Electrical
Test Confidence (Page 2 of 2)
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Several
retest cycles will eventually recover all the parts that have been
invalidly rejected by the initial rounds of electrical testing, since all of them
are good anyway. Such a process for recovering invalid rejects, however, is inefficient and not
cost-effective. The
value
of
recovering
invalid failures through retesting is, therefore, determined by the
system's test confidence as well.
The resulting
first pass yield, Y1, is the product of the real or actual yield of the
lot (denoted here as 'Y') multiplied by the test confidence of the ATE
system (denoted here by 'C'), or Y1 = Y x C. Thus, if a test engineer
knows the test confidence of a system, he can estimate the actual yield
of the lot through the equation
Y = Y1 / C.
If the
engineer decides to recover the invalid failures, he'll have to retest
all the fall-outs from the first test (denoted here by 'R2'). The
retest quantity R2 is equal to the initial quantity (Q) multiplied by
the first pass rejection rate, (1-Y1). Thus,
R2 = Q (1-Y1).
The resulting
yield of the retest (denoted here as 'Y2') is equal to the actual yield
of the retest multiplied by C. The actual yield of the retest
(denoted here as 'YY') equals the number of invalid failures Rinvalid
divided by the retest quantity R2. Thus, YY = Rinvalid/R2 and Y2 = (Rinvalid/R2)
x C.
The number of
invalid failures Rinvalid is equal to the initial test quantity Q multiplied
by the difference between the actual yield of the lot Y and first pass
yield of the lot Y1, or Rinvalid = Q x (Y-Y1). But Y = Y1/C, so Rinvalid = Q x (Y1/C - Y1).
Thus, Y2 =
{[Q(Y1/C-Y1)] / [Q (1-Y1)]} x C = [(Y1/C-Y1)/(1-Y1)] x C.
Simplifying,
Y2 =
(Y1(1-C))/(1-Y1),
where Y2 is
the expected yield of the retest based on the first pass yield Y1 and
the test confidence C.
This equation
can also be used by an engineer to
compute
for the test confidence exhibited by his test system, given first pass
and retest yield data: 1-C = Y2(1-Y1)/Y1, or
C = 1 -
[Y2(1-Y1) / Y1]
where C =
confidence of your test system, Y1 is the first pass yield, and Y2 is
the yield when the first pass rejects are retested.
According to
Christopher Jones, author of the article "Analyze test Confidence to
Enhance Throughput" upon which this article was based, their experience
in testing millions of RF IC's every week has taught them the following:
1)
never
run a test if the test confidence is
less than 85%;
2)
retest of the rejects is
not
necessary if the test confidence
exceeds 95%;
and
3)
retest
of the
rejects is
recommended
if the
test confidence is between
85%-95%.
Of course,
the above guidelines may
not
be applicable to every company, since different device groups and
package types are subject to different economic factors, as well as
exhibit different sensitivities to contactor degradation. It is
the task, therefore, of a Test Manager to determine for his company
how
they can best apply the concept of test confidence management in
improving their bottom lines.
Given the
very competitive atmosphere of the IC testing industry today, every Test
Manager must know when to do a retest, when it is not economical to do
so, and when a test system should not be used at all. Being able
to distinguish these situations from each other based on test confidence
data and reacting to each of them
appropriately
is an important aspect of test engineering management.
Primary Reference:
Christopher Jones, M/A-COM Division of AMP, Lowell, MA, "Analyze Test Confidence to Enhance Throughput";
Test & Measurement World, 9/1/1999;
through http://www.reed-electronics.com
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