- questions most frequently asked by visitors of www.siliconfareast.com
mechanism am I dealing with if the rejects recover (become good again)
mechanisms are considered to be bake-recoverable?
encounter a failing unit that becomes good again after it has been
subjected to bake (usually 24 hours at 125 deg C), then you can suspect
several possible scenarios that can explain the initial failure.
and this is the most common conclusion written in reports, is that the
device had an
internal or external package moisture that enabled ions on the package
to become electrically conductive, causing pin-to-pin leakage. This
scenario is often applicable only to slight parametric shifts, such as
higher-than-normal leakage, and isn't generally used to explain a
catastrophic failure (unless there is more in the samples than just
moisture). Baking drives away the moisture and makes the units
presence of a conductive foreign material on the die or the package that
bridges electrically active parts is the second possibility. This
scenario is different from the first one in the sense that a localized
contaminant not normally found anywhere on the package is the culprit
more than the presence of moisture in the device. Baking dries
up such a contaminant and either takes away its conductive properties or
reduces its size to the point that it can no longer act as a bridge.
third scenario is the presence of mobile ionic contaminants in the die.
Mobile ionic contaminants tend to 'localize' to certain areas with the
application of bias (such as during burn-in). Once they've
accumulated at a high enough concentration, they can exhibit a charge
that's large enough to shift the properties (such as threshold voltage)
of a nearby transistor, making the device fail. Baking disperses
the mobile ions in random directions, causing the failing unit to become
good again. Unfortunately, the failure can return when bias is
applied to the unit again.
effects, or the injection and entrapment of electrons or holes in oxide
layers and other areas of the device where they shouldn't be, is the
fourth possible cause of bake-recoverable failures. Hot carrier
injection, which is often caused by excessive bias voltages being
applied to the device, results in the formation of a volume charge that
can shift the characteristics of a device. Baking can 'dislodge'
these trapped carriers and disperse them, eliminating the volume charge
formed by their accumulation and making the affected device 'recover'
from its failure.
possible cause of failures that recover after bake is die surface
charging, which is often caused by bias over time. Surface
charging often manifests as excessive leakage or as a shift in
transistor characteristics. Since the charge accumulates on the
surface of the device, this mechanism is also die surface
wash-recoverable. Baking disperses the surface charge accumulated by
bias, making the device recover.
stress on the die is the last possible cause of bake-recoverable
failures. Studies have shown that a direct application of mechanical
stress on an active component of the die circuit can shift its
electrical characteristics by a factor that's large enough to make the
device fail. Sources of such package-induced die stresses include
the TCE mismatch between the die and the molding compound and the
molding compound fillers exerting force on the die. Baking can
shift the 'stressed' points and make the device recover. Package
stress-related failures should be permanently recoverable by
Back to the FAQ Page...
All Rights Reserved.