Functional Testing

Printed Circuit Board Assemblies (PCBA) need to be tested for functionality after IC programming, AOI and X-ray inspection. As a contract manufacturer, MADPCB specialized in turnkey PCB assembly, we provide value-added Functional Testing (FCT) Service to your assembly projects so that you have full assurance that the PCBAs will function in the field as expectations. PCBA functional testing is the most comprehensive test to determine the final pass/fail status of your board after manufacturing.

 

Applications of Functional Testing:

 

• Opens & Shorts
• Display Testing
• Power Supply
• Digital Sections
• Analog Sections
• Mixed Signals
• Communication/Interface Testing (Calling, Bluetooth…)
• Boundary Scan Testing
• Calibration

 

FCT is performed through a combination of connecting a cable to your PCBs on-board connectors, and the use of test probes contacting test points where necessary on your board. The testing software for the board is developed to conduct the tests per the specific requirements of the board’s function. The goal of the functional test is to look at your entire PCB assembly and its functionality to determine whether or not it has been manufactured according to your specifications. This requires a reliable EMS (electronic manufacturing service) provider who knows how to best work with your PCB assembly for testing.

 

How to Perform Functional Testing?

 

Functional testing is performed in the last step of PCB assembly manufacturing process. A functional test emulates the operational environment to ensure the PCBA’s correct functionality. The operational environment may consist of a device that communicates with the PCBA DUT. Typically, the DUT’s power supply and software test program will load the necessary parameters to make the DUT work properly.

Functional testing will verify the functionality of your PC board assembly. It will do this by applying a range of different signals and currents to stimulate the board’s behavior based on testing software developed for the board, and then measure how the PCBA reacts. It will detect both analog and digital signal problems, including distortions, incorrect currents, and signal timing issues.

The printed circuit board assembly (PCBA) may be subjected to a sequence of signals and power supply voltages. Responses are monitored at specific points to ensure correct functionality. All testing is performed according to the customer test engineer’s specifications and procedures.

 

PCBA Functional Testing Fixtures

 

Many different test fixtures have been developed for functional testing of a PCBA (PCB assembly). Electrical connections may be made with connectors present on the PCBA or by test probes that contact test points defined by the PCB designer. Connectors on a test fixture are usually modified to allow for ease of mating and de-mating of the fixture’s connectors and the connectors on the PCBA to be tested. Connectors and the connectors on the PCBA to be tested. Connectors on the test fixture are also usually made easily replaceable, since they can wear out quickly with the multiple mating and de-mating cycles of production testing.

When making direct contact to test points on a PCBA, probes under the PCBA are typically fixed and the test fixture applies hold-down force to the top of the PCBA for reliable electrical connections to the test probes. If the PCBA being tested has test points on both sides of the board, then two probe plates are required, both of which must be precision aligned to each other and to the PCBA under test. With double-sided probing, the PCBA under test is typically fixed in place and the probes move in from both top and bottom when the fixture is operated.

 

Probe-Based Fixture Testing

 

There are two main methods for making contact with a PCBA being tested using a probe-based test fixture, commonly called a “bed-of-nails” fixture. These methods include wired and wireless fixtures. Choosing the proper fixture approach for a specific application impacts total system costs and tester longevity. In addition, decisions must be made to allow for removable fixtures using some form of mass interconnection between the PXI test instruments and the fixture.

• Wired test fixtures have long been the industry norm for functional testing fixtures. On lower-probe-count PCBAs or prototypes where a design is subject to change, wired test fixtures provide an economical approach because they avoid nonrecurring-engineering (NRE) costs. The challenge with wired fixtures is that as signal frequencies increase, crosstalk and coupling problems can increase. This is due to how wires are routed under the probe land, with coupling and crosstalk increasing with increasing frequencies.
• Wireless test fixtures were developed to address the limitations of wired test fixtures. In a wireless fixture, a multilayer PCB, called a translator board (T-board), replaces the nest of signal wires on the underside of a wired probe plate. Automated PCB design software is used to route circuit traces when replacing wires. Double-ended probes connect a PCBA under test to the T-board.

 

Wireless fixtures generally perform more reliably than wire fixtures because PCB designers can control exactly how each trace is routed on the bottom side of the T-board, making it possible to isolate troublesome signals. Duplicate fixtures perform nearly identically and electrical parameters are very consistent because there are no wires and the variations that they cause. The reduced noise from grounding and crosstalk allows faster signal rebound, which is particularly important in low-voltage test applications.

A test fixture enclosure, often referred to as a pan, plays an important role in the functional testing of PCBAs. The enclosure pan defines the connectivity to the instrumentation through a mass interconnect, and to the actuation mechanism probing the PCBA. The strength of the enclosure pan is important because of the high forces that may be applied to a PCBA under test. Standard vacuum and pneumatic fixtures can subject to a PCBA under test (UUT) to damaging stress levels if the fixture is not properly constructed. Push fingers or vacuum pressure can place additional forces on the PCBA.

A unique method for lowering the costs of test fixture is to use an interchangeable adapter drop-in unit that allows the same actuation mechanism to be used with multiple PCBs. Such a configuration allows the same test system to be quickly reconfigured with different tooling and probe patterns for testing different PCB assemblies. This maximizes fixture reuse and minimizes fixture cost.

At MADPCB, we are committed to high-quality PCB fabrication, assembly and design. Our PCBA functional testing capabilities help us ensure that PCB assemblies delivered to you are in operating conditions. No matter how simple or complex a PCB board is, we care finish a custom functional testing as per the procedure. Our vast PCB related experience, coupled with our knowledge of various functional testing methodologies has helped us build a satisfied customer base.