IPC Physical Test Concerns Printed board assembly functional test equipment is usually very expensive and requires highly skilled personnel to operate. If printed board assembly testability is poor, the printed board assembly test operation can be very expensive. There are some simple physical considerations that can decrease the debug time and therefore the overall test costs.

IPC Physical Test Concerns The orientation of polarized parts should be consistent so that the operator does not get confused with parts being oriented 180° out of phase with other parts on the printed board assembly. Non-polarized parts still need to have the pin #1 identified so that the test operator knows which end to probe when guided probe software says to probe a specific pin.

IPC Physical Test Concerns Test connectors are much preferred over test points which require the use of test clips or test hook-up wires. However, test points such as riser leads are preferred over clipping on to the lead of a part. If riser leads are used for temporary testing, such as determining a select-by-test resistor, it is suggested that the risers remain after the installation of the selected component. This allows verification of the selected item without re-fixturing the assembly.

IPC Physical Test Concerns Signals that are not accessible for probing (such as can happen with leadless parts) can greatly increase fault isolation problems. If scan registers are not used, it is recommended that every signal have a land or other test point somewhere on the printed board assembly where the signal can be probed. It is also recommended that lands used for test points be located on grid and placed so that all the probing can be done from the secondary side of the printed board assembly.

IPC Physical Test Concerns If it is not feasible to provide capability for probing every signal, then (1) only the strategic signals should have special probing locations and (2) the test vectors need to be increased or other test techniques need to be utilized to assign fault isolation to one component or a small set of components.

IPC Physical Test Concerns Many faults are often due to shorts between the leads of adjacent parts, shorts between a part lead and an external layer conductor on the printed board or shorts between two printed board conductors on the external layers of the printed board. The physical design must consider these normal manufacturing defects and not impair the isolation of the faults due to lack of access or inconvenient access to signals. As with design for in-circuit testability, probe pad test points should be on grid to allow automated probing to be used in the future.

IPC Physical Test Concerns Partitioning of the design into functions, perhaps digital separated from analog, is sometimes required for electrical performance. Testing concerns also are helped with physical separation of dissimilar functions. Separation of not just the circuitry but also the test connectors or at least grouping the pins on the connectors can help improve testability. Designs that mix digital design with high performance analog design may require testing on two or more sets of test equipment. Separating the signals will not only help the test fixturing but will help the operator in debugging the printed board assembly.

IPC Physical Test Concerns As with in-circuit test fixturing, functional test fixturing can have a significant cost impact. Normally a standard board size or only a few board sizes are used for all designs on a program. Similarly one, or at most a few, test fixtures are typically used for a program. Generating test fixtures can be costly and debugging noise problems in the fixtures or tuning the fixtures to the tester can be expensive.

IPC Physical Test Concerns If the test fixturing is not adequately engineered, it may not be possible to accurately measure the board under test. Typically much effort is expended in generating a few test fixtures and it is expected that the fixtures will be used for all the printed board assembly designs. Therefore the test fixturing restrictions must be considered in the printed board assembly design.

IPC Physical Test Concerns The fixturing restraints can be significant. Such as (1) requiring ground and voltage supplies on specific connector pins, (2) limiting which pins can be used for high speed signals, (3) limiting which pins can be used for low noise applications, (4) defining power switching limitations, (5) defining voltage and current limitations on each pin, etc.