🚀 go-pugleaf

Iddq is a current-measurement technique for detecting defects.  Iddq, or
quiescent Idd, is the (leakage) current which remains after a transition.
In CMOS circuits, the quiescent current is very small (on the order of
micro or nano amperes), and so Iddq is a particularly effective method
for detecting defects such as gate-oxide shorts, which can raise the
Iddq into the milliampere range.  There is also widespread belief (with
some evidence to back it up) that the kinds of defects that Iddq detects
will often lead to infant mortality failures, and so Iddq could also be
considered to be a reliability test.

Iddq was first investigated and used (I Believe) by Ford Microelectronics in
the early 80's.  Other notable locations which have published results include
Sandia Labs, Phillips Research, CMU and National Semiconductor.  There are
lots of papers on Iddq in the proceedings of the International Test Conference,
for example.

In essence, Iddq testing could be considered to be essentially the same as
scan-based testing, except it uses the power & ground lines to observe the
defect mechanism rather than scanning out a voltage value using the scan
shift register; both techniques rely on having a high-coverage pattern to
sensitise the defect.  In addition, Iddq also requires that other sources
of leakage current are controlled (e.g. dynamic memories, pullup resistors
on input pads, etc) so that the Iddq value is as small as possible.  Iddq
values generally follow a normal distribution centred around a very small
value, and so the test method basically involves deriving a margined test
value similar to any other parametric test.  Scan-based patterns have
frequently been used for Iddq because they obtain high internal node
coverage, and they are quite predictable in their timing behaviour.  Many
vendors of scan & design-for-test tools have the capability to generate a
scan-based pattern for Iddq.
  
The main difficulty with the test method has been the measurement speed.
Since the values are so small, a certain amount of settling time has to
be allowed.  Some proponents have designed their own low-current measuring
instruments, others have used the tester parametric measurement unit, and
others have used the power supplies.

Regards,
Phil Pownall

philp@bnr.CA