Understanding DFM and its Role in PCB Layout

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DFM, DRC, DFF, DFA, DFwhat? These are all terms used daily in the PCB design world regarding manufacturing analysis, and they are often used interchangeably. But what exactly is DFM and why is it such an important, but often ignored aspect of the PCB design process?

Let’s start by clarifying some terms. DFM is short for “design for manufacturability.” It is the process of arranging a PCB layout topology to mitigate problems that could be encountered during the PCB fabrication and assembly processes required to manufacture an electronic system. Addressing fabrication issues is what’s known as design for fabrication (DFF), and addressing assembly issues during design is known as design for assembly (DFA). The two together mostly make up DFM analysis—mostly.

In many cases, the term DRC, which stands for design rule checking, is also used interchangeably with DFM and creates further confusion. That’s understandable, because DRC issues detected in manufacturing can indeed have a direct impact on the manufacturability of a PCB. However, DRC is markedly different from DFF and DFA. Think of DRC as a hard pass/fail detection of a problem in a PCB. Either a problem exists or it doesn’t. In engineering, DRC is used to ensure that PCB layout connectivity accurately reflects the connectivity defined in a board’s associated schematic diagram. But connectivity is only one aspect of DRC. The “R” stands for rules. The rules are used largely to define the minimum spacing allowed between various PCB objects for the entire PCB or for individual layers, nets or areas on the PCB. In engineering, the spacing may have direct impact on circuit performance. In manufacturing, spacing may play a pivotal role in the ability to fabricate or assemble a PCB. As a result, DRC becomes a subset of DFM, but only if the rules used reflect a manufacturer’s requirements for spacing. Otherwise, DRC is used solely for electrical verification.

DFM’s two primary components, DFF and DFA, are more nuanced than DRC. While DRC detects very specific discrepancies from the intended interconnect, DFM identifies issues in the PCB topology that have the potential to create manufacturing problems. What’s more, a DRC defect will be present in every copy of the PCB built, so if there is a short missed in DRC, every PCB will contain the short, no matter how many PCBs are produced. By contrast, if the same PCB quantities contain DFM issues, problems may only manifest in some of the PCBs while others perform correctly as expected.

For example, a PCB layout containing very thin pieces of copper created in the design tool by rule would be correct per the schematic. And if spaced properly it would pass DRC. However, that same sliver, being so thin, could potentially detach on the physical PCB and inadvertently connect itself to other copper elements during assembly, thus creating shorts on some PCBs but not on others. So, the sliver would pass DRC verification, but in real-world manufacturing the sliver could cause some PCBs to fail. Without DFM, this problem would go on undetected and would result in scrap or rework. 

This article originally appeared in the March 2015 issue of The PCB Design Magazine. To read this article in its entirety, click here.


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