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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.
Bed of nails tester
A bed of nails tester is a traditional electronic test fixture which has numerous pins inserted into holes in an epoxy phenolic glass cloth laminated sheet (G-10) which are aligned using tooling pins 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 and simultaneously made with hundreds or even thousands of individual test points within the circuitry of the DUT. The hold-down force may be provided manually or by means of a vacuum or a mechanical presser, thus 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 process: small dimples (from the sharp tips of the Pogo pins) can often be seen on many of the soldered connections of the PCB.
Bed of nails fixtures require a mechanical assembly to hold the PCB in place. Fixtures can hold the PCB with either a vacuum or pressing down from the top of the PCB. Vacuum fixtures give better signal reading versus the press-down type. On the other hand, vacuum fixtures are expensive because of their high manufacturing complexity. Moreover, vacuum fixtures cannot be used on bed-of-nails systems that are used in automated production lines, where the board is automatically loaded to the tester by a handling mechanism. The bed of nails or fixture, as generally termed, is used together with an in-circuit tester. Fixtures with a grid of 0.8 mm for small nails and test point diameter 0.6 mm are theoretically possible without using special constructions. But in mass production, test point diameters of 1.0 mm or higher are normally used to minimise contact failures leading to lower remachining costs.
This technique of testing PCBs is being slowly superseded by boundary scan techniques (silicon test nails), automated optical inspection, and built-in self-test, due to shrinking product sizes and lack of space on PCB's for test pads. Nevertheless ICT is used in mass production to detect failures before doing end-of-line test and producing scrap.
ICT failures & mechanical simulation
In-circuit testing has been known to cause mechanical failures such as capacitor flex cracking and pad cratering. This typically occurs on a bed of nails tester if there is excessive board flexure due to poor support placement or high probe forces. It can be challenging to optimize for ideal support locations and probe forces without spending resources designing and building an ICT fixture. Current methods typically employ strain gaging or similar techniques to monitor board flexure. More recently, some have looked at finite element simulation to proactively design or adjust an ICT fixture to avoid these mechanical failure modes. This approach can be implemented as part of a design for manufacturability methodology to provide rapid feedback on ICT design and reduce costs.
Example test sequence
- Discharging capacitors and especially electrolytic capacitors (for safety and measurement stability, this test sequence must be done first before testing any other items)
- Contact Test (To verify the test system is connected to the Unit Under Test (UUT)
- Shorts testing (Test for solder shorts and opens)
- Analog tests (Test all analog components for placement and correct value)
- Test for defective open pins on devices
- Test for capacitor orientation defects
- Power up UUT
- 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 Memory, EEPROM, and other device programming
- Discharging capacitors as UUT is powered down
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:
- Cameras to test for presence and correct orientation of components
- Photodetectors to test for LED color and intensity
- External timer counter modules to test very high frequencies (over 50 MHz) crystals and oscillators
- Signal waveform analysis, e.g. slew rate measurement, envelope curve etc.
- External equipment can be used for hi-voltage measurement (more than 100Vdc due to limitation of voltage that is provided) or AC equipment Source those have interface to PC as the ICT Controller
- Bead probe technology to access small traces that cannot be accessed by traditional means
While in-circuit test is a very powerful tool for testing PCBs, it has these limitations:
- Parallel components can often only be tested as one component if the components are of the same type (i.e. two resistors); though different components in parallel may be testable using a sequence of different tests - e.g. a DC voltage measurement versus a measurement of AC injection current at a node.
- Electrolytic components can be tested for polarity only in specific configurations (e.g. if not parallel connected to power rails) or with a specific sensor
- The quality of electrical contacts can not be tested unless extra test points and/or a dedicated extra cable harness are provided.
- 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.
The following are related technologies and are also used in electronic production to test for the correct operation of Electronics Printed Circuit boards:
- PCB electrical test of bare PCBs
- AXI Automated x-ray inspection
- JTAG Joint Test Action Group (Boundary Scan Technology)
- AOI automated optical inspection
- Functional testing (see Acceptance testing and FCT)
- "About Teradyne". Teradyne Corp. Archived from the original on 15 February 2014. Retrieved 28 December 2012.
- "Preventing Pad Cratering During ICT Using Sherlock" (PDF). DfR Solutions.
- Jun Balangue, “Successful ICT Boundary Scan Implementation,” CIRCUITS ASSEMBLY, September 2010. http://www.circuitsassembly.com/cms/magazine/208-2010-issues/10282-testinspection