sampled wafer test
**Sampled Wafer Test** is a production strategy that tests only a subset of die on a wafer to reduce test time and cost while maintaining statistical quality control.
## What Is Sampled Wafer Test?
- **Method**: Test representative die across wafer, not 100% coverage
- **Sampling**: Statistical patterns (systematic grid, random, or adaptive)
- **Purpose**: Reduce test time for low-risk, high-yield products
- **Risk**: Some defective die may ship untested
## Why Sampled Testing Is Used
For mature products with >99% yield, testing every die is economically inefficient. Statistical sampling provides adequate quality assurance.
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Sampling Patterns:
100% Test: Grid Sampling: Random Sampling:
● ● ● ● ● ● ● ● ● ● ● ●
● ● ● ● ● ● ○ ○ ○ ○ ● ○ ●
● ● ● ● ● ● ● ● ● ● ● ●
● ● ● ● ● ● ○ ○ ○ ● ○ ○
● ● ● ● ● ● ● ● ● ● ● ● ○ ●
● = tested ○ = not tested
Full coverage Statistical coverage
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**Sampling Decision Factors**:
| Factor | Full Test | Sampling OK |
|--------|-----------|-------------|
| Yield | <95% | >99% |
| Safety critical | Always full | Never sample |
| Margin to spec | Tight | Comfortable |
| Test cost | Low | High |