PCBA online testing (ICT) is a critical quality control process used to verify the assembly quality of printed circuit board assemblies before they proceed to functional testing or final assembly. Online testing is performed "online" as part of the production line, allowing for quick detection of assembly defects such as missing components, incorrect components, short circuits, open circuits, poor solder joints, and incorrect component orientation. The primary goal of ICT is to identify defects early in the production process, reducing the cost of rework and preventing defective PCBs from reaching the next stage of production. There are several common PCBA online testing methods, each with its own advantages, limitations, and applicability to different types of PCBs and production volumes.

In-Circuit Test (ICT) is one of the most widely used online testing methods for PCBA. ICT uses a bed-of-nails test fixture, which consists of an array of spring-loaded pins that make contact with specific test points on the PCB. The test fixture is connected to an ICT tester, which applies electrical signals to the test points and measures the response of the circuit. This allows the tester to verify the presence and functionality of each component, check for short circuits between adjacent traces, and detect open circuits in component leads or PCB traces. ICT is highly accurate and efficient, capable of testing multiple components simultaneously, making it suitable for high-volume production. It can detect a wide range of defects, including missing resistors, capacitors, or ICs, incorrect component values, reversed diodes or capacitors, and cold solder joints. However, ICT requires custom test fixtures for each PCB design, which can be costly and time-consuming to develop, making it less suitable for low-volume or prototype production.

Flying Probe Test (FPT) is another popular online testing method, particularly suitable for low-volume production, prototypes, or PCBs with complex designs that are not feasible for ICT test fixtures. Unlike ICT, FPT does not require a custom bed-of-nails fixture. Instead, it uses a set of movable probes (typically 2-4 probes) that "fly" to the test points on the PCB, guided by a computer-controlled system. The probes make contact with the test points sequentially, applying electrical signals and measuring the circuit response. FPT is flexible, as it can be programmed to test different PCB designs without the need for custom fixtures, reducing setup time and costs. It is also capable of testing PCBs with high component density, small test points, or complex layouts that may be difficult to access with an ICT fixture. However, FPT is slower than ICT, as it tests one test point at a time, making it less suitable for high-volume production. It is also less effective at detecting certain defects, such as cold solder joints, compared to ICT.

Boundary Scan Test (BST), also known as JTAG (Joint Test Action Group) test, is an online testing method designed for PCBs with complex integrated circuits (ICs) that have boundary scan capabilities. BST uses a standardized test interface (JTAG interface) built into the ICs, which allows the tester to access the internal nodes of the ICs and test the connections between components without the need for physical test points on the PCB. This is particularly useful for PCBs with high component density, where test points are limited or inaccessible. BST can detect defects such as open circuits, short circuits, incorrect component placement, and IC functionality issues. It is also useful for testing the interconnections between multiple ICs on the PCB. BST is fast and efficient, as it can test multiple ICs simultaneously, and it does not require custom test fixtures (only a JTAG interface). However, BST is only applicable to ICs that support the JTAG standard, and it cannot detect defects in passive components (such as resistors and capacitors) that do not have boundary scan capabilities.

Automated Optical Inspection (AOI) is a non-destructive online testing method that uses high-resolution cameras and image processing software to inspect the PCB for visual defects. AOI systems capture images of the PCB from multiple angles, compare the images to a pre-programmed reference image (of a good PCB), and identify any discrepancies. Common defects detected by AOI include missing components, incorrect component placement, reversed components, solder bridge (short circuits between solder joints), cold solder joints, and excess or insufficient solder. AOI is fast, capable of inspecting PCBs at high speeds, making it suitable for high-volume production. It is also non-contact, so it does not damage the PCB or components. However, AOI is limited to visual defects and cannot detect electrical defects (such as open circuits in component leads or incorrect component values). For this reason, AOI is often used in conjunction with ICT or FPT to provide comprehensive testing coverage.