Working for manufacturer engineering couple years. Sharing the manufacturing experience, skill, DFx, design, and information. Be care of this blog is personal share and content may not be 100% correctly.
Function verification tests (FVT), or call Function Circuit Tests (FCT), are crucial for detecting defects in PCBA (Printed Circuit Board Assembly). These tests simulate the full functionality of the circuit board as if it were part of the final product. The goal is to identify and remove faulty boards before they are fully assembled into the final device, preventing wasted time and materials from having to disassemble and reassemble defective products.
The appearance of these testing machines varies depending on the product’s size and shape. The testing methods also differ based on urgency, the number of boards to be tested, and cost considerations. According to Workingbear, the most commonly used function testing methods can be categorized as follows, excluding more automated or cabinet-based testing methods:
Functional Testing with Physically Assembled Boards:
Typically, a complete product consists of more than one PCBA. “Physically assembled” testing involves assembling all the PCBAs and necessary components of the final product without enclosing them in the casing. This approach allows for easy assembly and disassembly to test the boards’ functions.
The extent of the assembly depends on the functions being tested. Ideally, the test should cover all functions, but it should at least test the major functionalities, such as power-up, button functions, and screen display. Without such comprehensive testing, it might be more efficient to test the fully assembled product directly.
Before starting the tests, you need a fully functional set of PCBAs as the standard sample. also call as golden sample. For example, if the set includes boards A, B, and C, to test board A, use standard boards B and C as fixtures and only replace A for testing. Repeat this process for each board.
The main issue with this testing method is the potential damage to the standard sample. Frequent plug and unplug can wear out connectors or cables between PCBAs. To mitigate this, extension cables are used. They make assembly easier, prevent overcrowding of boards, and are cheaper and easier to replace than an entire circuit board. However, some components that require special electromagnetic Interference shielding, such as barcode scanner and touch screen devices, cannot use extension cables.
This test method is labor-intensive, so it is usually only used in the EVT (Engineer Verification Test) stage, when production quantities are low, or the circuit board design is not yet finalized.
Bed of Nails for Function Verification Tester
This test method uses test points that were pre-designed into the printed circuit board layout. Probes then connect to these test points to perform a complete functional test.
For testing, you place the board into the test machine. The machine’s mechanical action pushes the probes to contact the test points on the board. This setup simulates the board being connected as it would be in the final product. You can then operate and test all functions from the machine.
If the final product includes a display and buttons, their signals are routed out for command input and result viewing. If the product lacks a display and buttons, you’ll need to use an interface to connect to a computer for operation and result viewing.
When creating this type of function test machine, you usually need a fully functional final product. The fixture manufacturer will disassemble the product and install its components into the test fixture. Probes replace the original connectors and cables used in the final product, enabling quick loading and unloading of the test board. This method also minimizes damage to the standard sample, reducing the need for frequent replacements.
Below are some images related to this testing method:
▼The first photo shows an ICT test machine. FCT/FVT setups are similar but include additional display and buttons.
▼The second photo is a close-up of the fixture, showing some of the test probes. When the test board is placed onto the fixture and pressed down, the probes make contact with the board’s test points.
▼The third photo illustrates the internal wiring of an FVT fixture. It’s packed with cables, with each probe connected to a specific cable, each labeled for identification.
▼The fourth photo shows the exterior of a traditional function verification test fixture, with probes soldered to cables. Displays and buttons are usually connected for easy operation and result viewing.
▼The fifth photo reveals the internal wiring of a traditional function verification test fixture. While there are fewer cables than in an FVT fixture, cable management is still challenging. Maintenance, heat dissipation, and signal interference remain concerns.
Wireless Bed of Nails Function Verification Tester
This type of function verification test machine is an improved version of the traditional bed of nails tester. It replaces the wires originally connected to the test probes with a custom PCB. In this setup, both ends of the probes now contact a PCB—one end connects to the test points on the PCBA, and the other end connects to the custom PCB. This custom PCB routes the necessary connections and integrates the wiring methods of the entire system, similar to a big board. This design helps address issues with components sensitive to signal interference by using large ground planes to isolate signals. It also simplifies maintenance since there are no complex cables to manage or worry about breaking.
However, this method is more expensive, typically costing 40-70% more than traditional bed of nails testers due to the custom PCB. It is best suited for high-volume, stable products because any design changes in the PCBA that affect the test points would require a new custom PCB.
Below are some images related to this testing method:
▼The first image shows a wireless function test machine that replaces traditional cables with a custom PCB. The machine integrates the product’s functions on top.
▼The second image shows where the test board is placed. After placing the board and closing the lid, the probes contact the test points on the board, simulating the final product’s connections.
▼The third image reveals the internal structure of the wireless function verification test machine. Unlike traditional fixtures, there are no numerous cables running around inside. The visible cables are for external communication.
Leave a Reply