Choosing an In Circuit Tester
the steps to making a choice of the correct In Circuit Test ( ICT )
system
Choosing the correct type of test, and the correct type of
tester is essential to ensure that production costs are minimised. Although In
Circuit Test ( ICT ) has some disadvantages in today's highly compact
electronics manufacturing arena, ICT is nevertheless a very effective form of
testing. Many new developments and techniques have been applied to ICT
technology to ensure that it can meet the needs of today's electronics
manufacturing requirements.
Often a choice may need to be made between In circuit Test (
ICT ) and other types of test such as Automatic or Automated Optical Inspection
( AOI ), X-Ray Inspection, Boundary Scan and Functional Test. However it has
been found that even though new forms of test have been introduced, these have
not replaced ICT, but instead they have been used in conjunction with ICT to
provide a more comprehensive test. It is found that as boards and assemblies
have become more complicated that a variety of approaches have been required.
An In Circuit Testers can be categorised in a number of ways.
Choosing the most applicable type can be the first step in choosing the correct
tester for a particular application. Tests fall into four main categories:
- Low cost tester
- Large tester capable to handling large number of nodes
- Flying probe tester where probes move to the nodes to be tested This
negates the need for a bed of nails test fixture but is slower
- Combinational tester capable of testing boards using a variety of test
techniques
Selection considerations
The choice of the correct type of tester depends upon a variety of different
elements. These include the component technology used. Very densely packed
surface mount boards will need a different type of tester to one that might use
conventional leaded components. The production volume and throughput required
will also have a major impact on the choice. Also the mix of board types and
whether there are frequent changes of the type of board to be tested are
important factors. Further information that is essential is the anticipated
process yield and fault spectrum. While these two factors cannot be determined
from real figures for boards or assemblies that are being designed, they can
nevertheless be estimated quite accurately from existing boards that are in
production.
Collecting information
Determining the yield of a process is relatively straightforward. It is simply
the number of boards passing successfully through the process divided by the
total number entering. Collecting these figures is a simple number collecting
process. Once collected, these metrics give a good indication of the process.
The higher the yield, then the better the process. If yields are low then this
indicates that higher levels of testing are required.
Batch size is another important issue. Small batches require
frequent changes of fixture, reloading of programmes, and the like. Small
batches result in a reduced throughput, but this is often necessitated by the
requirement for the product, and the availability of the equipment. If batch
sizes are small, then a machine tailored to suit small batches needs to be
chosen. Often a roving probe machine may be suitable because fixturing costs are
less, although they run more slowly than machines that use a bed of nails. This
is because the probes physically move from one node to another to undertake the
test.
A knowledge of the fault spectrum is important in determining
which In Circuit Tester ( ICT ) to use. Unfortunately this is not always easy,
but it may be possible to gain data from similar boards that are currently being
produced. Adjustments can be estimated according to any knowledge of the board
to enter production. With a knowledge of the typical fault spectrum anticipated,
a suitable tester can be chosen. For example if most of the faults found are
short and open circuits, then the more expensive device testing facilities may
not be justified. It may be more cost effective to perform a different type of
test to locate the other tests.
Choices
It is necessary to adopt "production orientated" approach to testing. When
developing a test strategy there is often a tendency to want to test everything.
This approach is particularly expensive as test time and investment in the ICT
machine and the ICT programme increase to enable this over-comprehensive test to
be undertaken.
Instead, look at the throughput to see the level of machine
required, and the fault spectrum to judge the facilities required in the machine
to perform the tests that are needed. Also develop the test ICT test programme
to locate the likely tests. There is no point in spending time testing for
faults that never occur.
By adopting an approach that meets the needs to the
requirement, considerable amounts of money can be saved. However it is also
necessary to ensure that the whole test strategy fits together so that each
stage of testing performs the required tests most efficiently. In this way a
quality product can be assured at the lowest cost.
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