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78 PCB007 MAGAZINE I FEBRUARY 2024 Today, resistance values play into the overall performance of the cir- cuit and, further, the performance of the final product. We must remember that 4-wire Kelvin is different from standard continuity testing and enhanced equipment is required. We must also remember that fixture tes- ters are not a good solution either. e main reason is the continu- ity threshold capability. Honestly, most 1980s and '90s fixture testers will struggle to perform under a 5-ohm continuity thresh- old requirement unless they are in a strict maintenance program that refurbishes the cas- settes and machine interfaces. Otherwise, con- tamination and oxidation will hamper the abil- ity to test to the lower continuity requirement. Second, most fixture testers cannot perform a 4-wire Kelvin test due to the lack of system metering technology. e solutions are bench tests or flying probes. Technology exists now where passive com- ponents can be tested to a known value. is is common with resistors, capacitors, and induc- tors. ese values can be learned on machines and compared, or a predetermined value can be programmed. However, this is used in lower resolution requirements of usually 3 ohms and above. is is still a 2-wire measurement with better accuracy obtained by less wiring and wire resistance nulling on the machine. is is usually unavailable on fixture testers. To realize the full potential of the high-res- olution measurement requires the 4-wire Kel- vin bridge. e theory is that as the stimulus is applied to the unit under test (UUT), the stray/contact resistance is shunted, leaving only true resistance. is allows for very fine measurements in the milli- and micro-ohm ranges. Using this methodology, the very fine changes in resistance can be measured. Here is where it has been very beneficial identifying plating anomalies. in copper in the via bar- rels will have a change in resistance that can be detected using 4-wire Kelvin and thus removed from the supply chain and stopping the latent field failure. Build technology is advancing rapidly and 4-wire Kelvin has found a new niche in the high-speed substrate, HDI, and microwave markets. High-speed transmission and wave propagation are affected by resistance changes. ese changes can cause changes in the reac- tance of circuits. In this arena, it is bad news; 5 ohms continuity in a circuit may be way too much. Designers are now adding predictability to their designs, which includes circuit resis- tance. We also see this in ceramic substrates where circuit resistance must be predictable in the 300 mohm to 1.2 ohm range. is requires measuring with 4-wire Kelvin hardware. is can be pre-programmed so that the test is invis- ible to the operator. Reports can be generated so that predictability can be reviewed. is is why 4-wire Kelvin has emerged as an even more important tool in the ET arena. Standard testing, buried passive interrogation, and 4-wire Kelvin should be in the manufac- turer's toolkit. If not, seek a reputable test sup- plier to help you along the way. PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reli- ability issues. To read past col- umns, click here. Figure 2: Kelvin clips.