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64 DESIGN007 MAGAZINE I APRIL 2024 and then processing multiple plating cycles to hit the target. A typical plating cycle presum- ably adds one ounce of copper to the PCB sur- face (in order to plate a minimum of 1 mil in through-holes). At a lower amperage setting, this will take two hours in the copper plat- ing bath alone (resulting in an overall plating cycle of over three hours). Consequently, this is one of the longest cycle times of all the PCB fabrication processes. When all is said and done, plating-up to 3-ounce finished copper from a half-ounce base adds multiple days to the lead time. Since it uses standard DFM rules, plating-up allows for smaller minimum line and spacing requirements. Most fabricators have the capac- ity for plating up, even if they are not aware of it. Having said that, there is a much higher cost resulting from the repeated back-and-forth from imaging-to-plating. ere are also an increased number of failure modes, consider- able fluctuation in the final copper weight, and longer lead times. Additionally, the sidewalls will not be flush/straight. ey may look more like a Jenga wall. is method is a last resort to avoid higher costs and longer lead times. Layer Fill Ink Sometimes, we encounter a predicament where the desired inner layer copper thickness does not jibe with the desired maximum total thickness. e primary reason is that in order to have enough prepreg plies to fill the etched gaps requires a set minimum thickness allow- ance between layers. By the time we add all of them up, we are well beyond the maximum desired thickness. So, what do you do when overall thickness cannot be increased and inner layer copper weight cannot be decreased? e answer is layer fill ink (Figure 14). Figure 13: Plate up. Figure 14: Layer fill ink.