Importance of PCB DFM Checks — Steps Involved

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Key Steps in Designing Printed Circuit Board Layouts
July 12, 2019

Importance of PCB DFM Checks — Steps Involved

PCB

PCB assembly at Rush PCB involves several steps and processes, as we want to make sure in advance that our customers’ boards will not have any issues with manufacturing. Therefore, Rush PCB employs Design for Manufacturing or DFM checks to ensure we manufacture all PCB assemblies properly.

PCB DFM checks are totally cost-effective, as the system in use at Rush PCB is entirely automated for both DFM and DFA. Our system rapidly scans the manufacturing issues that may prohibit the PCB manufacturing process. Therefore, the application of this DFM/DFA check is beneficial to our customers, as it saves the PCB lead time while lowering the PCB cost.

It can become very expensive for the customer when they find errors in the fabrication or assembly process during the final stages of delivering the product to market. Anything may have gone wrong—PCB layout, material used, difference between prototype and original manufacturing files, and so on. By going through DFM checks it is possible to solve these problems before the Contract Manufacturer or CM starts fabricating and assembling their PCB.

Who Handles DFM?

Often, OEM design teams tend to look at DFM as something their CM should handle. In fact, most CMs perform DFM analysis prior to taking up production for identifying and fixing issues. This is a vulnerable step if the manufacturer does not share the changes they make with the design team and make the changes without a proper understanding of the requirements of the design and circuit performance. Any new revision by the design team will result in total chaos and probable failure of the PCB and finished devices.

Rush PCB recommends the design team performs their own DFM analysis prior to the prototyping stage to detect and weed out the issues, allowing them to incorporate the changes in the PCB design. This ensures the OEM lowers the cost by maintaining their design intent, and ensure the follow-on builds also work properly.

Involving the CM in the early stages of design works to the advantage of the OEMs. The CM, when reviewing the design, will match the parts on the board with their master component vendor inventory database, offering real-time recommendations on the availability of parts.

Design engineers at the CM will also review the design for its functional performance. They make recommendations for material the board should use and changes necessary at layer stackups for impedance control, suggesting routing changes for improvements. They may also suggest changes in component placement and routing for improving signal integrity.

Working closely at the early stages of the design is a win-win situation for both the OEM and the CM, resulting in smooth operation for the production of a successful and working PCB assembly, ready for market deployment.

Implementing DFM Checking Systems

It is easy to set up DFM checking systems as software packages are readily available. These packages work along with the CAD system for PCB design including schematic drawing and PCB track layout. The DFM system can detect errors that normally remain undetected during the reworking of the design. CAD systems often overlook issues such as acid traps, starved thermals, insufficient annular rings, slivers, and so on, but which can be disastrous during manufacturing. DFM software is usually equipped with fabrication analysis for detecting such issues.

Most DFM checking packages include easy-to-use PCB manufacturing analysis technology for identifying specific design issues that could be detrimental to PCB fabrication. DFM checking software, therefore, helps to reduce scrap, improve yield, while preventing setbacks from expensive time to market issues.

Major Issues Covered in DFM Checking

Major issues that DFM checking systems detect during design are:

  • Formation of Acid Traps
  • Possibility of sliver and island formation
  • Formation of solder bridges between pads
  • Paste mask openings for heat sinking
  • No solder connection or cold solder joints
  • Non-inclusion of test points
  • Proximity of copper to board edge
  • Optimization of drill size
  • Incorrect pad sizes
  • Component spacing
  • Component location and rotation

Formation of Acid Traps

Acute or odd angles of copper features on the PCB can form acid traps. These are areas where acid can remain trapped during the PCB etching process, and sometimes even cleaning chemicals are unable to clean them completely. The residual acid gradually erodes the nearby copper features, creating a smaller width of traces than intended, or even creating a discontinuity in the trace.

PCB designs with 4-5 mil traces are quite common today. Trapped acid can easily erode and open these thin traces. Designers can avoid such acid traps by not placing incoming traces at acute or odd angles to pads. Rush PCB advises keeping the angles of traces at 45 or 90 degrees to the pad, and the DFM software can flag non-conformances.

Possibility of Sliver and Island Formation

While designing, many plane layers can have slivers and islands or free-floating copper, which can create serious problems during etching. These freely floating copper specks can cause several issues as they find their way to other portions of the panel creating a short between closely spaced traces. Slivers may also cause noise and other interference as it is floating copper and may behave as an antenna does.

The only way to make sure there are no slivers or islands created is to check manually or let the DFM software detect them.

