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JANUARY 2024 I DESIGN007 MAGAZINE 55 tance invalid, it's just that the designer needs more than a datasheet to make the height pre- diction. As always, if the designer has a good relationship with the fabricator (or a specialist broker), then good predictions and models can be made. Longer supply chains, though, can make this a challenge. e isolation distance marked in Figure 3 shows the Z distance from the surface of a trace facing downwards from a core. It is pressed into the top of the three prepreg layers below. e stackup tool, in this case, is fed with the copper coverage percentage on each layer to assist in predicting how far the copper presses into the prepreg. A good sanity check with this kind of calcu- lation is to think of the limit situation. With 100% copper coverage (such as a ground plane), there will be no penetration into the prepreg. e same is true for the other limit situation: With zero percent copper coverage, again there is no penetration into the prepreg. e maximum situation is at 50% copper cov- erage where the resin in the prepreg will flow into all the gaps between the traces. I've written this from the perspective of PCB thickness and isolation distances, but there are many other areas where it is very valuable for PCB designers to have an insight into the PCB fabrication process. A visit to a PCB fabricator is an enlightening and educational experience. From time to time, industry bodies like EIPC arrange such plant tours, and one such tour is taking place in southern Germany later this month. Visit eipc.org for more information. If you miss this one, be sure to keep your eyes open for another one or call your local PCB supplier and ask for a visit. Conclusion PCB fabrication is quite different from PCB assembly. In assembly, the parts are joined electrically and mechanically by soldering, but the components (including the PCB) travel through the process essentially unchanged. In PCB fabrication, copper is etched and plated, Figure 2: Side-by-side schematic and proportional view of the same stackup.