Design For Test
Design-For-Test or DFT takes into account several major and minor considerations at the layout stage. In this limited webpage space, the key ones are described. However, the overall DFT success for 90 percent plus test coverage relies heavily on implementing all these critical layout considerations.
Initially, the PCB Layout engineer should lay out all test points on one side of the board. Another consideration is maintaining minimum test point distance from one test point to another. There should be a minimum distance of 100 ml., accounting for about 2.54 millimeters so that probes can perform their job properly.
Test point distribution is important, as well. A high density of test points or clusters should be avoided. Test points should be uniformly distributed throughout the board, making it easier to test with multiple probes and not having to rely on a specific area for all the probes to concentrate on. Moreover, there should be a free zone or a safe zone designated as a keep out area.
While doing the layout and when feasible, it is a good idea to consider using standard commercial electronic modules, which are available for testing. However, if there are redundant modules, are they being laid out so that they can be tested independently? Put another way, if there's a fault in one module and not in the second, third and fourth, each module should be available for independent testing. Accessibility to each address and data and bus line is important.
The PCB Layout must determine whether or not system level feedback loops are de-controllable. Also, while the board is being tested, are all system and subsystem specs made available? Also, test probe nodes should have access to one test node. Every separate node should have one test point and maybe multiple points.
These are just a few of the many DFT considerations critical for achieving 90 percent plus test coverage on your PCB Layouts.
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