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Today’s Circuit Board Test Reality

Circuit Boards Have Changed.
So Has The Board Test Fault Spectrum.

Over the past several years printed circuit board assembly yields have increased as component and assembly process defect rates have decreased as shown in the figure on the right. In addition, the distribution of fault types typically found on boards during the manufacturing and assembly process has shifted over the past several years as illustrated in the fault spectrum pie diagrams to the right.
  • Solder defect rate has increased as SMT opens predominate over through hole shorts.
  • Solder defects are now mainly quality-related such as cold solder.
  • SMT process defect rate is lower than through-hole rate.
  • Overall parts defect rate has decreased dramatically, especially for digital ICs.
  • Circuit boards are increasingly inaccessible by bed-of-nails fixtures.

Why Does the Shifted Fault Spectrum Matter? Because the Fault Coverage of Traditional In Circuit Testers Is Declining.

  1. Densely packed boards and higher device speeds effectively eliminate test pads needed for bed-of-nails fixtures.  Loss of Access (LoA) means less ICT coverage.
  2. New chip package types such as BGAs impair access as well.
  3. Increasingly complex digital and mixed signal chips such as SOC make digital vector tests impractical or extremely expensive and time-consuming to develop.
The Shifted Fault Spectrum also means that the faults that traditional ICT is adept at finding occur much less frequently—reducing overall tester effectiveness—but not cost.
  1. Out-of-tolerance analog components rarely happen.
  2. SMT has resulted in opens and other solder quality problems that ICT has difficulty finding.
  3. Digital backdrive is best at finding defects on “jelly bean” logic—but they almost never fail, so there’s not much reason to test them in the first place.
As a Result
Test engineers have eliminated almost all of the tests traditional ICT was designed to do well—resulting in simple in circuit programs that leave all that "big iron" ICT capability either under-used or unused. This mismatch between today's fault spectrum and yesterday's traditional ICT means high overhead costs for unused tester resources in the form of more complicated fixtures and programs and the higher support costs inherent in complex testers.

New Technologies Will Augment In Circuit Test (ICT)
While ICT remains the most economical means to identify manufacturing and component defects, new technologies such as Automated X-Ray Inspection (AXI) and Automated Optical Inspection (AOI) are being integrated into assembly lines to deal with the new defect classes that ICT can no longer—or simply never could—identify.

What Does the Shifted Fault Spectrum Mean For ICT?
If test engineers continue to use traditional "big iron" ICT for every test job, OEMs and contract manufacturers will keep generating unnecessary test costs. For North America alone, CheckSum estimates that more than $250 million annually is spent unnecessarily on testers, tester support, test programs and fixtures by using "big iron" where Low Cost ICT will do the job. Employing Tester Portfolio Management will reduce total ICT cost while maintaining superior fault coverage--and free up budgets to fund the new AOI and AXI technologies that today's new board technologies also require.
Changing Fault Spectrum Graph
Advances in circuit board and component technology, as well as the SMT manufacturing process have 'shifted' the typical fault spectrum over the past several years. The Fault Spectrum Shift has significant implications on both in circuit test strategies and on in circuit test equipment.