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Accepting the PCB Test and Inspection Challenge
December 31, 1969 |Estimated reading time: 7 minutes
The past couple years have witnessed a surge of interest in infrared (IR) technology as a mechanism for testing printed circuit boards (PCB) and assemblies. OEM and contract manufacturers (CM) are experimenting with innovative IR tests at various phases of the PCB production process.
By John Sexton
Figure 1. The important difference between physical and electrical inspection.
The surge of interest in IR technology stems from the unique and rather daunting challenges that PCB testing products must contend with today. The increased adoption of SMT is spawning more compact, denser assemblies especially in the telecom and computer industries. According to San Jose, Calif.-based marketing consultancy firm Frost & Sullivan, the phenomenal growth of the SMT equipment market worth $3.5 billion in 1999 and an expected $8.4 billion by 2006 is triggered by changing PCB product requirements. The firm says that the trend is toward miniaturization of electronic components. Newer PCBs have more joints per board, use finer pitch components and double-sided boards. Typically, they also host devices such as ball grid arrays (BGA).
Nonetheless, in some sectors there also is a reverse trend towards using a few large but more complex PCBs in place of many smaller ones. Both developments have placed new pressures on PCB testing pressures with which conventional technologies cannot cope.
Current Testing LimitationsIn-circuit tests (ICT) still are the most widely used PCB inspection tool today, yet they cannot address the challenges posed by smaller, more densely populated boards and increased PCB production speeds. This explains why ICT's share of the global PCB testing equipment market will shrink in the next five years from 50 to approximately 25 percent, according to a Frost & Sullivan report.
Frost and Sullivan cite other reasons as well for the decline of the commercial ICT market, including the high cost of the technology and its inability to link design and test environments.
Closing the Technology GapWhile ICT is losing ground, other established PCB inspection technologies like automated optical inspection (AOI) or X-ray cannot pick up the slack. Though both are excellent physical tests, they do not electrically stimulate the PCBs being tested. While defects such as fractures, misplaced components or missing solders are detected, more subtle flaws such as specific electrical shorts or unacceptable heat/cold concentrations caused by defective components, go unnoticed.
AOI equipment use has dwindled further with the increased presence of CMs in the market. These companies have tighter budget restraints and often are concerned about whether the benefits delivered by AOI sufficiently justify the substantial equipment costs (Figure 1).
The inadequacy of ICT, AOI and X-ray to face new challenges has, in turn, led to a massive increase in bone piles, large inventories of PCBs that cannot be salvaged. It is estimated that 40 percent or more of PCBs manufactured today end up as costly bone piles because conventional PCB technologies cannot salvage them.
The IR AdvantageWhere these technologies fall short, however, IR succeeds. This is because IR systems are not limited by PCB miniaturization (or expansion) and component density. Unlike an ICT system, which electrically stimulates boards and then probes them using several physical contact points under the board, IR is noninvasive and does not require direct access to the PCB.
IR screening, based on heat differentials, also is significantly faster than material density pattern analysis techniques like the X-ray. With IR, image capture and analysis is accomplished fast enough to synchronize with the speed of automated production (8 to 15 seconds). This enables inspection in real-time.
IR systems execute up to 153,000 high-resolution, "virtual" IR probes that sense heat flow differences within a PCB. Heat flow analysis is a reliable and accurate method for viewing workmanship or manufacturing defects, and consequently, a more precise measure of PCB integrity.
Several OEMs and CMs already are leveraging these powerful IR capabilities to improve the quality and manufacturing yield of PCBs, enhance customer satisfaction and reduce costs.
IR probes help them to detect one or more of the following:
- Actual defects including the exact location of shorts
- Persistent patterns thermal images of a failed PCB may, at times, disclose a historical pattern
- Latent (or potential) defects, under certain conditions.
How IR WorksIR technology and algorithm-based applications can be combined to detect PCB faults. Tested boards are compared to a statistically generated model PCB, derived from a composite image of 30 defect-free boards. Operators identify statistical deviations from this model and whether they are within acceptable limits or not.
