PCB FCT test: A Comprehensive Guide

What is the FCT test? This is a question that you may have if you are new to PCBs. In this comprehensive guide, we will introduce you to the FCT test and explain what you can use it. FCT test is a key component of quality control in the PCB manufacturing process.  And it is essential that your PCB manufacturer performs this test on every batch of boards they produce. We will discuss the benefits of FCT testing and explain how it can help improve the quality of your PCBs.

What is FCT in PCB?

Multimeter probes examining a computer circuit board

FCT test, or functional circuit test, is a type of testing performed on PCBs during the manufacturing process. And this test checks for shorts and open circuits in the PCB and verifies that the board is functioning correctly. In addition, FCT is an important step in quality control for PCBs, and it typically gets performed after the soldering process.

Why is FCT important?

Oscilloscope Probe on Electronic circuit

FCT is important because it helps ensure that the PCBs function correctly and there are no manufacturing defects. Besides, this test can identify problems with the PCB before shipping to the customer, which can save time and money.

What is Required For Functional Testing?

In order to perform an industrial FCT test, you’ll need the following;

Test Fixture

A test fixture helps keep the PCB in place during the test. It controls the DUT (device under test) as it allows exposure to external variables and punctually records its actions.

Key Elements of FCT Fixtures

An FCT fixture is usually custom-designed for every PCB to provide the following primary functions;

• Test point interfaces – these are interconnect mechanisms such as edge connectors, test probes, or pogo pins that are integrated to make electrical contact with standard pads or test points on a PCB. Some popular options include flying probe fixtures and a bed of nails.
• Physical board holding – fixtures incorporate plates or a frame that securely holds the PCB in a certain position and orientation. This could include fixtures such as clamps or vacuum channels to maintain consistent contact.
• Grading for safety – fixtures provide insulation or shielding around PCB to provide protection against electrical hazards while you’re testing. Covers and ground planes are commonly used.
• Durability – a test fixture is designed to withstand the rigors of repeated testing over countless test boards without failing. Often, materials such as stainless steel are used.
• Handling ease – a fixture is designed for quick unloading and loading by handling equipment (such as robots) or operators. Thus improving test throughput and efficiency.

Two Primary Categories of Fixtures

• Flying Probe Fixture

A flying probe fixture uses movable test probes on precision robots to target specific test points on a PCB. Thus providing improved versatility to reach points on a complex board geometry. Flying probes can test highly sophisticated analog and digital circuits.

Also, there are hybrid fixtures that offer both flying probes and a bed of nails. The type of fixture you choose depends on your PCB’s complexity, design, test access requirements and various other considerations.

Caption: Flying Probe Fixture

• Bed of Nails Fixture

This fixture uses an array of spring-loaded pins or nails to contact test pads on the PCB. It provides access to test points across the entire board area. Bed of nail fixtures can test simple analog and digital circuits.

Connective Interface

Usually, testers use probe cards to connect your test equipment to your PCB.

System and Sensors 

These create the necessary ideal conditions to simulate the working environment for your test target board, ensuring it functions in pre-planned states as it obtains parameters for every state.

Test Software

Often referred to as firmware, test software is an application that manages the test method and collects key information.

It enables operators to perform functional tests automatically via a computer. The software communicates with external programmable instruments like communication ports, digital multimeters, and I/O boards.

The software works with the fixture interfacing the instruments with the device under testing to make it possible to perform an FCT.

Caption: Digital Multimeter

Benefits of Performing FCT for Your PCB Assembly

Female digital electronic engineer testing computer PC motherboard in laboratory

There are some benefits of the FCT test for your PCB assembly.

First and foremost, FCT can improve product quality by detecting functional defects early in the manufacturing process.

Secondly, It allows you to make changes to the design quickly and easily without incurring high costs.

Lastly, FCT can save time by identifying potential problems with the PCB before shipping it to the customer.

PCB FCT Test Efficiency

The most productive PCB FCT tests are mostly semi-automated or fully automatic. Due to the demand to improve product efficiency and the fast development of technology, most functional testing methods are now fully automated. The testing software application (firmware) enables automated functional testing.

In order to interface with your test applications normally you should employ an external programmable instrument like a digital multimeter or I/O board.

You can perform FCT tests using instruments connected to devices under test via a test application and fixture. However, semi-automated or manual FCTs are popular when looking to decrease production costs and streamline design.

Controller

Embedded CPU controllers and MCU controllers – these are easy and quick FCTs for specialized programs and circuits for output and data display. 

PC controller – this is the most popularly used FCT as a result of its accessibility. Furthermore, its low prices contribute more to its use. In such a case, the file processing and data output of the test results could be accomplished on a workstation’s operating system. Also, the overall functioning of the test application is rather straightforward.

PCL controller- during development mode, it is a standard FCT focusing on the need to regulate induction components. In professional industrial control, the most commonly used controllers are PCLs.

What is the Process of the Functional Circuit Test?

Planning the tests you plan on performing is normally the first step before you start FCT testing. 

Functional testing involves connecting your electronic device to your test system and running a series of tests in order to make sure all the components are properly working.

The proper electrical connection between various parts is verified by applying signals and power and measuring the output in load and voltage.

Depending on the complexity of your PCB design, operators need to perform various tests, including logic tests, signal integrity tests, and power-on tests.

If the tests uncover any issues, professionals isolate the problem and determine the root of the problem. Any issues have to be addressed before the product is released into the market.

If the board is clear on all tests, it is cleared for further production operations and later released to the market.

Manufacturers must make sure that all FCT tests are done in accordance with industry regulations and standards.

This entails ensuring the test system is properly calibrated and maintained, and every element is tested according to its specification.

Furthermore, manufacturers have to document every test result and keep records of any modifications or changes made during the process.

Defects and Faults Found By FCT

FCT testing for different circuit issues and defects that could occur in the PCB fabrication stage. By using test signals and measuring the output response. The following are some of the issues that could be detected.

• Short circuits – unintended conductive bridges between 2 points not meant to be connected according to the design schematics. This could be caused by unwanted soldering, drill errors (such as plating through a hole wall), or copper smearing.
• Open circuits – breaks in the conductive path between points that should be connected according to the design schematics. This could be caused by incomplete holes, fabrication flaws, thinning and gaps in copper traces.
• Leakage – unwanted current flowing through points that should be isolated, thus indicating issues with the insulation resistance. This is often a result of contaminants such as residue on the PCB surface or moisture.
• Value faults – when impedances such as resistances or capacitances are out of tolerance from their designed value as a result of flaws or variations in fabricated parts on the PCB.
• Intermittent faults – defects appearing intermittently as you conduct the tests, indicating unstable or marginal connections that will likely cause reliability problems over time.
• Impedance faults – improper characteristic impedance of transmission lines, such as microstrips, that could impair signal integrity at high frequencies.
• Crosstalk – unintended signal coupling between neighboring components, traces, or vias as a result of fabrication variations. This could cause noise and interference. 
• Circuit logic faults – circuits failing FCT testing as a result of problems with the fabricated components or connections preventing proper logic function according to the design.

Summary

Technician inserting electronic components into a PDB for assembly

A PCB FCT test is essential during the PCB manufacturing process. This will help ensure that your boards are functioning as they should and that there are no errors or defects.

If you’re looking for a quality PCB manufacturer, be sure to ask about their testing procedures and whether or not they include a functional test of the boards.