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Step 8: Test/Inspection
December 31, 1969 |Estimated reading time: 5 minutes
By Michael J. Smith
Many types of defects can be found on today's modern printed circuit boards (PCB). The types of defects and the techniques used to inspect them change constantly. Increasing data and clock speeds will reduce in-circuit test (ICT) bed-of-nails access and, therefore, require an increased use of different and complementary inspection strategies.
When PCBs were first manufactured using plated through-hole (PTH) technology, the major fault class was shorts, with the wavesolder machine the major cause of the problem. With solder's high-surface tension, solder could be trapped between the legs of the devices that stuck through the PCB, thus causing an electrical short.
With the transition to SMT, the major fault class moved from shorts to opens, caused mostly by insufficient solder on the solder pad. In an attempt to reduce the number of open faults, some electronics manufacturers used techniques such as extra solder on solder pads to minimize open faults at the expense of creating additional solder shorts. The belief was that they could find solder shorts reliably, but had not yet developed techniques to find open faults easily.
In the last 15 years, test and inspection techniques have been refined and developed to address the open defect and the evolving fault spectrum. For example, boundary scan, XOR/NAND tree and analog-opens testing, such as Opens Xpress, DeltaScan and TestJet, are electrical techniques developed to find the open defect, while image-acquisition techniques, such as optical and X-ray, have been adopted to check for opens.
Meanwhile PCB and component quality have improved dramatically in recent years. Indeed, the larger contract manufacturers (CM) have achieved high yields by eliminating the common faults of opens and shorts via better design rules and improved production equipment. However, there has been a "cost": Solder quality and other, previously low-order defects (including cosmetics) have become important because they now represent a higher percentage of the fault spectrum.
Most defects fall in five fault classes (see sidebar). Manufacturing and component faults vary greatly from manufacturer to manufacturer and supplier to supplier. But if an industry average were taken, the percentage for each fault class would be as shown in Figure 1, with each consisting of numerous related faults.
Figure 1. Typical fault class distribution. Each class consists of many related faults.
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In-circuit Testing (ICT)
ICT has provided manufacturers with a powerful tool to verify the electrical integrity of a PCB and, indeed, has been the main workhorse of the testing process for the last 20 years. Traditional ICT systems use a bed-of-nails test fixture to access electrical nets on a board. With access to all nets, it is easy to check for electrical connections between them and to locate all potential shorts - including both joint shorts and distant shorts.
With sufficient access, ICT can transmit test signals in and out of a PCB to evaluate component value and device functionality. ICT is capable of finding all component faults, including bad and marginal components, together with most component-assembly faults. Further, an ICT system can find any open faults, including solder, leg and signal opens. To differentiate between "bad" devices and open faults, ICT manufacturers have developed tools such as "Scratchprobe" and the more useful "SoftProbe" to help with the diagnosis of digital opens. Recent tools such as the aforementioned TestJet, DeltaScan and Opens Xpress use analog techniques to detect opens.
Automated Optical Inspection (AOI)
Used even longer than ICT, optical inspection has been applied in various forms but traditionally was performed by a human inspector. Today, however, it is increasingly the job of fully automatic systems. Whereas a human inspector looks for missing and incorrectly placed components, solder quality and cosmetic factors (including solder joints, connectors, components and bare boards), AOI can duplicate those features but with greater objectivity, reliability and resolution.
AOI systems use camera-based imaging technologies (Figure 2) to check for missing components and can find most component-manufacturing faults such as misaligned parts, orientation flaws and wrong parts using color-coded or optical character recognition (OCR) software. Also, by inspecting solder joints, AOI can find the excessive- and insufficient-solder and cosmetic problems with visible solder joints. By inspecting the solder joint, AOI systems also are capable of detecting visible opens, lead opens and joint shorts. The major advantage of AOI systems is that faults typically are identified precisely by the image-processing software and not simply implied, as in the case of some faults found by ICT systems.
Figure 2. AOI systems rely on camera-based imaging technologies to identify visible component and solder defects.
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Automated X-ray Inspection (AXI)
AXI and its manual off-line X-ray cousin have become more prevalent with the advent of hidden solder joints as used in ball grid array (BGA) packages, which cannot be tested by AOI. AXI also has become most important with small pads to check not just the toe of a surface mount joint (which AOI can do), but also its heel for insufficient solder, which only AXI can do.
AXI inspects for solder quality problems such as excessive and insufficient solder and solder joint voids. Additionally, the systems can find joint opens, lead opens and joint shorts. By analyzing the solder joint profile, in most cases, an AXI machine can determine if a device is missing or misaligned. Although AXI cannot measure value or function, check cosmetic appearance, or find far shorts and signal opens, as with AOI systems, its image-processing software can identify faults directly (Figure 3).
Figure 3. ICT, AOI and AXI all have unique and overlapping fault-coverage ranges.
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Conclusion
The types of defects found on a modern PCB are broad. To understand the defect spectrum for different products against the potential to introduce defects across different manufacturing lines is very complex. There have been many changes in defects and inspection techniques over the past 15 years with many more expected. New assembly technologies such as lead-free solder will change the fault spectrum and possibly the defect types that must be detected. Increasing PCB data and clock speeds will reduce ICT bed-of-nails access and require increasing use of different and complementary inspection strategies such as AOI and AXI.
With the new inspection complexities, software tools are being developed to analyze the PCB, the manufacturing process capability, and the potential fault coverage of each test and inspection system to predict and verify the defect potential and defect coverage. Advances in accurately predicting and verifying defect coverage of various test strategies will simplify the deployment of ICT, AOI and AXI in a distributed test environment.
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Michael J. Smith may be contacted at Teradyne Inc., Assembly Test Div., 7 Tech-nology Park Dr., Westford, MA 01886; (978) 589-7000; Fax: (978) 589-7007; E-mail: michael.j.smith@teradyne.com.