Assembly
results, and a detailed understanding of aerospace and similar applications
how the tests actually proceed and not where potentially hazardous gas leaks
unwittingly undermine the test process. less than 0.01 sccm need to be
In leak testing, for example, the detected. In most applications requiring
choice between methods is driven by a detection of low leak rates, the lower
compromise between a leak detector’s cost mass flow testing method using
performance and cost of purchasing a customised sensors is actually
highly specified piece of equipment. preferable, since it has the shortest
There are various methods that one possible testing times, can readily
might choose from simple pressure compensate for temperature and
decay testing, differential pressure overall delivers the best accuracy for
Issue III 2009
decay testing; mass flow leak testing, applications where testing is required at
square4
and helium mass spectrometer testing. 0.05 sccm or larger.
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The lowest cost method, pressure decay The details of how a test is
testing, actually has lower initial costs performed can be all important in
for fixtures etc, but may in fact be more determining if the testing process is
costly due to longer test cycle times adequate. Often this means having the
and is wholly unsatisfactory for testing ability to test the test process as testing
for small leaks under 5sccm. proceeds. In downstream mass flow
Differential pressure decay testing testing, for example, where there is
oasiasemiconductor
methods are superior to simple zero pressure from atmosphere, it is not
.eur
pressure decay testing for small leak possible to know if a valve used in
www
testing or for testing at high pressures, testing is working or not unless extra Up to 70% reduction in test cycle times
but are similarly inadequate for testing steps are taken. A zero leak is standard in testcentric assemblies
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parts where temperature compensation measurement could otherwise be due
is an issue. Both pressure decay and to a dysfunctional valve or a line being
differential pressure decay test methods plugged. For that reason, accurate of the reasons why test centric
involve increased cycle times compared downstream mass flow test methods assemblies generally have significantly
to other dry air test methods. will always employ a bias leak that higher yields and shorter test times.
The most common mistake made verifies that the entire system is working
by those without testing expertise is to and that the test circuit’s integrity is not Differences in fixtures for
select the lowest cost instrument off the compromised. A bias leak is simply the assembly vs. testing
shelf. However, this does not imply that introduction of a known leak as a self It often comes as a surprise to those
higher cost test methods are directly check of the testing system. If valves familiar with building standard
correlated to the best test technology are not operating properly and test assemblies that the fixtures they take
choices. High cost helium testing, seals are insufficient the bias leak will pains to create for fastest assembly
especially costly in the current context not measure as a leak, indicating that throughput often have quite the
of rapidly rising helium prices remedial actions on the test system opposite characteristics of what one
worldwide, is rarely justified except for need to be taken. would want for failsafe testing in most
Similarly, test systems should be assembly and test operations. This is
frequently and accurately calibrated and why one is best advised to consult with
validated. Mistakes are often made by applications laboratories dedicated to
relying on antiquated mechanical developing best match test processes
calibration methods that have inherent to help with test fixture designs.
limitations such as pneumatic valves and For example, if you have a part that
other moving parts that can stick or is driving a screw you generally want a
wear, and/or small orifices that can clog. lot of float. However if you are leak
Today’s test centric assemblies rarely testing that same part you would have
use human error prone mechanical to seal it on dead nuts so the sealing
calibration methods but rather rely on location would interfere with other
electronic technology for calibration steps of the assembly operation. This is
and validation of testing. These why one generally does not use the
In process testing minimizes rework and electronic test methods are not only same pallets during leak testing as in
scrap costs more reliable but are fast, which is one other phases of assembly and test.
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