engineering

In-circuit testing

In-circuit testing is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand In-circuit testing rather than just read about it. In short: In-circuit testing (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" test fixture and specialist test equipment, or with a fixtureless in-circuit test setup.

Key takeaways

In-circuit testing belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect In-circuit testing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of In-circuit testing from memory before moving on to harder problems.

Reference excerpt

In-circuit testing (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" test fixture and specialist test equipment, or with a fixtureless in-circuit test setup. In-circuit test (ICT) is a widely used and cost-efficient method for testing medium- to high-volume electronic printed circuit board assemblies (PCBAs). It has maintained its popularity over the years due to its ability to diagnose component-level faults and its operational speed. Using in-circuit test fixtures is a very effective way of maintaining standards when carrying out tests. It can help to reduce production downtime by identifying faults early in the testing process, ensuring that defective products are removed from the production line and fixed.

Fixtures for in-circuit testing

A common form of in-circuit testing uses a bed-of-nails tester. This is a fixture that uses an array of spring-loaded pins known as "pogo pins". When a printed circuit board is aligned with and pressed down onto the bed-of-nails tester, the pins make electrical contact with locations on the circuit board, allowing them to be used as test points for in-circuit testing. Bed-of-nails testers have the advantage that many tests may be performed at a time, but have the disadvantage of placing substantial strain on the PCB. An alternative is the use of flying probes, which place less mechanical strain on the boards being tested. Their advantages and disadvantages are the opposite of bed-of-nails testers: the flying probes must be moved between tests, but they place much less strain on the PCB. There are a range of companies who specifically create in-circuit test fixtures and test systems, including companies such as Teradyne & Keysight who build and manufacture test systems. There are also a range of independent fixture houses which supply and manufacture in-circuit test fixtures such as INGUN (who provide fixture kits), Forwessun, Rematek & Moteco.

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

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:

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)

References

External links In-Circuit Test Tutorial

Illustrations

Worked examples

Example 1 — a first encounter with In-circuit testing

Start with the simplest possible case. Write down what In-circuit testing claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to In-circuit testing before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about In-circuit testing ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of In-circuit testing

In research: In-circuit testing appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses In-circuit testing in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: In-circuit testing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic test equipment, Hardware testing, Printed circuit board manufacturing, so understanding it makes those chapters shorter.
In everyday life: Look for In-circuit testing outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study In-circuit testing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what In-circuit testing means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain In-circuit testing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is In-circuit testing in simple terms?

Why does In-circuit testing matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study In-circuit testing?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on In-circuit testing.