Why are Advanced DFM (Design for Manufacturability) Checks Necessary?

Most PCB layout software will display your PCB design perfectly on the screen, and pass it thru their Design Rules Check (DRC) with 'flying colors'.  At this point you should be confident enough to generate Gerber/NC files and send them out for fabrication... Right?

Unfortunately Wrong... There are two common mistakes which may cause your boards to fail and become very costly to your wallet and reputation:

1. Don't Automatically Trust the Gerber files generated by your PCB Layout Software.

PCB layout software has always had a history of not being able to output "100% reliable" Gerber files (plane layers especially). 

Why is there a problem outputting reliable Gerber files? The Gerber Format is just unintelligent X,Y coordinate data with apertures (that's right apertures which were used in early plotters back in the 70's).  To output Gerber files, your PCB layout software must dismantle and break apart its' own intelligent design database - down to simple X,Y coordinates and apertures. Generating Gerber files for plane layers (power/ground) and mixed technology are the most prone to hidden problems and mistakes.

What about ODB++ and IPC-2581, will these new formats eliminate the problem?   Unfortunately no, the same problem still exists when outputting to other manufacturing formats such as ODB++, IPC-2581, etc.  These formats are more advanced but still built upon a foundation which uses X,Y coordinates with shapes (i.e Apertures).  The same mistakes that appear in Gerber commonly appear in these newer formats.

 Important: It really doesn't matter what your PCB layout software displays on the screen or the results from their DRC checks... it's the outputted gerber files which determine the final quality of your boards.   Here's a more detailed article about these potential pitfalls: Don't blindly trust your Gerbers
  

2. Lack of true Manufacturing Checks (DFM).

A PCB layout software's primary focus is to check for design integrity and not necessarily manufacturing integrity.

Manufacturing specific issues are much more difficult to detect and can quietly cause your boards to fail or have poor performance. Common manufacturing checks (DFM) include: Acid traps, Copper Slivers, Antennas, Solder Bridging, Starved Thermal, Ground Plane Check, Thermal Conflicts and much more.
 An Independent Third Party DFM Check is Essential: Avoid using DFM software and PCB Layout software created by the same company.  Chances are if their PCB Layout software missed a manufacturing problem, those same mistakes will get overlooked or unnoticed in their DFM software.  An independent third party DFM solution is always recommended for the most reliable results.


Advanced DFM Check Sections:  

  1. Signal

  2. Plane

  3. Silkscreen

  4. Solder Mask

  5. Drill

  6. Assembly

  7. SMT Stencils

  8. Netlist

  9. Test Points

  

Advanced DFM Check Severity Level:

 Critical:  The PCB will fail and/or have very low manufacturing yields.
 Elevated Risk: May result in poor PCB performance.
 Common:   Cause delays and/or additional problems during PCB fabrication.


 Signal DFM Checks

 Acid Traps

Due to the surface tension during the etching process, larger deposits of acid may get trapped into certain areas - resulting in over-etching. 

Critical:  Over-etching will directly effect your board yields and performance.

DFM Check - Acid Traps

 Find Net Discrepancy

Search for overlapping copper objects which have different nets defined. This net discrepancy typically occurs when the PCB layout software has incorrectly outputted intelligent formats such as ODB++ and IPC-2581.

Critical:  Indicates a potential short originating from the PCB design.  May result in poor board performance, failure, or other undesired effects.

DFM Check - Net Discrepancy

 Copper Slivers

Narrow areas in the copper where the potential for over-etching is great. This typically happens when trace angles are 30 degrees (or less), when composites are used, and on plane layers. 

Critical:  A board can fail if certain copper area are over-etched.  Resulting in shorts and decrease in board yield.

 Antennas / Dangling Traces

Search for Non-terminating traces which may behave as an antenna.  Such traces may be intentional, but a warning is reported as a precaution.

Elevated Risk:  May result in poor board performance, short, or other undesired effect.


 Minimum Clearance: Outer Layers

May cause under-etching on copper areas and/or exceed PCB fabricators capabilities. Resulting in possible opens or shorts.

