Specification limits define what counts as good product: the upper spec limit (USL), the lower spec limit (LSL) and often a target in between. They come from function, design margins or a customer drawing, not from process data. Product outside them is nonconforming. Some characteristics have only one limit, such as a maximum particle count or a minimum breakdown voltage, and get one-sided capability indices.
Spec limits on gate CD are 45 plus or minus 3 nm. We're running at 46.8, in spec but way off target.
Also heard as
- spec limits
- specs
- tolerance limits
- USL and LSL
Do not mix it up with
- Control limits: Control limits describe what the process actually does; spec limits describe what the product must do.
Related terms
Control limits
Boundaries on a control chart computed from the process's own variation, marking how far routine noise normally reaches.
Process capability
How well a stable process's natural spread fits inside the specification limits, usually expressed as an index.
OOS
Out of specification: a measured result outside the spec limits, meaning the product itself is nonconforming.
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.
Opening the spec
Widening a specification limit so more product passes, instead of fixing the process that makes it.