Infrared (IR) verification can find PCB defects other detection methods often miss.

Estimated reading time: 9 minutes

By Jerome Gazdewich

Screening printed circuit board (PCB) defects quickly and effectively on the production line is essential to minimizing manufacturing costs. However, defects come in many shapes and sizes and often cannot be spotted easily via conventional methods. Added to that is the current proliferation of components and joints, which results in increased opportunities for defects and complicates the process of screening for them. Further, this comes at a time when the market is pressing manufacturers to make quality checks without sacrificing throughput.

Conventional Testing Drawbacks

Problems arise when existing methods cannot identify a specific type of defect. For example, visual inspection cannot discern if a component is defective or if a board has a hidden short. Problems also arise when existing methods cannot cover large or critical parts of the board, such as the integrity of hidden solder joints under a ball grid array (BGA). Likewise, users of in-circuit testers (ICT) are finding it increasingly difficult to check large sections of a PCB because of limited access to test points.

Equipping all production lines with all existing inspection and test equipment at all possible stations simply is too expensive a solution for most manufacturers. Even with the various inspection and test technologies currently available, debugging technicians still labor over defective boards, which still may end up on the scrap heap. Something is being missed.

However, there now is an alternative that can help fill the gaps in fault detection. That alternative is IR verification.

Quality Verification Figure 1. Electrically related defects such as a broken trace (a), lifted lead

IR verification finds defects by looking for anomalies in the thermal images of assembled PCBs using mature IR imaging technology. It can inspect the entire board at once regardless of component density and without the need for physical test probes to make contact. In essence, each pixel of the IR image is a virtual test probe. Thus, without slowing the production line, the system can check both leaded and area-array (BGA, CSP, etc.) devices for defects spanning the full fault spectrum, especially defects undetectable by other means.

(b) or solder bridge

IR verification systems consist of three main parts: a test chamber, an IR camera and a computer that runs thermal analysis software. In the test chamber, power is applied to an assembled PCB, which then radiates heat in a characteristic manner, i.e., its "thermal signature" that the IR camera records. Each pixel in the camera array provides a measure of the heat radiated by the corresponding point on the surface of the board. In total, the camera makes tens of thousands of simultaneous measurements across the entire surface of the assembly.

(c), affect the normal flow of current and distort a board's thermal signature.

Anything that disrupts the flow of current through the board will distort its thermal signature, causing parts of a defective board to run hotter or colder than normal. These parts can be detected by comparing the test board's thermal signature to that of defect-free boards. Thermal anomaly detection results in board failure. With no operator intervention, the task generally is completed within 30 seconds. Failed boards are off-lined, where, using the IR images, a technician determines the cause of rejection without affecting production line throughput.

Fault Coverage

Any electrically related defect affects the normal flow of current and thus distorts the thermal signature of the board. This includes defects due to solder joint irregularities, placements and component problems. It also includes some types of functional and latent defects. Mostly, these problems result in opens or shorts, which either block or divert current. For example, excess or missing solder generally leads to short circuits or opens at component joints. These can take the form of solder bridging, lifted leads, tombstoning or billboarding (Figure 1). Other examples include placement problems, such as missing components, backward or rotated components, and bent leads. Some problems, such as wrong components, may not be seen necessarily as a gross open or short, but rather as an incremental change in power dissipation.

Functional problems can affect the thermal signature because of their influence on a circuit's electrical activity at power-up. Further, some defects can distort the thermal signature and yet escape detection by functional test. These include stressed components that still function yet run hotter than normal, high-resistance shorts and poor attachment of mechanical components such as heat sinks or potting material. Such defects will be detectable via IR verification provided they affect the operation of an electrically stimulated component when power is applied to the board. An electrically stimulated part is any part (active or passive) that has current flowing through it and dissipates heat.

Figure 2. IR verification can increase the fault coverage of traditional EOL inspection and test strategies.

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End-of-line (EOL) Detection

Once assembled, there are numerous inspection and test strategies to choose from to check for defects before functional circuit test (FCT). These include automated optical inspection (AOI), automated X-ray inspection (AXI) and ICT. AOI and AXI are proficient in looking primarily for placement and gross joint defects. AOI's coverage, however, is limited to what it is programmed to see and what it cannot see. And whereas AXI handles the hidden-joint problem, particularly BGAs, it can be a complex technology and have problems with locating subtle shorts and defective or wrong components.

To test for component integrity, components must be stimulated. ICT is capable, but only at the component level. However, while ICT can discover many component problems, this becomes an increasingly difficult task as board designs become more complex and access to test points more restricted. For example, shorts become inaccessible when the circuit contains numerous components between test nodes. Problems also can become masked by "out of tolerance" indications because of poor probe contact.