Areas on the board where the temperature variation is ±0.2°C show up in the IR image as red (hotter) or blue (colder). As operators become skilled in the technology, they recognize variations and identify defects more easily. Additionally, some available products may identify specific patent defects or latent (potential) ones such as high-resistance shorts or a cracked capacitor. These defects then can be rectified.
IR Inspection StagesThere are three successive stages to a typical IR test: heat flow analysis, imaging and test verdict. In the first phase, one or more high-resolution infrared cameras sense heat-flow differences in a device (these are indicative of workmanship or manufacturing defects). In the next phase, imaging, this information is transmitted to a computer data file and is image-processed with the help of algorithms that represent acceptance criteria. Finally, depending on whether these criteria are met or not, the test verdict phase assigns a pass or fail status to the PCB.
Test result information can be provided in a variety of ways, from a simple warning light on the assembly line to physically routing the PCB off the production line to engineering data on a computer screen for design or troubleshooting purposes.
IR Business AdvantagesIRs salvage PCB bone piles. IR systems most commonly are used for the reclamation of bone piles. PCBs that cannot be reworked end up in a bone pile unless salvaged with the help of IR. As PCB bone piles often are worth millions of dollars, their recovery brings immediate and substantial financial benefits. In some instances, IR technology has enabled OEMs and CMs to salvage 80 percent or more of their PCB bone piles. The payback period for this semiautomated defect analysis of failed PCBs can be as short as two to three months.
Figure 2. IR is useful because it picks up defects often missed by conventional tests, including backward components (a); broken traces (b); lifted leads (c); missing components (d); reverse polarity capacitors (e); and solder bridges (f).
In addition to such immediate and measurable benefits, however, IR tests provide long-term strategic advantages. Board failures often are symptomatic of a larger pattern. They may, for instance, be indicative of problems with the assembly process or with the design itself and these issues, too, can be identified using IR and quickly rectified.
For instance, if PCB failures stem from design problems rather than the assembly process, a CM can recommend a simple redesign to their customers possibly moving a part a few millimeters or lengthening lead spacing. CMs can enhance their collaborative relationship with customers by communicating larger diagnostic issues.
IR plays important role of "gatekeeper" in the production line. IR technology also can be used at every stage of the production process. At the prototype phase, for example, IR may be used to create designs that avoid heat concentration and optimize thermal distribution. This enhances the PCB's reliability and life.
During the production phase, IR testing may be positioned as a gatekeeper, pre-screening PCBs before other inspection technologies kick in. Alternatively, it may be used as a post-screen test. The payoff in both cases is significant.
When used as a pre-screen tool, IR reduces the amount of time spent on other tests like ICT. This, in turn, reduces inspection equipment and manpower costs. Accordingly, the benefits are present when IR is used after conventional tests because it picks up defects like high-resistance shorts, bad thermal contact, faulty transformers or broken traces that these tests may have missed (Figure 2).
IR's cost effectiveness and speed improve bottom line in PCB production. IR inspections generate cost savings wherever they are positioned in the manufacturing process because of shorter test times, reduction in the number of test stations required or improved PCB yield.
Their relatively short and simple deployment process is yet another reason why IR screening tools are being used more widely during the PCB production. Programming an ICT probe can take more than a day. Preparing for AOI/X-ray PCB tests also is a long and laborious task. Conversely, because IR systems have no moving parts, they can be deployed very quickly.
The shorter deployment times directly impact the speed and frequency of new product launches. Today increased competition and shorter product life cycles are forcing OEMs to get their products to the end-user as quickly as possible. This calls for a faster assembly process, the ability to detect flaws and initiate corrective action quickly. The time to get a PCB tool up and running, and the speed of the actual test are vitally important in the time-to-market equation.
In addition to bone pile recovery and in-line testing, IR tools can be used by OEMs to inspect PCB consignments on a semiautomated basis when they come in from CMs. This is one of the many likely future uses of this technology.
JOHN SEXTON, senior vice president, industrial systems, may be contacted at ART Advanced Research Technologies Inc., 10999 Reed Hartman Highway, Suite 206-208, Cincinnati, OH 45242; (513) 793-8300; Fax: (513) 793-8380; Web site: www.irscreening.com.