In-circuit testing (ICT) – fault localization at the component and net level

Detect faults before they become system-level problems

In-circuit testing is an electrical test method for printed circuit board assemblies (PCBAs) that verifies functionality at the net and component levels. It bridges the gap between automated optical inspection (AOI) and functional testing and allows manufacturing defects to be located and corrected at the individual solder joint or component level.

In-circuit testing is therefore not a replacement for functional testing but rather a targeted complement that identifies manufacturing defects before they appear as system-level faults during functional testing.

Your advantage: development and manufacturing from a single source

This is where the key difference from pure EMS providers becomes apparent. At EPnP, we both develop and manufacture your assemblies. As a result, testability is not a downstream compromise but rather an integral part of the design from the outset.

Design for testability (DFT) starting at the circuit design stage

We incorporate ICT requirements into the planning phase as early as the development phase.

• Test points and test pads are incorporated into the printed circuit board (PCB) layout from the outset—in the correct size, with sufficient spacing, and on the side best suited for probing.
• Direct netlist comparison between the schematic and the ICT program is possible because both are developed in-house.
• Keep-out zones and accessibility for the test fixture are considered during component placement rather than addressed later.
• Boundary scan (JTAG) interfaces can be incorporated for complex ICs and BGAs from the outset in order to increase test coverage in a targeted manner.

The result: Instead of debating later whether and where test points can still be accommodated, the design supports high ICT test coverage from the start.

MCU programming via the ICT adapter

Another practical advantage: In many cases, the ICT fixture can also be used to program microcontrollers and programmable devices (in-system programming via JTAG, SWD, or manufacturer-specific interfaces). This eliminates a separate manufacturing step, reduces handling, and enables programming and electrical testing to be combined in a single operation.

Typical areas of application

In-circuit testing delivers the greatest value in applications that require process stability, traceability, and precise fault localization:

• Medical devices – regulatory requirements for traceability and process validation

• Aerospace – the highest requirements for reliability and documentation

• Vehicle manufacturing – production-grade quality with stable, reproducible test results

• Industrial electronics – medium- and high-volume production with stable designs

The traditional ICT fixture is particularly effective for medium- and high-volume production with frozen designs for which investment in the fixture and test program is offset by production volume.

Flying probe testing: ICT-level quality even for low production volumes

Particularly in our target markets of medical devices and aerospace, production volumes are often too low to justify a dedicated test fixture economically while the requirements for test coverage and traceability remain exceptionally high. This is precisely where flying probe testing provides an effective solution.

How flying probe testing works

Instead of an assembly-specific test fixture with fixed contact pins, a flying probe tester uses freely movable probes that can be software-controlled to access virtually any point on the PCB. The test program is generated directly from CAD and netlist data, thereby eliminating the need for a mechanical test fixture.

Advantages for medical devices and aerospace

• No fixture investment required – the test is cost-effective even for a batch size of one, making it ideal for prototypes, low-volume production, and variants with frequent design revisions.
• Fast availability – new test programs can be quickly created and modified without having to wait for fixture fabrication.
• Equivalent test coverage – short circuits, open circuits, component values, and polarity are verified at the net level, comparable to conventional ICT.
• Flexible when designs change – for revisions or variants, only the test program—and not the hardware—needs to be updated
• Dual-sided probing – many flying probe systems can test the top and bottom sides simultaneously, improving test access for double-sided assemblies.

Limitations compared with fixture-based ICT

• Longer test time per assembly – because the probes are positioned sequentially, testing is slower than parallel probing with a test fixture.
• Not cost-effective for high production volumes – beyond a certain production scale, fixture-based ICT offers superior throughput and lower per-unit testing costs.
• No MCU programming – ISP integration via the test fixture is not available, so programming must be carried out in a separate step.

Our recommendation

We use flying probe testing specifically where high quality requirements and low production volumes do not justify a fixture-based ICT. In practice, this applies to many of our medical device and aerospace projects. As production volumes increase, the transition to fixture-based ICT can be made seamlessly while the test strategy and DFT preparation remain fully intact.

When the limits of ICT are reached

In cases involving extensive miniaturization, double-sided assembly with limited access, or BGAs without testable vias, neither ICT nor flying probe testing alone is sufficient. In these situations, we evaluate which additional test methods are appropriate:

• Boundary scan (JTAG) for digital ICs and BGAs without physical test access
• Specialized functional tests for parameters that ICT cannot verify because of design constraints

Consistent results through process control

In-circuit testing delivers reliable results only when the entire chain is aligned: test program, probing, and documentation.

• Fixture qualification and maintenance: Regular inspection of test pins for wear and contact resistance
• Threshold management: Test limits are calibrated based on production data to minimize false failures.
• Traceability: Test results are documented for each individual assembly and integrated into the traceability system.