IR verification, however, provides the process engineer with another valuable option that can increase fault coverage simply and in a user-friendly manner (Figure 2).

Finding Elusive Defects

IR verification has demonstrated its usefulness in detecting hard-to-find defects through its success in board recovery. These were boards that technicians have spent a great deal of time debugging with little or no success. Accordingly, the defect composition of this population of boards is different from those that would be seen at EOL. In other words, the easily located defects, such as misplaced parts and other gross opens, already have been screened out in the scrapped boards.

For their part, IR verification systems typically have demonstrated a detection rate of 70 to 80 percent on scrapped boards, and reaching as high as 90 percent in some cases. Usually, the defects detected are associated with power-to-ground shorts or bad components. Had IR verification technology been used at the EOL, those boards would have been screened out and repaired long before they hit the scrap pile, with a corresponding increase in yield at FCT. Examples of elusive detects found by IR verification:

Defective BGA. Figure 3 shows an example of a defective BGA from a computer board detected by IR verification. The device had two internal shorts that could not be found using X-ray or other means.

Figure 3. IR verification can detect internal short circuits on BGAs.

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Defective Voltage Control Oscillator. IR verification found a defect in a VCO that was oscillating at the wrong frequency for a given voltage control signal. This defect was undetectable via ICT, and FCT only provided a "board not responding" result.

Decoupling Capacitors. Finding defects in banks of these components is difficult. In one case, ICT tests exposed a short but could not isolate it. After IR verification test, however, a hot spot was uncovered immediately on one of 150 tiny capacitors.

Cable Defect. FCT indicated a board problem but could provide no causal information. Rather, the defect halted the functional test. IR verification was able to track the problem to an interlayer short in a flat cable connected to the board.

Power-to-ground Shorts. IR verification isolated an interlayer short within a 16-layer communication board. While ICT indicated a short circuit, it could not isolate the defect, in part due to test shutdown following short indication.

Programming Errors are impossible to see via visual inspection and difficult to detect by ICT and (sometimes) FCT. In one case, a PCB's flash memory contained an outdated version of its programming, but the FCT was programmed for the updated version. ICT and FCT tests rejected the boards but could not isolate the problem, which was done only with IR testing. After reprogramming, 90 percent of 100 scrap boards passed IR verification and FCT.

Heat Sink Attachment. In the case of a poorly attached heat sink (Figures 4a and 4b), which would not be spotted by AOI, ICT or FCT, the heat sink appears blue because its heat dissipation is less than normal.

Stressed Components. PCBs can pass inspection, ICT and FCT, yet contain stressed components, which can fail early in the field and present reliability hazards. These components may run hotter than normal and thus be identifiable using IR verification. The technology clearly identified one such quad flat pack (QFP) that was damaged when another defect on the board caused a 5 V bias line to go to 22 V, destroying other components in the process. After replacing the bad parts, the board with the stressed QFP passed both ICT and FCT. Although the board was completely functional, the internal damage causing the QFP to overheat likely would have led to an early field failure.

Conclusion

IR verification offers today's manufacturer an economical way to screen problem boards quickly at EOL and before functional test. It has demonstrated that it can catch defects that lead to excessive debugging cost and scrap write-offs. Not only is it effective in filling these gaps, but it also can detect defects found by other technologies, covering a broad range of the fault spectrum.

Manufacturers who benefit most from IR verification include:

• Those currently using AOI, AXI and/or ICT, but who are not getting sufficient fault coverage in a cost-effective manner as measured by expenditure of technician time in the repair loop and the size of scrap piles.
• Those who currently cannot afford to use AOI, AXI and ICT, but still would benefit financially from some form of functional pre-screening and repair.
• Those concerned about quality and reliability and who want to purge marginal product without incurring extensive environmental stress-screening costs.

IR verification is an innovative tool that provides a valuable alternative to current EOL strategies. As electronics assembly technology continues advancing, current inspection and test strategies will be taxed to the limit. IR verification fills the gaps in fault detection left by traditional AOI, AXI and ICT systems, resulting in productivity and cost-savings benefits.

A missing heat sink compound can change a PCB's thermal signature, but often goes undetected with traditional EOL inspection and test configurations.

Jerome Gazdewich, vice president, may be contacted at Photon Dynamics 125 Columbia, Aliso Viejo, CA 92656; (949) 448-0443; Fax: (949) 448-0445; E-mail: Jerome.Gazdewich@photon.dynamics.com; Web site: www.photondynamics.com