Related article: Assembly Testing
Fabrication testing relates to the testing of a printed circuit board at the pre-assembly phase of manufacture, before any components have been placed onto the board. As such, it is often referred to as bare-board testing. This method of testing might typically involve the use of a flying probe device to perform net-by-net testing. Essentially, two probes are programmed to operate in concert, one to pulse a signal through a net and the other to detect the presence (or absence) of that signal.
Two common test scenarios for bare-board fabrication testing are:
- Test to ensure that intentional end-to-end contiguous copper carries a clean signal (under a maximum permissable impedance threshold) without any open-circuits.
- Test to ensure that nets are not shorted to each other.
Testpoint Location Constraints
- Dedicated testpoint pads/vias are usually not necessary for fabrication testing. As the board is not yet loaded with components, the probes enjoy free access to component pads. Since the pads tend to be free of solder mask at this stage, the testpoints are typically located directly on the pads themselves.
- Flying probe tests are not constrained by a grid, as flying probe devices are capable of quite precise locations.
- Each location pair is constrained by a minimum separation – due to the physical body size of the probe heads.
Exceptions and other Considerations
- For fine pitch devices, it may not be possible (or even practical) to probe a single component pad. Such copper features can be miniscule and delicate, so it is occasionally recommended that they avoid contact with test probes. In these instances, it can be a good idea for the designer to add dedicated test pads/vias to the board.
- A bed-of-nails fixture can also be used during bare-board fabrication testing. This is typically the case for high-volume manufacturing runs, where use of such a fixture involves less testing time than a flying probe. However, bed-of-nails testing can involve more rigid testpoint location constraints and, if the test fixture is not a customized one, will often require that the testpoints conform to a specific grid.
- There are occasions where specific pairs of testpoint locations are not tested in-concert by a flying probe. This is typically the case when there is no danger of a short (or crosstalk) between nets, as the nets are not adjacent to each other (all objects of both nets are mutually separated by their physical proximity).