EC-25.3 Testing a Wafer, and Testing a Package
You can describe what happens at wafer sort and at final test and why both exist, explain how test time and test coverage trade against each other, and interpret the escape and overkill that any test limit produces.
Before:PE1-1. VLSI TechnologyEC-8. Measurements, Instrumentation and Experimental MethodEC-21. Hardware Verification, FPGA and SoC Design
The verification module put scan chains into the design; this is what the factory does with them. Every die is tested before the wafer is cut, because packaging a dead die wastes the package, and every packaged part is tested again, because packaging breaks things. Test is a large fraction of the cost of a chip, so almost every decision here is about buying the most confidence per second of tester time.
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Wafer sort: probing every die, and inking the ones that fail
Testing before the wafer is cut avoids packaging parts that were never going to work, which is the largest single saving available in the flow. The map of which dies passed is also the first yield data anyone sees.
NPTEL: Digital VLSI Testing (IIT Kharagpur) · CourseWhat the tester does: patterns, timing, and measuring at speed
Automatic test equipment applies patterns through the scan interface and compares responses, and it can also run the part at its rated speed to catch delay faults. The second kind of test costs far more machine time.
NPTEL: Digital VLSI Testing (IIT Kharagpur) · CourseTest time as a cost, and compression as the answer
Tester seconds are expensive enough to appear directly in the price of a part, and scan patterns are long. Compression logic on the chip lets far fewer tester pins deliver the same coverage.
NPTEL: Digital VLSI Testing (IIT Kharagpur) · CourseFinal test, and why a packaged part is tested again
Bonding, moulding and handling all introduce their own failures, so a die that passed at sort can fail after packaging. Final test also measures the parameters that only exist once the package is on.
NPTEL: Electronic Packaging and Manufacturing (IIT Kharagpur) · CourseSetting a limit: escapes, overkill and the guard band between them
A limit tight enough to catch every bad part will also reject good ones, and the measurement uncertainty has to be added as a margin. Where the limit sits is a commercial decision informed by what a field failure costs.
Known good die, and testing parts that will be assembled together
When several dies are packaged together, one bad die scraps the whole assembly, so each must be proved before assembly. That requirement is what makes advanced packaging a test problem as much as a mechanical one.
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