The Printed Circuit Board Assembly (PCBA) process has multiple testing stages, each with its specific implementations and goals. Functional Testing (FCT) powers the device and validates expected operations.
This is part 1 in a series that covers PCBA functional testing. This first part goes over general PCBA testing procedures and what their specific goals are.
PCBA Functional Testing series
- PCBA Testing Overview
- Functional Test System Overview
- Writing a Functional Test Specification
- Test Software Development
This guide briefly outlines the printed circuit board assembly process, then gives examples of Automated Optical Inspection, X-Ray Inspection, Flying Probe Tests, In-Circuit Testing, and Functional Testing.
Printed Circuit Board Assembly Test Stages
The printed circuit board assembly (PCBA) process typically consists of the following steps.
After the components have been soldered onto the PCB, a series of inspection and test processes are performed on the printed circuit board assembly. The PCBA that is being tested is commonly referred to as the device under test or DUT. This typically includes the following steps.
Automated Optical Inspection
Automated Optical Inspection uses a camera to take images of the circuit board and then use computer vision to verify that all components are populated and that the solder joints look good.
Images are taken using various light sources and can also use cameras at various angles. Common issues that AOI identify include missing components or tombstoned resistors.
A tombstoned resistor
Automated X-Ray Inspection
Automated X-Ray Inspection is very similar to AOI, however instead of processing optical images, it uses computer vision with X-Ray images. This allows for the test to validate all of the solder connections on the board, including solder joints that are hidden underneath a component. This is useful for validating that ball grid array (BGA) components have been correctly assembled.
An X-Ray capture of a BGA component
In the image above, some small solder voids can be seen, look for the lighter grey areas within the darker grey pad. This device passed, however, larger voids can indicate that a solder joint was not made during the reflow process.
Flying Probe and In-Circuit Testing
Both flying probe and in-circuit testing (ICT) use the same methodology for testing a DUT. These test methods use test probes to measure the resistance, capacitance, and other basic qualities between test points on the DUT.
Ideally, a test point is available on each net of the DUT, which will then enable this type of test to fully validate that no shorts or opens are on the DUT and that the components have all been placed in the correct locations.
A Flying Probe tester uses robotic test probes to quickly iterate through each test point and take measurements between it and every other test point, this process can take up to 30 seconds.
No custom fixtures are needed to use a flying probe tester, which makes it a great way to perform some electrical testing on small runs of circuit boards.
When larger volumes of boards are being produced, an in-circuit tester (ICT) is typically used. This type of system uses a bed-of-nails, containing many different probes which contact every test point on the DUT.
The bed-of-nails fixture that is used in an ICT system has to be custom designed for each DUT, and this added cost and development time make them common for only mass-produced PCBA’s. These fixtures perform the same tests that a flying probe system performs but in a fraction of the time.
Both the flying probe and ICT tests are typically performed without having to apply power to the DUT.
A functional test (FCT) also uses a bed-of-nails fixture, however instead of testing the electrical connection between various nets, it validates that the DUT operates as expected.
Typical tests that are performed during functional tests include validating that the voltage rails generate the correct voltages when the DUT is powered, programming microcontrollers and memory, and validating various communication buses on the device.
In the next part of this series, we will cover Functional Test Systems, what components are needed to make a turn-key system, and FixturFab’s approach to designing them.