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.

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 upto a maximum of around 600mA to do the node-forcing. Of course, this is done for a very short amount of time to ensure that the tests done are within the safety limits.

Whole Board Functional testing, on the other hand, is characterized by powering up the board and the application of input stimuli and measurement of the output signals on the whole board (usually through pins on the edge connector). The measured output is compared against an expected result from a Known Good Board already stored in the database. 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 tested.

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 upto 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 a time consuming affair. It could take upto a day to test all ICs and the passive components one by one. Whole board testing is much faster as the complete functionality of the board is tested in one go, in a matter of minutes.

4. Clip on testing is meant for testing boards for which there is limited data. 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 of board. The effort to develop the test program is only once, but the number of boards that can be recovered by the use of this methodology of test is large and in quick time.

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

Written By: Sukumar
Website: Estec