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68 PCB007 MAGAZINE I AUGUST 2022 William ompson to the rescue. If you are familiar with 4-Wire Kelvin testing, you might know that ompson invented this test in about 1861. is high-resolution measure- ment system is based on the Kelvin Bridge which ompson (later known as Lord Kel- vin) invented (Figure 3). Using this theory, high resolution measurements can be taken while eliminating the stray or parasitic resis- tance of the leads and electronics. 4-Wire Kel- vin can detect finite differences in resistance in the milli- and micro-ohm ranges. is test will detect taper plate anomalies and poor bonding of micro-vias. It is common to perform destructive anal- ysis on plated barrels during the manufactur- ing process. ese potted micro-sections can identify plating anomalies if they are a systemic or gross error to the entire order. Remember that these sections are a sample of just a few holes and a PCB may have many thousands of holes. erefore, this is not always a good indi- cator of an isolated issue such as a low volt- age flight bar in plating or a bad agitation cycle which results in an isolated set of suspect pan- els. is is where 4-Wire Kelvin can save time and money by detecting these types of issues. If 4-Wire Kelvin is integrated into your manu- facturing theatre, a sampling of product with high aspect ratio drilled and plated holes is indicated. A random sample of panels from different flight bars should be tested to iden- tify any possible anomalies. If an anomaly is found, that flight bar can be isolated and the panels fully tested under 4-Wire Kelvin guide- lines to validate integrity. Conforming groups can be released for further processing. What this does is remove the threat of latent defects in the field due to poor barrel plating or bond- ing of the micro-vias. Although this might take some time in the manufacturing process, it far outweighs the cost of returned product and reputation damage with your customer base. Here, the cost of quality has a valuable argu- ment against the cost of non-conforming prod- uct and loss of customers. is is especially true today when time to market is crucial and PCBs are increasingly more expensive to man- ufacture. PCB007 Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues. To read past columns, click here. Figure 3: The Kelvin meter.