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An A.T.E. Solutions, Inc. Internet Publication
Volume 10 Number 13 July 16, 2006

The Testability Director Version 3.2



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This Issue's Feature Articles

Considerations for Selecting Test Systems Using Flying Probe Access

 By: Louis Y. Ungar, Editor, The BestTest Newsletter

Product/Service Focus

This issue's focus is Flying Probe
You can view and add to our existing list of Test Products/Services, Test Literature, Test Definitions, Test Vendors, containing "flying probe"

What's New in Test
  7/13/2006 Verigy touts IC-test focus and Cadence link
  7/12/2006 JTAG Technologies Supports Altera Stratix II FPGA Design Security
  7/12/2006 Nextest Magnum Grande tests 720 devices simultaneously
  7/10/2006 XJTAG boundary scan to debug and test Ethernet network cards
  7/3/2006 GÖPEL electronic opens branch in UK
Magazine Articles
  7/14/2006 Testing the Military Machine - Using Boundary Scan
  7/1/2006 A Test Strategy for Pre-Reflow AOI
  7/1/2006 Integrating Boundary Scan With Various ATE
  7/1/2006 Multisite Test Strategy For SIP Mobile Technologies
  7/1/2006 Scopes Don’t Just Display Waveforms
  7/1/2006 Socketless Probe Testing
  7/1/2006 Universal NOT= Simple
Product Releases
  7/12/2006 EMI test speed jumps hundredfold with FFTs
  7/10/2006 Agilent Technologies' flat-panel-display test system increases multisite and high-resolution capabilities
  7/7/2006 Automatic X-ray inspection - New system generation
  7/1/2006 Boundary-scan system gains SPI and I2C interfaces
  7/1/2006 Mobile Wi-Fi power consumption tester debuts
  7/7/2006 ATE industry could see 'pause' in 2H
  7/7/2006 Capital Equipment to Rise and Fall
Web Postings
  7/16/2006 LXI Live Web Demo
  7/1/2006 Boundary Scan Tools for Design Verification and Prototype Debug
  7/1/2006 You've Decided to Get Into Boundary Scan... Now What?
Considerations for Selecting Test Systems Using Flying Probe Access

By: Louis Y. Ungar, Editor, The BestTest Newsletter

The Flying Probe Tester

Intuitively, a robotic arm accessing circuit nodes and feeding the information to a central computer seems like a good choice for automatic testing.  But when we talk about a “Flying Probe Tester” we have to remember that we are referring to the manner in which we access the circuit nodes, rather than provide information about what we test and how we make measurements.  Theoretically, flying probe access, like bed-of-nails fixtures, can be used by any ATE type – Connectivity, Manufacturing Defects Analyzer (MDA), In-Circuit (ICT), or Functional Board Test (FBT).  Practically, however, the latter two provide some complications because accurate measurements have to be made and an appropriate amount of time is required to make them.  Some of the measurements, as well as stimuli may also require synchronization to come along – all of which would further burden the probe time.  For these reasons, when one refers to a Flying Probe Tester, we can usually assume it to be a Connectivity or MDA tester.  

Historical Perspective

The flying probe tester found its niche during the 1990s.  While it was still slow compared to grid testers, it did not require a test fixture.  Also, flying probe could probe small targets on close centers.  

Flying probe access can be made to contact pads as small as 6 mills with accuracies down to 75 micrometers.  Test fixtures typically do not access pads smaller than 25 mills.

The major advantage of going fixtureless was the cost of fixtures, but there were also other advantages.  Prototype testing could be performed without waiting for a fixture to be constructed.  Changes in the circuit (and therefore in the fixture) required much less time.  Also, the flying probe tester enabled direct inputs from computer aided design (CAD) data, allowing changes to the test program to go into effect much faster.

Initially, flying probe was slow – about 10 minutes to test both sides of a fairly large and complex inner layer using 16-head flying probes.  As such, they were primarily used to confirm automatic optical inspection (AOI) detection.  Initial results confirmed that opens and shorts were indeed escaping AOI, but the flying probe introduced some false errors.  Upon closer examination, it was determined that AOI results were subjective and was greatly influenced by the tolerances set by operators.  When probing speeds were improved to 30-60 sec typically, flying probe became more widely used. (History of Inner-Layer Testing, by Brian Marcinak and Gary Stoffer, Surface Mount Technology, June 2006).

Flying Probe Capabilities

In their early development, flying probe testers provided only simple connectivity or MDA capabilities.  Thus only shorts, opens, and discrete components could be tested.  Now, they can provide power on capabilities used by in-circuit testers (ICTs).  As ICTs these days tend to be equipped with boundary scan, flying probe test can be combined with boundary scan to provide a greater set of test information for fault detection and diagnoses.

Though not common, flying probe and functional test is also possible.  When available on a functional tester, the user can have the best of all worlds.  When functional test detects a failure on a circuit board output, the flying probe can be invoked to provide diagnostic capabilities.  With boundary scan also available, the tester can be provided with additional circuit state information, thus assisting fault isolation and diagnoses.

Flying Probe Limitations

Flying probe is not a complete answer.  It’s slow test time is the major concern.
Some improvements are being made to speed up probing.  To optimize Z-plane motion as well as X-Y movement, flying probe software calculates the highest component level along the path of motion and raises the probe just above this level rather than to a generic PCB area fly height.

Another concern is the possibility of damage caused by the probing.  Several techniques, such as Digitaltest’s “Soft Landing” technology, provides a level of protection.  As a buyer of test systems you should make sure you get some assurances on this point.  You should especially be careful of systemic damage that can affect a large portion of your production volume.

Even without causing permanent damage, probing can create false test results due to flexing and probing pressures masking opens.  As a design for testability effort, circuit board layout should take into account flying probe physics and ensure the validity of the tests.


Flying probe methods are certainly catching on as a way to access hard-to-reach places.  They have improved in speed and probing accuracy.  They found their way to ICT applications and have made a bridge to boundary scan tools.  A new bridge to FBT is also very likely.  The limitations, however, still exist and as a result one should consider flying probe as a useful tool rather than a comprehensive test solution.

Next Issue's Product/Service Focus
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