Issue link: https://iconnect007.uberflip.com/i/1101604
26 DESIGN007 MAGAZINE I APRIL 2019 be used. These are decisions that only the fab- ricator can make, and yet many CAD profes- sionals will feel duty bound to give some sort of answer, which will inevitably be wrong. Clearly, CAD developers will try to avoid such scenarios by going through their chosen NC format and picking what they believe is most appropriate for their software and the CAD to CAM data transfer process. This is not easy, as the language is dense, sometimes re - dundant, and IPC-NC349 and Excellon 2 con- tain legacy Excellon 1 code—most of which CAD developers don't need. So, for fear of leaving out parts of the format that could be of value, developers tend to include more than necessary, which just adds more confusion for the CAD user. Despite the developers' best efforts, it's not always clear how to use parts of the NC for- mats or if they are even capable of transferring certain data. CAD users will simply leave these parts out of their drill files and express the rel- evant information as sidecar information in comments or in separate text files. But the biggest problem with these NC spe- cifications is that, thanks to an age-old space- saving convention, the drilling coordinates lack a decimal point. That's okay if there's a com- mand or header in the file that indicates where the decimal point must be—the so-called fixed "point format." But in Excellon files, there is no such instruction or standard for saying where it should be, so designers are on their own. Similarly, there is no standard for expres- sing whether the designs are in imperial or metric measurements. The Excellon specifica- tion does mention defaults, but these are also confusing and can end up being used in diffe- rent ways. All of this places the final responsibility on the CAM engineer to try different possible variants until the drill files fit with the copper files. This is fine—if not great—when handling normal boards, but is not okay when handling RF boards, for example, where there are no cle- ar pads to fit the holes. An additional issue in the NC stakes is that the specification is no longer published, so the industry has relied for decades on copies of copies that may be infringing copy- right protections. And the IPCNC-349 specifi- cation—which is dense, overcomplicated, and Figure 1: An example of the many steps of the process from PCB design to manufacturing.