X-Ray Systems Reveal Hidden Defects

X-ray systems reveal hidden problems to help you overcome solder defects.

Jon Titus, Editorial Director -- Test & Measurement World, 2/1/1998

When you’re producing PCBs, you must visually inspect the solder connections between component contacts and pads on the board. The results of those inspections then help you reduce defects and maintain a high-quality production line. But today’s densely packaged products now routinely use ball-grid arrays (BGAs) and other packages that hide solder connections from view. By using an x-ray inspection system, though, you can “see” through opaque packages and observe the characteristics of minute solder connections. And manufacturers offer a variety of x-ray systems specifically meant for such inspection tasks.

Until recently, x-rays were not an obvious choice for examining PCBs with hidden connections. X-ray systems cost a great deal and have found more use in labs than on production lines. So, to overcome the lack of “visual access” to a BGA’s contacts, designers routed many of the connections to accessible electrical test pads. But those pads took space, and thus offset the space saving of using BGAs in the first place.

Even when test pads provided connections to BGA contacts, in-circuit testers and manufacturing defect analyzers (MDAs) could not detect critical soldering problems such as voids in solder, cold solder joints, and poor solder adhesion. But those defects could not go undetected. They might not cause a failure at final test, but they could lead to costly problems later in a product’s life. (Keep in mind that x-ray systems find use beyond inspecting hidden solder connections. They also inspect packaged ICs, assist in failure analysis, examine PCB layer registration, and so on.)

Look at Typical Problems
The typical solder problems that an x-ray system can help identify include those briefly described below:

Missing solder can include solder improperly placed on a PCB pad or a missing solder ball on a BGA or a flip chip. Solder balls (Fig. 1) may be knocked off during chip packaging or knocked off because of rough handling of the packages. Electrical testing can eventually detect missing solder (an open connection), but in an x-ray image, the absence of solder shows up immediately.

Figure 1. This magnified x-ray image represents the tiny solder balls that melt to make connections between a BGA and a PCB. A dislodged solder ball can cause two problems: a missing connection on the BGA and the dislodged solder showing up where it shouldn’t. (Courtesy of Feinfocus.)

Improper solder placement results when solder appears where it shouldn’t. This condition results when a solder ball that was knocked off a BGA gets lodged where it doesn’t belong—under or near another component. Improper solder placement may also result in solder bridges.

Solder bridges occur between adjacent contacts because one or both contacts received too much solder or the solder got put in the wrong place. Bridges often show up on reworked PCBs because precisely placing—manually—the right amount of solder on a board undergoing repair proves difficult. An x-ray image shows bridges as dark patterns where none should exist (Fig. 2). If you perform a powered-up test on a board with solder bridges, the short circuit may destroy the board and some of its components.

Figure 2. The x-ray image of a BGA on a PCB shows several defects: solder bridges in four places, excess solder (the large dot to the left of the center), and solder balls in the center that may not have melted properly (small black dots). (Courtesy of FeinFocus.)

Poor registration appears when component leads don’t line up properly with solder or pads or when solder balls on a BGA don’t align properly with pads. An x-ray image clearly shows registration defects (Fig. 3).

Figure 3. The two images show computer-processed images that compare good connections (left image) and the connections on a misaligned BGA (right image). The computer enhances the areas of misalignment. (Courtesy of Hewlett-Packard.)

Solder voids within a solder joint show up as light spots on an x-ray image. Optical inspection and electrical tests cannot find this type of defect. Although a joint that contains a void may operate properly, the void can indicate process problems and may lead to a later failure. Voids result from heating volatile compounds trapped inside the solder. During solder reflow, the compounds vaporize and may pop through the solder, causing an effect called “popcorning.”

Poor solder contact shows up when solder doesn’t flow properly between a component’s contact and a PCB pad. Poor contact also results from a cold-solder joint in which a reflow process failed to heat the solder enough to make it properly flow between the pad and the contact.

Open connections occur when a contact and the solder on the corresponding pad do not touch. For example, the solder balls at one end of a BGA melt properly and make good contact, but the balls at the other end do not and make no connection. Some x-ray systems can detect the unconnected balls, but some cannot tell the difference between a whole solder ball and one that wets the BGA contact and the pad. Not all opens show up in x-ray images. X-ray systems can have a difficult time finding hairline cracks that may cause an intermittent connection in solder.

