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40 DESIGN007 MAGAZINE I MARCH 2024 the board. Above all, they have eliminated the need for those complex wired buss configura- tions that C-3PO was forced to endure. e purpose of this article is to answer the most frequently asked questions that design- ers have concerning heavy copper PCBs. By the same token, we seek to stoke the designer's imagination in creating heavy copper designs. Additionally, this article will cover the fabrica- tion pitfalls as well as define the design solu- tions and manufacturing methods used in Heavy Cu builds. Heavy Copper PCB Design Issues Product designers are oen flustered by cer- tain DFM rules pertaining to heavy copper fab- rication. I have listed the most common issues in Table 1. Table 1 Note: Failure to address these issues prior to completing your design can result in considerable rework. Manufacturing Pitfalls e best way to avoid design problems is to examine the ensuing manufacturing pitfalls of each feature (Figure 1). Design issue: Stackup dielectric thicknesses too thin Manufacturing pitfall: Delamination/resin voiding Designers frequently call out core and pre- preg placement. Ideally, the designer should stipulate minimum dielectric thicknesses and trust the fabricator to decide where a core or prepreg will be used. A common rule of thumb for electrical insulation is 750–1,000 volts DC per 1 mil (0.001"/25 micron) of FR-4 material. For example, if the designer requires 5,000 volts DC electrical insulation, then the manufacturer should use a minimum of 0.005"–0.007" dielec- tric thickness in their stackup design. Calling out your bare needs allows the fabricator to satisfy your design requirements, avoid manu- facturing pitfalls, and ensure reliable builds. Stackups are the most overlooked element of heavy copper design. Standard PCB designs are simple: Design your board with as many layers as necessary and inform the fabricator of the maximum allowable thickness. Gener- ally, we can accommodate. However, in regard to heavy copper, we must consider the resin- filling capability of the prepreg. A multilayer PCB is made up of two primary components: cores and prepregs. Core mate- rial is copper foil pre-bonded to each side of the fully-cured fiberglass epoxy sheets. ese make up the inner layer circuit pairs. For example, the foil construction of the inner layer pairs would be Layer 2-Layer 3, Layer 4-Layer 5, etc. Yet, prepreg is just the fiberglass epoxy sheet used to make a core—merely a sheet of woven fiber- Figure 1: Heavy copper manufacturing pitfalls.