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Strip Testing (Page 3 of 3)
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Another problem associated
with strip testing is difficulty of retesting or rescreening fall-outs
from an initial round of electrical testing. Some of these initial
fall-outs are invalid failures for one reason or another, which can be
recovered by simply retesting them. Depending on the level and
nature of the yield loss, there comes a point wherein it is more
profitable to retest the failures and recover the invalid failures.
This is easy to do when dealing with singulated units, since one can
simply segregate and retest singulated rejects.
Strip retesting,
unfortunately, is just like a second round of strip testing - the entire
strip will have to go through the strip test process again, because
that's how this parallel testing approach is set up. Thus, even
the good units in the strip get an extra 'handling' that they no longer
need, potentially affecting their quality.
One solution offered for
this problem is to just perform the retest on the rejects after
singulation, this time using a test system for singulated units. Leaving
the rejects unmarked makes them identifiable for this retest.
Another
drawback of strip testing is the higher risk of units being damaged
after electrical testing, which can reach the customer if OQA is not
able to detect the problem. The higher risk is simply due to the
number of critical manufacturing steps that the units still need to
undergo after they've been subjected to final test. Thus, it is
necessary to conduct strict process evaluations and qualifications to
ensure that steps following strip testing (notably singulation) will not
induce any damage to the units.
Other
challenges
for strip testing include the following:
1) design and
fabrication of lead frames that would allow proper debussing (trimming
or isolation) of all the leads of
2) design of
contactors with very precise alignment capabilities;
3) design of
strip test modules that can adapt to as many packages as possible;
4) resolution
of high mechanical force problems associated with connecting a high
number of pins at the same time;
5) design of
effective temperature soaking systems for correct thermal conditioning
of units prior to temperature testing;
6) power and
thermal management for parallel testing of multiple high-power devices;
7) reduction
of cross-talk between the devices under test;
8) better
understanding of ESD phenomena associated with the new strip test
set-ups.
The following
excerpts from a press release taken off the internet illustrates how
strip testing is currently implemented by its originator, Amkor:
"In a cooperative effort, the three companies will be
jointly exhibiting the Amkor-developed strip test process using an MCT
Tapestry(TM) automated strip handling system integrated with a Nextest
Maverick II GT digital tester....
By combining standardized strip handling tooling and
robotics, new generation test systems, and simplified contactor
methodologies, this new process allows massively parallel test at full
device operating speeds. Devices are tested in groups of 16, 32, 64 and
up to 128 devices at one time while also increasing quality and test
yields. Test costs can be reduced by 50% or more due to the high
parallelism and fast handler index times. Test yields can increase by 5%
due to more reliable contact methodologies, and bent leads can be
virtually eliminated, since the contacting of the device leads during
test now occurs prior to the lead form operation.
Amkor currently has several of the combined (MCT/Nextest)
strip test systems in production and has tested over 200 million units
in strip test form."
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First posted on October 20, 2004
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