Friday, July 29, 2016
VOLUME -23 NUMBER 12
Publication Date: 12/1/2008
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ARCHIVE >  December 2008 Issue >  Special Feature: Test and Measurement > 

The Future of In-Circuit Test
"Analyst" low-cost ICT test system.

Distinct eras define the 40-year history of circuit board test equipment. Digital functional testers such as the Teradyne L115 and General Radio 1796, supported by LASAR and CAPS simulators respectively, dominated electronics manufacturing test from the late 1960s to the late 1970s. In 1978 GenRad introduced the GR2270, and Hewlett-Packard (now Agilent) rolled out its 3060 in-circuit tester (ICT).

The GenRad development legitimized what had been up to that point a backwater of automatic test served by small companies such as Faultfinders and Zehntel. Faultfinders was purchased in 1978 by Fairchild, which was in turn acquired by Schlumberger in the early 1980s. Teradyne purchased Zehntel in 1988.

By virtue of its ability to directly and quickly identify common assembly and parts defects that occurred on assembled circuit boards, ICT quickly supplanted digital functional test as the preferred manufacturing test technology. Test coverage of digital ICs via "backdrive" vector test was available on most ICT systems by the early 1980s. As circuit board complexity and density grew, so did ICT's capabilities. It's not an exaggeration to assert that ICT became an important enabler of the volume and technology growth of electronic products — especially personal computers and telecommunications infrastructure — throughout the 1980s and 1990s when worldwide ICT system sales peaked at nearly $750 million in 2000. (Source: Prime Research Group.)

Now in the early 21st century telecommunications and personal computers are no longer the influencers of electronics progress. Today, the technology leadership role belongs to consumer electronics — especially handheld products that combine personal computers, MP3 players, GPS devices and cell phone features. Just compare a cell phone from 1999 with a 3G iPhone or Blackberry. This order-of-magnitude increase in functionality in less volume has had a profound impact not only on circuit board designs but on the processes to build them, as well. There are several key trends.

  • Physical size constraints necessary for handheld electronics mean increasing component complexity and board density.
  • Rapid improvement of high-volume SMT processes with higher manufacturing yields.
  • Improved parts quality — especially digital ICs.
  • Ever-faster integrated device operating speeds — and the circuits surrounding them.
  • Increasing use of programmable parts (ISP) in circuit designs. Not surprisingly, this technology and process evolution affects circuit board test and inspection in significant ways. Until around the year 2000, ICT test coverage was well aligned to diagnosing the typical defects that occurred on circuit boards. The PC motherboard served as the high technology exemplar of this era. Although there was lots of discussion among test cognoscenti about impending "loss of access," the classic bed-of-nails test fixture could contact almost every circuit node on every board. Primary defect classes of the late 1990s era included:
  • Solder shorts and IC pin opens.
  • Component and process problems that could be detected electrically (out of tolerance, reversed, etc.).
  • Low speed (<5MHz) digital ICT faults and open pins.

However, as circuit board and component technologies continue to evolve, traditional ICT strategies and equipment have not. Almost every ICT supplier has retrenched since its 2000 market heyday. Most Big Iron systems of that time are no longer in production. Although ICT remains — and will continue to remain — a widely used manufacturing electrical test technology, new test and inspection technologies such as boundary-scan, automated inspection both optical (AOI), and automated x-ray (AXI) have enjoyed rapid growth. There are numerous reasons for the gradual decline of ICT, but two technical realities have played the most significant role.:

Loss of electrical access to circuit nodes. The theoretical loss-of-access discussions of the 1990s have turned to reality as smaller, increasingly complex components are packed more densely on boards that are in turn shrinking in size. Declining accessibility means declining ICT coverage.

The fault spectrum shift. Even as board density and component complexity have increased, automated assembly and improved component quality have increased manufacturing yields. But at the same time the relative proportion of typical part and assembly process defect categories have changed. This shifting landscape of fault types means shrinking ICT test coverage, in turn altering what in-circuit test needs to do on the manufacturing floor.

To see how ICT requirements are changing, we need to unpack the implications of the fault spectrum shift.

