What is a Test Coupon?
A test coupon is a small section of the PCB that is designed with specific dimensions and electrical properties, such as trace widths, trace spacing, and impedance values, to detect manufacturing defects.
Some of the common types of test coupons used in PCB manufacturing include electrical test coupons, peel-strength electrical test coupons, microsection test coupons, and impedance test coupons. To expand on the different types of test coupons, let's take impedance testing as an example. It's important to design test coupons that include trace configurations representative of the different impedance requirements of the PCB.
Figure 1. Types of Test Coupons
The Location of Test Coupons
Test coupons are typically located on the same PCB panel as the main PCB, usually at the edges or corners, to minimize interference with the signal traces on the main PCB. Alternatively, test coupons can be located directly on the PCB, which provides a more accurate representation of the manufacturing process and the electrical properties of the PCB. However, this approach can be more challenging in design and may interfere with signal traces. The decision of whether to place test coupons separately or integrate them into the main PCB layout depends on specific requirements of the PCB design including the complexity of the PCB design, the size of the PCB, and the required level of accuracy.
Figure 2. PCBs with Test Coupons
Advantages and Limitations
Test coupons are highly recommended for PCB designs that require high reliability, complex designs, or high-speed signal traces. However, for simpler PCB designs, other testing methods may be sufficient, such as flying probe testing and in-circuit testing. By including test coupons, manufacturers can identify manufacturing defects or issues during the manufacturing process, before it is assembled into the final product, which can reduce the time and cost involved in debugging and redesigning a PCB. However, it's important to keep in mind that the test environment may not accurately reflect the actual conditions that the product will be exposed to in real-world applications. Differences in scale, exposure, and stress can all impact the performance of the PCB in real-world conditions, which test coupons may not detect. Therefore, test coupons should be used in conjunction with other testing methods to ensure the highest level of quality and reliability in PCB design.
