Models for Power-Aware Testing - Models in Hardware Testing - page 190

Low test throughput . In order to avoid the above degradation mechanisms, heat
dissipation has to be kept under a given safety limit. This limit is determined
from the knowledge of the thermal capacity of the package, the use of cooling
systems, etc. Restricting the amount of power that can be dissipated will have a
negative impact on the level of parallelism that can be used during wafer or chip
testing, or on the test frequency that can be used. A reduced test concurrency or
test frequency will therefore lead to a lower test throughput.

7.3.2 Issues due to Elevated Peak Power

As for average power, excessive peak power consumption may occur during test
and lead to undesirable and abnormal behavior of the circuit. These issues may
occur when testing the circuit at the wafer level or at the chip level. Excessive peak
power consumption comes with a high instantaneous current demand due to high
switching activity during test, and may lead to considerable drops in voltage levels
at power grid nodes.
Voltage drop in the power grid, also referred to as Power Supply Noise (PSN),
is mainly due to two components: IR-drop and L(di/dt) ( Arabi et al. 2007 ). IR-
drop refers to the amount of decrease in the power rail voltage and is linked to
the existence of a non negligible resistance between the rail and each node in the
circuit under test. R represents the resistances of the power mesh network, power
pads and device package. L(di/dt) is an inductive noise and refers to current varia-
tions occurring during switching through inductive connections. In this expression,
L represents the inductances of the power mesh network, power pads and device
package, and di/dt represents the magnitude of the variation of the current flow-
ing through this connection. These two types of noise are illustrated in Fig. 7.6 .
Note that L(di/dt) also refers to voltage glitches or surge/droop phenomena known
as Ground Bounce or Simultaneous Switching Noise (SSN) ( Chang et al. 1997 ) .
Crosstalk is another noise phenomenon that refers to capacitive coupling between
neighboring lines within a circuit. Crosstalk is known to be less significant during
test compared to IR-drop and L(di/dt) ( Saxena et al. 2003 ) .
With high peak current demands during test, PSN may become much higher than
during functional mode and then is no longer negligible. With increased PSN (see
Fig. 7.7 ) , the voltages at some gates in the circuit are reduced and these gates exhibit
higher delays (performance degradation), possibly leading to test fails (good dies

Vdd Vdd

U=R.I U=Ldi/dt

R

Vdd-U Vdd-U

I

i(t)

L

Fig. 7.6 Illustration of power supply (IR-Drop, Ldi/dt) noise