Critical:  May result in poor board performance, short, or other undesired effect.

DFM Check - Outer Layers

 Minimum Clearance: Inner Layers

May cause under-etching on copper areas and/or exceed PCB fabricators capabilities. Resulting in possible opens or shorts.

Critical:  May result in poor board performance, short, or other undesired effect.

DFM Check - Minimum Clearance Inner Layers

 Minimum Annular Ring: Drill-Pad

Search for all plated drills and ensure they have a minimum copper annular ring. Having a minimum copper annular ring around each drilled hole is required to assure it will be properly fabricated.  

Critical:  In many cases, drilled holes are not perfectly registered within the exact center of each copper pad.  Not having enough of a copper annular ring, may make plating on vias, as well as solder-ability on component holes more difficult. In extreme cases, may result in poor board performance, failure, or other undesired effects.

DFM Check - Annular Ring Drill to Pad

 Minimum Clearance: Track-Track

Locate tracks (with different nets) on the same signal layer that are placed too close to each other.

Critical:  May cause under-etching on copper areas and/or exceed PCB fabricators capabilities. Resulting in poor board performance, short, or other undesired effects.

DFM Check - Minimum Clearance Track to Track

 Minimum Clearance: Track-Pad

Locate tracks and pads (with different nets) on the same signal layer that are placed too close to each other. 

Critical:  May cause under-etching on copper areas and/or exceed PCB fabricators capabilities. Resulting in possible opens, shorts, or other undesired effects.

DFM Check - Track to Pad

 Minimum Clearance: Pad-Pad

A copper pad on the signal layer is placed too close to other copper pads (with a different net).

Critical:  May cause under-etching on copper areas and/or exceed PCB fabricators capabilities. Resulting in possible opens, shorts, or other undesired effects.

DFM Check - Pad to Pad

 Non-Functional Internal Pads

Essentially these are unconnected internal pads typically found on plane layers.   

Elevated Risk:  May break loose and cause potential drilling problems, risk of shorts, and more.

DFM Check - Non Functional Internal Pad

 Minimum Pad Size

A very small diameter pad placed on a signal layer.  This typically occurs when incorrect pad stacks are defined within the PCB layout software, or an error has occurred when PCB layout software has generated the Gerber data.

Elevated Risk:  May cause etching or soldering problems during fabrication. Smaller pads also provide less mechanical support for component leads.

DFM Check - Minimum Pad

 Clearance from Board Edge

Objects are placed too close or outside the PCB border.

Elevated Risk: May result in exposed copper at the board edge which could lead to corrosion and long term reliability problems.

DFM Check - Signal Clearance from Board Edge

 Minimum Width: SMT Pad

Locate SMT pads on top/bottom signal layers that have a smaller width than permitted.

Elevated Risk:  May cause under-etching and/or uneven pad widths. May also result in poor soldering performance & misalignment during assembly.

DFM Check - Minimum SMT Pad Width

 Minimum Clearance: Rout-Copper

Search for Rout paths that are too close to signal objects.

Elevated Risk:  During NC routing process, copper may be damaged or result in other undesired effects.

 Minimum Clearance: Same Net

Locate objects on the same net and signal layer that are too close to each other.

Elevated Risk:  May cause under - etching on copper areas and / or exceed PCB fabricators capabilities.Resulting in possible opens or shorts.

DFM Check - Same Net Clearance

 Hole Registration

Find drill holes that do no align with copper pads, due to potential rounding errors found in the drill data, or other causes. 

Elevated Risk:  This may make plating on vias, as well as solder-ability on component holes more difficult. In extreme cases, may result in poor board performance, failure, or other undesired effects.

 Redundant Pads

Duplicate pads are commonly included in the manufacturing data generated from PCB layout software.

Common:  This duplication may result in bloated file sizes, potential issues during DFM verification, NC drilling, and your wallet.  That's correct your wallet.  Many PCB fabricators use pad counts and drill counts as a factor when quoting prices to manufacture a PCB.  Having excessive pad and/or drill counts could lead to increased manufacturing costs.

