New Process or Product
Semiconductor manufacturing involves processes that need a high degree
of control. As such, every
significant change in these processes, or the introduction of any new
process, must undergo proper
prior to its implementation.
This will ensure that the affected process flow will still be
capable of consistently meeting the specifications of the form, fit, and
function of the final products, even after the change.
process changes will have different effects on the process, so there can
be no single, standard procedure for qualifying a change. The
qualification procedure defined for a process change must therefore be
custom-made for the change being qualified.
Furthermore, the qualification of a change usually requires that no
degradation in the quality and reliability of the final products will be
introduced, even if the change is being implemented to serve another
purpose, e.g., productivity or cycle time.
Thus, in the context of this discussion, process qualification
refers to the process of ensuring that the change will not result in any
degradation in the quality and reliability of the final products.
semiconductor companies follow a basic flow when defining the
qualification plan for a process change or a new process, namely:
identify potential failure modes and failure mechanisms that the
process change may bring about;
subject samples to the appropriate reliability stresses to
accelerate these potential failure mechanisms;
test the samples to determine if they are still acceptable after
completing the reliability stresses.
basic flow works on the premise that the process change may be qualified
for implementation if the final products will not exhibit any failures
associated with the change.
key to a good qualification plan is the excellent anticipation or
prediction of what potential failure mechanisms the process change will
trigger. Once the potential
failure mechanisms have been defined, the selection of the reliability
tests will follow, based on the accelerating factors pertaining to the
instance, consider the process change wherein a new molding compound will
be qualified. What potential
failure mechanisms would a change in molding compound bring about?
These would include:
- package cracking, because the molding compound may turn out
to have thermomechanical characteristics that are less compatible with
the intended package and application;
- die cracking, because the molding compound may turn out to
be too stressful on the die;
bond lifting/wire breaking,
because the molding compound may turn out to be too stressful on the wires
die/wire/leadframe corrosion, because the molding compound
may contain corrosive elements.
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these failure mechanisms, the reliability engineer would, more or less,
select the following
reliability tests to
accelerate these mechanisms: solder heat resistance test, temperature
cycle or thermal shock test, and temperature humidity bias (THB) test. The new molding compound may be considered as qualified only
if the final products using this compound pass all the reliability tests
testing is statistical in nature, so the sampling scheme used for process
qualification also has a significant bearing on the success of the
qualification. In theory, the
greater the sample size used in reliability testing, the higher the
confidence one can have in the reliability data generated.
Unfortunately, more samples mean higher costs, so the reliability
engineer needs to strike a balance between extremes when designing his
qualification plan. A good guideline is to base the sample size on the
quality objectives of the company using industry-standard sampling methods
LTPD and AQL sampling.
sampling scheme should take into consideration process interactions and
variabilities, as well as the failure rates acceptable to customers,
versus the cost of samples and the cost of performing the reliability
tests. Getting qualification
samples from several vehicle devices and several production lots is normal
to ensure that the reliability assessment covers the extent of the
variability of the process and products.
analysis is an integral part of any process qualification.
Failures that are not related to the process change may be
encountered during the qualification, so these must be differentiated from
valid qualification failures. Invalid
failures or failures not related to the qualification should not affect
the results of the qualification and, subsequently, the management
decision on whether the process change must be implemented or not.
valid qualification failures must be taken seriously, no matter how few,
because it would be very expensive in the long run to implement a process
change that is doomed to fail from the very start.
Modeling; Life Distributions;
LTPD and AQL Sampling
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