What is a Test Coupon?

A test coupon is a small section of a PCB designed with specific dimensions and electrical properties, such as trace widths, trace spacing, and impedance values, to detect potential manufacturing defects.

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 further elaborate on the different types of test coupons, let’s focus on impedance testing as an example. It is essential to design test coupons that represent the diverse impedance requirements of the PCB through specific trace configurations.

Figure 1. Types of Test Coupons

The Location of Test Coupons

Test coupons are generally positioned on the same PCB panel as the main PCB, typically at the edges or corners, to minimize interference with the signal traces on the main board. Alternatively, test coupons can be integrated directly onto the PCB itself, providing a more accurate reflection of the manufacturing process and the PCB’s electrical properties. However, this method can be more challenging to design and may interfere with the signal traces. The decision of whether to place test coupons separately or incorporate them into the main PCB layout depends on various factors, including the complexity of the PCB design, its size, and the required level of precision.

Figure 2. PCBs with Test Coupons

Advantages and Limitations

Test coupons are highly recommended for PCB designs that demand high reliability, complex layouts, or high-speed signal traces. However, for simpler PCB designs, alternative testing methods such as flying probe testing or in-circuit testing may be sufficient. By incorporating test coupons, manufacturers can identify potential defects during the manufacturing process, before the PCB is assembled into the final product. This helps reduce time and costs related to debugging and redesigning a PCB. That being said, it’s important to note that the test environment may not fully replicate the real-world conditions the product will face. Variations in scale, exposure, and stress can affect the PCB’s performance in actual applications, and these factors may not be captured by test coupons. Therefore, test coupons should be used in conjunction with other testing methods to ensure the highest standards of quality and reliability in PCB design.

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