In-circuit Testing Methods in PCB Manufacturing

PCB manufacturers use various in-circuit testing methods to detect errors. Today’s post offers insight into these tests and the steps employed by quality control inspectors to complete them. It also compares the bed-of-nails and flying probe tests.

What is an In-Circuit Test?

An in-circuit test in PCB manufacturing is a procedure that checks PCBs using electrical signals. In-circuit means that the checking occurs with the components still on the printed circuit board (PCB).

The testing machine has electrical probes. The probes contact designated test points. It may also use an interface to connect to the board, enabling a quality control expert to send signals and receive their output.

The test is usually manual or automated. Manual testing is a low-cost process for small production runs. Automated versions improve the testing speed, allowing manufacturers to check more boards.

Why is In-circuit Testing Important?

In-circuit testing (ICT) enables component-level checks. It helps quality control engineers to uncover PCB defects that other methods may not detect.

The test is electrical, which means it occurs quickly and provides circuit-based results. In other words, it assesses PCBs for manufacturing quality and structural integrity in the application device.

Some ICT systems power PCBs to make them functional, just like they would in electronic devices. The simulated condition allows the operator to verify their operation.

Because the test uses low-power electrical signals, it is non-destructive. It also checks many defects, making it a versatile verification process. The procedure identifies these defects: opens, shorts, and incorrect components.

In-circuit Testing Methods

There are various in-circuit testing techniques. PCB manufacturers and testing service providers choose them based on the specific project requirements. They primarily include the bed of nails test and the flying probe test. In addition to these probe-based methods, other structural tests like Boundary Scan are also used, which serve as a powerful supplement or alternative.

Bed of Nails Test

This bed of nails method is the conventional ICT technique. It uses multiple spring-loaded probes held on a fixture to contact test points. The test probes carry electrical pulses to the board, enabling the ICT machine to check components and circuits.

The method, although less flexible, checks many test points simultaneously, making it fast. However, it requires a fixture for every PCB type. This need makes it one of the most expensive and time-consuming ICT checks to set up.

Flying Probe Test

The flying probe method uses “flying pins” mounted on a platform. The platform can move up, down, and sideways. This motion allows it to fly the probes above the board, landing them at pre-determined positions.

Upon landing, the probes complete electrical circuits, passing test signals to individual components and groups. The signal allows the probes to take measurements. These measurements must match with the board’s specifications.

The flying probes method is highly adaptable. That’s because it doesn’t use fixed pins or custom fixtures. However, the moving pins reduce the checking speed, making it less suitable for high-volume production.

Boundary Scan Test – Probe-less Alternative

Boundary Scan Test is a method for testing interconnects on printed circuit boards or within integrated circuits. It is often considered a powerful alternative or supplement to traditional In-circuit Testing (ICT), rather than a type of ICT itself.

The key difference is that Boundary Scan does not use physical probes to access the PCB’s components. Instead, it utilizes a dedicated test access port (TAP) and special logic cells built into the ICs, known as boundary scan cells, to electronically test connections. The tester sends signals through this interface to set pin states and detect responses, allowing it to identify faults in IC pins, connections between ICs, and other structural defects.

The boundary scan technique requires no fixture and only requires access to a few specific test points (the Test Access Port). It also takes less time to complete. Manufacturers can also re-use the test’s program, which reduces costs.

In-circuit Testing Benefits

ICT testing is a popular quality control process. It’s also one of the most crucial testing methods, given its speed, coverage, and several other benefits. The following are its key advantages.

Non-destructive

It doesn’t harm the PCB compared to other quality control methods, such as thermal cycling. The sample remains intact to undergo any other checks. It also means fewer losses during quality checks, making the procedure safer and repeatable.

Fast Results

It uses electrical signals to check components, providing faster results than most other quality control methods. Automated types increase speed and let manufacturers check PCB boards in batches. However, the speed depends on the specific technique.

High Coverage

It detects a wide range of production defects, such as soldering faults and PCB assembly errors, making it a comprehensive quality control method. High coverage means manufacturers can quickly identify a broad range of potential production errors.

Cost Effectiveness

It costs less to check PCBs electrically than using other methods, such as X-rays and environmental testing. The equipment is simpler, and the testing duration is shorter. Overall, it reduces the amount spent controlling a PCB’s quality.

Bed of Nails Test vs. Flying Probe in-circuit Test

The bed-of-nails and flying probes are critical in-circuit testing methods in PCB quality control. They are also almost identical, utilizing pins to probe components and circuits. The two techniques differ in execution, fixture requirements, and several other ways.

The bed-of-nails method uses fixed probes to contact test points, making it inflexible for design changes. It also requires a custom fixture. That’s because the probes must be in the right place based on the specific PCB’s structure, raising costs.

On the other hand, flying probes move above the board, assisted by a custom computer program. This design allows the manufacturer to automate the process. It also eliminates the need for a custom fixture, reducing costs.

Additionally, flying probes can check different PCBs without requiring a fixture change. This benefit makes it suitable for multiple board types in busy production facilities.

Conclusion

Various in-circuit testing methods are available today. They include the bed-of-nails, flying probe, boundary scan, and power-on. Each technique has benefits and drawbacks. The testing engineers select the most appropriate option. They base their choice on various factors, such as the number of boards, PCB type, and project budget.