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MARCH 2024 I DESIGN007 MAGAZINE 41 glass yarn surrounded by epoxy. Be that as it may, a closer inspection reveals that the epoxy is not fully cured. We refer to this as "B-Stage" while the fully-cured core is referred to as "C-Stage." Prepreg is available in a variety of glass thicknesses and epoxy resin content. Gener- ally, the thicker the glass, the more you reduce your epoxy content in proportion to the over- all thickness. Prepreg is the "glue" that holds the individual copper cores together. Hence, we align the inner layer cores and place the most applicable prepreg between them. Next, we insert these materials between heavy steel plates before placing the stack into a multilayer press. e press draws a vacuum before heating up to a controlled temperature, which liquefies the resin in the prepreg. is resin then flows in between the etched inner layer copper features on the opposing cores until achieving final cure. Once cured, the epoxy becomes rigid, match- ing the "C-stage" state of the inner layer cores. Your chief concern is the amount of free epoxy required to displace the inner layer copper portions that were etched away. In conclusion, the heavier the copper, the greater the Z-axis fill requirement (i.e., thicker dielectric spacing in stackup). e fabricator's prime directive is selecting the prepreg quantity and style accordingly to insure proper resin fill of opposing copper. For instance, a 6-ounce design will have etched recesses of 8.4 mils (in the extreme situation where an etched area opposes an etched area, there are 16.8 mils of void that require prepreg resin filling). Indeed, we have been known to push our fair share of boundaries. Figure 2: Material layup shown on the left is pressed into a single package with multilayer lamination. Table 2: Standard readily available copper weights. Table 3: Common FR-4 based resin system prepregs.