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|Application Note 112
PCB test — how to approach fault finding using a Polar Fault Locator
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|Locating faults on PCBs requires some practice to become
successful. Polar Fault Locators allow you to take an
alternative approach to the 'scope and DMM techniques you
may be used to. However, don't be surprised if it takes
you a few weeks to become accomplished.
do I start?
Hint: Try to avoid practising on boards which have defied all previous attempts at repairing! Those are best set aside until you are accomplished at using the fault locator.
all differences faults?
You then need to examine the difference displayed and decide if this comparison is failing because:
do I determine the true fault cause?
If you are working on microprocessor based boards and you have a good understanding of their operation (i.e. you are familiar with the principle of guarding and address and data bus operation) then use both Nodal Impedance and ICT in combination.
If you are not too familiar with microprocessor principles then it is recommended you use the Nodal Impedance and Links tests and disregard the ICT logic test. You will still find 50-80% of faults by using Nodal Impedance and Links tests but you won't need to understand or use guarding.
Hint: When looking at the Nodal Impedance signatures of data buses (or address buses) usually all lines should show similar signatures.
Whether you use Nodal Impedance and Links test or Nodal Impedance, ICT and Links test you need to remember that on a bus node faults appear to be on all the components attached to that bus node. Use the information in AP107 and AP108 to learn more about how to decide which component is actually faulty.
using Quicktest it is important to understand the
difference between AutoTest and Library compare.
Library Compare is used to compare good and faulty boards or a good and faulty channel on the same board. Library Compare makes no attempt to predict mathematically the IC's logical function. It simply drives the input with a valid set of test vectors for that device and records the output directly from the board under test.
Remember that the first time you clip and test in any of the Quicktest compare modes the test information is saved as a reference. Once you have finished comparing a given component on a good and faulty board use the Erase function to clear the reference data. The PFL is now ready to learn a new set of test data.
If you have a trouble-shooting technique (related to PFL or T-Series) or have a Polar Fault Locator application you would like to see published please fax or email Polar Instruments on the number below. Every 6 months we will give away a free SMD probe to the best solution.
Polar Instruments Ltd
Tel: +44 1481 253081 Fax: +44 1481 252476 © Polar Instruments 2002
|© Polar Instruments 2002. Polar Instruments pursues a policy of continuous improvement. The specifications in this document may therefore be changed without notice. All trademarks recognised.|