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62 PCB007 MAGAZINE I JUNE 2019 This process can also be utilized to pin- point the through-hole vias that fail custom- er specification for plating thickness. Theo- retically, these vias could then be processed through rework plating cycles and retested to determine the efficacy of the rework pro- cess. Since this is a non-destructive process, products would then be viable for shipment. There are cases where we have customer- finished hole diameter requirements where the PTH to inner layer spacing prohibits over - sizing the PTH drill as much as our design guidelines indicate we should. This limits our drill diameter and the amount of plating that we can plate in the via and still meet the finished requirements. Since our plating window is drastically reduced, we utilize Kel - vin resistance measurements to disposition the copper thickness in the final product so that we are certain that the minimum plating thickness criteria are met. Finally, we utilized this process to improve the reliability of a conductive bonding process that we use. We baselined our current pro- cess before and after component assembly and made improvements to the manufacturing and assembly processes by comparing the impact that these changes had on the Kelvin resis- tance measurements. In addition, Kelvin test- ing assisted our company in qualifying a more thermally robust dielectric material to use in this process. Different materials were tested, and the percent change in the Kelvin resistance measurements was reviewed and compared through the assembly reflow process. Conclusion Four-wire Kelvin testing can provide signifi- cant benefits to your plating and testing pro- cesses provided that it is adequately set up to your process. Following the steps that have been outlined, this testing process will be set up properly and have the ability to accurately predict the copper thickness in plated through- holes. This allows you to adequately disposi- tion plating voids without costly cross section analysis. It also allows you to determine exact- ly which holes do not meet specifications and may require rework and ensures that there are no escapes to customers, which may result in costly field failures. Overall, Kelvin testing pro- vides an excellent way to characterize, base- line, and improve your process. PCB007 Brandon Sherrieb is currently the process engineering manager at Integrated Test Corporation in Dallas, Texas. He has a degree in mechanical engineering and 15 years of process engineering experience in the industry with a focus on mechanical and thermal processes. by Andy Shaughnessy As a field applications engineer at TTM Technologies, which has fabrication and manufacturing locations around the world, Julie Ellis sees a wide variety of customer design requirements. In this interview with the I-Connect007 Editorial Team, Julie explains how PCB designers can influence the development of the PCB. She shares a variety of tips and tricks that designers can imple- ment early in the design process to help optimize fabrication and assembly later on and keep small issues from becoming big problems downstream. To read this entire article, which appeared in the June 2019 issue of Design007 Magazine, click here. Design Is a Pivotal Piece of the Puzzle Julie Ellis