The ‘bathtub curve’ (blue, upper solid line) for Semiconductor Reliability is a combination of a decreasing Early Failure (red dotted line) and an increasing Wear-out Failure (yellow dotted line), plus some Constant (Random) Failure (green, lower solid line).
The Bathtub curve is a reliability model well matched to the Semiconductor industry, it is not one curve but the consequence of 3 curves or factors that make a bathtub shape. In its most basic terms, the curve provides an expectation of product performance over time. However, the Curve can be significantly different for related products and can change over time for the same product. The Bathtub curve does not specifically take Event failures into consideration.
Bathtub
What makes up the Curve
Early / Infant mortality failure
Infant mortality rates are devices that fail early in the initial lifetime of the product as indicate by the name but consist of several fails influenced largely by the manufacturing processing.
The various (wafer probe, final test, process control) product test programs are developed at NPI during a time limited period, and although there are methods to maximise the effectiveness of this period, limitations such as: Product availability for test development, resources and changes (by the nature of development), results are less than perfect. So, there will be a higher-than-expected failure rate until higher volumes of product are tested, results analysed, and programs adjusted, this should be a continuous improvement process.
Additionally, the product will experience initial operating stress potentially in less than perfect (out of provided specification) conditions continuously for the first time and a likely time for devices with present defects not yet defective (marginal) to fail.
The rates of fail in this area can be significantly reduced with a number of good practices, increase in test coverage, reduction in Defect density and introduction of stress testing early on.
Constant Random fails
A result of the random fails still able to escape process control and sometimes referred to as inherent manufacturing defects.
Again, primarily manufacturing processing & wafer defect density related concerns not found during at screening, may also be affected by Design match & process robustness.
Although stated as a constant this can be significantly improved with process control, continuous improvement and Defect density reduction
Wear out Failures
Primarily the breakdown of product due to stress over its lifetime. At Chip level: gradual degradation and breakdown of gate oxide layers, Hot carrier injection leading to degraded performance, resulting in slower transistor switching as well as electromigration resulting in shorts or opens.
Whilst at the package level: wire bond breakages or lifting, due to mechanical stresses, thermal expansion/contraction or contamination and likewise with fatigue cracking of solder joints of Flip chip or BGA packages. Environmental degradation (moisture or temperature etc) resulting in Corrosion and Delamination.
These can be improved with healthy process and product design margins, better materials and optimisation of design rules to minimise stress.
Conclusion
In summary, the Bathtub Curve Model serves as a valuable tool in understanding semiconductor reliability by illustrating the three key phases of a product’s life cycle: early failure, constant random failure, and wear-out failure.
This model highlights how reliability evolves throughout a product’s lifecycle, starting with a higher rate of failures due to manufacturing imperfections, followed by a period of stable performance, and concluding with an increase in failures due to wear and tear. emphasizing the importance of robust manufacturing processes, rigorous testing, and continuous improvements to reduce failures at each stage.
By recognizing the factors that contribute to each phase of the curve, the Semiconductor Industry and their Customers can better anticipate potential failures, optimize product performance, and extend the overall lifespan of their devices.
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