 Minimum Width: Track

Locate tracks which have a smaller width than the allocated distance.

Common:  May cause etching problems during fabrication. The smaller the trace width, the lower the yield for the PCB fabricator and the more costly your boards may be.

DFM Check - Minimum Trace Width

  Plane DFM Checks

 Power/Ground Short

Determine any potential shorts between Power and Ground planes.  

Critical: A board will fail if there is a short between the power and ground planes.

DFM Check - Power Ground Short

 Starved Thermal

Check whether a thermal connection to the copper plane has been constricted by overlapping or adjacent data that is way too close.

Elevated Risk:  May result in poor board performance, short, or other undesired effect.

 Neg. Plane Thermal Conflicts

Determine if any via or plated hole is connected to more than one negative plane together.

Elevated Risk:  May result in poor board performance.

 Isolated Connections

One of the most common problems is to determine whether your PCB CAD system has properly outputted good plane connections to the copper plane.   

Critical:  A board will fail if any plane connections is not properly connect the copper plane.

 Drill Registration

Locate any plated drill holes that do not center over an existing pad. Most likely the Drill layer has either been generated or imported incorrectly. 

Elevated Risk:  This will lead to problems during PCB fabrication. 

 Minimum Annular Ring: Drill-Plane

Search for all plated drills on plane (GND/VDD) layers and ensure they have a minimum copper annular ring. Having a minimum copper annular ring around each drilled hole is required to assure it will be properly fabricated. 

Critical:  In many cases, drilled holes are not perfectly registered within the exact center of each pad.  Not having enough of an annular ring, may result in poor board performance, failure, or other undesired effects.

 Minimum Clearance: Plane

May cause under-etching on copper areas and/or exceed PCB fabricators capabilities. Resulting in possible opens or shorts.

Critical:  May result in poor board performance, short, or other undesired effect.

 Minimum Width: Plane

Locate metal areas on the plane which have a smaller width than the allocated distance.

This typically occurs when PCB layout software has incorrectly generated complex polygons with holes (i.e. dark/clear areas) for the plane layers in the Gerber data.

Elevated Risk:  May result in poor board performance, short, or other undesired effect.

 Minimum Thermal Air gap

 

 Minimum Thermal Spoke Width

 

 Minimum Clearance: Plane to Drill

Search for all plated drills and ensure they have a minimum clearance from the plane. Having a minimum spacing around each drilled hole is required to assure there will be no shorts or other undesired affects.  

Elevated Risk:  In many cases, drilled holes are not perfectly registered within the exact center of each clearance pad.  Not having enough of a clearance, may result in poor board performance, failure, or other undesired effects.

DFM Check - Minimum Clearance: Plane to Drill

 Minimum Clearance: Plane to Rout

Search for Rout paths that are too close to plane objects.

Elevated Risk:  During NC routing process, copper may be damaged or result in other undesired effects.

 

  Silkscreen DFM Checks

 Minimum Clearance: Silk Screen to Board Edge

The minimum clearance allowed from Silkscreen to board edge.  Any silkscreen objects outside the board edge will also be flagged.

Elevated Risk:  This will lead to problems during PCB fabrication. 

DFM Check - Silkscreen to boarder edge

 Minimum Width: Silkscreen Line

Check for the minimum width for each object on the silkscreen. 

Common:  If the line width is too thin it will not be legible on the fabricated PCB.

DFM Check - Minimum Silkscreen Line width

 Minimum Silkscreen Text Height

Determine whether the Text height is large enough on the silkscreen layers.

Common:  If the text height is too small it will not be legible on the fabricated PCB.

DFM Check - Minimum Silkscreen Line width

 Silkscreen over Solder Mask

Locate if there are any areas where the silkscreen overlaps with openings in the solder mask layers. 

Elevated Risk:  May cause contamination of the pads and make electrical testing plus SMT soldering more difficult.

 Minimum Clearance: Silk Screen to Copper

Verify the minimum clearance between silkscreen objects and copper objects (performed on top and bottom signal layers only).