  • The automated SMT process is subject to fewer process defects in placing parts on boards (missing, skewed, reversed, etc.) than older through-hole processes. Placement errors have become a smaller proportion of the overall fault spectrum.
  • Solder-related connectivity problems (opens) caused by insufficient solder or solder paste issues tend to occur more frequently in the SMT process than do the solder bridges (shorts) that predominated in through-hole processes.
  • Many solder-related defects are now latent or quality-related, e.g. cold solder or component "tombstoning" and "billboarding." Taken as whole, solder-related defects now make up a larger proportion of the SMT fault spectrum.
  • Smaller, denser connectors and tighter physical tolerances that characterize handheld products have resulted in mechanical defects becoming a proportionally greater percentage of today's fault spectrum.
  • The component parts defect rate has also shrunk substantially, especially for digital ICs.
    10 to 15 years ago digital failures accounted for upwards of 35 percent of total faults. Today, digital defects usually make up less than 1 percent of board faults. The shifted fault spectrum means that the test and inspection strategy must also be realigned in order to maintain overall test coverage. If the ICT strategy — and the in-circuit testers that support that strategy — remains unmodified and optimized for yesterday's fault spectrum in today's rapidly changing technology environment, there is increasing misalignment between what needs to be tested and what is actually being tested. Test coverage misalignment means higher cost per fault detected, or more ominously, more undetected faults that escape — either to functional test or, worse, to the field, resulting in increased warranty and service costs due to untested latent defects.
Eliminating Digital Testing
As we noted, today's fault spectrum requires far greater attention to solder- and connectivity-related defects, best served by optical (AOI) or X-ray (AXI) inspection. On the other hand, in-circuit digital testing can be eliminated since digital faults have virtually disappeared.

It all appears so simple: just add new AOI or AXI systems to the process. But while the technology solution appears obvious, the economic solution is less so. A very real cost problem looms. Just adding new inspection technology like AOI on top of existing Big Iron ICT's cost will break the budget barrier. Exacerbating the cost problem is that in today's reality of shrinking resources, the budget is also probably shrinking. Nevertheless, we'll assume the budget is at least constant. The budgets that served so well for a test strategy centered on Big Iron ICT will not accommodate a host of new equipment and technologies. Either the test budget must be increased or expense must be reduced somewhere.

The key to finding budget dollars for new investments in test and inspection technology is possible when it is recognized that the existing Big Iron ICT — even though it's old enough to have been written off and often viewed as a "solved problem" — is not "free" at all. Continuing to implement new test jobs (fixture + test program) on traditional ICT, with its expensive fixturing requirements and complex test programming infrastructure, creates an ongoing "expense bleed" that's actually a lot more costly than newer ICT alternatives.

When we reexamine today's fault spectrum we see that the in-circuit test task is actually less complex than it once was. Why? Primarily because digital testing is no longer required. Less comprehensive requirements means ICT should take up less budgetary space. In short, the way to free up budget for new test and inspection technologies is to invest in Low-Cost In-Circuit Test with its lower capital cost and even more important, its lower ongoing cost of test fixtures, test programs and system support.

Lowering the Cost of Test
One major electronics OEM who was testing more than 9 million boards a year recognized the potential savings of Low-Cost ICT. Recognizing that the Big Iron testers were becoming too costly to support and that capabilities such as digital vector test were no longer required, this manufacturer transitioned to CheckSum Low Cost in-circuit testers as new board types came on line. Within two years of its decision, production volume had increased 20 percent to more than 11 million boards per year, and the company's tester inventory stood at 35 Low-Cost in-circuit testers and just 5 Big Iron systems, down from the previous 30 Big Iron testers. By using Low-Cost ICT for all but the few boards still requiring digital ICT, this OEM reduced per board test cost from $0.20 per board to just over $0.08 a board, a savings of 60 percent.

It turns out that the prospects for ICT are actually quite bright. Its future does not lie in increasingly complex technology, but in its growing cost-effectiveness. For those OEMs and contract manufacturers willing to recognize, not deny, the opportunities created by loss of electrical access and a shifting fault spectrum, making the move from Big Iron to Low-Cost ICT provides an even better return on investment. Low-Cost ICT will continue to be the centerpiece of successful manufacturing test strategies for a long time to come.

Contact: CheckSum, 6120 195th St. NE, Arlington, WA 98223 360-435-5510 fax: 360-435-5535 Web:
http://www.checksum.com

 1) Teradyne L115 automated test system
 2) Article

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