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66 The PCB Magazine • May 2014 by Todd Kolmodin GardIen serVIces usa Testing: Everything You Wanted to Know, but Were Afraid to Ask c o l u m n testiNg todd This month, Testing Todd presents some readers' questions about the basics of electri- cal 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 revis- it 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 con- nectivity 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 vali- date 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 intermedi- ate 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. Reader: when and why do your customers decide to use the different kinds of testing? todd: Class III is the strongest requirement. In this class, basically zero downtime of the product is allowed—usually medical or aero- space. The utmost care in test and reliability is expected.