Elevated Risk:  May cause contamination of the pads and lead to problems during PCB fabrication.

 Minimum Clearance: Silk Screen to Drill

Verify the minimum clearance between silkscreen objects and drilled holes (both plated and unplated).

Common:  May cause problems during PCB fabrication.

 Minimum Clearance: Silk Screen to Rout

Verify the minimum clearance between silkscreen objects and routed paths.

Common:  May cause problems during PCB fabrication.

 Minimum Clearance: Silk Screen (Line to Line)

 Verify the minimum clearance between silkscreen line objects.

Common:  May cause problems during PCB fabrication.

 Minimum Clearance: Silk Screen (Line to Pad)

Verify the minimum clearance between silkscreen line and pad objects.

Common:  May cause problems during PCB fabrication.

 Minimum Clearance: Silk Screen (Pad to Pad)

Verify the minimum clearance between silkscreen pad objects.

Common:  May cause problems during PCB fabrication.

 

  Solder Mask DFM Checks

 Mask Slivers

Areas in the solder mask where the resist is so narrow that it may cause small pieces of the resist to flake off and present soldering problems later.  

Critical:  A board can fail if the resist lifts in an area that needs to be soldered later on.

DFM Check - Mask Slivers

 Solder Bridges

Typically when the mask layers, contain openings which are too large, and may expose other adjacent conductive areas. 

Critical: During fabrication, copper may get too close and form a bridge to other adjacent conductive areas.

 Undersized Mask Clearances

 

 Missing Mask Clearances

 

DFM Check - Missing Mask Clearance

 Minimum Clearance: Solder Mask to Rout

 

 Minimum Clearance: Mask (Draw to Draw)

 

 Minimum Clearance: Mask (Draw to Pad)

 

 Minimum Clearance: Mask (Pad to Pad)

 Partial Clearances

 

 

  Drill DFM Checks

 Drill-Plane Shorts

 

 Minimum Drill Size

 

 Double Drill Hits

Duplicate drill hits are commonly included in the manufacturing data generated from PCB layout software.

Common:  This duplication may result in potential issues during DFM verification, blowouts during NC drilling, and your wallet.  That's correct your wallet.  Many PCB fabricators use drill counts as a factor when quoting prices to manufacture a PCB.  Having excessive drill counts could lead to increased manufacturing costs.

 Pads without Drills

 

 Plated Drill without Pads

Search for all plated drills and ensure they have a minimum copper annular ring on all required metal layers. Having a minimum copper annular ring around each drilled hole is required to assure it will be properly fabricated.  

Elevated Risk:  In many cases, drilled holes are not perfectly registered within the exact center of each copper pad.  Not having enough of a copper annular ring, may make plating on vias, as well as solder-ability on component holes more difficult. 

 Touching Drill Hits

Occurs when drilled holes are placed too close to other drilled holes on the circuit board.

Common:  When trying to drill holes that are too close together, it may cause the drill bits to break during the drilling process. This effect is also known as blowouts.

DFM Check - Drill hit touching

 Minimum Clearance: Plated Drills to Copper

 

 Minimum Clearance: Unplated Drills to Copper

 

 Minimum Clearance: Drills to Rout

 

 Minimum Clearance: Rout to Rout

 

 Minimum Drill Hole Spacing

 

 

  Assembly DFM Checks

 Minimum Clearance: Component

Verify spacing between components is greater than the minimum permitted value.  The component outline is determined by either (1) the extent of its pins and any geometric objects drawn in silkscreen, or (2)  The extents provided when importing an intelligent ODB++ or IPC-2581 file.

Critical: Components that are placed too close together may prevent the board from properly being assembled.  

DFM Check - Minimum Clearance between components

 Maximum Component Height

Check each component height and determine if it exceeds the maximum permitted value.

Elevated Risk: If a component exceeds the maximum permitted height it may prevent a  fully assembled PCB from properly fitting into a designed enclosure.

