Issue link: https://iconnect007.uberflip.com/i/1190860
64 DESIGN007 MAGAZINE I DECEMBER 2019 change the starting copper weight. Remember, most U.S. fabricators shoot for an additional ounce of copper for the through-hole plating process to meet the IPC mandate of a mini- mum of eight-tenths of a mil of plated copper in the barrel of the hole. Then, let's change from a half-ounce start- ing copper weight and 1.5-oz. plated finish to a 1-oz. starting copper weight callout with a 2-oz. finish (with the same starting trace width of 0.00675" with 0.004" g-sep achieving 43 ohms for a 50-ohm trace). Now, we are at 41.88 ohms, which is also well below the tar- get of 50 ohms based on the original calcula- tions (Figure 8). To add insult to injury, this is not possible for many manufacturers, as achieving 0.004" g-sep means we will add an etch compensa- tion for the 2-oz. finish of 0.001" taking the g- sep down to 0.0035". Remember, for a g-sep change, it will be half that of a space change, making the part not possible for many manu- facturers without reducing the copper weight. How to Make Sure This Does Not Happen to You First, when communicating what your sig- nal integrity needs are to your chosen PCB fab- ricator, ask them to provide you a calculation based on material type, starting copper weight, color of mask, and whether or not the part will have any close copper pour, creating a copla- nar model. Second, if your trace size will be based on component constraints that the fab- Figure 5: After changing from 0.00675" traces with 0.004" g-sep to 0.00365" traces and 0.0055" g-sep. Figure 6: The same trace after changing from Np-175 to P-370HR.