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14 The PCB Design Magazine • March 2015 So why move DFM into the PCB design flow? There are several reasons: cost of finished PCBs, maintaining design intent, and the po- tential for future design failure. It can cost a PCB manufacturer as much as 20% of the cost of the PCBs for CAM engineering, which is the processing and tooling of design data to pre- pare it for manufacture. This additional cost is built into the end price that users pay to have physical PCBs fabricated. So, theoretically, de- signs submitted without DFM defects are less expensive to manufacture than ones with DFM defects. One could deduce that it's better to pay a little more to have a manufacturer ensure the design can be built. However, this creates other issues that are not so desirable. To take a design that has DFM issues and make it comply with the manufacturing pro- cess, a CAM engineer may need to modify the design data. What this means is that the layout provided to manufacturing may not be 100% consistent with the finished PCB. Issues with electromagnetic interference, signal integrity, cross talk, etc., which are commonplace in to- day's high-tech electronics and are addressed in design engineering, may be unknowingly re- introduced into the design as it's reworked for manufacturing. There is also no guarantee that a CAM engineer will communicate the design changes back to engineering to be incorporated into the original PCB design database. So not only is the design layout different between engi- neering and manufacturing, but what happens when a second manufacturing build is required or the design is released to a different manufac- turer for volume production? Consider this real-life scenario: a design en- gineer designs a PCB, runs DRC analysis and determines that the design is correct. He cre- ates PCB manufacturing files and sends the files off to a manufacturer to have prototypes made. The manufacturing engineer runs his analysis on the PCB files to ensure the design can be fab- ricated and identifies defects in the design that could result in scrap or low yields. Wanting to deliver a good product, the manufacturer fixes UnDERSTAnDInG DFM AnD ITS ROLE In PCB LAyOUT continues feature Figure 2: Acid traps have the potential of trapping acid during the PCB etching process longer than intended and can eat away a connection, making the circuit defective.