1. IPC-2221 3.5.1 Printed Board Assembly Testability Design of a printed board assembly for testability normally involves systems level testability issues. In most applications, there are system level fault isolation and recovery requirements such as mean time to repair, percent up time, operate through single faults, and maximum time to repair. To meet the contractual requirements, the system design may include testability features, and many times these same features can be used to increase testability at the printed board assembly level.

2. IPC-2221 3.5.1 Printed Board Assembly Testability The printed board assembly testability philosophy also needs to be compatible with the overall integrations, testing and maintenance plans for the contract. The factory testers to be used, how integration and test is planned, when printed board assemblies are conformal coated, the depot and field test equipment capabilities and personnel skill level are all factors that must be considered when developing the printed board assembly test strategy.

3. IPC-2221 3.5.1 Printed Board Assembly Testability The test philosophy may be different for different phases of the program. For example, the first unit debug philosophy may be much different than the test philosophy for spares when all the systems have already been shipped.

4. IPC-2221 3.5.1 Printed Board Assembly Testability Before the PWB design starts, requirements for the system testability functions should be presented at the conceptual design review. These requirements and any derived requirements should be partitioned down to the various printed board assemblies and documented. The system and program level test criteria and how they are partitioned down to the printed board assembly requirements are beyond the scope of this document.

5. IPC-2221 3.5.1 Printed Board Assembly Testability The two basic types of printed board assembly test are functional test and in-circuit test. Functional testing is used to test the electrical design functionality. Functional testers access the board under test through the connector, test points, or bed-of-nails. The board is functionally tested by applying pre-determined stimuli (vectors) at the printed board assembly's inputs while monitoring the printed board assembly outputs to ensure that the design responds properly.

6. IPC-2221 3.5.1 Printed Board Assembly Testability In-circuit testing is used to find manufacturing defects in printed board assemblies. In-circuit testers access the board under test through the use of a bed-of-nails fixture which makes contact with each node on the printed board assembly. The printed board assembly is tested by exercising all the parts on the board individually. In-circuit testing places less restrictions on the design. Conformal coated printed board assemblies and many Surface Mount Technology (SMT) and mixed technology printed board assemblies present bed-of-nails physical access problems which may prohibit the use of in-circuit testing.