In-circuit testing

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This technique of testing PCBs is being slowly superseded by boundary scan techniques (silicon test nails), automated optical inspection, and built-in self-test, due to shrinking product sizes and lack of space on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)

• I've edited this information before & somebody deleted it. I keep copies & will paste in everyday &every evening I'll check.
• This is good for I'm wanting to improve situations, time saved, & cost. Thank you.
• Important Notes Concerning the test: * Shorts testing (Test for solder shorts, trace shorts, and opens) Replacement / Changes / Important Level High
• Reasons / Advantages for the change at ICT:
• 1. Reduce man hours
• 2. Test will remain finding solder & trace shorts, opens, & defective components. All from the measuring of test nodes, & without PCB being powered on.
• 3. This test results should report when PCB failed but not report the following, For it's the results that when the test fails that should report there's a possibility for either open or defective device has been detected. The failure analysis technician will then need to determine which of the 2 possibilities the reason for failing. There's a simple technique using a flow chart that I can have an ICT programmer to create. Same technique I've used when performing failure analysis.
• When this test finds an issue, testing should continue to completion

• Editor: Robert Eugene Peterson, Jr
• Senor Electronics Failure Analysis Technician Specialist 5
• 10/1982 ~ Electronic Technical Institute US-CO-Denver #2 School in Nation vs DeVry #1 School in Nation
• "Associate Degree in Applied Science in Electronic Technology".
• Assignment: Replace Shorts testing (Test for solder shorts and opens)
• Why? To reduce failures that occur at Function Test, & Final Assembled Product Testing "Manufacturing"
• Shorts testing (Test for solder shorts and opens) This test needs to be replaced with what is known as "IRTG Testing" In reference to ground.
• Setting Limits: There no Limits (Contact me for details why there's no limits). There are Rules that apply.
• Shorts testing (Test for solder, & trace shorts and opens)
• During my first job there's only one PCB I'll never forget. This failure occurred only as a fully functional end user type test.
• This failure had me reconsidering changes my career path. The analysis was known, but couldn't be seen. I had no access

 to a microscope. This is during the time~frame for through hole components. 

• The answer is this failure was due to a 400Ω "ohms" short at D1 & another pin on IC U5. "400Ω "ohms" Honestly it had to been a trace short.
• While at the vendor in San Jose California is when I had asked the ICT programmer. When testing for shorts, what is the limit set at?
• The answer was 10 ohms. For the fact that there was only one PCB failure I didn't disclose to the programmer the reason I asked that question.* The failure was due to a 400Ω "ohms" short at D1 & another pin on IC U5. "400Ω "ohms"
• I've performed at least 100,000 PCBAs in my 31 years.
• When the same vendor in California called the Test Engineer & said they have 68 failures @ functional test. I replied that I cannot do analysis over the telephone.

 They over night those PCBAs & I completed failure analysis < 6 hours.

• What I discovered as the Rules at ICT for In Reference to Ground aka: zero potential reference point. ",⏚, ,0 volt,
• I've performed at least 100,000+ PCB's in my 31 years.
• What I discovered as Rules can be explained & ICT Programmer interested should contact me at by leaving a message here about my edit.
• Thank all of you

• 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

Advantages

In-circuit testing is best suited to stable products with a mature design and higher volumes. It offers the following advantages:

• Fastest test per unit
• Tests components individually
• Tests logic functionality
• Measures tolerance of components
• Finds shorts and opens
• Power-up ability to take measurements

Limitations

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.

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

• AXI Automated x-ray inspection
• JTAG Joint Test Action Group (Boundary Scan Technology)
• AOI automated optical inspection
• Functional testing (see Acceptance testing and FCT)

External links

• In-Circuit Test Tutorial