PCB DFM Guidelines and Why They Matter

January 5, 2021 Cadence PCB Solutions

PCB DFM Guidelines and Why They Matter

 A PCB

It is important for PCB designers to follow DFM guidelines to ensure a smooth manufacturing process.

To create printed circuit boards (PCBs), designers must do much more than input a template into a computer or 3D printer. The design process for PCBs is extensive, and a design must be checked carefully before a prototype is made. 

There are multiple types of PCB quality checks, from those that check for electrical viability to those that manage physical clearances of component parts. The design for manufacturing (DFM) process is an important one of those checks. 

PCB DFM guidelines ensure that the physical, electrical, and other properties of a proposed PCB can be manufactured effectively. A PCB design without any obvious physical or electrical errors may still be difficult or impossible to manufacture. 

PCB DFM guidelines also ensure that designers identify a potentially problematic situation before prototypes are put into production. Following these guidelines prevents designers from investing a lot of time into complex designs that cannot be built as expected.

Closeup of a PCB

Following DFM guidelines helps ensure designers identify problematic PCB designs before the production process begins.

What Do PCB DFM Guidelines Cover?

PCB DFM guidelines identify a variety of potential problems, including the following:

  1. Physical problems, such as component crowding, are comparatively simple to find and correct, but other problems can be more insidious. 

  2. Electrical problems can occur even when following DFM guidelines. Layouts that have correct current flow may be difficult to solder evenly, which could in turn lead to uneven heating when current flows through the design. 

  3. Design document problems can come from discrepancies within a bill of materials (BOM) or within schematics, leading to uncertainty over which components to use during manufacturing. 

  4. Manufacturing difficulties can come from layouts that are difficult to create without causing damage to components, such as overlapping mounting holes that could jam and break drill bits.

There are many problems that robust PCB DFM guidelines can catch, and many of them aren’t found in design software. One of these problems is the acid trap, which happens when acid used for etching a PCB collects in undesirable areas of a board through surface tension. These areas can be tiny and easy to miss with the naked eye, but they can cause electrical problems or weaken the structure of the board. 

Another problem that can be difficult to see is solder mask slivers. These happen when two soldered areas are too close together, causing the areas to connect via tiny solder threads. These unexpected connections can cause solder to flake off or cause regions of a PCB to short out. A similar problem can also happen with unconnected slivers of copper after the etching process.

How Should Designers Use PCB DFM Guidelines?

Despite how important PCB DFM guidelines are, there are no generic guidelines that apply to every single type of PCB design. This means that designers must often use their best judgment concerning which guidelines to follow. 

Most of the time, however, designers don’t have to guess, since there are some guidelines that work well for most PCB types. For example, a designer can be confident that a PCB with copper traces should be checked for disconnected traces and trace violations, regardless of that PCB’s application. On the other hand, high-speed digital PCBs have DFM guidelines that don’t apply to other PCB types. If a PCB manufacturer recommends following certain DFM guidelines, designers should be ready to perform those checks.

Contract manufacturers (CMs) often have rules they would prefer designers to follow when building PCBs. Although some designers consider these rules to be restrictive, CMs include them to maintain a high standard of quality. Many DFMs enforced in this way result in a lot of time reworking a board, but even multiple reworkings usually take less time than fixing a board after the fabrication process starts. 

It is possible to identify many DFM problems in real-time so designers don’t have to wait before fixing problems as they come up. Designers should remember, however, that even the best DFM guidelines from CMs may miss something. Therefore, if designers want to follow other DFM rules in addition to those the CM requires, they should not hesitate to do so.

How Do PCB DFM Guidelines Work with DRCs?

Design rule checks (DRCs) identify electrical problems in a PCB before fabrication. They are a good starting point for checking the quality of PCBs, but they can’t find every possible problem. PCB DFM guidelines are more extensive, as they analyze many more types of problems. 

DRC constraints can form the basis for PCB DFM guidelines, and DRC violations can signal DFM problems. DRC and DFM guidelines can be focused on different parts of the manufacturing process, designated as design for fabrication (DFF) and design for assembly (DFA). Ideally, checks should focus on fabrication, assembly, and quality.

There is some overlap between DRC and DFM guidelines concerning electrical functionality. This is not necessarily a bad thing, however, because small problems with an electrical flow can cause a cascade of problems through PCBs and the machines that contain them. 

PCB DFM guidelines can save designers hours of headaches by helping them catch design problems that might otherwise be missed. The PCB Design and Analysis overview page at Cadence Design Systems provides tools ideal for analyzing PCB designs, including the Allegro PCB Editor

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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