Combining Grid and Flying Probe test to achieve higher pass rates and better test quality

In the past, there was a strict separation between testing unpopulated circuit boards on a Universal Grid tester and Dedicated tester (both fixture based test systems) and a Flying Probe Test system (FPT) (i.e. fixtureless test system).  While high-volume productions were tested using a Universal Grid tester or Dedicated tester, the Flying Probe Tester was mainly used for prototypes or small series production. New developments let PCB manufacturers rethink the role of the FPT.

Over the time, the Flying Probe Test systems became faster and more automated. With that the lot sizes for which a FPT can efficiently be used increased, avoiding the cost and the lead times of fixture building. But apart from this, there are also other factors which let PCB manufacturers rethink the role of the FPT:
Due to the increasing density of PCBs and Substrates and also due to the smaller pad sizes, it becomes ever more difficult to test the products with fixtures. Especially in areas with BGAs, the required deflections on the universal grid are testing the limit. There is also an increase in the number of false faults due to a misalignment of test pads and the test pins of the fixture (e.g. resulting from distortions in the printed circuit board). Next to these contacting problems, also measurement capabilities limit the application of fixture based tests. Small structures can require resistance tests down to a few µOhm while a fixture based tester barely can test values of a few mOhm reliably.

Two ideas arose.  On the one hand, to perform a kind of split test: to test on a fixture based tester all the parts of a PCB or Substrate which safely can be tested on such a system and to make all other measurements on a FPT. The latter measurements are predominantly those which require higher mechanical accuracy, better alignment or higher measurement resolution.

On the other hand, to perform a verification of the reported faults of the fixture based tester on the FPT. While this was in the past only feasible for small quantities, with highly automated FPTs, this can now be done on a large scale. An enabling factor for this is also the integration of data between systems and platforms which is advancing rapidly with the spread of Industry 4.0.

With all these options, the PCB manufacturer now has the possibility to set up a far more cost-efficient and more effective test environment than it was possible a few years ago.