertical slice planes that can only be achieved with 3-D digital tomosynthesis or computed tomography test systems.
 Figure 8. a.)Transmission image. b.) Slice at top side. c.) Slice at bottom side. |
The primary advantage that 3-D provides over 2-D AXI is increased diagnostic resolution because the solder connection can be more finely viewed and measured. This is significant for area array packages because the solder can be tested at the board interface - providing the best opportunity to detect open, insufficient and cold solder joints. However, this increased diagnostic capability also increases the cost of the test where the only major benefit may be the testing of certain area array devices and high-density double-sided assemblies. In practice, the type of faults typically found during the inspection of fine-pitch ICs and discrete components do not necessarily warrant the need for 3-D AXI, unless these devices are obscured by other components.
Double-sided Challenges
Figure 7 shows a view of a double-sided assembly with an IC on the topside and a resistor on the bottomside of the assembly. When a PCB is manufactured as a double-sided assembly, 2-D systems may not be able to test the assembly to find the full spectrum of errors because a large percentage of the solder connections normally overlap on these assemblies and, therefore, cannot be automatically tested using 2-D AXI. If all the pins in Figure 8 were inspected using a 2-D AXI system, a bridge would be detected. This would be a false call. To overcome this obstacle, many manufacturers are successfully using an alternative strategy with 2-D AXI by inspecting Side A of the assembly with 100-percent access first, and then inspecting Side B, still providing overall test coverage.
On the other hand, the major advantage of 3-D X-ray systems is the ability to inspect double-sided assemblies without adopting alternative manufacturing considerations where increased solder-pin density produces significant overlapping of solder connections; 3-D methods can additionally separate superimposed solder joints (top/bottomside-mounted devices). 3-D AXI inspection is a necessity, especially on complex telecommunication PCBs. The significant loss of test accesses, either by design intent or sheer connection density, renders all techniques - except for 3-D AXI - less effective. Only a 3-D system could generate the necessary top and bottom image plane required to provide full test access to all these pins.
Conclusion
Companies are developing integrated testing solutions to keep pace with the accelerating miniaturization and ever-changing package and process technologies used to assemble printed circuit cards. Both test access and fault coverage are driving the need to adopt AXI test strategies at an ever-increasing rate. With the advent of high-speed networking and related software tools, the post-reflow testing of soldered connections is not only minimizing potential field failures but also providing fast and reliable monitoring of the SMT process to drive toward an improved operation and increased yields. Ultimately, this provides value and assistance for managing the inherent variation of a complex manufacturing process.
Digital tomosynthesis and laminography 3-D X-ray inspection techniques offer manufacturers a different diagnostic capability. Laminography, with its analog imaging, will always be prone to lower contrast and blurring of edges. This is not necessarily a problem for the detection of presence/absence, solder bridges or excess solder. But it is critical to correctly detect insufficiencies, opens and solder voids while minimizing false calls and potential defect escapes. A digital tomosynthesis-based 3-D system will provide the greatest assurance that such defects can be found while minimizing false failures. Digital tomosynthesis provides better diagnostic capability than laminography for those assemblies that require 3-D X-ray imaging. However, 2-D AXI is just as capable of inspecting complex PCBs, providing full-test access on single-sided assemblies and near-total access on double-sided assemblies at a lower cost compared to 3-D AXI.
AP
FRANK SILVA, AXI technical support manager, can be contacted at Nicolet Imaging Systems/SRT (a division of GenRad), 8221 Arjons Drive, San Diego, CA 92024; 858-635-8662; Fax: 858-695-9902; E-mail: [email protected].