DFM Check - Maximum Component Height

 Component Overlaps

Verify components do not intersect.  The component outline is determined by either (1) the extent of its pins and any geometric objects drawn in silkscreen, or (2)  The extents provided when importing an intelligent ODB++ or IPC-2581 file.

Critical: Components that overlap will prevent the board from properly being assembled.  

 Invalid RefDes

 

 Invalid Component Rotation

 

 Footprint to Pad Registration

 

 

  SMT Stencil DFM Checks

 Minimum Width: SMT Stencil Pad

Locate SMT pads on top/bottom paste layers that have a smaller width than permitted.

Elevated Risk: May cause under-etching and/or uneven pad widths during SMT stencil fabrication. May also result in poor soldering performance & misalignment during assembly.

 Minimum Aspect Ratio: SMT Stencil Pad

Locate SMT pads on top/bottom paste layers that have a smaller Aspect Ratio than permitted.  The Aspect ratio equals the paste pad width divided by the stencil foil thickness.

Elevated Risk: May cause under-etching and/or uneven pad widths during SMT stencil fabrication. May also result in poor soldering performance & misalignment during assembly.

 Minimum Area Ratio: SMT Stencil Pad

Locate SMT pads on top/bottom paste layers that have a smaller Area Ratio than permitted.  The Area ratio equals the paste pad area divided by the paste pad height and stencil foil thickness.

Elevated Risk: May cause under-etching and/or uneven pad widths during SMT stencil fabrication. May also result in poor soldering performance & misalignment during assembly.

 Minimum Volume: SMT Stencil Pad

Locate SMT pads on top/bottom paste layers that have a smaller paste volume than permitted.  The paste volume equals the paste pad area and stencil foil thickness.

Elevated Risk: Excessive paste volume may result in poor soldering performance & bridging during assembly.

 SMT Stencil Pad to Pad Registration

 Locate SMT pads on top/bottom paste layers and verify the align with pads located on top/bottom metal layers. 

Elevated Risk: A SMT stencil pad misalignment (or registration issue) will cause problems during SMT stencil fabrication or pcb assembly.

 

  Netlist DFM Checks

 Open Net Detected

Check all test points and verify the same net is found at each defined location.  A violation is highlighted if the extracted CAM net (which is generated from the manufacturing data) differs from the external net defined in the IPC Netlist file.  

Critical: There is a very good chance your manufacturing data (Gerber, NC, ODB++, etc.) differs from your original PCB Design. A serious error has occurred either in your original PCB design or while exporting to your manufacturing data.  A thorough review of your design in the highlighted areas is recommended.  We also recommend you verify the export settings used while generating your manufacturing data.

 Shorted Net Detected

Check all test points and verify the same net is found at each defined location.  A violation is highlighted if the extracted CAM net (which is generated from the manufacturing data) connects with two or more different external nets defined in the IPC Netlist file.  Essentially the same CAM net is found at two (or more) different external net locations.

Critical: There is a very good chance your manufacturing data (Gerber, NC, ODB++, etc.) differs from your original PCB Design. A serious error has occurred either in your original PCB design or while exporting to your manufacturing data.  A thorough review of your design in the highlighted areas is recommended.  We also recommend you verify the export settings used while generating your manufacturing data.

 Missing Copper

Search each external net location (defined from the IPC Netlist file), and determine if an extracted CAM net (which is generated from the manufacturing data) exists.

Elevated Risk:  Essentially an external net location was defined, but there was no CAM net found.  This may be non-critical (caused by a single point net or CAM object without a net).

 External Net Missing

An extracted CAM net (which is generated from the manufacturing data) exists where no external net was defined (from the IPC Netlist file).

Elevated Risk:  Essentially a CAM net was detected without an external net defined.  In many cases, this may not be considered a critical error.

 

  Test Point DFM Checks

 Minimum Test Point Diameter

 

 Minimum Distance: Test Point Center to Center

 

 Minimum Clearance: Test Point to Board Edge

 

 Minimum Clearance: Test Point Pad to Pad

 

 Minimum Clearance: Test Point to Mask

 

 Minimum Clearance: Test Point to Rout

 

 Find Test Point without Net