X-ray systems can also find other types of production defects such as missing or misaligned components. Tombstoned or skewed SMT components fall into the latter category. In general, an x-ray system can’t tell when the wrong component has been put on a PCB. You can check the orientation of polarized capacitors, though. These capacitors have an offset “slug” that an x-ray system can detect.

X-Ray Systems Answer Many Needs
After you decide that you must inspect hidden connections, you need to examine the many types of x-ray systems that manufacturers offer. These systems range from basic manual units to sophisticated automatic production-line systems. You can choose from systems that let you examine PCBs one at a time to those that accept PCBs on a production line. You can select a system that provides an image of a PCB and supplies a few metrology tools, or you can select a system that “learns” a PCB and can automatically locate, inspect, and judge each solder connection.

Finally, you can choose systems that take a 2-D image of a PCB or systems that can produce 3-D images of a PCB. And, you can find combinations of these types of systems. Although the choices sound bewildering, a basic understanding of how these systems operate and what their capabilities are will help you narrow your choices.

At its simplest, an x-ray system provides a source that generates high-energy x-rays that penetrate through a PCB and its components. The resulting image shows everything in the path of the x-ray beam. Older systems used film to record images (See “Film Isn’t Dead Yet,” below), but most x-ray inspection systems now rely on electronics to convert the x-rays into an image that people view or that a computer processes.

Take a Manual Approach
Manual x-ray inspection systems require you to manually insert a PCB into the system and then operate the system to obtain an image. In most cases, the system can take an image of only a portion of the PCB, so you first select the area of interest and then acquire an image. The image shows up on a monitor screen. Depending on the sophistication of the system, you can get just the basic image, metrology information, or complete quantitative information about features in the image (Fig. 4).

Figure 4. An x-ray inspection system can make dimensional measurements and then convert them into quantitative information about the quality of a solder connection between a PCB pad and a component lead.

Nicolet offers such a basic system, the NXR-1002, which lets you examine bare PCBs to observe the registration of multiple layers. Other companies such as CR Technology, Micro Focus Imaging, Glenbrook Technologies, and FeinFocus also sell manual systems that can inspect bare PCBs, assembled PCBs, and complete assemblies.

If you choose a manual system that doesn’t automatically analyze the x-ray image, the operator must use his or her experience observing other boards to pass or fail a board undergoing inspection. That means that “inspectors” must undergo training so they can distinguish the good connections from the bad. But even sophisticated systems require training so they can be used properly.

Operators Must Make Choices
Operators who use just video images must maintain a mental picture—or have comparison guides—of what good connections should look like. To help in such inspections, the Dual VU system from Glenbook Technologies provides an x-ray image and a visual image of the same area on a PCB so you can observe the image both ways (Fig. 5). And you can compare the images side by side. If an area shows visual problems, an operator can view an x-ray image, too, or vice versa.

The word manual doesn’t imply lack of sophistication, though. More elaborate manual systems use computers to enhance images, and companies offer add-on image-analysis hardware and software packages. Several systems let you control the position of a PCB in the x and y planes so you can quickly move from one part of a PCB to another. These systems can also move the board up or down relative to the x-ray source and the detector. Because the x-rays radiate from their source in a cone, moving the source relative to the PCB changes the magnification. The closer the PCB gets to the source, the greater the magnification. As the PCB gets closer to the detector, the magnification approaches 1X.

The FXS-160.30 series of systems from FeinFocus also rotates the x-ray source and detector so you can take oblique views of a PCB. That type of view lets you examine the interior of plated-through connections, for example. Instead of moving the source and detector, the CRX Series of systems from CR Technology rotates and tilts a PCB so you can observe components and solder connections from almost any angle.

Automate X-Ray Inspection
If you plan to use an x-ray inspection system as part of a high-volume production line, an automated system will probably meet your needs better than a manual system. Automated systems can accept PCBs directly from a production line, from cartridges or magazines, or from robotic handlers. If you plan to include an x-ray system in a production line, it should comply with the specifications established by the Surface Mount Equipment Manufacturer’s Association (SMEMA).

An automated system’s computer controls the board-loading and image-acquiring steps. Depending on the system’s sophistication, the computer may also make quantitative decisions about quality, thus making pass/fail decisions and transmitting test data to a networked computer for statistical analysis. Because the inspection system cannot usually take an accurate x-ray image of the entire board at one time, the computer automatically positions the board and acquires images. Then, the computer analyzes the image information and determines that the board requires rework or that it passes inspection.

