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PCBD-Apr2016

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48 The PCB Design Magazine • April 2016 operating temperatures. We also talked about how hybrids are manufactured using an addi- tive screen print process. This differs from the standard PCB manufacturing process which is a subtractive (copper etching) process. And now, let's press on. Design software for hybrids is similar to de- sign software for PCBs, albeit with some impor- tant differences and enhancements. Due to the differences in fabricating hybrids from regular PCBs (screen printing conductors and dielectric materials onto a substrate instead of compos- iting separate layers of copper and dielectric material), the need for a specific CAD layer to match a specific board layer does not apply. However, a hybrid design still requires separa- tion between the different conductive and di- electric materials, so the standard method of setting up layers in design software is still the foundation of the design database. Therefore a hybrid design still has layers designated in the layout application, but the names of the layers will be different then what you might see in a PCB layout. Instead of "Top," "Bottom," and "Inner" signal layers, you will instead see layers described as "Metal_Top," "Wire_1," "Wire_2," and "Metal_Bottom." There may also be layers devoted completely to power or ground as in a PCB, but remember that they will be conductive material that is screen printed on instead of be- ing an actual copper layer. Routing conductors (wires) in a hybrid de- sign on positive layers is done essentially the same way as routing traces in a standard PCB design. The conductors will carry intelligent net information the same way as a trace does in a PCB design, and the CAD application will account for their connectivity and net rule re- quirements. But instead of only designing lay- ers for connectivity as a PCB would require, a hybrid also requires the design of its dielectric material as well. Just as in a PCB, hybrid conductors need to be isolated from other conductors that are printed above or below them. And just as in a PCB, a dielectric material is used to provide this insulation. The difference is that in a hybrid the dielectric material will not be an actual compos- ited layer as it would be in a PCB. Instead it will be added by another screen printing process. Once it is in place, screen printing of conduc- tive material can be added on top of it to con- tinue the hybrid buildup. Solid layers of dielectric material are de- signed in the CAD application in a similar way to how an area fill or a power plane is designed for a PCB. Therefore the positive area fill or negative power plane algorithms in PCB design software lend themselves well for this, with one exception: They must be enhanced to account for the lack of intelligent net information that they would carry in a PCB. Remember, this is dielectric material which has no need for net information to be assigned to it. The key here for PCB designers is to remember that when you are looking at a hybrid design's dielectric lay- ers in your CAD application, they are not fills or planes even though they may look like what you are traditionally used to seeing as a fill or a plane. And to further complicate matters, there may still be layers in a hybrid design that are devoted to being an actual conductive power fill or plane. One thing that PCB designers may not be aware of that is very different in hybrid design is the use of crossover dielectric material. As we have discussed, full layers of dielectric material can be designed into a hybrid. But in a hybrid design you may have a situation where there are only a few spots between two adjacent con- ductor layers where wires are crossing over each other. Obviously these points need to be isolat- ed from each other with dielectric material to keep them from shorting, but to add another layer of full dielectric material for only these the Basics of hyBriD Design, part 2 " Design software for hybrids is similar to design software for PCBs, albeit with some important differences and enhancements. "

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