Define Test Strategy

Decide what test strategy to use on your PC board. When determining the test strategy, consider the items listed in Table 3. It is not an exhaustive list but is intended to show the kinds of factors to consider.

Possible test strategies include:

in-circuit only,
in-circuit and functional,
functional, with in-circuit for diagnostics only, and
functional only.

(Refer to Analog Testing and Digital Testing for information about the test methodologies involved.)

Also consider the additional test strategies and methodologies in Table 4.

To define and document the test strategy:

1   Learn the function of the board and its components.

2   Identify the clusters to be functionally tested (digital testing only).

It is better to have several small clusters than a few large ones. Smaller circuits let the development software make more efficient use of testhead resources and require fewer module cards to implement the test. Also, diagnosing faults within small clusters is easier than diagnosing faults within large clusters.

3   Determine device dependencies.

Arrange the testing according to device dependencies. For example, if device A will not work unless device B is working, device B needs to be tested first.

4   Determine digital device intrinsics.

Determine digital device intrinsics, for example, determine the internal resistance between two pins of the device. Model the device in the board description with an internal device entry or a Part Description Library.

5   Determine which fault types to cover.

Decide which fault types your test will identify. For example, do you need to diagnose which RAM cell is faulty or is it enough to know that the RAM chip is not working? If your board has a device that can operate at a higher speed than the circuit requires, is it necessary to test that device to its high-speed specification?

6   Determine fixture strategy.

Possible fixture strategies are:

Top side probing
Wireless fixtures
Short wire fixtures
Long wire fixtures
Fixture electronics
Throughput multiplier
Dual well shared wire
Dual stage fixturing
Vacuum fixtures
Pneumatic fixtures

Also consider:

Which fixture default files are needed.
If user-modifiable fixture component files are needed.

7   Document the strategy.

Table 3    Factors in determining test strategy

Consideration	Issues
Board volume	If many boards of a given type are tested, the per-board cost of developing the test is relatively small. This makes it practical to develop a comprehensive test that includes both in-circuit and functional testing. When only a few boards of a given type are tested, the test cost per board is high, so an in-circuit only test might be preferable. If test time is a concern, high volume applications may need a faster test than low volume applications. A faster, in-circuit test usually means less fault coverage and accuracy than a slower, more thorough test. One advantage of having a large volume of a particular board is that you can quickly collect meaningful fault data and apply SQC methods to improve your production process.
Fault spectrum	The kinds of faults on the PC boards depend on such things as the types of components and how they are used, the manufacturing process, and how the board was designed. Process faults and digital device problems are best found by in-circuit testing. Timing faults in high speed circuits require functional testing.
Yield into test system	This is the percentage of your boards which you expect to be good when they arrive at the test system. If your yield into the test system is low, you will want a thorough in-circuit test because of its ability to diagnose faults easily. If your yield is high, you can rely on less in-circuit testing. In either case, you should enable datalogging to collect data that can be used to produce SQC reports.
Programming capacity	The size of your programming department and its current workload should be considered. In-circuit tests require less programming time than functional tests and are easier to generate and debug.If desired, you can develop only an in-circuit test and start testing boards as soon as possible, then develop the functional part of the test later. To avoid modifications to the fixture, provide at least setup-only tests so that resources will be assigned and built into the fixture for any future testing.
Quality objectives and company test strategy	These two considerations are closely tied together, with the strategy generally based on such quality objectives as the required test system yield. Different companies will have different objectives and strategies. The environment in which the product is to be used can also affect the test strategy. If the boards are to be used in a critical environment, exhaustive in-circuit and functional testing may be justified.

Table 4    Optional test strategies and methodologies 

Option	Description
Multiple Board Versions	Develop and maintain one test and fixture for multiple versions of a PC board.
Panel Test	Develop board tests and fixtures for multiple-board panels.
Throughput Multiplier	Test up to four boards of a single-board-type panel simultaneously.
Dual-Well Shared Wiring	Develop a dual-well fixture that has parallel wiring (helps to eliminate handling time on high-volume lines).
Connect Check	(Mux systems only) An unpowered test of the connectivity of device pins.
Polarity Check	An unpowered test of the orientation (polarity) of electrolytic capacitors.
DriveThru Test	Use VTEP technology to test devices through series components, such as resistors, capacitors, and inductors.
MagicTest	(Mux systems only) Test passive analog components that cannot be tested in-circuit due to insufficient electrical access to nodes.
Vectorless Test EP (VTEP) and TestJet1	An unpowered test which targets manufacturing defects such as connectivity problems (e.g. lifted pins, non-wet pins) and missing devices.
iVTEP	Extension of VTEP capability targetting at pins of integrated circuits which are too low for VTEP measurement.
NPM	Extension of VTEP capability targetting at power and ground pins of connectors and sockets.
Cover-Extend Technology	Perform Cover-Extend tests on connectors and sockets.
Advanced Boundary Scan	Develop tests for digital devices that comply with IEEE Standard 1149.1 and 1149.6.
Silicon Nails	Perform Silicon Nails tests on non-Boundary-Scan devices.
Device Programming	Use flash and PLD programming.

1  See TestJet and VTEP v2.0 Powered!.

Note the Additional Considerations for Optional Test Methodologies if these test strategies and methodologies are to be implemented.