X-ray inspection offers unique capabilities that address today’s increased demands on process quality at both the bare and populated PCB levels.
The emphasis on process quality is partly due to more stringent requirements that the use of SMT components places upon bare PCBs—board flatness and solder- plating thickness must be better controlled, for example. Part of the emphasis is created by greater circuit density that increases the number of board layers and reduces copper trace width. Higher trace density requires better solder-resist performance and tighter control of the etching and solder-plating processes.
XRF accurately and nondestructively measures coating and plating thickness and composition on PCBs, ICs, connectors, and lead frames. Figure 1 demonstrates the degree of dimensional and compositional detail available. Manufacturers of these parts use XRF test results as inputs to their SPC programs as well as to meet ISO/QSO reporting requirements and provide process certification for customers.
The largest factor affecting the quality of PCB assemblies is improper manufacture, whether it involves the wrong parts, incorrectly placed parts, or faulty solder joints. As components become smaller, are more densely packed on PCBs, and the use of AAP expands, X-ray inspection is becoming the preferred means of determining the quality of hidden solder joints.
Applications of Transmissive X-Ray Systems
In high-volume, low-cost, consumer hand-held products such as portable computers and phones where size and ruggedness are major requirements, the use of CSP is growing. CSP is defined as one less than 1.2× the size of the chip itself. BGAs, technically more than 1.2× the size of the chip, and CSPs have many advantages. For example, single BGAs achieve high-connection density of 300 to 700 leads in computer applications.1 The relatively large ball contacts conduct heat away from the chip; make, good high-frequency connections; and are less susceptible to handling damage than are very fragile, precisely formed QFP leads. Most of all, BGAs and CSPs are considerably smaller than QFPs.
On the downside, it’s impossible to inspect reflowed BGA and CSP solder connections visually because they are beneath the package. Boundary scan or other BIST methods may prove that the device has been properly positioned and connected, but purely electrical tests cannot determine the quality of solder joints until after marginally faulty ones have actually begun to fail. The only practical choice is X-ray inspection.
Desirable X-Ray System Features
According to Collin Charrette, product manager of the HP 5DX series of X-ray inspection systems at Hewlett-Packard, the following features are desirable in an X-ray system:
Display of cross-sectional images.
Automatic analysis of joints.
Repeatable measurements.
Repeatable and accurate good/bad decisions.
Reputable vendor with a good service record.
A proven track record of good performance for a particular system.
Low-noise comparison of images is a prominent feature of Digiray’s reverse geometry systems. Transmitted X-rays are directly counted at each pixel site, producing so-called first-generation data. Because this approach results in low-noise, high-contrast data, image subtraction from a known-good picture is particularly clear for first-generation data.
Mr. Charrette of HP compared the pros and cons of X-ray inspection. On the positive side:
High fault coverage, typically 98% of process defects.
Exact pinpointing of faults.
Quick to program once a library is established.
No fixturing required.
Faults caught that no other method can find, including defects which may gradually lead to failure in the field.
Good throughput and cost-effectiveness for automatic systems.
Complementary coverage to ICT.
But there are some drawbacks. On the negative side:
Limited identity of dead, bad, or wrong components.
Relatively expensive compared to AOI.
Heavy because of required shielding.
Automatic systems have two main advantages over basic manual systems. The obvious difference is the capability to keep up with the production-line beat rate. The more important difference from an inspection point of view is the good/bad-decision repeatability and accuracy afforded by using computer-image processing algorithms instead of relying upon the operator’s experience and judgment. Manual systems cost considerably less and are a good choice for prototype inspection and periodic production sampling.
Of course, there are differences among X-ray technologies. See the sidebar for a brief discussion of laminography, tomography, and fluorescence spectroscopy.
For example, most systems have the capability to learn what constitutes a good joint by examining known-good parts. The “s” in parts is important because the rules used to distinguish good from bad may be distilled from many examples of correctly soldered joints, all of which differ from each other in small ways. Not only does this technique reduce programming time, it also ensures that the system is comparing test joints against a realistic range of criteria.
“Current software is strongly based on an image-analysis technique referred to as a-priori knowledge,” explained HP’s Mr. Charrette. “This is the approach of memorizing data from previous known-good boards and comparing the data to the board being tested. This improvement has had a dramatic effect on the accuracy and the ease-of-setup for today’s systems. Previous systems would catch approximately 90 to 95% of the shorts, but the new approach has resulted in detection rates near 100%.”
