In-circuit test

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In-circuit test (ICT) is an example of white box testing where an electrical probe tests a populated printed circuit board (PCB), checking for shorts, opens, resistance, capacitance, and other basic quantities which will show whether the assembly was correctly fabricated. It may be performed with a bed of nails type test fixture and specialist test equipment, or with a fixtureless in-circuit test setup.

There are many different test platforms for performing in-circuit test (Agilent 3070,i5000,i3070, i1000 Genrad (now part of Teradyne), Teradyne, TRI to name a few).

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[edit] Bed of nails tester

A bed of nails tester is a traditional electronic test fixture which has numerous pins inserted into holes in an acrylic plate which are aligned to make contact with test points on a printed circuit board and are also connected to a measuring unit by wires. Named by analogy with a real-world bed of nails, these devices contain an array of small, spring-loaded pogo pins; each pogo pin makes contact with one node in the circuitry of the DUT (Device Under Test). By pressing the DUT down against the bed of nails, reliable contact can be quickly, simultaneously made with hundreds or even thousands of inidividual test points within the circuitry of the DUT. The hold-down force may be provided mechanically or by means of a vacuum pulling the DUT downwards onto the nails.

Devices that have been tested on a bed of nails tester may show evidence of this after the fact: small dimples (from the sharp tips of the pogo pins) can often be seen on many of the soldered connections of the PCB.

Typically, four to six weeks are needed for the manufacture and programming of such a fixture. Fixture can either be vacuum or press-down. Vacuum fixtures give better signal reading versus the press-down type. The bed of nails or fixture as generally termed is used together with a in-circuit tester such as i3070, 3070 from Agilent (USA), Teradyne Spectrum Series and continuation of the old Genrad Teststation series under the Teradyne flag (USA), IFR 4200 series, was Marconi Test prior to acquisition by IFR (USA), TRI (Taiwan), Digitaltest(Germany), Okano (Japan), Checksum (USA).

The Checksum system is an entry level machine selling for approx. USD$10K and finding approximately 95-98% of manufacturing defects. The Teradyne system is a system designed for high end manufacture and military use, and sells for approx. USD$200K plus.

"During board layout, one of the primary concerns is that accurate tooling holes and appropriate test pad size and location are all critical to increasing the probability that the spring-loaded probes in a bed-of-nails test fixture will reliably make contact and transfer signals to and from the board under test." Blackwell, The Electronic Packaging Handbook

This technique of testing PCB's is being slowly superseded by Boundary Scan techniques (Silicon Test Nails) and Automatic Optical Inspection, due to shrinking product sizes and lack of space on PCB's for test pads.

[edit] Example test sequence

  • Shorts Testing (Test for solder shorts and opens)
  • Analog Tests (Test all analog components for placement and correct value)
  • Testjet (Agilent 3070) or Opens Express (Genrad / Teradyne) (Test for correct soldering on an IC)
  • Powered Analog (Test for correct operation of analog components such as regulators and opamps)
  • Powered Digital (Test the operation of digital components and Boundary scan devices)
  • JTAG Boundary scan tests
  • Flash and other device programming
  • Agilent Medalist Beadprobe - Bead probe technology
  • Agilent Vectorless Test Solution – VTEP v2.0

While in-circuit testers are typically limited to testing the above devices, it is possible to add additional hardware to the test fixture to allow different solutions to be implemented. Such additional hardware includes:

  • External programming hardware
  • Cameras to test for presence and correct orientation of components
  • Photodetectors to test for LED color and intensity
  • External Timer counter modules to test crystals and oscillators

[edit] Limitations

While in-circuit test is a very powerful tool for testing PCBs, it has these limitations:

  • Parallel components can only be tested as one component.
  • Electrolytic components can not be tested for polarity
  • The quality of electrical contacts can not be tested
  • It is only as good as the design of the PCB. If no test access has been provided by the PCB designer then some tests will not be possible. See Design For Test guidelines.

[edit] Related technologies

The following are related technologies and are also used in electronic production to test for the correct operation of Electronics Printed Circuit boards

[edit] External links

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