Issue link: https://iconnect007.uberflip.com/i/1348195
MARCH 2021 I DESIGN007 MAGAZINE 31 send the design to multiple companies and the PCB that is provided goes into the design validation, tuning, and qualification stage. e debugged design then gets sent to the other potential sources who are told to build it exactly the same, even if it does not fit well into their manufacturing process, because they just know it works but not necessarily why. Nonsensical Requirements Once, a data package was brought to me for review, complete with documentation requir- ing a 5-ohm impedance specification on a sig- nal trace. I looked at the salesperson and asked if the designer was under 22 years old. He said, "Yes, how did you know?" We sat down with the engineer and I asked why he speci- fied 5 ohms. e response was that the simu- lation program results yielded that answer to make the circuit work. Aer we reviewed the model setup, we discovered that the simula- tion model had been set up wrong. e young engineer didn't realize that the answer didn't make sense. Mechanical tolerances used in sheet metal production will oen be called out on the fabrication print. Examples are +0/-1-mil tol- erance for hole diameters of ±1 mil for loca- tional tolerances. Sheet metal does not expand or contract during production like polymers used for PCBs. is is generally found when an inexperienced mechanical designer is involved in their initial PCB layout. Violating Company Specifications New companies or third-party design com- panies may not have a detailed fabrication acceptance specification. ey will just refer- ence IPC specifications. More experienced companies may have a detailed acceptance specification that has been updated as fabri- cation and design issues were encountered over time. Many new designers will not read or understand their internal acceptance speci- fication and provide designs that violate these documents. Larger PCB fabrication companies have regionally located application engineers who can assist new and experienced designers in selecting proper design rules and materials. It is also common for 25–50% of a fabrica- tor's front-end engineering capacity to be used to perform pre-design and incoming design DFM reviews as well as required adjustments. Conclusion e industry needs to change the NPI process to build designs as they are provided instead of assuming that every design needs to be reviewed and modified. is will happen when layout soware tools can incorporate all the design rules that front-end engineers use, fabricators provide these rules to their customers, and the industry completes its conversion to using intel- ligent data, like IPC-2581, instead of non-intel- ligent Gerber-based design packages. Education can also play a big role here. ere are a variety of DFM courses available which can provide good information on how designers can alleviate or eliminate these kinds of mistakes. One silver lining of the past year: Many of the PCB design classes that were once only available at conferences are now online. But in the meantime, don't be surprised if your fabricator's CAM engineer tells you, "Oh, there's just one more thing…" DESIGN007 Dana Korf is the principal consultant with Korf Consultancy. New companies or third-party design companies may not have a detailed fabrication acceptance specification.