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March 2015 • The PCB Design Magazine 15 UnDERSTAnDInG DFM AnD ITS ROLE In PCB LAyOUT continues feature Figure 3: small copper slivers can detach themselves during assembly, float around during soldering and inadvertently reconnect themselves anywhere on the PCB, potentially tying multiple nets together. the issues, builds the PCBs and ships back the finished prototypes without communicating what changes were made. Back in the lab, the design engineer tests the prototypes and finds that they work successfully. That's great; how- ever, unbeknownst to the design engineer, his prototypes are different from his PCB manufac- turing files. Now the design engineer releases the manufacturing files for high-volume pro- duction from a different manufacturer who spe- cializes in production PCBs. This manufacturer, for one reason or another, chooses not to run an analysis prior to manufacturing and there- fore doesn't detect the same issues as the pro- totype manufacturer. They build and ship the finished PCBs back to the customer. The boards are assembled and tested and, oddly enough, some, most, or all of the PCBs fail. Why? Because the design data still con- tained the manufacturing file's original DFM er- rors that were corrected in prototype, but never incorporated for production. The result was thousands of dollars in material being scrapped but—even worse and more costly—lost time-to- market. Had the design engineer had the ability to perform his own DFM analysis prior to proto- type, the same issues could have been detected and addressed in engineering and incorporated in the PCB design where they belong, lowering the cost, maintaining design intent, and ensur- ing that follow-on builds also work correctly. Just a few more minutes with the design in engineering would have prevented a whole de- sign and manufacturing iteration, and the costs associated with it. So what are DFM issues? Mostly these are issues in the PCB topology that create adverse effects in manufacturing and are typically not detected in the CAD software that creates the design. Table 1 provides a short list of typical DFM issues that pass detection in the CAD system but can result in PCB failures in the real world. This is just a short list of DFM is- sues. Good DFM tools will analyze for not only these issues, but also many more that most PCB design systems are not architecturally designed to detect.