Also Read:  How to Detect Circuit Board Faults?

Formation of Solder Bridges Between Pads

With designers using fine-pitch components more commonly than before, it is essential they also include solder mask in between adjacent pads. Excluding this essential solder mask can allow excess solder on pads to join during reflow, creating a solder bridge and an unwanted electrical short.

To get around this issue, designers need to check the alignment and spacing of solder mask from pads to neighboring shapes. They should also consult their CM for the minimum webbing space and alignment they allow in a design. The DFM system software can easily flag issues such as non-existing solder mask between adjacent pads, and if solder mask covers a pad.

Paste Mask Openings for Heat Sinking

A heat sink is essential for absorbing and dissipating heat from an electronic component. The thermal contact is usually through a metal base that attaches to a large copper surface on the PCB. For proper heat transfer, the electronic component is also soldered to the copper surface. However, a large opening in the paste mask allows deposition of a substantial amount of solder paste, allowing the component to float off the pads during reflow.

To prevent the above, designers must limit the amount of solder paste the paste mask can deposit on the copper pads. Rather than have a single large paste mask opening, the designer can break it up into several smaller openings, thereby preventing the component from floating away during the reflow process.

Rush PCB suggests designers should consult their CM for the proper size of the paste mask opening. Essentially, DFM systems check for proper paste mask openings.

No Solder Connection or Cold-Solder Joints

Vias placed within pads can create an issue, as the via can allow molten solder to flow down the opening and leave the pad with too little solder to form a strong joint. This forms a no solder connection or a cold-solder joint. The DFM software checks if the percentage of the via within the pad is within permissible limits or else flags it for plugging.

Non-Inclusion of Test Points

Inclusion of test points in the PCB during design allows testing the board after assembly and in the field. This allows an easy way to tell whether the board functions as intended. DFM systems allow checking test points for clearance from components, their pad size, placement along a grid, and so on, to allow a fixture to locate them easily.

Designers must Include test points in the PCB during the initial layout phase of the design. Adding test points later on could lead to creating noise and crosstalk in sensitive circuits. The primary requirement of test points is they are easily accessible and not hidden below components. Their spacing must also be adequately apart to allow test pins to access them.

Proximity of Copper to Board Edge

Manufacturers prefer making boards in panels, as it is cost effective to make multiple boards simultaneously. They separate the individual boards from the panel by various means such as shearing, breaking along a V-cut and other methods. Essentially, a minimum gap must exist between the edge and the copper surfaces on the board, to prevent the copper from damage when the fabricator separates the boards.

Another reason exists for maintaining a proper distance between the copper on the board and the board’s edge. Motorized transport must grip the boards properly during the various processes. For instance, the etching process requires application of electric current to the panel, and presence of copper too close to the edge of the board can create shorts.

Designers must consult their CM for various equipment they use and the spacing they require. The DFM system checks for and flags the issue if it finds the spacing inadequate.

Optimization of Drill Size

Designers often use several drill sizes for their boards. Changing drills during board manufacturing not only wastes time, it increases the expense for board fabrication. The DFM software checks for drill sizes in the board and flags the designer to consolidate them to an optimum number.

Incorrect Pad Size

Sometimes the designer may overlook a wider trace leading to different sized pads for an electronic component. While reflow, the different size of pads can lead to uneven heating of the component, resulting in the chip component lifting up on one of its ends, a condition known as tombstoning. DFM systems check for and flag instances of incorrect or mismatched pad sizes.

Component Spacing

Pick-n-place machines need a minimum gap between components to enable proper placement. The designer must consult their CM to know the minimum their machines need. While placing components during layout, if the designer places components too close, the CM will have a problem during assembly, as the pick-n-place machine may not function optimally. DFM systems check for this spacing and flags if it finds the spacing inadequate.

Component Location and Rotation

Placing smaller components very close to large ones may cause soldering problems during reflow. The larger components absorb more heat leaving the smaller components not hot enough for proper soldering. DFM systems predict this shadow effect and warn designers.

Conclusion

It is not only frustrating but also expensive for the customer to detect failure when getting ready to enter the market. According to Rush PCB, one of the ways of avoiding such failures is by planning for the future by designing for manufacturing. The availability of several good DFM software packages makes it easy to implement such checks and prevent failures from cropping up. Although it is always possible to manually identify and resolve such issues, the DFM software systems make checking independent of the human.

Rush PCB UK
Rush PCB UK
Rush PCB Inc, UK is one of the leading PCB manufacturing, fabrication and assembly company in UK. Get your PCB design in Uk from the finest PCB manufacturer.