Issue link: https://iconnect007.uberflip.com/i/1464168
APRIL 2022 I DESIGN007 MAGAZINE 35 issues, our traces need to be as spread as pos- sible, with return path guard traces where pos- sible. If we need to have traces close together, we want to reduce any parallelism as much as possible by fanning out wherever possible. Finally, we want to group together traces that are a part of the same interface as this will assist with our noise immunity. Armed with this knowledge, how can a designer or engineer start utilizing this new fabrication method within our designs? One of the first things to do is talk to your fabricator. At present there are a few different additive methods on the market, like A-SAP™ (Aver- atek semi-additive process) and mSAP (modi- fied semi-additive process). Each of these has different potential, from trace width to trace heights, so an understanding of your fabrica- tor's capabilities is important to enable design to the appropriate additive specification. e fabricator's capabilities define our minimum trace size, and we already have our crossover point of around a 0.075 mm trace width. Now, it is possible to have additive designs with larger trace sizes, just like it is possible to have subtractive designs with sub-0.075 mm traces; however, that 0.075 mm trace size is a good crossover point to consider switching to using additive design technology. From experience, designing with additive traces is best suited for those tight or micro- component package breakouts. It results in a less expensive, more reliable PCB by enabling access to more of the component pins, with- out needing to increase the number of layers in the PCB. However, the focus needs to be within the package area; once you can route to free space, both trace width and gap should increase. is allows for a small impedance mismatch between the regular trace across much of the PCB, and the small breakout sec- tion within the package area. A New Route When routing boards that feature additive technology, a designer may experience a slight mental shi away from the traditional routing method. Typically, once your board's shape, stackup, and constraints have been added into your design, each block or section of the design is then placed and routed on the outer layers of the PCB, with the intent of connecting most of these different routed sections using the inter- nal layers of the PCB. With additive design, coplanar waveguides can be utilized, allow- ing for increased routing on outer layers of the PCB. is decreases layer count, reduces the number of vias, and maximizes the utilization of every layer. Conclusion Additive and semi-additive processes offer a variety of advantages compared to traditional subtractive etch fabrication. ese include spaces and traces as small as 0.010 mm, far smaller than anything possible with subtrac- tive methods, and nice, rectangular-shaped traces instead of the old trapezoidal traces. But with increased density comes the potential for greater crosstalk and other signal integrity issues. Additive and semi-additive PCBs are likely to become more mainstream. And any designer or design engineer can begin designing addi- tive PCBs—with a little prior planning. DESIGN007 Tomas Chester is a hardware designer and founder of Chester Electronic Design. With additive design, coplanar waveguides can be utilized, allowing for increased routing on outer layers of the PCB.