Common DFM Mistakes to Avoid in Product Design

SAI’s TransferAssist team works with customers to ensure their projects are ready for outsourcing. The process includes a review of fabrication and Gerber files. Afterwards, the team makes recommendations for improving the design and/or documentation to better accommodate the requirements of a volume manufacturing process optimized for increased throughput with minimal handling.

In the design for manufacturability (DFM) realm, seven common mistakes stand out:

• Panelization issues: this is an area where working with your contract manufacturer can be critical, as many contract manufacturers have specific guidelines on preferred printed circuit board (PCB) sizing and orientation for fast setups and maximum throughput. Additionally, the panelization strategy should be reviewed to determine if it is the most efficient layout in terms of minimizing wasted FR4.
• Connector placement: Edge mounts must be kept on PCBs through processing. Depaneling tools cannot be used if a right angle connector is placed on a side of the PCB containing an edge mount. The only option for that type of layout is to depanel using manual force, which can negatively impact solder joint integrity. Consequently, it is important to consider panelization strategy in PCB layout when determining connector placement.
• Documentation: Gerber files are often transferred as single-up designs instead of as a panelized design. This adds additional work during project transfer.
• Stencil paste file errors: Optimum paste deposition often requires an aperture design that varies from the component footprint. Many Gerber stencil paste files are prepared 1:1, which doesn’t account for the impact of component mass in paste deposited via the screen aperture. Component manufacturers list recommended specifications and many contract manufacturers have an aperture design preference for specific types of components. For example, SAI has found that LED diodes are best placed when a triangular shaped solder paste aperture is used. Its team reviews the parts with critical paste deposition requirements prior to ordering the screen.

• Fiducials: Fiducials are marks placed on the PCB to enable machine vision systems to understand the position of the component placement head relative to the PCB. Most PCB layout teams are good at adding fiducials to the PCB layout. However, some FR4 masking colors can make fiducials hard to read or invisible because the color of the mask is the same or lighter contrast than the fiducials. Machine algorithms can be adjusted to change fiducial brightness in most cases. Another option is to avoid masking over the fiducial. The most difficult color to compensate for is yellow. In this case, the “no mask” area should have a wide clearance for fiducials.
• PCB finish mismatch: In some cases, a designer will specify a HASL finish for use with lead-free solder. HASL finishes do not have a flat coplanarity and in a lead-free reflow process can create coplanarity issues. An ENIG finish is generally preferred for use with lead-free solder. The team at SAI most often sees this issue when working with consigned materials.
• Failure to utilize recommended pad layout size: Misalignment and other solder joint integrity issues can occur if the correct pad layout size for the specified component package is not used.