Knowing what a good solder joint looks like is only part of the solution. Automatic X-ray inspection systems download data from PCB CAD/CAM files to find out where a component should be located and what kind of joints it should have.
Manufacturing Process Control
Information about the faults is fed back to correct the manufacturing process. PCB assemblies rejected by X-ray inspection are analyzed as part of the ongoing SPC effort to determine and eliminate recurring types of problems.
Some systems accumulate inspection fault data and can report statistics such as the location and types of components having the highest fault level or the areas of the board with the most faults. An important financial argument to be made for buying an X-ray inspection system is significantly reduced rework and scrap because of projected improvements in the manufacturing process.
Factors Affecting Throughput
Overall throughput of an inspection station depends upon how much needs to be inspected and how quickly it can be accomplished. This also is true for X-ray systems. But because a single image may contain many solder joints, more joints in an assembly does not mean much higher inspection times if the system has sufficient resolution and computational speed. As an example, the Nicolet NXR-1410HR has a 1.8″ maximum FOV. Thousands of joints could be present on a high-density SMT board, but all those within the FOV would be inspected from the data gathered in one exposure.
Some manufacturers claim that their X-ray system throughput increases with connection density. It’s true that joints/seconds increases, but if you measure throughput as boards/seconds then, at best, throughput is much less dependent on joint density than in other types of inspection systems.
Mr. Charrette of HP offered other factors affecting throughput:
Component Pitch—A small pitch implies that magnification is needed, which reduces the FOV. A smaller FOV requires multiple repositioning to cover the entire board being inspected.
Board Warping—Excessive warping slows down the methods used to form cross-sectional views of both sides of a populated PCB.
Mechanical Board Movement—In some systems, the PCB is moved around at high speed, a process that can excite mechanical resonances that have to die down before meaningful measurements can be made.
References
1. Crum, S., “Trends in Advanced Component Technologies,” Electronic Packaging and Production, January 1998, pp. 48-53.
2. Teska, M., “2-D and 3-D X-Ray Inspection Systems—What Do They Mean?,” EE-Evaluation Engineering, February 1998, pp. 28-29.
NOTE: This article can be accessed on EE’s TestSite at www.nelsonpub.com/ee/. Select EE Archives and use the key word search.
Glossary of Terms
AAP Area Array Packaging
AOI Automated Optical Inspection
BIST Built-In Self-Test
BGA Ball Grid Array
CSP Chip-Scale Packaging
FOV Field of View
ICT In-Circuit Test
PCB Printed Circuit Board
QFP Quad Flat Pack
QSO QS 9000 is Automotive Industry Equivalent of ISO 9000
SBL Scanned-Beam Laminography
SMT Surface Mount Technology
SPC Statistical Process Control
XRF X-Ray Fluorescence Spectroscopy
X-Ray System Differences Exposed
The various 3-D technologies available today all produce focal plane slices through the DUT by combining different, multiple views. The techniques range from mechanical image integration used in SBL to totally stationary, computed solutions.
Conventional SBL produces a cross-sectional image by using a steerable X-ray source and a synchronized rotating detector.2 Figure 2 shows how a focal plane is established and how objects above or below this level are smeared. The result is a focused cross-sectional image cut through all components, or the board itself, at the focal plane height.
To view objects on the other side of the board or at a different height on the same side of the board, for example, the board position has to be changed relative to the focal plane. This is the scheme shown in Figure 2 and used by HP in the 5DX series.
Data is generated indirectly by SBL systems. The image is formed on a large-area X-ray detector that is viewed by a vidicon or CCD camera. The scanned video output is digitized to produce image data.
Synthetic tomography or tomosynthesis, also known as computed laminography, differs from SBL because digital processing is required to produce the focal-plane image. In SBL systems, because the DUT features lying in the focal plane reinforce as rotation progresses, a slice through the DUT at focal-plane height appears directly on the X-ray detector.
Synthetic tomography also forms images of DUT cross sections, but does it computationally by processing more than one view through the DUT from different angles. There are different types of systems.
Both the beam and detector remain stationary, while the DUT rotates to a series of positions in a plane perpendicular to the beam but offset from its center. An offset is required to produce a change in the viewing angle as rotation progresses. This method is used by CR Technology.
