In-circuit functional tester

Clip-On Test Versus Whole Board Functional Test

In-circuit functional testing (or clip-on testing as it is popularly called) is best described as testing the functionality of each component on the board using test clips. Here, the inference is that the overall board functionality can be verified if each and every component on the board is tested and found to be good. This is ideal for component-level replacement, where replacing an entire board is expensive or impractical.

Clip-on testing uses a technique called backdriving to force the desired logic state to the input pins of the Device under test (DUT). The tester hardware is equipped with powerful pin drivers that can source or sink currents high enough to force nodes on the DUT to a required state. Of course, this is done for a very short amount of time to ensure that the tests done are within the safety limits of the devices in the circuit. This can often be difficult in circuits where the components are normally operating near their current handling limits or node capacitance is high.

Whole Board Functional testing, a.k.a. "Black Box" testing, is characterized by powering up the board, applying input stimuli, and measuring the output signals on the whole board (through connectors or test points). The measured output is compared against an expected result, developed from a "Known Good Board", or Engineering Analysis of the circuit. This is aimed at verifying the functionality of the entire board. Here, there is no need to test individual components on the board, the entire board is taken as a single device and replaced. However, some component-level fault detection may be possible, usually requiring replacement of several components across a signal path. Diagnostic probing may reduce component callouts to more manageable levels, but slows overall testing. Whole board testing is most useful for identifying failures in functionality where the board is easily and inexpensively replaced, and/or component level testing can be done at a later time. This testing may also be performed after board fabrication, for quality control purposes.

Limitations of clip on testing as compared to Whole Board testing

1. In Clip on testing, the Ics are tested only for its functionality. It is assumed that if all the components on the board are good, the board should work when put in the machine. The track connectivity between the components on a board are not checked. Although the links between the pins of the DUT are tested during clip on test, the interlinks between the various components on the board as well as to the edge connectors are not tested. If, due to spillage, corrosion, or rework, a track fault exists, this fault cannot be traced using the clip on test method. Where as, in whole board test, since the input/output signals are driven/sensed from the edge connector, any track opens/shorts will be easily detected.

2. Let us look at another example, where the PCB is faulty because of a low impedance of around 50ohms w.r.t. GND at one of the input pins of an IC. In clip on test technique, since the tester hardware is driving this faulty input pin, its powerful pindrivers (that can source & sink currents up to a max. of 600ma.) can easily drive the required Test patterns to this input pin during test and declare the IC as good. But when this test is carried out through the edge connector during whole board test, the board fails, as the output stage of the previous IC will be unable to drive the required current to override the low impedance fault. Thus using the whole Board Test, faults can be zeroed in on much quickly and efficiently.

3. Testing a device in-circuit can be relatively time consuming. Isolation of component faults is difficult and often results in multiple-component replacement on a single test failure.

4. Clip on testing is useful for testing boards for which there is limited data, or board layout is expected to change without component changes. For Whole board testing, availability of complete schematic diagram is necessary. Developing whole board test program is done after understanding the complete working of the PCB. The program developed would then fully test all the functions. Special Test jigs are to be developed for each type/revision of board. A single test program development can be extended to a larger number of boards, reducing test development costs for high-revision-count or low-production-quantity boards.

5. Using Whole Board testing methods, higher quantities of boards can be made serviceable more quickly, compared to the clip on method. The test time is reduced and board serviceability percentage increases.