Because an automated system relies on a computer to control the inspection steps, you must first “train” the system in many ways. The system must know about the types of solder connections it will inspect, and it must receive quantitative information about those connections. Systems come with libraries of lead types and typical measurement thresholds for the various types of solder connections.

Customize Settings
Because your PCB may include some nonstandard contacts, the system must allow you to enter new information about contacts and components. The system should also let you adjust the thresholds for connections so you can change the parameters to adapt to your processes. Without such flexibility, the system might mark as defective a connection you would accept.

Second, the system must know about your board—the location of solder pads and PCB traces, and the types of contacts—BGA solder balls, J-leads, and so on—expected at those points. Most PCB CAD tools provide that information in standard files. The automated inspection system accepts the CAD files and converts them into their own inspection-layout maps. The system should let you modify connection types or even exclude connections from the inspection plan.

The sophisticated test-development software that x-ray system manufacturers supply lets you start inspecting boards quickly. Putting together a program for a production run, a prototype run, or even a one-off prototype takes only a few days. And an x-ray inspection system requires no fixtures, so you won’t need to modify fixtures or build new ones as your product moves from development, to prototypes, to production.

Take Images in Slices
You probably realize that an x-ray image shows everything in its path. Thus, a transmission image can be a curse when you examine double-sided PCBs. You get images of components from one side superimposed on images of components from the other. Simultaneously displaying components on both sides complicates finding defects.

An alternative x-ray imaging technique called tomography or laminography lets you view slices in a PCB in much the same way that an MRI scan produces clear images of the internal parts of living things. In PCB tomography, the system takes slices in the board’s plane. To obtain an x-ray tomograph, the inspection system moves the detector or the x-ray beam relative to the PCB and takes an image. Then it moves again and takes another image, and so on, taking multiple images of the same portion of the PCB from different positions.

Then the system uses the sequential images to compile one image. Each sequential image includes the component of interest—for example, a BGA that you want to examine—as well as components on the opposite side of the PCB. But because the detector moves, the “interfering” components do not always show up in the same positions in the images. So they don’t add as much to the final image as the BGA, which shows up clearly. In the same way that repetitive signal sampling averages accompanying noise, the moving images help reduce the “noise” from interfering components in the final composite image.

Manufacturers Offer 3-D Systems
Hewlett-Packard and Nicolet both provide 3-D systems for inspecting PCBs. In Hewlett-Packard’s HP-5DX system, the detector moves in a circle around the center of the section of board that the system images. And the system mechanically steers the x-ray beam (Fig. 6). Nicolet’s MV-6000 system uses a set of eight stationary detectors along with a steerable x-ray beam.

After a 3-D system inspects the components on one side of the board, it shifts the PCB slightly so it can image the components on the opposite side. On each side, the plane slices through the solder connections.

But PCBs are not perfectly flat, so the 3-D inspection systems must account for slight differences in board height across the surface of the entire PCB. If they did not, the inspection plane might cut through solder connections at some points, and components or the board at others. HP’s system overcomes board warping by first scanning the PCB with a laser to accurately map its surface heights. Then it adjusts the board slightly in the imaging plane to properly inspect the solder connections as it scans the PCB. Nicolet uses a dynamic surface-mapping technique that lets it avoid the step of laser scanning.

Although not strictly a PCB inspection system, the CRX-3D 3-D inspection system from CR Technology can image components. That system rotates the component through its x-ray beam rather than moving the source or the detector.

Data Help Set Sample Sizes
Although x-ray systems can reveal problems, it’s up to you to decide how to use the information they provide. Many people use x-ray systems to provide quality-control information for solder-paste printing and component placement. Placing an x-ray system on a production line lets them examine all the boards and quickly feed back solder-defect information for process control.

After you have a production line working, you may not need 100% inspection. If you’re producing the same type of board for long periods, your manufacturing may work well enough that you can just x-ray sample lots. You may not need an automated system for production line use. Some people report excellent results with sample sizes from 10% down to 1% of their production runs. In such cases, the test engineers use x-ray systems mainly to monitor their processes rather than to identify defects. T&MW
 

FOR FURTHER READING
Rooks, Stephen, “X-Ray Laminography Examines Solder Joints,” Test & Measurement World, Newton, MA, February 1996, p. 15.