Similar to SBL, a steered X-ray source is used, but a large, stationary detector replaces the synchronously rotating detector in Figure 2. Nicolet uses this method in the MV-6000 system. The steered X-ray source can be used to provide the optimum viewing angle for conventional transmission imaging, or it can be used to create multiple views of the DUT from which a focal plane slice can be computed.
Digiray arranges the tube, detector(s), and DUT as shown in Figure 3a. A large X-ray detector/image intensifier and a video camera are not used in these systems. The term “reverse geometry” refers to locating the DUT very close to a large, raster-scanned X-ray source. Typically, a number of small detectors are used to obtain different views of the DUT from a single exposure, although only one detector is shown in the figure. The classical location of the DUT immediately in front of a large X-ray detector, some distance from the source, is shown in Figure 3b.
According to Dr. Thomas Albert, director of technical communications at Digiray, the system has these advantages:
No Moving Parts—Depth information does not depend upon relative motion of the steered X-ray beam and the detector.
Much Lower Noise—Small detectors positioned relatively far behind the DUT are not struck by scattered X-rays. Most of the signal recorded is due to X-rays passing through the DUT so the contrast is improved by up to an order of magnitude.
Direct Digital Output—The X-ray beam is deflected to 1,000,000 discrete positions in a raster pattern covering the selected field of view. The count value from each scintillation crystal detector corresponds directly to the beam position at a particular time. Magnification is accomplished by reducing the size of the raster. Panning is done by selectively repositioning the shrunken raster.
One-Second Scanning—Multiple focal plane images can be developed from one scan. All pixels are scanned in one second.
X-Ray Fluorescence Spectroscopy
A DUT that is irradiated re-emits X-rays characteristic of its composition. This is the principle behind X-ray systems that measure coating or plating thickness and composition. These systems are trained by observing the emitted X-rays from a number of known, standard coatings.
The results of a real test are only valid if the X-ray system is focused in the same way as it was during the training procedure. To address this set of conditions, precise mechanical height adjustment is facilitated in CMI’s XRX system by using a small laser to ensure repeatable height repositioning from sample to sample.
X-Ray Inspection Products
Data-Presentation Software
Reports on Coating Thickness
SmartDocs is a data presentation program that generates reports based on any user-defined combination of tables, text, graphics, images, statistics, or charts. The data-base program can organize test data generated by the company’s X-ray fluorescence coating inspection and thickness measurement system. SmartDocs processes the data base responses to user-defined sort criteria. Reports can be used to display production and quality data, satisfy ISO/QSO reporting requirements, and provide process certification for customers. CMI, (800) 678-1117.
Combined Vision and X-Ray
System Increases Test Coverage
In addition to 100% inspection of BGA, flip chip, and other hidden solder joints, the XRV System identifies correct part orientation and positioning. Three CCD video cameras are combined with an X-ray source and X-ray-sensitive video camera. The system self-learns a PCB by extracting part information from the CAD pick-and-place file, automatically stepping through a known-good board. The system can reduce test cost, board handling, and floor space; improve quality; and increase throughput. CR Technology, (949) 448-0443.
Reduced Footprint System
Now Inspects Larger PCBs
The improved HP 5DX Series II family of automated X-ray test and inspection systems now handles PCBs up to 18″ × 24″. Overall floor line length has been reduced by more than 40%, and the accuracy of the good/bad inspection-decisions improved. Earlier 5DX system benefits have been retained, including fixtureless, high-speed inspection of densely populated PCBs; short programming time aided by CAMCAD for test CAD transfer software; and more than 98% test coverage regardless of access. From $349,000. Hewlett-Packard, (800) 452-4844, ext. 5764.
Component and Joint Inspection
Suits High-Resolution System
The 2-D, manual NXR-1410HR Real-Time X-Ray Imaging System with magnification of up to ×240 and maximum spatial resolution >40 line pairs/mm can be used to inspect CSP and flip-chip devices. The 1,300-lb machine has an 18″ × 20″ inspection area and variable 0.045″ to 1.80″ FOV. Conversion to an NTSC video format following the X-ray detector/image intensifier is via a 512 × 480 pixel CCD camera with a zoom lens. Features include split screen display, a video micrometer, text annotation, histograms, drill offset, and void measurement. Nicolet Imaging Systems, (619) 635-8600.
Copyright 1998 Nelson Publishing Inc.
November 1998