At Rush PCB UK, we ensure all printed circuit boards for our customers are fully functional before delivery—as the consequences of failure are pricey. We are as much concerned as the customer is about non-functional PCBs because of a QA or a design issue causing a shortened life span.
For us, testing PCBs during assembly is an integral part of our manufacturing process. As a reputable electronics contract manufacturer, we offer a variety of PCB testing methods, to make sure all our boards are dependable and of high quality. To let our customers know the PCB testing methods we use, here is a short primer.
Automated Optical Inspection or AOI
AOI methods of testing typically use a single 2-D camera or two cameras for a 3-D photo of the PCB. A computer program compares the photos of the board to a detailed reference photo. If the photo does not match the reference photo to a certain degree, the program flags the board for inspection by a technician.
AOI methods are useful for detecting issues early in the manufacturing stage. We use AOI methods at various stages of manufacturing—after solder paste deposition, after component placement, and after reflow soldering. However, as the test does not require powering up the board, it is not a completely foolproof method.
Automated X-Ray Inspection or AXI
X-ray testing is necessary for inspecting parts of the PCB that may not be visible under normal conditions. This includes soldering under BGA and similar ICs and internal PCB layers. BGA and similar ICs hide solder joints underneath the chip package. However, X-ray inspection methods require trained and experienced operators. Also, not all the PCB can be examined or inspected using X-ray methods, but it is possible to view:
- Hidden Solder Joints
- Traces on Internal Layers
- Barrels of Plated-Through-Holes
- AXI tests can be 2-D or 3-D, with the latter offering faster testing.
While we restrict the above two testing methods primarily to assembly processes, we use other testing methods after completing the PCB assembly.
One of the most robust PCB testing methods, its cost depends on the board and type of fixture necessary. In-circuit testing is also known as a bed-of-nails testing, as the fixture we use has several probes protruding from a test bed. We place the board on these probes and power it up. The layout of the probes matches the design of the board,
The probes check the integrity of the solder connections. A computer program measures various parameters using the probes and compares the readings with a reference chart. Any mismatch raises a flag for further inspection by a technician.
Flying Probe Testing
This method is a variation of the in-circuit testing method with the difference that no fixture is necessary. Also, during the testing with flying probes, it is not necessary to power up the board. As there is no fixture necessary, this testing is less expensive as compared to in-circuit testing.
A computerized program allows two or three probes to connect to different parts of the board for testing various parameters like:
- Diode functions
Similar to the in-circuit testing method, the computer compares the results of the flying probe tests to a reference chart. Any mismatch raises a flag for further inspection by a technician.
This test requires the board to be powered up. A test jig or fixture records various parameters of the board, and a computer compares the results to a reference chart. Inspection by a technician is necessary if there is any mismatch.
This is a more intense form of testing for PCBs that have passed the above tests. This test detects early failures and establishes a load capacity. We power up the board to its maximum-specified capacity and allow it to operate continuously for a specified number of hours. This test weeds out boards with high infant mortality rates.
As all tests carry a cost factor, we always consult our customers before setting up a testing scheme for boards. However, we always perform the automated optical inspection and automated X-ray inspection (if necessary) during the assembly processes to ensure high quality.