What is DFM Statistical Analysis?

July 16, 2020 Cadence PCB Solutions

Key Takeaways

  • Why is DFM important?

  • What is DFM statistical analysis?

  • How can I implement DFM statistical analysis?

A cartoon graphic embodying analyzing process data and cost-effectiveness

Using statistical analysis to improve efficiency

Repetition breeds complacency. Although I hate to admit it, I am indeed guilty of falling into the complacency trap. One good example is putting a dish in the oven and thinking that I can estimate the elapsed time without the use of a clock. At times, I seem to do well with this technique, but at other times, I end up ordering a pizza. This just proves what any good engineer or scientist knows--without accurate data you likely have no idea what is going on.

The need to collect and analyze data in order to have a good understanding can be applied to virtually any endeavor, and certainly to circuit board production, whether PCB prototyping or high-volume. For example, without tracking and analyzing yield rates and board failure statistics you would be unaware of problems that exist within the board development process that may require redesign and/or manufacturing adjustments. 

For these statistics to be useful, you must be able to associate them with some aspects or steps of the design and/or manufacturing process. The most logical candidate is your contract manufacturer’s (CM) Design for Manufacturing (DFM) rules and guidelines for design constraints. Let’s explore this DFM statistical analysis by explicitly defining what it is and then looking at how to best utilize it to improve your process efficiency and maximize yield rates and board reliability. But first, let’s see why DFM is important and provides the most direct means of minimizing the creation of ineffective or unusable boards.  

DFM: Is It Really That Important?

In order to answer this question fully, we must be clear about what DFM is and the purpose that it serves for PCBA development. 

  • What is DFM?

DFM includes all design choices by the circuit board designer or engineer that are made such that the design specifications fall within the capabilities of the contract manufacturer (CM) that will build the board. These specifications can usually be classified as either Design for Fabrication (DFF) or Design for Assembly (DFA). 

  • How is DFM used?

DFM is interjected into the design stage of PCBA development as design rule check (DRC) constraints. These constraints are based upon rules and guidelines furnished by the CM. The designer then uses these constraints to inform the limitations or parameter ranges for design specifications. 

As emphasized above, your CM’s DFM rules should be utilized, as opposed to design package defaults or generic industry standards. Failure to follow the best DFM rules and guidelines may result in quote delays, long manufacturing turnaround times, or even designs that cannot be manufactured at all.

Is DFM important? Yes, it is really important as it affects design manufacturability. Now, let’s see how DFM can be used to analyze and improve manufacturing process performance.  

What Is DFM Statistical Analysis?

Design for Manufacturing (DFM) constraint management

Types of DFM constraints

As the figure above illustrates, DFM rules include constraints on board size, solder mask, annular ring size, copper weight, trace spacing, silkscreen elements, and other PCB attributes. Depending upon the CM you work with, and the PCBA design package, the list of constraints can be quite extensive. 

As demonstrated in the previous section, these constraints are determinants of whether your development and production processes yield usable and reliable boards. The overall effectiveness of a set of DFM rules can be measured. For example, the yield rate provides a ratio or percentage of usable boards to built boards. Once manufactured and deployed, there are many causes for electronic circuit failure; however, premature failure is almost always a result of design and/or manufacturing error, and both yield and failure rate are directly measurable. 

What if these rates can be linked to specific DFM rules? Then it would be possible to assess the effectiveness of the rules themselves and their impact on manufacturing and reliability. This is a form of DFM statistical analysis. Below is a more general definition:

  • What is DFM Statistical Analysis?

DFM statistical analysis is the usage of statistics or data measured and/or derived from a part of or the entire PCBA development process that is used to assess the impact of DFM utilization in order to quantify the significance of implementation. 

DFM, in this definition, may be an individual rule or a set of rules, and the statistics or data may be from a specific step, stage, or as previously mentioned, a metric that reflects the overall development process. Having this information is great! But how can it be used?

How to Use DFM Analysis to Improve Your PCBA Development

There are several ways in which DFM statistical analysis results can be put to work to improve your development process yield and/or premature failure rate, as listed below.

📈    Determine the cost/benefit of using the DFM rule or rules.

📈    Refine DFM specification(s).

📈    Identify area(s) for redesign.

📈    Evaluate your QC process.

📈    Compare CMs prior to engagement.

The level of effectiveness for all of these options begins with getting good DFM from your CM. One of the easiest and efficient ways to acquire DFM data is by having a design package, such as OrCAD, where you can request and receive these rules and guidelines from multiple CMs as shown in the following clip. 

By using Cadence’s PCB Design and Analysis package you have all the functionality for acquiring, integrating, and analyzing DFM in one tool. And with DesignTrue Technology, you can easily check DFM in real-time during design. 

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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