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This month, Testing Todd presents readers’ questions about the basics of electrical test, including the different types of testing available today.
Reader: I recently found out that all points are not necessarily checked during electrical test. Why is that and which types of boards do not get all of their points tested? Isn’t there a chance that something will be missed? Is there a type of test that hits all of the points?
Todd: When a board is programmed for test, certain points of the board are de-selected. To properly answer this question, we need to revisit IPC-9252A, Amendment 1, which stipulates that Class I and II can remove the mid-points of a net on the board. What this means is that we are only concerned with the end-to-end connectivity of the net. With IPC Class III we need to include the mid-points of the net to quickly isolate any problems in the chain. However, this is cautioned by the additive properties of solder mask encroachment or via-fill. In these cases, although Class III, they cannot be tested and are allowed the waiver under the 9252A with Amendment 1 release.
Within the Class III requirement of 9252A with Amendment 1, we can add probes to validate each landing pad. With Class I and II we are only validating the end points of the net to make sure the signature is intact. By adding mid-points with Class III we validate that the connectivity is valid to all landing pads. This is in case there may be contamination to any of the intermediate landing pads in the chain of the net.
Adding all pads in the net is referred to as Class III per IPC-9252A with Amendment 1, which hits all of the points.
Read the full column here.
Editor's Note: This column originally appeared in the May 2014 issue of The PCB Magazine.