SMA Technology—Current Practices and Criteria for Testability

Automatic in‐circuit testing is a cost effective facility for not only testing assembled printed circuit boards but for providing component level diagnostics very quickly with an average PCB test time of less than 30 seconds. The in‐circuit tester relies on a customised fixture (essentially a bed‐of‐nails) to interface particular PCBs to the system and a computer programme to control the hardware to test all the individual components on the PCB to their own specification irrespective of how the circuitry is configured. So if all the individual components work and are interconnected correctly, the complete assembly should function correctly. Traditionally the fixture requirements for most PCBs have been straightforward. In general, components are mounted on only one side of the board allowing easy access to the other side where all the tracks may be accessed by a bed of nails. However, the introduction of surface mount technology introduces mechanical restraints on this concept. Components can be mounted on both sides of the PCB, possibly causing an access problem. Not all circuit interconnections may be available on a single side of the PCB, necessitating dual side fixturing. The pitch of components together with pad and track sizes has dramatically reduced, possibly requiring the use of smaller test probes and certain SMDs do not allow direct probing at all. All these things can present problems but they do not mean that in‐circuit testing is no longer the solution to the test and repair problem. On the contrary, it is particularly well suited to the production problems being experienced by manufacturers. And where components are difficult to remove and replace on a PCB, it is particularly important that the fault diagnosis is to component level. This paper describes the problem in detail and goes on to demonstrate that if a code of practice is followed within the design of a new PCB, then there need not be a problem. Much can be achieved very simply within the design that can improve the testability of PCBs so that in‐circuit testing remains a very cost effective system. Experience demonstrates that most designs could be fixtured with traditional techniques and that means cheap and reliable fixturing.