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At the time, Integrated Test Corporation did have Kelvin probes and a flying probe tester that was capable of performing the testing pro- cess. However, it had not been properly set up to accurately predict what the resistance mea- surements should be based on the aspect ratio of the via and the amount of copper in the hole. The first few times the process was used, all that was accomplished was to indicate which holes had resistance measurements outside of the normal distribution of results. These were then sectioned, and it was found that they would have met the minimum copper criteria. After a few instances of performing destruc- tive analysis on nondiscrepant products, it was decided that this process required testing for proper characterization. A quick search of articles and white papers yielded comprehensive descriptions of the pro- cess itself but not guides on how to set it up in pr oduction. Most of the papers available de- scribed building a baseline of resistance mea- surements with known good panels. Ultimately, the pr ocess that we desired was to accurately predict the resistance measurements based on drill aspect ratio and copper thickness. This would require a correlation between these mea - surements and a set of equations. These equa- tions could then be used to set the maximum r esistance specification during testing or to determine the copper thickness within plated through-holes without destructive analysis. Characterization At Integrated Test Corporation, very high as- pect ratio vias are common in production. So, we decided to design a test panel that was 0.300" that could be drilled with coupons including 0.010", 0.012", 0.015", and 0.020" vias to characterize the process for aspect ratios up to 30:1. For each diameter, via coupons were included that were copper plated with 0.0002", 0.0004", 0.0006", 0.0008", and 0.001", respectively. To ensure that the proper amount of copper plating was depos - ited in the holes, boards were processed through many different plating cycles, covering and un- covering coupons with resist during each cycle. This w as found to be more cost-effective than building panels exclusively for each copper plat - ing thickness. However, doing the characteriza- tion in this manner would simplify the plating and imaging operations. After the copper plating and etching pro- cesses were completed on the test panels, they were processed through four-wire Kelvin test- ing. Resistance measurements were graphed based on drill diameter (Figures 2 and 3). After reviewing the results and observing that there was little variation in the resistance mea- surements with respect to aspect ratio and cop- Figure 2: Resistance measurements (0.010" drill and 30:1 aspect ratio). 56 PCB007 MAGAZINE I JUNE 2019