Issue link: https://iconnect007.uberflip.com/i/1503357
20 DESIGN007 MAGAZINE I JULY 2023 cavities, several special processes need to be considered. Among these are inner layer plat- ing, particularly the effects of plating and sur- face finish on inner layer impedance for signal integrity; sequential lamination; surface fin- ish on exposed inner layer pads; and the use of controlled depth milling to form the cavity, just to name a few. One of the first items of concern is the effect of plating and surface finish on impedance of the inner layers. When cavities are formed without sequential lamination, the plating and surface finish needed to support the soldering process will add conductive material thickness to the exposed areas of the inner layer. is in turn will cause an impedance discontinuity due to these different thicknesses of the traces. Additionally, the removal of the reference planes from one side of the conductor changes the structure of the transmission line from a stripline on inner layer to a microstrip in the cavity. Most ECAD and SI tools do not sup- port different transmission line structures on the same layer of the board. ere are some solutions to this conundrum: Rather than rely- ing on the built-in transmission line structures of ECAD tools, the designer can manually cal- culate the controlled impedance trace widths for the stripline and microstrip sections and then use area constraint rules to define the widths both in the cavity and on the remainder of the layer. One possible way around this issue is to use sequential lamination to allow plating on the entire layer. But sequential lamination poses its own problems. First, it is a more costly and time-consuming process. Second, it adds fab- rication allowances. It may also require the use of blind and/or buried vias to make the con- nections between layers. e multiple plating cycles needed for sequential lamination can cause other fabrication issues. It is recom- mended that the fabricator be consulted on any sequential lamination PCB design. Other design challenges exist in the forma- tion of the cavity using milling methods. One of these is the need for a large radius on the inside corners of the cavity. In Figure 1, you can see a simplistic view of the cavity from the design tool. In Figure 2, you can see what the cavity will need to look like with the radiused corners from the milling operation. Design tools can define these inside radii; however, the fab shop should be consulted to determine what the appropriate radius needs to be for cavity formation. One way to avoid completely milling out the area of the cavity is to pre-form the prepregs, cores, and copper foils. Figure 3 shows an example of what the pre-formed Figure 2: A view of a cavity showing radiused corners from milling process. Figure 3: Cavity created with pre-formed prepreg.