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60 DESIGN007 MAGAZINE I APRIL 2024 etching per 1 ounce of base copper. Base cop- per for outer layers is 1 ounce less than the desired finished copper while the base copper for inner layers is the same as the desired fin- ished copper weight (excluding a termination layer for a plated blind or buried vias). Final spacing should be at least 0.004"/100 microns for good yield results. Fabricator Solution for Inner Layers e inner layers require more scrutiny as they tend to feature thicker amounts of copper. Do not worry if your design does not allow for this amount of spacing (typically in windings, via pad diameters, etc.) as larger fabricators employ two distinct methods for etching the inner and outer layers. e ideal method for inner layer etching features the etchant chemistry cupric chloride. As it etches down, it leaves a coating against the now-exposed sidewall of active features to inhibit further etching. A good rule of thumb states that for every ounce of copper etched down, the etch into each feature's sidewall is only one-quarter to one-half of 1 mil (0.00025"– 0.0005"/6.25 to 12.5 microns). For good yield results, final spacing should be at least 0.004"/100 microns. With that in mind, we will now recalcu- late the 3-ounce inner layer copper design using cupric chloride as the etchant (Table 7). Table 6. Table 7.