Issue link: https://iconnect007.uberflip.com/i/1515990
76 PCB007 MAGAZINE I FEBRUARY 2024 As William omson (Lord Kelvin) once said, "If you cannot measure it, it cannot be improved." is is truer today than ever before. With the advances in microelectronics and nanoelectronics, HDI, and buried active/pas- sive components, the signature of the printed circuit is critical. High-speed substrates rely on signal transmission to be as pristine as possible. Minor changes can cause impedance changes in standing wave ratio (SWR) elevation, which leads to degraded performance. As we know, there is a difference between low resistance threshold continuity testing and 4-wire Kelvin. Wait, maybe we don't. Let's take a look. Continuity resistance in the standard electri- cal test (ET) theater relates to the end-to-end resistance of a single circuit. is is outlined in IPC-9252 for testing printed wiring boards. Why 4-wire Kelvin? In this specification, referenced in many others as the standard for ET, Table 4-1 out- lines continuity and isolation parameters for Class 1, 2, and 3 builds. ese are referenced to Test Levels A, B, and C, accordingly. Here, continuity resistance for Class 3/Test Level C is 10 ohms, meaning point-to-point resis- tance in the circuit cannot exceed 10 ohms to be compliant. However, this just means that if the resistance is ≤ 10 ohms, all is well with the world. However, with via plating, microwave cir- cuits, and critical timing circuits, this can be a problem. is is where the high-resolution measuring of the 4-wire Kelvin bridge is rec- ommended. Historically, 4-wire Kelvin has been used to identify plating anomalies where sight resistance changes may indicate taper plate or insufficient bonding in the microvias. Testing Todd by Todd Kolmodin, GARDIEN SERVICES USA Figure 1: Lord Kelvin.