The Best Methods to Perform a DFM Check

Back in my youth, I used to enjoy shooting the bow and arrow. I did not use the bow to hunt for food, which is probably a good thing, as I probably never would have eaten it, but I did a fair amount of target practicing. I found it both fun and challenging to attempt to mentally create the correct projectile arc of motion that would end at the bullseye. Outside of pure happenstance, this result is only obtained when the entire trajectory is optimized for that end.

When designing circuit boards, the objective is to build PCBs that accurately reflect the design intent and meet the highest quality. This is only achieved when the entire PCB development process is optimized for this end. The first step towards this realization is the generation of a design file package that comprehensively describes the board you want to build. Most importantly, it must incorporate design for manufacturing (DFM) specifications that fall within the capabilities of your contract manufacturer’s (CM’s) fabrication and assembly processes. Let’s review what these specifications are and how to perform a DFM check to ensure they are included in the design file package.

Review of Design for Manufacturing (DFM)

Employing good DFM as part of your design is essential to the manufacturability of boards. The benefits of DFM include faster turnaround times, higher quality boards, faster bring up, and lower costs. Although DFM is instituted during the design stage of PCB development, it should be done in collaboration with your CM and tailored to their equipment capabilities and manufacturing techniques. Doing so allows you to create the best PCB layout and is critical for optimizing the effectiveness of your PCB development. Specifications that should be included as part of your DFM include the following:

DFM Essentials

• Complete design file
• Minimum component to component clearances observed
• Minimum trace clearances and spacing observed
• No broken nets or disconnected traces
• No landless pads
• Adequate solder mask clearances
• Trace widths and copper weights within tolerance
• No non-plated through holes (NPTHs) to copper
• No edge clearance violations
• Copper balance
• Controlled impedance consistency
• Readable silkscreen
• No overlapping of silkscreen on non-solder mask areas
• No missing solder dams
• No component to pad mismatches
• Adequate solder mask relief
• Present and legible polarity and reference indicators
• Adequate support for flex circuits
• Adequate thermal relief for SMDs and plated through holes (PTHs)

The absence or inadequacy of the above items may render your board non-manufacturable without redesign, which will extend turnaround time and may add additional cost. Furthermore, there are other considerations like avoiding moisture sensitive and thermal sensitive components, as these could cause problems during PCB assembly, or PCBA.

How to Perform DFM Checks

Now that we know what should be included as part of our DFM, the next question is how to ensure that our design incorporates these essentials and they meet our CM’s requirements? The first thing to remember is: DO NOT RELY ON DESIGN PROGRAM DEFAULTS. It is highly unlikely that your software’s default specifications will be totally correct and adequately organized to prioritize the correct specifications for your chosen CM. Instead, you should incorporate your CM’s DFM rules and guidelines that will ensure your board’s manufacturability. This can be accomplished by uploading design rule check (DRC) files, if available from your CM, or manually setting the DRC rules within your package so they align with your CM’s specifications. Care must be taken to organize rule priorities, as they would be instituted during manufacturing. Most PCB design software packages provide a great amount of latitude here, which should simplify this process.

Once your CM’s DFM rules are incorporated into your program, you should make liberal use of your DRC to verify that your design includes no violations. To maximize effectiveness and facilitate the correction of violations, it is suggested that DFM checks are performed as follows.

When to perform DFM checks

1. After stackup
2. After component placement
3. After trace routing
4. After labeling

Some software programs are capable of in-process DFM checking, which provides you with real-time violations or errors. If available, this is preferable to waiting until completing a process and having multiple errors to correct.