PCB impedance Test

PCB impedance testing doesn't test every single trace 100%. Here's a look at the two most common methods.

1. Using an Impedance Test Coupon

This is the most common method. PCB manufacturers create test samples, or coupons, on the unused process edge of a production panel. These coupons are designed with the exact same trace structures (like microstrips and striplines) as the main board.

Since they go through all the same manufacturing processes (lamination, etching, solder mask, surface finishes, etc.), the coupons are a perfect representation of the impedance control for that entire batch of PCBs. Once complete, a Time Domain Reflectometer (TDR) is used to precisely measure the impedance of these coupons.

After testing, the process edge is cut off and discarded, leaving only the main boards.

2. On-Board Testing

For high-end products like servers or communication backplanes, or when a customer specifically requires it, a flying-probe TDR is used to test specific critical signal traces directly on the board.

While this method is expensive and slow, it provides the most authentic data and directly verifies the performance of critical signal traces.

Key Points to Remember

• Timing: The best time for impedance testing is after all processes that can affect impedance—especially the solder mask and surface finishes—have been completed.

• What's Tested: The primary focus is on impedance test coupons on the process edge. Direct on-board testing of critical traces is less common.

• Purpose: The goal is to ensure the PCB's impedance values meet the design requirements (e.g., 50Ω, 90Ω, 100Ω), which guarantees signal integrity and minimizes reflection and loss.

• Relationship to Electrical Test (ET): Impedance testing and ET are two completely different but equally critical tests. Impedance testing checks for signal quality, while ET verifies that the connections are correct.