Test coupons are added to PCBs mainly to help with testing and ensuring quality during production. They act as a way to double-check that the board meets the required standards at different stages of manufacturing.

Here’s why they’re important:

1. Electrical Testing: Test coupons let manufacturers test the board’s electrical performance, like signal integrity and resistance. This helps catch any potential electrical issues early on.
2. Quality Assurance: They help spot any design or construction flaws in the PCB, such as problems with trace widths or hole sizes, making sure everything is up to par.
3. Verifying the Manufacturing Process: Test coupons are used  to make sure the PCB was built correctly. They help confirm things like etching quality or the application of soldermask, ensuring the production process is running smoothly.
4. Checking Trace and Hole Sizes: Some test coupons have specific trace widths and hole sizes that allow the manufacturer to confirm they’ve followed the design specs accurately.
5. Reliability Testing: Coupons can also be used to see how the board performs under certain stresses, like heat or electrical loads, giving a sense of how reliable the PCB will be over time.
6. Meeting Standards: Finally, test coupons help ensure that the PCB meets industry standards, like IPC-2221 or IPC-6012, which lay out quality requirements for PCB manufacturing.

Essentially, test coupons are a handy way to check that the PCB is up to spec and will perform as expected once it’s in action.