Evaluation Engineering - 19

Signal with and without ripple

consists of four different functional units,
you should have an eye on:
* Current source
* Cabling
* High-current matrix/multiplexer
* DUT (fixture/needle adapter/wafer
prober)
The used current source itself is very
different in design, depending on whether
one is performing DC or pulsed testing.
High-current DC sources are designed to
supply energy continuously, and therefore
also have a correlating cooling capability
which both result in a larger form factor.
Since they are based on switching power
supplies, these provide output currents,
which do have a certain ripple. It is important to take care of when performing
measurements.
Beside the current source, it is the cabling, multiplexer, and the DUT adapter
which will all generate-although they
are stressed-ohmic and thermal losses
when performing high-current tests.
Performing these tests with DC currents
significantly increases the required effort
for these system components. They have to
be capable of handling 1.000A DC or more,
and therefore the thermal power losses generated have to be cooled constantly. This
results in devices like DC capable power
sources, heavy and very low resistance
cabling, big high current switching relays,
and the need for many contacting needles.
Every component has to be able to handle
the system DC currents, so in consequence

they are bigger, need a lot of rack and floor
space, and are more cost-intensive for operating costs for all the years the DUT should
be produced and tested.
The second way is smarter. By using
very fast high current pulse sources,
most components of a test system can
be run more easily. Testing a device with
1.000A with a pulse of e.g. 300µs greatly
changes the requirement for the overall performance of system components
like matrix or cabling. To realize such a
"smart" test system, one challenge that
has to be overcome is the reduction of
parasitic inductances. By using a pulse
source and therefore the possibility to
use more smart system components
like a smaller matrix, it is possible to reduce the system inductance. Designing
low-inductive system components-like
a low inductive matrix-needs a lot of
know-how but is feasible. In particular,
by optimizing the system cabling, there
is another great opportunity to reduce
parasitic inductances. In addition, the
proper selection of connectors, their type,
and the amount of contact needles are
significant for the system.
As a rough estimate, one can say that
initial system hardware cost of DCcapable test system and smart pulse
test system may differ not very much.
The big advantages of smart test systems
are a much smaller space requirement

and the ability to achieve a much higher
system throughput due to the high test
speed, which will additionally reduce
cost for testing.

Ohmic resistance and parasitic
inductance...
Since the size of these system components are mainly defined by their thermal capability, the Ohm's Law is the
major problem to fight. Thermal losses
rise exponentially with rising currents:
P = R * I * I (Ohm's Law)
This means: raising the test current by
a factor of 2 will raise ohmic power losses
in relays, cable, and needles by a factor of
4. Raising the test current by a factor of
10 will then rise ohmic power losses by a
factor of 100.
As if that wasn't enough, forcing a certain current into a resistance-inductancenetwork (RL network)-what a series of
cable, matrix and DUT represents-is taking linearly longer. That means 10 times the
current takes 10 times the time: E = P * t
What does that mean for the relays,
cables and needles? When bringing these
two physical laws together, you can estimate that raising the test current from
500A to 1.000A by a factor of 2 will raise
the thermal energy within the relays,
needles, and cables roughly by a factor
of 8. Raising the current by a factor of 3
rises the thermal energy by a factor of 27,

"By using a pulse source and therefore the possibility
to use more smart system components like a smaller
matrix, it is possible to reduce the system inductance."
- Christian Degenhart, CEO of VX Instruments

Current curve

MAY 2019 EVALUATIONENGINEERING.COM

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Evaluation Engineering

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